Got a heating element in a wallpaper stripper which is blowing 30mA RCD.

Testing live to earth on an old (but "electronic" rather than wind-up)
Megger (which seems to provide 320 volts DC) gives ~ 5 MegOhm.

Testing on a DVM gives ~ 200 kOhm, corresponding to a little over 1 mA
leakage current on mains.

So why the trip, especially given the Megger reading? Or is the megger
f**ed?

Whatever, I guess I need a new megger, any views on the £30 Chinese
models from eBay, or should I be looking at a second hand Avo / Robin /
Fluke etc? This is not for "professional" use, it probably only comes
out a couple of times a year.

On Thursday, 4 January 2018 21:01:21 UTC, newshound wrote:
Got a heating element in a wallpaper stripper which is blowing 30mA RCD.

Testing live to earth on an old (but "electronic" rather than wind-up)
Megger (which seems to provide 320 volts DC) gives ~ 5 MegOhm.

Testing on a DVM gives ~ 200 kOhm, corresponding to a little over 1 mA
leakage current on mains.

So why the trip, especially given the Megger reading? Or is the megger
f**ed?

Whatever, I guess I need a new megger, any views on the £30 Chinese
models from eBay, or should I be looking at a second hand Avo / Robin /
Fluke etc? This is not for "professional" use, it probably only comes
out a couple of times a year.

A microwave turntable motor can do that. Crank it at suitable speed to put out upto 1kV, use 2 meters to measure V & i. The downside is output voltage is unsteady, affecting accuracy.


NT

On Thursday, 4 January 2018 21:01:21 UTC, newshound wrote:
Got a heating element in a wallpaper stripper which is blowing 30mA RCD.

Testing live to earth on an old (but "electronic" rather than wind-up)
Megger (which seems to provide 320 volts DC) gives ~ 5 MegOhm.

Testing on a DVM gives ~ 200 kOhm, corresponding to a little over 1 mA
leakage current on mains.

So why the trip, especially given the Megger reading? Or is the megger
f**ed?

Whatever, I guess I need a new megger, any views on the £30 Chinese
models from eBay, or should I be looking at a second hand Avo / Robin /
Fluke etc? This is not for "professional" use, it probably only comes
out a couple of times a year.

The steam may be getting in somewhere and effecting things.
When it drys out,it may give good reading.

newshound explained :
Whatever, I guess I need a new megger, any views on the £30 Chinese models
from eBay, or should I be looking at a second hand Avo / Robin / Fluke etc?
This is not for "professional" use, it probably only comes out a couple of
times a year.

I bought one years ago, unlikely the same model, but it did 1,000, 500
and 250v, with a low voltage resistance test, plus LCD display. I
checked the voltage droop under load and I must say it proved to be
quite well speced for holding up its voltage and accurately calibrated.

newshound wrote:

Got a heating element in a wallpaper stripper which is blowing 30mA RCD.

Testing live to earth on an old (but "electronic" rather than wind-up)
Megger (which seems to provide 320 volts DC) gives ~ 5 MegOhm.

Testing on a DVM gives ~ 200 kOhm, corresponding to a little over 1 mA
leakage current on mains.

So why the trip, especially given the Megger reading? Or is the megger
f**ed?

Whatever, I guess I need a new megger, any views on the £30 Chinese
models from eBay, or should I be looking at a second hand Avo / Robin /
Fluke etc? This is not for "professional" use, it probably only comes
out a couple of times a year.

Can you test the leakage to the casing of the element on its own removed
from the stripper? If it is still 5 megohms then it is defective. The
discrepancy in the readings is surprising but the cause is uncertain.
Can you test both instruments on a one megohm high voltage resistor?



--

Roger Hayter

On 05/01/2018 08:52, Roger Hayter wrote:
newshound wrote:

Got a heating element in a wallpaper stripper which is blowing 30mA RCD.

Testing live to earth on an old (but "electronic" rather than wind-up)
Megger (which seems to provide 320 volts DC) gives ~ 5 MegOhm.

Testing on a DVM gives ~ 200 kOhm, corresponding to a little over 1 mA
leakage current on mains.

So why the trip, especially given the Megger reading? Or is the megger
f**ed?

Whatever, I guess I need a new megger, any views on the £30 Chinese
models from eBay, or should I be looking at a second hand Avo / Robin /
Fluke etc? This is not for "professional" use, it probably only comes
out a couple of times a year.

Can you test the leakage to the casing of the element on its own removed
from the stripper? If it is still 5 megohms then it is defective. The
discrepancy in the readings is surprising but the cause is uncertain.
Can you test both instruments on a one megohm high voltage resistor?



That test was with the element removed. I agree that 5 meg is defective,
but wouldn't necessarily trip an RCD. Didn't have any suitable resistors
to hand when checking, will have to drag out a deeply buried box of
"bits too useful to throw away".

Real purpose of post was to get views on cheap Chinese instruments.

On 05/01/2018 08:50, Harry Bloomfield wrote:
newshound explained :
Whatever, I guess I need a new megger, any views on the £30 Chinese
models from eBay, or should I be looking at a second hand Avo / Robin
/ Fluke etc? This is not for "professional" use, it probably only
comes out a couple of times a year.

I bought one years ago, unlikely the same model, but it did 1,000, 500
and 250v, with a low voltage resistance test, plus LCD display. I
checked the voltage droop under load and I must say it proved to be
quite well speced for holding up its voltage and accurately calibrated.

Thanks, that is encouraging.

On Thu, 04 Jan 2018 21:01:19 +0000, newshound wrote:

Whatever, I guess I need a new megger, any views on the £30 Chinese
models from eBay, or should I be looking at a second hand Avo / Robin /
Fluke etc? This is not for "professional" use, it probably only comes
out a couple of times a year.


I bought a brand new Megger multimeter last year and it really wasn't
expensive; came in at well under 100 quid.


--
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protocols constitutes acceptance of this condition.

On Fri, 05 Jan 2018 15:43:19 +0000, newshound wrote:

Real purpose of post was to get views on cheap Chinese instruments.

Well, they're a *hell* of a lot better than they used to be. They're even
making some respectable oscilloscopes suitable for pro use, too.



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protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.

Cursitor Doom wrote
newshound wrote

Whatever, I guess I need a new megger, any views on the £30
Chinese models from eBay, or should I be looking at a second
hand Avo / Robin / Fluke etc? This is not for "professional" use,
it probably only comes out a couple of times a year.

I bought a brand new Megger multimeter last year and
it really wasn't expensive; came in at well under 100 quid.

Thats stupidly expensive IMO.

I've got a Fluke 19 that didnt cost anything like that.

On 04/01/2018 21:01, newshound wrote:


So why the trip, especially given the Megger reading? Or is the megger
f**ed?

Did you test it full of boiling hot water - or when drained?


--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

newshound wrote:

On 05/01/2018 08:52, Roger Hayter wrote:
newshound wrote:

Got a heating element in a wallpaper stripper which is blowing 30mA RCD.

Testing live to earth on an old (but "electronic" rather than wind-up)
Megger (which seems to provide 320 volts DC) gives ~ 5 MegOhm.

Testing on a DVM gives ~ 200 kOhm, corresponding to a little over 1 mA
leakage current on mains.

So why the trip, especially given the Megger reading? Or is the megger
f**ed?

Whatever, I guess I need a new megger, any views on the £30 Chinese
models from eBay, or should I be looking at a second hand Avo / Robin /
Fluke etc? This is not for "professional" use, it probably only comes
out a couple of times a year.

Can you test the leakage to the casing of the element on its own removed
from the stripper? If it is still 5 megohms then it is defective. The
discrepancy in the readings is surprising but the cause is uncertain.
Can you test both instruments on a one megohm high voltage resistor?



That test was with the element removed. I agree that 5 meg is defective,
but wouldn't necessarily trip an RCD. Didn't have any suitable resistors
to hand when checking, will have to drag out a deeply buried box of
"bits too useful to throw away".

Real purpose of post was to get views on cheap Chinese instruments.

The trouble with cheap Chinese imports is that they may be good,
mediocre or plain fraudulent, and it is hard to tell in advance.

The market for insulation testers is mainly to electricians and I would
suggest buying one from a UK based wholesaler who has to take some
responsibility for what is being sold. e.g.

http://cpc.farnell.com/c/test-equipm...earchlookahead

BTW, your element might leak a lot more when it is hot. I don't know
the full failure mechanism, but they tend to fail abruptly with an arc
to earth internally at some point.



--

Roger Hayter

On 2018-01-05 15:43, newshound wrote:
On 05/01/2018 08:52, Roger Hayter wrote:
newshound wrote:

Got a heating element in a wallpaper stripper which is blowing 30mA RCD.

Testing live to earth on an old (but "electronic" rather than wind-up)
Megger (which seems to provide 320 volts DC) gives ~ 5 MegOhm.

Testing on a DVM gives ~ 200 kOhm, corresponding to a little over 1 mA
leakage current on mains.

Can you test both instruments on a one megohm high voltage resistor?

That test was with the element removed. I agree that 5 meg is defective,
but wouldn't necessarily trip an RCD. Didn't have any suitable resistors
to hand when checking...

You've already told us you've got one. Your DVM should have a constant
input resistance (usually 10 MOhms, but check the manual). So, while
you're reading the 320 V output of the Megger on your DVM also check the
Megger to see what resistance it sees looking in to the DVM.


--
Graham Nye
news(a)thenyes.org.uk

On 05/01/2018 21:31, Roger Hayter wrote:
newshound wrote:

On 05/01/2018 08:52, Roger Hayter wrote:
newshound wrote:

Got a heating element in a wallpaper stripper which is blowing 30mA RCD.

Testing live to earth on an old (but "electronic" rather than wind-up)
Megger (which seems to provide 320 volts DC) gives ~ 5 MegOhm.

Testing on a DVM gives ~ 200 kOhm, corresponding to a little over 1 mA
leakage current on mains.

So why the trip, especially given the Megger reading? Or is the megger
f**ed?

Whatever, I guess I need a new megger, any views on the £30 Chinese
models from eBay, or should I be looking at a second hand Avo / Robin /
Fluke etc? This is not for "professional" use, it probably only comes
out a couple of times a year.

Can you test the leakage to the casing of the element on its own removed
from the stripper? If it is still 5 megohms then it is defective. The
discrepancy in the readings is surprising but the cause is uncertain.
Can you test both instruments on a one megohm high voltage resistor?



That test was with the element removed. I agree that 5 meg is defective,
but wouldn't necessarily trip an RCD. Didn't have any suitable resistors
to hand when checking, will have to drag out a deeply buried box of
"bits too useful to throw away".

Real purpose of post was to get views on cheap Chinese instruments.

The trouble with cheap Chinese imports is that they may be good,
mediocre or plain fraudulent, and it is hard to tell in advance.

The market for insulation testers is mainly to electricians and I would
suggest buying one from a UK based wholesaler who has to take some
responsibility for what is being sold. e.g.

http://cpc.farnell.com/c/test-equipm...earchlookahead

Good point, I suppose this one

http://cpc.farnell.com/tenma/72-9400...ohm/dp/IN07406

is just about affordable (for my very limited use)


BTW, your element might leak a lot more when it is hot. I don't know
the full failure mechanism, but they tend to fail abruptly with an arc
to earth internally at some point.



Realise that, but all this testing was done "cold". Took the element out
when it failed open circuit, just did the insulation resistance test out
of interest.

On 05/01/2018 20:49, alan_m wrote:
On 04/01/2018 21:01, newshound wrote:


So why the trip, especially given the Megger reading? Or is the megger
f**ed?

Did you test it full of boiling hot water - or when drained?


This was cold, it went open circuit first (on a supply without RCD),
only discovered the 30 mA trip when I took it home to test.

On 09/01/2018 16:48, Graham Nye wrote:
On 2018-01-05 15:43, newshound wrote:
On 05/01/2018 08:52, Roger Hayter wrote:
newshound wrote:

Got a heating element in a wallpaper stripper which is blowing 30mA RCD.

Testing live to earth on an old (but "electronic" rather than wind-up)
Megger (which seems to provide 320 volts DC) gives ~ 5 MegOhm.

Testing on a DVM gives ~ 200 kOhm, corresponding to a little over 1 mA
leakage current on mains.

Can you test both instruments on a one megohm high voltage resistor?

That test was with the element removed. I agree that 5 meg is defective,
but wouldn't necessarily trip an RCD. Didn't have any suitable resistors
to hand when checking...

You've already told us you've got one. Your DVM should have a constant
input resistance (usually 10 MOhms, but check the manual). So, while
you're reading the 320 V output of the Megger on your DVM also check the
Megger to see what resistance it sees looking in to the DVM.


Didn't think of that! I have a collection of DVMs but I think the
manuals are long gone. Could of course do some cross calibration.

On 2018-01-09 21:45, newshound wrote:
On 09/01/2018 16:48, Graham Nye wrote:
On 2018-01-05 15:43, newshound wrote:
On 05/01/2018 08:52, Roger Hayter wrote:
newshound wrote:


Can you test both instruments on a one megohm high voltage resistor?

That test was with the element removed. I agree that 5 meg is defective,
but wouldn't necessarily trip an RCD. Didn't have any suitable resistors
to hand when checking...

You've already told us you've got one. Your DVM should have a constant
input resistance (usually 10 MOhms, but check the manual). So, while
you're reading the 320 V output of the Megger on your DVM also check the
Megger to see what resistance it sees looking in to the DVM.


Didn't think of that! I have a collection of DVMs but I think the
manuals are long gone. Could of course do some cross calibration.

If you have multiple DMMs you can use one to check the input resistance
of another at the low voltage used by a DMM. You can then use the
checked one with the Megger.



--
Graham Nye
news(a)thenyes.org.uk

newshound wrote:

On 05/01/2018 21:31, Roger Hayter wrote:
snip


BTW, your element might leak a lot more when it is hot. I don't know
the full failure mechanism, but they tend to fail abruptly with an arc
to earth internally at some point.



Realise that, but all this testing was done "cold". Took the element out
when it failed open circuit, just did the insulation resistance test out
of interest.

Did you mention it had gone open circuit? When they do so they often
seem to have a brief arc internally and sputter metal all over the
insulation, which is generally mineral granules. Then the leakage to
earth is anything from like yours to a dead short.

--

Roger Hayter

On 09/01/2018 23:06, Roger Hayter wrote:
newshound wrote:

On 05/01/2018 21:31, Roger Hayter wrote:
snip


BTW, your element might leak a lot more when it is hot. I don't know
the full failure mechanism, but they tend to fail abruptly with an arc
to earth internally at some point.



Realise that, but all this testing was done "cold". Took the element out
when it failed open circuit, just did the insulation resistance test out
of interest.

Did you mention it had gone open circuit? When they do so they often
seem to have a brief arc internally and sputter metal all over the
insulation, which is generally mineral granules. Then the leakage to
earth is anything from like yours to a dead short.

Yes I thought that might be what had happened. Just interested that a
megger with 320V DC seemed to give about 1 mA while 240V AC gives 30
mA. But I guess the AC was kicking off an arc.

Aren't mergers limited to 1mA to prevent being used to discipline apprentices?