Insulation tester fault

[Leuven]

I have a Metrel multitester and when I put it to insulation testing, it only goes up to about 30-50 MOhm. The meter is capable to go to 999 MOhm and that's what it should display with no leads connected.

I took the case apart and looked for obvious things, re-soldered all the wires going to the test socket and back, changed the fuses but no luck. The interesting thing is, with no fuses on, the instrument returns 0 Ohm on the insulation test.

Also if I leave the meter on for a while, it starts going higher and higher - up to 300 MOhm. Presumably as things warm up. If I do multiple tests one after the other, the values will also creep up a meg or two each test.

Is this a case of a cracked solder joint somewhere on the hight voltage side, or the contrary - a sign of a short condition on the board?

[Leuven]

I'm focusing on the high voltage output section (even attempted to make it into a diagram) and pulled all the blue decoupling capacitors and resistors along the way in the hope one of them is duff. No luck, still 30Mohm (worse from cold).

Any ideas?

[I2C]

That's really strange? I'm a qualified electrician (UK - mainly domestic) and I've never had an issue with the insulation resistance test mode. Like you say, if there's no internal short-circuit/leakage then I'm not sure what the problem could be. Are you using it at 500V for domestic 230V R1-R2 & R1-Rn tests? If so, as long as you're over 2MOhm then it's compliant in the UK. Some approved multi-function testers only measure up to 10MOhm anyway, 999MOhm is probably overkill in this case.

Another point (I'm sure you've checked this, not insulting you're inelegance in any way as I've done it myself) sometimes if neon's (among other things such as PIR's, etc) are present on circuits, they can have a fairly high impedance and can fool you into thinking the test equipment is faulty when it's not. Perhaps try a test on a bare piece of cable to make sure this is not the case... I'm sure you already have, but I was fooled years ago.

My advice, test it against another insulation resistance tester on the same circuit (not at the same time, obviously) and compare results to check the accuracy - if all is good, carry on and maybe look at getting it calibrated; however if it's not then perhaps look at buying a new multi-function tester. I'd personally recommend the Di-Log testers, they are far cheaper than Fluke or Megger brands but conform to the same standards and often have a lot more features in terms of value-for-money (like auto-RCD testing, combined PFC, PSC & Zs/Ze, etc....)

Hope it all works out well anyway!

[Leuven]

That's what I thought too at the beginning. It didn't look faulty, knowing that something like single digit MOhm is a pass for a domestic circuit. But I happened to have access to another one and that one went all the way to 999Mohm. I spoke to a place that services and repairs these meters and apparently it's quite a common issue with these Metrels to loose high MOhm and usually it's a capacitor on the high voltage side, or the transformer. Actually the guy was almost sure there is no voltage actually being put out for the test (regardless what the meter is showing on the screen). That might be how they usually fail, but this one is indeed putting out full 500V (on the 500V test) so I don't know where else to look. I suppose I can leave well alone and hopefully won't get worse, but it gnaws at me.

By the way, all tests were done with the leads disconnected.

I have an older Di-log insulation meter and I like it, it's a solid piece of equipment. I'm only a hobby-ist but if I were on the market for a new unit, I wouldn't hesitate to buy a Di-log, especially over Metrel.