Megger "funny"

[newshound]

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.

[tabbypurr]

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

[harry]

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

[Harry Bloomfield]

newshound explained :

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.

[Roger Hayter]

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?

[newshound]

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.

[newshound]

Thanks, that is encouraging.

[Cursitor Doom]

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

[Cursitor Doom]

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.

[alan_m]

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

[Roger Hayter]

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.

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.

[Graham Nye]

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.

[newshound]

Good point, I suppose this one

is just about affordable (for my very limited use)

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.

[newshound]

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.

[newshound]

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.

[Graham Nye]

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.

[Roger Hayter]

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.

[newshound]

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.

[Cynic]

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