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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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Home-made Earth Leakage Meter
Hi, all.
I've been plagued with intermittent nuisance trips on one of the 2 RCDs in the new 'cheapest way to achieve 17th Edition'CU that our clueless sparky installed during recent building work. In due course, I plan to install a new CU which contains all RCBOs. But in the meantime, I wanted to make some diagnostic measurements to see what the actual residual leakage was, and where it was coming from. Clamp meters are quite expensive, so I thought I'd D-I-Y... I basically needed the current transformer with sense coil. An old RCD was the donor for this. I got an old Merlin Gerin 2-pole RCD, and carefully opened it. I removed all the actual moving parts: toggle, contacts, springs etc. I soldered the output braids directly to the output terminals. It's now just a feed-thru. All that was retained was the housing, current transformer, terminals and a small PCB where the fragile sense coil wires terminated. The PCB was held in place inside the housing by the actuation solenoid, which was no longer required, so the tracks to it were cut. The sense coil was in fact centre-tapped, and a pair of diodes provided full-bave rectification to drive the solenoid. I did not use any of this, and simply ran a pair of wires directly from the outer ends of the coil, and ran these out to a pair of 4mm sockets. During calibration, I found that the output voltage drifted about a bit high when open-circuit, so I measured the original solenoid at 7k-Ohm DC, and so I added a 10k load resistor across the coil. That settled things right down. Calibrarion consisted of using a handy 24v transformer to provide an AC input, and a pair of Fluke meters. A series of resistors was chosen to provide nominal calibration points at 0,1,2,3,4,5,6,7,8,9,10 mA, then 15,20,25,30,35,40 mA. The resistors were hooked up in the usual way to provide an imballance ( 1 leg before the current transformer, one after. The actual mA imballance was measured with one fluke, and the mV AC output measured with another. The calibration was very linear, that's why I went to bigger steps after 10mA. In my example, I got around 45 mV / mA. After calibration at 24v ( keeps the power dissipation in the test resistors down ), I did a couple of check points at 240v, and it was spot on. It's easily accurate to within 0.5mA against the fluke, with a resolution better than 0.1mA. I'll post a photo. OK, not the convenience of a clamp-on, but at least I can now make meaningfull measurements and start fault-finding! -- Ron |
#2
Posted to uk.d-i-y
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Home-made Earth Leakage Meter
On Mar 3, 6:31*pm, Ron Lowe wrote:
Hi, all. I've been plagued with intermittent nuisance trips on one of the 2 RCDs in the new 'cheapest way to achieve 17th Edition'CU that our clueless sparky installed during recent building work. In due course, I plan to install a new CU which contains all RCBOs. But in the meantime, I wanted to make some diagnostic measurements to see what the actual residual leakage was, and where it was coming from. Clamp meters are quite expensive, so I thought I'd D-I-Y... I basically needed the current transformer with sense coil. An old RCD was the donor for this. I got an old Merlin Gerin 2-pole RCD, and carefully opened it. I removed all the actual moving parts: toggle, contacts, springs etc. I soldered the output braids directly to the output terminals. It's now just a feed-thru. All that was retained was the housing, current transformer, terminals and a small PCB where the fragile sense coil wires terminated. The PCB was held in place inside the housing by the actuation solenoid, which was no longer required, so the tracks to it were cut. The sense coil was in fact centre-tapped, and a pair of diodes provided full-bave rectification to drive the solenoid. * I did not use any of this, and simply ran a pair of wires directly from the outer ends of the coil, and ran these out to a pair of 4mm sockets. During calibration, I found that the output voltage drifted about a bit high when open-circuit, so I measured the original solenoid at 7k-Ohm DC, and *so I added a 10k load resistor across the coil. * That settled things right down. Calibrarion consisted of using a handy 24v transformer to provide an AC input, and a pair of Fluke meters. * A series of resistors was chosen to provide nominal calibration points at 0,1,2,3,4,5,6,7,8,9,10 mA, then 15,20,25,30,35,40 mA. * The resistors were hooked up in the usual way to provide an imballance ( 1 leg before the current transformer, one after. The actual mA imballance was measured with one fluke, and the mV AC output measured with another. * The calibration was very linear, that's why I went to bigger steps after 10mA. In my example, I got around 45 mV / mA. After calibration at 24v ( keeps the power dissipation in the test resistors down ), I did a couple of check points at 240v, and it was spot on. * It's easily accurate to within 0.5mA against the fluke, with a resolution better than 0.1mA. I'll post a photo. OK, not the convenience of a clamp-on, but at least I can now make meaningfull measurements and start fault-finding! Neat idea. Fancy posting this on the wiki? http://wiki.diyfaq.org.uk/index.php?...ecial:Allpages To create an article just type the name of it you want into the search box If you wanted to post a pic of the current transformer, it would make it easier for people to copy it without pulling an rcd apart. NT |
#3
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Home-made Earth Leakage Meter
In article ,
Ron Lowe writes: During calibration, I found that the output voltage drifted about a bit high when open-circuit, so I measured the original solenoid at 7k-Ohm DC, and so I added a 10k load resistor across the coil. That settled things right down. The clue here is in the name - it's a _current_ transformer, and so the output you want is the current, and not the voltage. You can turn the current into a voltage by passing it through a resistor, as you did. Calibrarion consisted of using a handy 24v transformer to provide an AC input, and a pair of Fluke meters. A series of resistors was chosen to provide nominal calibration points at 0,1,2,3,4,5,6,7,8,9,10 mA, then 15,20,25,30,35,40 mA. The resistors were hooked up in the usual way to provide an imballance ( 1 leg before the current transformer, one after. The actual mA imballance was measured with one fluke, and the mV AC output measured with another. The calibration was very linear, that's why I went to bigger steps after 10mA. In my example, I got around 45 mV / mA. After calibration at 24v ( keeps the power dissipation in the test resistors down ), I did a couple of check points at 240v, and it was spot on. It's easily accurate to within 0.5mA against the fluke, with a resolution better than 0.1mA. I'll post a photo. OK, not the convenience of a clamp-on, but at least I can now make meaningfull measurements and start fault-finding! It's prudent to issue a warning about current transformers. This goes back to the output being a current. If you leave the output open circuit, then the current transformer will need to develop thousands of volts in order to breakdown the insulation so it passes the necessary output current. In yours, the core will saturate limiting the output current as soon as the insulation breaks down, but you might still get a nasty belt off it. In larger ones used to measure large load currents, you can get them arcing between terminals if left open circuit, and they are lethal. The moral here is never leave the output of a current transformer open circuit. Always leave a resistor across it (or short circuit it) to prevent the output voltage heading off to infinity when the output resistance is infinity. In your case, I would solder the resistor into the old RCD carcuss, so it remains connected if the external circuit is disconnected. Also, bare in mind what happens if there is a fault, and you get, say, a few hundred amps of inbalance for the time it takes for the fuse/MCB to trip. Again, in your case the core will saturate limiting the output power, but your circuitry still needs to handle this. The core will have been designed for only very short exposure to these currents as it would have tripped the RCD, but you've take the rest of the RCD away, so you're now relying on something else to disconnect the supply, and it might not act as quickly. BTW, I did something similar about 15 years ago to measure earth leakage in a computer room, but I was using a 5VA 160:1 current transformer, and that could have done some real damage if I got it wrong. (That CT was used to measure the current on our 500kVA supply, before it was upgraded to 2MVA and needed bigger ones.) -- Andrew Gabriel [email address is not usable -- followup in the newsgroup] |
#4
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Home-made Earth Leakage Meter
On Mar 4, 9:46 am, (Andrew Gabriel) wrote:
In article , It's prudent to issue a warning about current transformers. This goes back to the output being a current. If you leave the output open circuit, then the current transformer will need to develop thousands of volts in order to breakdown the insulation so it passes the necessary output current. In yours, the core will saturate limiting the output current as soon as the insulation breaks down, but you might still get a nasty belt off it. In larger ones used to measure large load currents, you can get them arcing between terminals if left open circuit, and they are lethal. The moral here is never leave the output of a current transformer open circuit. Always leave a resistor across it (or short circuit it) to prevent the output voltage heading off to infinity when the output resistance is infinity. In your case, I would solder the resistor into the old RCD carcuss, so it remains connected if the external circuit is disconnected. I was once asked to make a bunch of 5000:1 current transformers (i.e. 4999 turns). To test them I had a large transformer with a one turn secondary consisting of aluminium bars totalling 6x1 inches in cross section. I never dared to have the CTs open circuit. |
#5
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Home-made Earth Leakage Meter
NT wrote:
On Mar 3, 6:31 pm, Ron Lowe wrote: Hi, all. I've been plagued with intermittent nuisance trips on one of the 2 RCDs Clamp meters are quite expensive, so I thought I'd D-I-Y... Neat idea. Fancy posting this on the wiki? http://wiki.diyfaq.org.uk/index.php?...ecial:Allpages To create an article just type the name of it you want into the search box If you wanted to post a pic of the current transformer, it would make it easier for people to copy it without pulling an rcd apart. NT OK, I'll write it up with some pics. Will take a few days. I'm busy chasing faults with it now... -- Ron |
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