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UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions. |
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#1
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Another RCD puzzler
Hi everyone,
I've got a "Steeple" split load consumer unit installed in 1997 with 2 * ring mains on the 30ma 80A RCD side. On the non-RCD side there are a number of other circuits, including an outside socket, protected by a "Powerbreaker" 30ma integral RCD socket. We moved into the house in July. Ever since we moved in, we've been having nuisance RCD trips, seemingly following no pattern. Everything would be fine for a while, then all of a sudden the RCD would trip - usually when using the shower, or maybe switching the kettle on. Lately the problem seems to have been getting worse - it tripped during the night, a period of low activity, apart from the immersion heater which is on the non-RCD side anyway. I thought it might be mouse activity biting through the cable, but then... ....this morning I went to switch on the vacuum cleaner, a double insulated device, i.e. no path to earth. Immediately the RCD tripped. The happens on the upstairs and downstairs rings. As I couldn't see how there could possible be an earth fault in the vacuum cleaner, I plugged it into the outside socket protected by its own RCD but NOT protected by the RCD in the consumer unit. It worked fine and did not trip the outside socket's own RCD. Am I right to suspect the RCD itself in the CU rather than any sort of appliance or wiring fault? FYI the house was rewired in 1997 along with the CU replacement - I've had a lot of floorboards up following CH install and it all looks in good nick. I am contemplating chancing £30.00 on a new RCD and wanted to get the group's general opinion. Thanks Luke |
#2
Posted to uk.d-i-y
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Another RCD puzzler
Luke wrote:
Hi everyone, I've got a "Steeple" split load consumer unit installed in 1997 with 2 * ring mains on the 30ma 80A RCD side. On the non-RCD side there are a number of other circuits, including an outside socket, protected by a "Powerbreaker" 30ma integral RCD socket. We moved into the house in July. Ever since we moved in, we've been having nuisance RCD trips, seemingly following no pattern. Everything would be fine for a while, then all of a sudden the RCD would trip - usually when using the shower, or maybe switching the kettle on. Lately the problem seems to have been getting worse - it tripped during the night, a period of low activity, apart from the immersion heater which is on the non-RCD side anyway. I thought it might be mouse activity biting through the cable, but then... ...this morning I went to switch on the vacuum cleaner, a double insulated device, i.e. no path to earth. Immediately the RCD tripped. The happens on the upstairs and downstairs rings. As I couldn't see how there could possible be an earth fault in the vacuum cleaner, I plugged it into the outside socket protected by its own RCD but NOT protected by the RCD in the consumer unit. It worked fine and did not trip the outside socket's own RCD. Am I right to suspect the RCD itself in the CU rather than any sort of appliance or wiring fault? FYI the house was rewired in 1997 along with the CU replacement - I've had a lot of floorboards up following CH install and it all looks in good nick. I am contemplating chancing £30.00 on a new RCD and wanted to get the group's general opinion. Thanks Luke Yes, you should definitely suspect the RCD. Also, you may have an insulation fault that is pushing the RCD very close to its trip level. This means that the slightest imbalance or increase in humidity will set it off. The vacuum cleaner sets it off because of the inrush on starting (John Rumm might be along later to explain why). You can buy a Hager 63A/30mA RCD from TLC for £16.80 +VAT http://www.tlc-direct.co.uk/Products/HGCD263U.html Also, get your installation checked out at some point to see if you can find where the leakage current is coming from. |
#3
Posted to uk.d-i-y
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Another RCD puzzler
Dave Osborne wrote:
Luke wrote: Hi everyone, I've got a "Steeple" split load consumer unit installed in 1997 with 2 * ring mains on the 30ma 80A RCD side. On the non-RCD side there are a number of other circuits, including an outside socket, protected by a "Powerbreaker" 30ma integral RCD socket. We moved into the house in July. Ever since we moved in, we've been having nuisance RCD trips, seemingly following no pattern. Everything would be fine for a while, then all of a sudden the RCD would trip - usually when using the shower, or maybe switching the kettle on. Lately the problem seems to have been getting worse - it tripped during the night, a period of low activity, apart from the immersion heater which is on the non-RCD side anyway. I thought it might be mouse activity biting through the cable, but then... ...this morning I went to switch on the vacuum cleaner, a double insulated device, i.e. no path to earth. Immediately the RCD tripped. The happens on the upstairs and downstairs rings. As I couldn't see how there could possible be an earth fault in the vacuum cleaner, I plugged it into the outside socket protected by its own RCD but NOT protected by the RCD in the consumer unit. It worked fine and did not trip the outside socket's own RCD. Am I right to suspect the RCD itself in the CU rather than any sort of appliance or wiring fault? FYI the house was rewired in 1997 along with the CU replacement - I've had a lot of floorboards up following CH install and it all looks in good nick. I am contemplating chancing £30.00 on a new RCD and wanted to get the group's general opinion. Thanks Luke Yes, you should definitely suspect the RCD. Also, you may have an insulation fault that is pushing the RCD very close to its trip level. This means that the slightest imbalance or increase in humidity will set it off. The vacuum cleaner sets it off because of the inrush on starting (John Rumm might be along later to explain why). You can buy a Hager 63A/30mA RCD from TLC for £16.80 +VAT http://www.tlc-direct.co.uk/Products/HGCD263U.html ISTR Steeples are rebadged Volex sold by Denmans Also, get your installation checked out at some point to see if you can find where the leakage current is coming from. If the RCD is not at fault then this is almost certainly a N-E fault or possibly a L-E fault on the RCD side of the installation. I would suspect that a N-E fault is more likely than a faulty RCD. I would start by inspecting the back of all the sockets to check that a fixing screw is not touching a neutral cable. -- Adam |
#4
Posted to uk.d-i-y
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Another RCD puzzler
In article
s.com, Luke scribeth thus Hi everyone, I've got a "Steeple" split load consumer unit installed in 1997 with 2 * ring mains on the 30ma 80A RCD side. On the non-RCD side there are a number of other circuits, including an outside socket, protected by a "Powerbreaker" 30ma integral RCD socket. We moved into the house in July. Ever since we moved in, we've been having nuisance RCD trips, seemingly following no pattern. Everything would be fine for a while, then all of a sudden the RCD would trip - usually when using the shower, or maybe switching the kettle on. Lately the problem seems to have been getting worse - it tripped during the night, a period of low activity, apart from the immersion heater which is on the non-RCD side anyway. I thought it might be mouse activity biting through the cable, but then... ...this morning I went to switch on the vacuum cleaner, a double insulated device, i.e. no path to earth. Immediately the RCD tripped. The happens on the upstairs and downstairs rings. As I couldn't see how there could possible be an earth fault in the vacuum cleaner, I plugged it into the outside socket protected by its own RCD but NOT protected by the RCD in the consumer unit. It worked fine and did not trip the outside socket's own RCD. Am I right to suspect the RCD itself in the CU rather than any sort of appliance or wiring fault? FYI the house was rewired in 1997 along with the CU replacement - I've had a lot of floorboards up following CH install and it all looks in good nick. I am contemplating chancing £30.00 on a new RCD and wanted to get the group's general opinion. Thanks Luke Sounds like you may have a Neutral to Earth short somewhere. Switch off at the main switch and pull the fuses or trip the MCB's and do a high ohms test from the neutral of each circuit to Earth and you may well find it there. Whilst at it there should be no ohms from Live to earth either if there is and you do find which circuit then trace it down from there .. easier said then dun;!... -- Tony Sayer |
#5
Posted to uk.d-i-y
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Another RCD puzzler
In article , Dave Osborne
scribeth thus Luke wrote: Hi everyone, I've got a "Steeple" split load consumer unit installed in 1997 with 2 * ring mains on the 30ma 80A RCD side. On the non-RCD side there are a number of other circuits, including an outside socket, protected by a "Powerbreaker" 30ma integral RCD socket. We moved into the house in July. Ever since we moved in, we've been having nuisance RCD trips, seemingly following no pattern. Everything would be fine for a while, then all of a sudden the RCD would trip - usually when using the shower, or maybe switching the kettle on. Lately the problem seems to have been getting worse - it tripped during the night, a period of low activity, apart from the immersion heater which is on the non-RCD side anyway. I thought it might be mouse activity biting through the cable, but then... ...this morning I went to switch on the vacuum cleaner, a double insulated device, i.e. no path to earth. Immediately the RCD tripped. The happens on the upstairs and downstairs rings. As I couldn't see how there could possible be an earth fault in the vacuum cleaner, I plugged it into the outside socket protected by its own RCD but NOT protected by the RCD in the consumer unit. It worked fine and did not trip the outside socket's own RCD. Am I right to suspect the RCD itself in the CU rather than any sort of appliance or wiring fault? FYI the house was rewired in 1997 along with the CU replacement - I've had a lot of floorboards up following CH install and it all looks in good nick. I am contemplating chancing £30.00 on a new RCD and wanted to get the group's general opinion. Thanks Luke Yes, you should definitely suspect the RCD. Also, you may have an insulation fault that is pushing the RCD very close to its trip level. This means that the slightest imbalance or increase in humidity will set it off. The vacuum cleaner sets it off because of the inrush on starting (John Rumm might be along later to explain why). Prolly thats causing the leakage to earth, via live or neutral to alter.. You can buy a Hager 63A/30mA RCD from TLC for £16.80 +VAT http://www.tlc-direct.co.uk/Products/HGCD263U.html Also, get your installation checked out at some point to see if you can find where the leakage current is coming from. Blimey .. they've dropped in price since I last bought one;!.. -- Tony Sayer |
#6
Posted to uk.d-i-y
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Another RCD puzzler
On Nov 6, 2:01*pm, Luke wrote:
Hi everyone, I've got a "Steeple" split load consumer unit installed in 1997 with 2 * ring mains on the 30ma 80A RCD side. On the non-RCD side there are a number of other circuits, including an outside socket, protected by a "Powerbreaker" 30ma integral RCD socket. We moved into the house in July. Ever since we moved in, we've been having nuisance RCD trips, seemingly following no pattern. Everything would be fine for a while, then all of a sudden the RCD would trip - usually when using the shower, or maybe switching the kettle on. Lately the problem seems to have been getting worse - it tripped during the night, a period of low activity, apart from the immersion heater which is on the non-RCD side anyway. I thought it might be mouse activity biting through the cable, but then... ...this morning I went to switch on the vacuum cleaner, a double insulated device, i.e. no path to earth. Immediately the RCD tripped. The happens on the upstairs and downstairs rings. As I couldn't see how there could possible be an earth fault in the vacuum cleaner, I plugged it into the outside socket protected by its own RCD but NOT protected by the RCD in the consumer unit. It worked fine and did not trip the outside socket's own RCD. Am I right to suspect the RCD itself in the CU rather than any sort of appliance or wiring fault? No, most likely is an electrically leaking appliance. Spend your notes on a multimeter first. http://wiki.diyfaq.org.uk/index.php?title=RCD http://wiki.diyfaq.org.uk/index.php?title=Multimeter NT |
#7
Posted to uk.d-i-y
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Another RCD puzzler
On 06/11/2010 20:08, tony sayer wrote:
Whilst at it there should be no ohms from Live to earth either [...] Hmm, I'd prefer to have quite a lot of ohms between line and earth :~) -- Andy |
#8
Posted to uk.d-i-y
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Another RCD puzzler
On 06/11/2010 15:46, Dave Osborne wrote:
Yes, you should definitely suspect the RCD. I've never known an RCD fail by becoming over-sensitive. I've known a few though that have become insensitive and failed to trip when they should. The test button is there for a purpose... For this case a N-E short or leakage, which could be in an appliance or in the fixed wiring, is no. 1 suspect, as others have said. I'd put the RCD itself a long way down the list. -- Andy |
#9
Posted to uk.d-i-y
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Another RCD puzzler
In article , Andy Wade
zen.co.uk scribeth thus On 06/11/2010 20:08, tony sayer wrote: Whilst at it there should be no ohms from Live to earth either [...] Hmm, I'd prefer to have quite a lot of ohms between line and earth :~) OooH!, you pedant;!. Woss wrong with no ohms /indicated/ on yer meter;?.. -- Tony Sayer |
#10
Posted to uk.d-i-y
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Another RCD puzzler
On 06/11/2010 23:12, tony sayer wrote:
OooH!, you pedant;!. Woss wrong with no ohms /indicated/ on yer meter;?.. Milliohms, megohms, what's the difference :-) If my meter indicated no ohms I'd think it needs a new battery. -- Andy |
#11
Posted to uk.d-i-y
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Another RCD puzzler
Andy Wade wrote:
On 06/11/2010 15:46, Dave Osborne wrote: Yes, you should definitely suspect the RCD. I've never known an RCD fail by becoming over-sensitive. Ah, I wasn't suggesting that the RCD becomes over sensitive, rather that is is and has always been over sensitive. An RCD that trips at 17mA is likely to be noticeably more of a nuisance than an RCD that trips at 27mA. I've known a few though that have become insensitive and failed to trip when they should. The test button is there for a purpose... The test button is of limited facility. There's no substitute for testing with an RCD Tester. I've had RCD's that didn't trip on the test button but did trip on a "proper" fault and vice versa. |
#12
Posted to uk.d-i-y
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Another RCD puzzler
Dave Osborne wrote:
Andy Wade wrote: On 06/11/2010 15:46, Dave Osborne wrote: I've never known an RCD fail by becoming over-sensitive. Ah, I wasn't suggesting that the RCD becomes over sensitive, rather that is is and has always been over sensitive. An RCD that trips at 17mA is likely to be noticeably more of a nuisance than an RCD that trips at 27mA. I wouldn't have thought so. Surely if there is a leakage (sorry, residual) current of 17mA then there is something seriously wrong somewhere and it's worth finding it and fixing it. |
#13
Posted to uk.d-i-y
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Another RCD puzzler
On 07/11/2010 02:01, Dave Osborne wrote:
Ah, I wasn't suggesting that the RCD becomes over sensitive, rather that is is and has always been over sensitive. An RCD that trips at 17mA is likely to be noticeably more of a nuisance than an RCD that trips at 27mA. Fairynuff, but you shouldn't consider one that trips at 17 mA as over-sensitive - the standard allows them to trip anywhere between 15 and 30 mA. In my experience though they all tend to trip at around the 20 mA mark, ±2 or 3 mA. The test button is of limited facility. Agreed, but it will (usually) alert you to a device that isn't going to trip at all. If the test resistor circuit has gone o/c you'll get a false negative result, which errs on the safe side. There's no substitute for testing with an RCD Tester. Agreed, but, OOI, how many times have you found an RCD that failed on elongated trip times but did actually trip on the test? -- Andy |
#14
Posted to uk.d-i-y
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Another RCD puzzler
Andy Wade wrote:
On 07/11/2010 02:01, Dave Osborne wrote: Ah, I wasn't suggesting that the RCD becomes over sensitive, rather that is is and has always been over sensitive. An RCD that trips at 17mA is likely to be noticeably more of a nuisance than an RCD that trips at 27mA. Fairynuff, but you shouldn't consider one that trips at 17 mA as over-sensitive Given that Hager (for example) sell RCDs that consistently trip at 27-28mA, I can (and I do) consider one that trips at 17mA as over sensitive. To be clear, though I meant over sensitive in the sense of "10mA more sensitive than it needs to be" rather than "failing the 1/2 Idn test". - the standard allows them to trip anywhere between 15 and 30 mA. Actually, the standard says that they must not trip at 15mA and must trip at 30mA (within 200ms). It is entirely with the manufacturer to decide what actual trip level they set their product at. In my experience though they all tend to trip at around the 20 mA mark, ±2 or 3 mA. Well, in my experience 'most' 30mA RCDs trip at 25-28mA. The test button is of limited facility. Agreed, but it will (usually) alert you to a device that isn't going to trip at all. If the test resistor circuit has gone o/c you'll get a false negative result, which errs on the safe side. There's no substitute for testing with an RCD Tester. Agreed, but, OOI, how many times have you found an RCD that failed on elongated trip times but did actually trip on the test? I do temporary power distribution for outdoor events. I don't know exactly how many RCDs are in my care but it's in the region of 80~100 and of a variety of makes. They all get fully tested at least twice (and often several times) a season. I expect to get failures at the start of the season (bearing in mind that they may have over-wintered un-powered in an unheated shed, garage or container). To answer your question, which I take to mean "how many times have I found an RCD that tripped at the required fault current, but *not* within the prescribed time" then I would say at least a couple each season. Sometimes you get a duff batch and (usually after a season or two of being fine) most or all of them fail to survive over-wintering. |
#15
Posted to uk.d-i-y
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Another RCD puzzler
A clamp meter on the CUs earth wire can be useful (mA range).
A 30mA RCD will tend to trip at 23-25mA. Several appliances with mains filters each leaking circa 1.5mA can create a "background" leakage of 15mA making it very easy for another appliance to push the total over the RCD trip threshold. If the house does have a single RCD (TT supply or "whole house") then conversion to a dual RCD CU would make life a lot easier when the fault is found. |
#16
Posted to uk.d-i-y
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Another RCD puzzler
In article , Andy Wade spambucket@maxw
ell.myzen.co.uk scribeth thus On 06/11/2010 23:12, tony sayer wrote: OooH!, you pedant;!. Woss wrong with no ohms /indicated/ on yer meter;?.. Milliohms, megohms, what's the difference :-) If my meter indicated no ohms I'd think it needs a new battery. Well if there's no indicated 'omes then theres no problem.. -- Tony Sayer |
#17
Posted to uk.d-i-y
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Another RCD puzzler
John Rumm wrote:
On 07/11/2010 10:00, Ronald Raygun wrote: Dave Osborne wrote: Andy Wade wrote: On 06/11/2010 15:46, Dave Osborne wrote: I've never known an RCD fail by becoming over-sensitive. Ah, I wasn't suggesting that the RCD becomes over sensitive, rather that is is and has always been over sensitive. An RCD that trips at 17mA is likely to be noticeably more of a nuisance than an RCD that trips at 27mA. I wouldn't have thought so. Surely if there is a leakage (sorry, residual) current of 17mA then there is something seriously wrong somewhere and it's worth finding it and fixing it. It depends a bit on what you have on the circuit. Lots of IT kit and other electronic items for example will typically "leak" a small amount due to the mains input filtering. With enough items on the circuit the cumulative "normal" leakage can add up and take a lump out of your available trip threshold. And more to the point, if there is any step surge or interference on the mains input, can transfer quite large currents resulting from the harmonics higher than 50Hz. |
#18
Posted to uk.d-i-y
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Another RCD puzzler
The Natural Philosopher wrote:
John Rumm wrote: On 07/11/2010 10:00, Ronald Raygun wrote: Dave Osborne wrote: An RCD that trips at 17mA is likely to be noticeably more of a nuisance than an RCD that trips at 27mA. I wouldn't have thought so. Surely if there is a leakage (sorry, residual) current of 17mA then there is something seriously wrong somewhere and it's worth finding it and fixing it. It depends a bit on what you have on the circuit. Lots of IT kit and other electronic items for example will typically "leak" a small amount due to the mains input filtering. With enough items on the circuit the cumulative "normal" leakage can add up and take a lump out of your available trip threshold. And more to the point, if there is any step surge or interference on the mains input, can transfer quite large currents resulting from the harmonics higher than 50Hz. I don't find either of these two answers particularly illuminating. Would you guys mind explaining why mains input filtering, or harmonics, would "leak" current to earth? Surely *no* equipment should have *any* connection to earth other than from the enclosure, and there should be no current path to earth from any of the "gubbins". And why would interference harmonics cause a leak? Doesn't in all circumstances "what goes in must come out" apply? In other words why should the live current ever (at any instant) differ from the neutral current (except in direction)? Or is it that there is never a real leak, but the breaker is fooled into thinking there is, in which case what is the mechanism for fooling it? |
#19
Posted to uk.d-i-y
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Another RCD puzzler
Ronald Raygun wrote:
Would you guys mind explaining why mains input filtering, or harmonics, would "leak" current to earth? Surely *no* equipment should have *any* connection to earth other than from the enclosure, and there should be no current path to earth from any of the "gubbins". And why would interference harmonics cause a leak? No idea about the harmonics (that was skipped over during my course), but IT equipment has a measurable line to earth leakage. It is something to do with the transformers in them, which leak a little. Add 10 computers in one room, and there is potential to trip an RCD. There are special rules in the 17th Ed. Regs. for rooms containing IT equipment, especially concerning their earthing - not earthed properly, and you then touch the casing of a computer, you can get a shock, so all earth connections have to be in separate terminals,and must be in a ring, even if the line circuit is a radial. Alan. -- To reply by e-mail, change the ' + ' to 'plus'. |
#20
Posted to uk.d-i-y
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Another RCD puzzler
On Nov 7, 12:12*am, John Rumm wrote:
On 06/11/2010 15:46, Dave Osborne wrote: Yes, you should definitely suspect the RCD. Also, you may have an insulation fault that is pushing the RCD very close to its trip level. This means that the slightest imbalance or increase in humidity will set it off. The vacuum cleaner sets it off because of the inrush on starting (John Rumm might be along later to explain why). Covered he http://wiki.diyfaq.org.uk/index.php?...nsitising_RCDs Also, get your installation checked out at some point to see if you can find where the leakage current is coming from. Following some of the tests described here can find many causes: http://wiki.diyfaq.org.uk/index.php?...cate_the_cause.... -- Hi everyone and thanks for the input so far. This morning I connected the vacuum cleaner directly up to the RCD in the consumer unit. Nothing else connected. Immediately I turned on the vacuum cleaner, the RCD tripped. So, I have reproduced the fault having isolated the wiring. Bearing in mind the above, and the fact that I have seemingly random trips occuring on all RCD protected circuits, at inexplicable times (i.e. no load), I am inclined to suspect the RCD as being over- sensitive, and am minded to buy a replacement. I am pretty sure I am right, that a vacuum cleaner, which presents no path to earth at all, and hence no means of providing a leakage current, should not under any circumstances trip the RCD. That's correct isn't it? Luke |
#21
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Another RCD puzzler
Luke wrote:
On Nov 7, 12:12 am, John Rumm wrote: On 06/11/2010 15:46, Dave Osborne wrote: Yes, you should definitely suspect the RCD. Also, you may have an insulation fault that is pushing the RCD very close to its trip level. This means that the slightest imbalance or increase in humidity will set it off. The vacuum cleaner sets it off because of the inrush on starting (John Rumm might be along later to explain why). Covered he http://wiki.diyfaq.org.uk/index.php?...nsitising_RCDs Also, get your installation checked out at some point to see if you can find where the leakage current is coming from. Following some of the tests described here can find many causes: http://wiki.diyfaq.org.uk/index.php?...cate_the_cause... -- Hi everyone and thanks for the input so far. This morning I connected the vacuum cleaner directly up to the RCD in the consumer unit. Nothing else connected. Immediately I turned on the vacuum cleaner, the RCD tripped. So, I have reproduced the fault having isolated the wiring. Bearing in mind the above, and the fact that I have seemingly random trips occuring on all RCD protected circuits, at inexplicable times (i.e. no load), I am inclined to suspect the RCD as being over- sensitive, and am minded to buy a replacement. I am pretty sure I am right, that a vacuum cleaner, which presents no path to earth at all, and hence no means of providing a leakage current, should not under any circumstances trip the RCD. That's correct isn't it? If your facts are correct,, and your assumption that the vacuum cleaner is blameless is true. My washing machine did this. 3k ohms showed between a motor frame and its windings. Even stripped down. New motor fixed it. Luke |
#22
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Another RCD puzzler
Would you guys mind explaining why mains input filtering, or harmonics,
would "leak" current to earth? Surely *no* equipment should have *any* connection to earth other than from the enclosure, and there should be no current path to earth from any of the "gubbins". And why would interference harmonics cause a leak? Its usually capacitors connected between Neutral and earth and Live and earth. Not a lot of current is intended to flow but some does. These are mainly used to stop or alleviate interference (radio frequency) problems often referred to as EMC compatibility. That's it in brief.. Doesn't in all circumstances "what goes in must come out" apply? In other words why should the live current ever (at any instant) differ from the neutral current (except in direction)? Thats the idea .. what goes into the "house" on one conductor must come out on the other, any discrepancy then thats "leakage" sometimes caused by capacitors in equipment, sometimes caused by insulation breakdown and sometimes caused by YOU when you might grab hold of a live conductor and the leakage makes its way to Earth by you .. not a good thing then the RCD trips out and disconnects you from Earth.. Or is it that there is never a real leak, but the breaker is fooled into thinking there is, in which case what is the mechanism for fooling it? No theres no fooling at all as such, what comes in must be the same as what goes out more then 30 milliamps difference - then trip out time. Sometimes an Earth Neutral short can be rather difficult to comprehend as normally with no current flowing then nothing will happen as Earth and Neutral are bonded together at the incoming point on most supply instances. However when some current starts to flow then as the current increases what is going in on the live isn't all coming out on the Neutral thru the RCD as the current thru that increases due to load then there will come a point when it gets past the trip value then out will go the trip!.. If no current is being consumed in the connected circuits then the current imbalance isn't enough to fire off the trip. Switching anything on that will cause sufficient current to flow will .. after the tripping current point is reached then out it goes. Put that in a simplistic way with nothing switched on then nothing will be flowing thru the live to come out on the neutral so no imbalance. However switch something on then the current flowing Into the RCD on the live it should see an equal current flowing OUT on the Neutral which it won't as its flowing away through the Earth system, not necessarily to earth as such, just the earth part of the incoming supply connected to the neutral now flowing around and past the RCD on the wrong conductor When there is sufficient current flowing thru the live conductor and not out of the neutral side then once that reaches the tripping current then it will trip out... -- Tony Sayer |
#23
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Another RCD puzzler
In article , A.Lee
scribeth thus Ronald Raygun wrote: Would you guys mind explaining why mains input filtering, or harmonics, would "leak" current to earth? Surely *no* equipment should have *any* connection to earth other than from the enclosure, and there should be no current path to earth from any of the "gubbins". And why would interference harmonics cause a leak? No idea about the harmonics (that was skipped over during my course), but IT equipment has a measurable line to earth leakage. It is something to do with the transformers in them, Capacitors I'll think you'll find.. which leak a little. Add 10 computers in one room, and there is potential to trip an RCD. There are special rules in the 17th Ed. Regs. for rooms containing IT equipment, especially concerning their earthing - not earthed properly, and you then touch the casing of a computer, you can get a shock, so all earth connections have to be in separate terminals,and must be in a ring, even if the line circuit is a radial. Alan. -- Tony Sayer |
#24
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Another RCD puzzler
I am pretty sure I am right, that a vacuum cleaner, which presents no
path to earth at all, and hence no means of providing a leakage current, should not under any circumstances trip the RCD. That's correct isn't it? Luke No its not if its a Neutral Earth short, then that can cause that with no problems at all as can a lamp or anything else that causes currents to flow. The only exception might be a faulty or leaky "suppression" capacitor that is permitting a current to flow from the Live wire in it to Earth that can trip it. Way to find out is to tray another load such as an electric fire on the same circuit, tripping then Earth Neutral short very likely, no tripping then prolly faulty caps in the vacuum cleaner.. -- Tony Sayer |
#25
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Another RCD puzzler
On Nov 7, 8:40*pm, tony sayer wrote:
I am pretty sure I am right, that a vacuum cleaner, which presents no path to earth at all, and hence no means of providing a leakage current, should not under any circumstances trip the RCD. That's correct isn't it? Luke No its not if its a Neutral Earth short, then that can cause that with no problems at all as can a lamp or anything else that causes currents to flow. The only exception might be a faulty or leaky "suppression" capacitor that is permitting a current to flow from the Live wire in it to Earth that can trip it. Way to find out is to tray another load such as an electric fire on the same circuit, tripping then Earth Neutral short very likely, no tripping then prolly faulty caps in the vacuum cleaner.. -- Tony Sayer Yes, but the vacuum cleaner has no earth cable and in any case I wired it directly to the CU. There was no earth wire involved at all. |
#26
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Another RCD puzzler
Luke wrote:
On Nov 7, 8:40 pm, tony sayer wrote: I am pretty sure I am right, that a vacuum cleaner, which presents no path to earth at all, and hence no means of providing a leakage current, should not under any circumstances trip the RCD. That's correct isn't it? Luke No its not if its a Neutral Earth short, then that can cause that with no problems at all as can a lamp or anything else that causes currents to flow. The only exception might be a faulty or leaky "suppression" capacitor that is permitting a current to flow from the Live wire in it to Earth that can trip it. Way to find out is to tray another load such as an electric fire on the same circuit, tripping then Earth Neutral short very likely, no tripping then prolly faulty caps in the vacuum cleaner.. -- Tony Sayer Yes, but the vacuum cleaner has no earth cable and in any case I wired it directly to the CU. There was no earth wire involved at all. Was this wired directly into the RCD with all the other connections to the RCD removed (apart from the incoming L and N supply) or was it wired into the RCD with the outgoing neutral still connected. It does make a difference. -- Adam |
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Another RCD puzzler
Ronald Raygun wrote:
The Natural Philosopher wrote: John Rumm wrote: On 07/11/2010 10:00, Ronald Raygun wrote: Dave Osborne wrote: An RCD that trips at 17mA is likely to be noticeably more of a nuisance than an RCD that trips at 27mA. I wouldn't have thought so. Surely if there is a leakage (sorry, residual) current of 17mA then there is something seriously wrong somewhere and it's worth finding it and fixing it. It depends a bit on what you have on the circuit. Lots of IT kit and other electronic items for example will typically "leak" a small amount due to the mains input filtering. With enough items on the circuit the cumulative "normal" leakage can add up and take a lump out of your available trip threshold. And more to the point, if there is any step surge or interference on the mains input, can transfer quite large currents resulting from the harmonics higher than 50Hz. I don't find either of these two answers particularly illuminating. Would you guys mind explaining why mains input filtering, or harmonics, would "leak" current to earth? Surely *no* equipment should have *any* connection to earth other than from the enclosure, and there should be no current path to earth from any of the "gubbins". And why would interference harmonics cause a leak? Doesn't in all circumstances "what goes in must come out" apply? In other words why should the live current ever (at any instant) differ from the neutral current (except in direction)? Or is it that there is never a real leak, but the breaker is fooled into thinking there is, in which case what is the mechanism for fooling it? OK, so the RCD measures the current flowing in the live wire and measures the current flowing in the neutral wire and compares the two. Notionally, if the current in the neutral wire is less than the current in the live wire, then some of the current from the live wire *must* be flowing to earth, as the neutral is bonded to earth at the substation and therefore earth provides an alternative return path to neutral for the current from the live wire. If the neutral current is lower than the live current by a threshold value, then the RCD trips out the supply. Some types of equipment can inject some nasty signals into the mains which cause interference with other equipment, so to comply with the EMC directive, suppression components have to be installed to stop the interference from getting out of the unit. One of the techniques for limiting the interference is to connect small capacitors between live and earth and between neutral and earth. These are called Class Y capacitors[1]. Now, some small current flows in these capacitors. As the nominal potential difference between live and earth is 240V and the nominal potential difference between neutral and earth is 0V, then the current flowing from live to earth is 240V/Xc and the current flowing from neutral to earth is 0V/Xc (i.e. zero), where Xc is the impedance of the capacitor. So, with this type of interference suppressor, current (say 0.5mA) legitimately flows from live to earth and this current is detected by the RCD. These currents all add up, so, for example, thirty pieces of IT equipment[2] may have a leakage of 15mA. If the nominal 30mA RCD actually trips at 25mA then effectively, RCD protection is sensitised to 9mA of unacceptable fault current instead of 24mA of unacceptable fault current. Now, the impedance of the suppression capacitor (Xc) is a function of the frequency of the signal passing through it. As you will see from http://en.wikipedia.org/wiki/Capacitor the impedance is inversely proportional to the the frequency, which is another way of saying that when the frequency rises, the impedance falls. So if the device being protected generates harmonic interference (i.e interference at multiples of the mains frequency), then the harmonic interference is going to cause an even higher current to flow between live and earth than that just due to the 50Hz mains. In summary, there are circumstances where a legitimate current flows from live to earth; these circumstances are commonplace in a modern house and they cumulatively serve to "sensitise" the tripping characteristic of the RCD. [1] http://my.execpc.com/~endlr/line-filter.html [2] How many pieces if IT equipment in your home? Computers? Laptop power supplies? TVs? DECT phones? DVD players? Satellite boxes? Printers? Scanners? Monitors? ADSL modem? Phone chargers? Microwave oven? etc. |
#28
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Another RCD puzzler
I am pretty sure I am right, that a vacuum cleaner, which presents
no path to earth at all, and hence no means of providing a leakage current, should not under any circumstances trip the RCD. That's correct isn't it? Luke The only exception might be a faulty or leaky "suppression" capacitor that is permitting a current to flow from the Live wire in it to Earth that can trip it. Yes, but the vacuum cleaner has no earth cable and in any case I wired it directly to the CU. There was no earth wire involved at all. Was this wired directly into the RCD with all the other connections to the RCD removed (apart from the incoming L and N supply) or was it wired into the RCD with the outgoing neutral still connected. It does make a difference. Adam Thank you. To answer your question, I left all the neutrals connected to the neutral side of the RCD but only connected one live, which was to the vacuum cleaner. I would take a photo so you could see exactly what I did, but I have disconnected it now and don't want to cut the power at this time of night to recreate it. So, I decided to simplify matters yet further by wiring the RCD into a socket: (Health and safety fanatics look away now) http://tinyurl.com/LukeRCD And this time the RCD didn't trip, and the vacuum cleaner worked fine. I must admit I am confused as to how leaving all those neutrals connected could have been a contributory factor, as only 1 live was connected. Is it therefore possible there is a leak between a Live on the non-RCD side and Neutral on the RCD side then? If you like, I will re-execute my original experiment tomorrow morning and post a picture. Luke |
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Another RCD puzzler
In article
s.com, Luke scribeth thus On Nov 7, 8:40*pm, tony sayer wrote: I am pretty sure I am right, that a vacuum cleaner, which presents no path to earth at all, and hence no means of providing a leakage current, should not under any circumstances trip the RCD. That's correct isn't it? Luke No its not if its a Neutral Earth short, then that can cause that with no problems at all as can a lamp or anything else that causes currents to flow. The only exception might be a faulty or leaky "suppression" capacitor that is permitting a current to flow from the Live wire in it to Earth that can trip it. Way to find out is to tray another load such as an electric fire on the same circuit, tripping then Earth Neutral short very likely, no tripping then prolly faulty caps in the vacuum cleaner.. -- Tony Sayer Yes, but the vacuum cleaner has no earth cable and in any case I wired it directly to the CU. There was no earth wire involved at all. Won't be a leak from Live to Earth then!, prolly you do have an Earth Neutral short... -- Tony Sayer |
#30
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Another RCD puzzler
Luke wrote:
I am pretty sure I am right, that a vacuum cleaner, which presents no path to earth at all, and hence no means of providing a leakage current, should not under any circumstances trip the RCD. That's correct isn't it? Luke The only exception might be a faulty or leaky "suppression" capacitor that is permitting a current to flow from the Live wire in it to Earth that can trip it. Yes, but the vacuum cleaner has no earth cable and in any case I wired it directly to the CU. There was no earth wire involved at all. Was this wired directly into the RCD with all the other connections to the RCD removed (apart from the incoming L and N supply) or was it wired into the RCD with the outgoing neutral still connected. It does make a difference. Adam Thank you. To answer your question, I left all the neutrals connected to the neutral side of the RCD but only connected one live, which was to the vacuum cleaner. I would take a photo so you could see exactly what I did, but I have disconnected it now and don't want to cut the power at this time of night to recreate it. So, I decided to simplify matters yet further by wiring the RCD into a socket: (Health and safety fanatics look away now) http://tinyurl.com/LukeRCD OK. So if that is the RCD from your CU then what socket did you plug this Darwin Medal into? And this time the RCD didn't trip, and the vacuum cleaner worked fine. I must admit I am confused as to how leaving all those neutrals connected could have been a contributory factor, as only 1 live was connected. Is it therefore possible there is a leak between a Live on the non-RCD side and Neutral on the RCD side then? If you like, I will re-execute my original experiment tomorrow morning and post a picture. I'd video it, the coroner may want a copy. You have the symptoms of a classic neutral earth fault on the RCD side of your house electrics. The RCD needs the live and neutral that pass through it to have the same current, any imbalance more than 30mA and the RCD will trip. If the neutral and earth are shorted after the RCD (in the T&E house cable not the vacuums flex) then some of the current that should flow down the neutral cable back to the RCD is flowing to earth and the RCD will trip. -- Adam |
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Another RCD puzzler
On Nov 7, 7:45*pm, Ronald Raygun
wrote: Would you guys mind explaining why mains input filtering, or harmonics, would "leak" current to earth? *Surely *no* equipment should have *any* connection to earth other than from the enclosure, and there should be no current path to earth from any of the "gubbins". * Older equipment, with transformers, doesn't usually do this. The real problems began with the advent of switch mode power supplies (c.1980): a way of building power supplies out of complex silicon (which is cheap these days) and small high-frequency transformers, rather than the old heavy 50Hz transformers. As these are high-frequency devices, they make electrical interference that can be transmitted to, and interfere with, other equipment. The solution to this (for both interfererence generator and susceptible equipment) is to fit input filters. These act as short circuits to high-frequency noise, shorting it out and dissipating it between live, neutral and earth. In practice they're usually five components: a pair of inductors or chokes in the live & neutral lines act as an open circuit to high frequencies whilst a triangular delta network of capacitors between the three wires, including the earth, acts as short circuits to high frequencies. Capacitors are an open circuit to DC and nearly so to low frequency 50Hz AC, so they don't usually trigger RCDs, certainly not when it's an older RCD of 100mA. However, as John Rumm points out, it's a cumulative effect and 30mA isn't a huge leakage allowance to begin with. And why would interference harmonics cause a leak? Harmonics isn't the right term. The thing is that if you switch an inductive load (such as a transformer or motor) and you don't switch exactly on the zero-crossing point (which only happens with sophisticated transistor or thyristor switches) then you're attempting to switch a sizable voltage instantly. With an inductive load, that would require an infinite rate of change in the current, which an inductor will resist. So the outcome instead is a "ringing" current, where the current and voltage oscillate until damped out. The extra current drawn during these oscillations is also out of phase between live & neutral (as they're on opposite legs of the inductive load), so it's seen by the RCD as if it was a leakage current (it isn't, it's purely between live and neutral, but it's still picked up by the RCD's differential transformer). This is a relatively slow oscillation, compared to that of a switched-mode supply's oscillator, so RCDs are still fairly sensitive to it. |
#32
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Another RCD puzzler
Andy Dingley wrote:
On Nov 7, 7:45 pm, Ronald Raygun wrote: Would you guys mind explaining why mains input filtering, or harmonics, would "leak" current to earth? Surely *no* equipment should have *any* connection to earth other than from the enclosure, and there should be no current path to earth from any of the "gubbins". Older equipment, with transformers, doesn't usually do this. The real problems began with the advent of switch mode power supplies (c.1980): a way of building power supplies out of complex silicon (which is cheap these days) and small high-frequency transformers, rather than the old heavy 50Hz transformers. As these are high-frequency devices, they make electrical interference that can be transmitted to, and interfere with, other equipment. The solution to this (for both interfererence generator and susceptible equipment) is to fit input filters. These act as short circuits to high-frequency noise, shorting it out and dissipating it between live, neutral and earth. In practice they're usually five components: a pair of inductors or chokes in the live & neutral lines act as an open circuit to high frequencies whilst a triangular delta network of capacitors between the three wires, including the earth, acts as short circuits to high frequencies. Capacitors are an open circuit to DC and nearly so to low frequency 50Hz AC, so they don't usually trigger RCDs, certainly not when it's an older RCD of 100mA. However, as John Rumm points out, it's a cumulative effect and 30mA isn't a huge leakage allowance to begin with. And why would interference harmonics cause a leak? Harmonics isn't the right term. The thing is that if you switch an inductive load (such as a transformer or motor) and you don't switch exactly on the zero-crossing point (which only happens with sophisticated transistor or thyristor switches) then you're attempting to switch a sizable voltage instantly. With an inductive load, that would require an infinite rate of change in the current, which an inductor will resist. So the outcome instead is a "ringing" current, where the current and voltage oscillate until damped out. The extra current drawn during these oscillations is also out of phase between live & neutral (as they're on opposite legs of the inductive load), so it's seen by the RCD as if it was a leakage current (it isn't, it's purely between live and neutral, but it's still picked up by the RCD's differential transformer). This is a relatively slow oscillation, compared to that of a switched-mode supply's oscillator, so RCDs are still fairly sensitive to it. Er no. Thats bull****. That's not what happens. Current cant be 'lost' in an inductor or a capacitor. What goes in comes out. Its purely down to the capacitative coupling between L&N, and E. There is a current path via any interference suppression capacitors between those and earth. Now at 50Hz sine, that's rated as a very low current BUT if you slam a big load on the mains, you get a sharp step in voltage (equivalent to a much higher frequency than 50Hz being applied) on the mains, and that translates to a big surge current through those capacitors. In short, if you have maybe 20 electronic bits plugged into the mains, and you connect a powerful motor and switch it on, you are almost guaranteed to flip a 30mA RCD. That, and neutral earth shorts are the most common causes of phantom tripping. When I had a lorry sized generator instead of a proper mains supply, I could reliably flip the RCD any time switching on a hoover or kettle. |
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Another RCD puzzler
On Nov 7, 10:13*pm, The Natural Philosopher
wrote: Its purely down to the capacitative coupling between L&N, and E. Cable capacitance (which is pretty damned small), or (as I described) the filter capacitors? BUT if you slam a big load on the mains, you get a sharp step in voltage We're talking about inductive loads here - so you _don't_ get this sharp step (which would indeed require those harmonics), you get a slower rise instead, with overshoot and ringing. |
#34
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Another RCD puzzler
ARWadsworth wrote:
Luke wrote: I am pretty sure I am right, that a vacuum cleaner, which presents no path to earth at all, and hence no means of providing a leakage current, should not under any circumstances trip the RCD. That's correct isn't it? Luke The only exception might be a faulty or leaky "suppression" capacitor that is permitting a current to flow from the Live wire in it to Earth that can trip it. Yes, but the vacuum cleaner has no earth cable and in any case I wired it directly to the CU. There was no earth wire involved at all. Was this wired directly into the RCD with all the other connections to the RCD removed (apart from the incoming L and N supply) or was it wired into the RCD with the outgoing neutral still connected. It does make a difference. Adam Thank you. To answer your question, I left all the neutrals connected to the neutral side of the RCD but only connected one live, which was to the vacuum cleaner. I would take a photo so you could see exactly what I did, but I have disconnected it now and don't want to cut the power at this time of night to recreate it. So, I decided to simplify matters yet further by wiring the RCD into a socket: (Health and safety fanatics look away now) http://tinyurl.com/LukeRCD OK. So if that is the RCD from your CU then what socket did you plug this Darwin Medal into? And this time the RCD didn't trip, and the vacuum cleaner worked fine. I must admit I am confused as to how leaving all those neutrals connected could have been a contributory factor, as only 1 live was connected. Is it therefore possible there is a leak between a Live on the non-RCD side and Neutral on the RCD side then? If you like, I will re-execute my original experiment tomorrow morning and post a picture. I'd video it, the coroner may want a copy. You have the symptoms of a classic neutral earth fault on the RCD side of your house electrics. The RCD needs the live and neutral that pass through it to have the same current, any imbalance more than 30mA and the RCD will trip. If the neutral and earth are shorted after the RCD (in the T&E house cable not the vacuums flex) then some of the current that should flow down the neutral cable back to the RCD is flowing to earth and the RCD will trip. Here is a crappy picture for you that might help. http://s428.photobucket.com/albums/q...urrent=rcd.jpg Note that any current flowing through earth does not go through the RCD and so causes an imbalance in the RCD that causes it to trip. -- Adam |
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Another RCD puzzler
On 07/11/2010 14:23, Dave Osborne wrote:
Given that Hager (for example) sell RCDs that consistently trip at 27-28mA, I can (and I do) consider one that trips at 17mA as over sensitive. To be clear, though I meant over sensitive in the sense of "10mA more sensitive than it needs to be" rather than "failing the 1/2 Idn test". OK, understood. I've no experience of Hager products, so thanks for the info. Do they guarantee a minimum tripping current though, other than that implied by compliance with EN 60898? Actually, the standard says that they must not trip at 15mA and must trip at 30mA (within 200ms). It is entirely with the manufacturer to decide what actual trip level they set their product at. Indeed, but I still think that the prudent fixed installation designer should assume a possible trip current of 15 mA and divide the installation such that no RCD should see more than (say) 10 mA of Idn leakage under normal load conditions (insofar as 'normal' can be predicted). Bear in mind BS 7671's requirements for maintainability - your Hager '27 mA' RCD could get replaced by a different make that trips at 18 mA, say. Your own interest is more specialised and under 'skilled person' supervision, and I can see your need to avoid spurious tripping at all cost. [snip] To answer your question, which I take to mean "how many times have I found an RCD that tripped at the required fault current, but *not* within the prescribed time" then I would say at least a couple each season. Sometimes you get a duff batch and (usually after a season or two of being fine) most or all of them fail to survive over-wintering. Interesting. I take it you've seen the ESC's reliability study: http://short.zen.co.uk/?id=faa -- Andy |
#36
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Another RCD puzzler
Andy Wade wrote:
On 07/11/2010 14:23, Dave Osborne wrote: [snip] To answer your question, which I take to mean "how many times have I found an RCD that tripped at the required fault current, but *not* within the prescribed time" then I would say at least a couple each season. Sometimes you get a duff batch and (usually after a season or two of being fine) most or all of them fail to survive over-wintering. Interesting. I take it you've seen the ESC's reliability study: http://short.zen.co.uk/?id=faa No haven't seen that before; thanks for the heads-up. It makes an interesting read and correlates closely with my observed experience. I had assumed that problems with my RCDs were due to the fact that they were un-powered (and therefore cold and damp) over the winter months, but it seems that there is a general issue with detrimental ageing. I quite often find that if you test an (in-service rather than new) RCD without first exercising it on the trip button two or three times, it fails on trip time, but it's thereafter OK. I have got into the habit of exercising the mechanism a few times on the trip button before testing on the RCD tester, so I don't have to record a test fail. |
#37
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Another RCD puzzler
On 8 Nov,
Dave Osborne wrote: I quite often find that if you test an (in-service rather than new) RCD without first exercising it on the trip button two or three times, it fails on trip time, but it's thereafter OK. I have got into the habit of exercising the mechanism a few times on the trip button before testing on the RCD tester, so I don't have to record a test fail. I've got into the habit of routinely testing mine when the clocks change. It saves having to reset all the clocks an extra time. I know they /should/ be tested twice as often but at least it's done twice a year. -- B Thumbs Change lycos to yahoo to reply |
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Another RCD puzzler
SNIP EXPLANATION AND PICTURE OF NEUTRAL TO EARTH FAULT Here is a crappy picture for you that might help. http://s428.photobucket.com/albums/q.../?action=view¤... Note that any current flowing through earth does not go through the RCD and so causes an imbalance in the RCD that causes it to trip. -- Adam Right! Thank you to you everyone for your very interesting and humourous discourse into RCDs and their operation. My wife and I had a good laugh about the Darwin Award reference, and the suggestion of a vacuum cleaner in a bucket of salt water! Just to clear a couple of things up: 1) I was able to test the Consumer Unit's RCD in a socket by temporarily re-jigging the CU so that none of the circuits were RCD protected. Good job I didn't post a picture of that! 2) I have (from what I can gather) a TT system, although it is not an overhead supply. The only earth provision to the property is via the incoming water pipe. This may seem unusual, but when I first moved in, I had an electrician check over the CU, and he confirmed that this was the case. He said if he were rewiring he would convert to a TN-C-S, but having spent a long time reading this newsgroup, I am not sure you can necessarily do this without consultation with the DNO. After examining Adam's picture, a light bulb went on, somewhat belatedly I must admit. It finally dawned on me that because all the neutrals are commoned together, then a path to earth can therefore exist on a _DIFFERENT_ circuit to the one where a load is applied, even if that _DIFFERENT_ circuit's MCB is actually switched off. So, filled with enthusiasm, I isolated the neutrals one by one, until finally the RCD no longer tripped, which was when I disconnected a defunct cable to an old bathroom fan heater. I am pretty sure the cable is just lying under the floorboards, no doubt with all its conductors haphazardly twisted together - although the Live had long since been disconnected, the neutral was still commoned together with the other RCD protected circuits. BINGO! There was my problem and now I have completely disconnected this circuit the system works perfectly. Thanks again Luke |
#39
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Another RCD puzzler
In article
s.com, Luke scribeth thus SNIP EXPLANATION AND PICTURE OF NEUTRAL TO EARTH FAULT Here is a crappy picture for you that might help. http://s428.photobucket.com/albums/q.../?action=view¤... Note that any current flowing through earth does not go through the RCD and so causes an imbalance in the RCD that causes it to trip. -- Adam Right! Thank you to you everyone for your very interesting and humourous discourse into RCDs and their operation. My wife and I had a good laugh about the Darwin Award reference, and the suggestion of a vacuum cleaner in a bucket of salt water! Just to clear a couple of things up: 1) I was able to test the Consumer Unit's RCD in a socket by temporarily re-jigging the CU so that none of the circuits were RCD protected. Good job I didn't post a picture of that! 2) I have (from what I can gather) a TT system, although it is not an overhead supply. The only earth provision to the property is via the incoming water pipe. This may seem unusual, but when I first moved in, I had an electrician check over the CU, and he confirmed that this was the case. He said if he were rewiring he would convert to a TN-C-S, but having spent a long time reading this newsgroup, I am not sure you can necessarily do this without consultation with the DNO. After examining Adam's picture, a light bulb went on, somewhat belatedly I must admit. It finally dawned on me that because all the neutrals are commoned together, then a path to earth can therefore exist on a _DIFFERENT_ circuit to the one where a load is applied, even if that _DIFFERENT_ circuit's MCB is actually switched off. So, filled with enthusiasm, I isolated the neutrals one by one, until finally the RCD no longer tripped, which was when I disconnected a defunct cable to an old bathroom fan heater. I am pretty sure the cable is just lying under the floorboards, no doubt with all its conductors haphazardly twisted together - although the Live had long since been disconnected, the neutral was still commoned together with the other RCD protected circuits. BINGO! There was my problem and now I have completely disconnected this circuit the system works perfectly. Thanks again Luke Neutral Earth short then;!... -- Tony Sayer |
#40
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Another RCD puzzler
Luke wrote:
SNIP EXPLANATION AND PICTURE OF NEUTRAL TO EARTH FAULT Here is a crappy picture for you that might help. http://s428.photobucket.com/albums/q.../?action=view¤... Note that any current flowing through earth does not go through the RCD and so causes an imbalance in the RCD that causes it to trip. -- Adam Right! Thank you to you everyone for your very interesting and humourous discourse into RCDs and their operation. My wife and I had a good laugh about the Darwin Award reference, and the suggestion of a vacuum cleaner in a bucket of salt water! Just to clear a couple of things up: 1) I was able to test the Consumer Unit's RCD in a socket by temporarily re-jigging the CU so that none of the circuits were RCD protected. Good job I didn't post a picture of that! 2) I have (from what I can gather) a TT system, although it is not an overhead supply. The only earth provision to the property is via the incoming water pipe. This may seem unusual, but when I first moved in, I had an electrician check over the CU, and he confirmed that this was the case. He said if he were rewiring he would convert to a TN-C-S, but having spent a long time reading this newsgroup, I am not sure you can necessarily do this without consultation with the DNO. After examining Adam's picture, a light bulb went on, somewhat belatedly I must admit. It finally dawned on me that because all the neutrals are commoned together, then a path to earth can therefore exist on a _DIFFERENT_ circuit to the one where a load is applied, even if that _DIFFERENT_ circuit's MCB is actually switched off. So, filled with enthusiasm, I isolated the neutrals one by one, until finally the RCD no longer tripped, which was when I disconnected a defunct cable to an old bathroom fan heater. I am pretty sure the cable is just lying under the floorboards, no doubt with all its conductors haphazardly twisted together - although the Live had long since been disconnected, the neutral was still commoned together with the other RCD protected circuits. BINGO! There was my problem and now I have completely disconnected this circuit the system works perfectly. I am delighted that the RCD trips have stopped, but I am afraid that your new post now gives me even greater concern about your electrical installation than a tripping RCD. Quote " 2) I have (from what I can gather) a TT system, although it is not an overhead supply. The only earth provision to the property is via the incoming water pipe. This may seem unusual, but when I first moved in, I had an electrician check over the CU, and he confirmed that this was the case. He said if he were rewiring he would convert to a TN-C-S, but having spent a long time reading this newsgroup, I am not sure you can necessarily do this without consultation with the DNO" Any chance of a few piccies of your incoming supply and CU? 1. There is no way an electrician would pass an incoming water supply as a means of earthing. 2. TT is very rarely from underground cables (I have seen it only once) 3. ALL circuits need RCD protection on TT supplies (not all at 30mA see http://wiki.diyfaq.org.uk/index.php?title=TT_Earthing and http://wiki.diyfaq.org.uk/index.php?...Earthing_Types) -- Adam |
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