symptoms of lost neutral?

[Marvo]

I wasn't taking the mickey I thought you were being serious with the test result you stated. That's also why I asked what instrument you were using. If you'd have said it was an insulation tester I'd have picked it up as a typo.

 

[Gavin A]

His original reply stated 0.33ohms he's amended it to 0.33Mohms - the electronics in the meter would clearly account for this and the last thing Id really want to do is Megger 500v through a £500 set-up. the meter shou;d have been isolated before testing, the meter is a load you can damage it, it will have electronic circuitry!

my original post after the tests gave the results as 0.33Mohms, I missed the M out once in a single post as a typo, and already stated that we tested the IR at 250V in response to your previous query, precisely because we didn't want to risk damaging the meter.

if you have some reason to think that a 250V IR test will damage a 3 phase meter rated to 100Amps per phase, then I'd be interested to hear your logic on why as I can't see why it should be an issue.

Note this was the 100Amp 3 phase meter that we tested in this way, not the £500 monitoring unit where we just used a basic continuity test, and did this only after the unit had been identified as faulty and removed, not before.

 

[Gavin A]

I wasn't taking the mickey I thought you were being serious with the test result you stated. That's also why I asked what instrument you were using. If you'd have said it was an insulation tester I'd have picked it up as a typo.

I had given the reading as 0.33Mohms twice in the response immediately above that post, but fair enough if you weren't taking the ****, just how it read to me, and I've had load of hassle to deal with from a supplier today so might be a bit prickly.

 

[Darkwood]

sorry gav its just the way the thread splits the pages and lays out the post and at the top of the page was your typo ... I gathered it was a typo but missed your 250v selection post, now as you asked; in some cases it can still be an issue as certain setups can be sensitive to DC when they usually run on AC ...your meter give a DC output hence PATesters have a soft test option for computers etc (although this is more about test currents usually) and why alot of things say 'Do not do insulation test' ... its not always about the voltage.

 

[Gavin A]

It's obv been converted to PME at some point if it is TNCS. Worth checking the earthing/bonding arrangements for the other boards after that.

Thinking about it, it probably is TN-S rather than TN-C-S, which would presumably make a significant difference to this if there were a fault on the incoming neutral. I didn't do the test sheets, and was posting from memory without thinking about the difference that would make to this.

I'm coming back around to thinking that the issues with the meter and monitoring unit could be problems that really stem from an underlying high resistance neutral situation that's then causing power to flow between the phases where the phases are imbalanced, and there's an underlying weakness in the insulation between the phases to be exploited and worsened.

But if there is, it's probably just on the neutral connection of our circuit in the main distro board as there's no evidence of any problems with any other kit on the site. The original testing was done out of hours in the evening after a full day on site, so it's more likely than usual that our spark at the time could have forgotten to retighten the neutral connection after testing the circuit.

I may be over thinking things, but I'll not really be happy until I've had the lid off the main distro board to check it properly, the problem being that means either a shut down of the factory or out of hours work with the owner having to hang around in the dark waiting for us.

my brain hurts now though.

 

[Gavin A]

sorry gav its just the way the thread splits the pages and lays out the post and at the top of the page was your typo ... I gathered it was a typo but missed your 250v selection post, now as you asked; in some cases it can still be an issue as certain setups can be sensitive to DC when they usually run on AC ...your meter give a DC output hence PATesters have a soft test option for computers etc (although this is more about test currents usually) and why alot of things say 'Do not do insulation test' ... its not always about the voltage.

not worries, you could be right, but I don't remember this ever being a problem previously on any meters we've tested through, and I'm pretty sure we'll have tested the circuit through a fair few. Could be wrong though as I've barely done any AC circuit testing myself, so I'm not 100% sure how our previous spark was doing these tests.

 

[topquark]

My assumption was that all of the boards were using the same type of earthing system and bonding was done to that same earth. Remember, we have no idea how large the factory area is, it's entirely possible that a remote part of the factory could be TT given the age of install​ (or even another supply...but the latter not so likely in this case).

If it is TNCS then you should have Neut and Earth with very low impedance at the head, with TNS not necessarily so. No difference really on how to proceed testing though!

Could you not arrange to arrive earlier ie pre factory start up to test?

WRT the shocks don't assume it's neutral t->earth PD/current flow, it may not be.

Food for thought: Just because a Meter/inverter hasn't completely failed when IR testing it, doesn't mean it hasn't been damaged. For example, the decoupling of the AC/DC sides is accomplished with capacitors internally and they have various states of well being between working/failed completely. You could end up with your Meter, or the inverter, dumping a nice DC current on to your neutral, post IR testing. Think about the symptoms then, when testing the PD on the phases to neutral.

 

[Gavin A]

Quick update,

I've redone the test on the faulty monitoring kit, and it's not full continuity between phases, but a very low insulation resistance between L2 to L3 of 0.83 Mohms, with the L1 to L2, and L1 to L3, and all phases to neutral at something like 450-500 Mohms.

The voltage swing to 80V vs 380V between the 2 phases occured on this circuit in a situation with no load on the circuit.

Bit of a head scratcher really, I still think I need to get into the main board to check the neutral connections, hopefully I can get access on the weekend.

 

[Marvo]

..... I still think I need to get into the main board to check the neutral connections, hopefully I can get access on the weekend.

I'd agree, I can't help thinking that you're over-thinking the problem. You definitely need to get access into the main board, establish what the earthing arrangements actually are and start again with all the basic tests before you go any further. Really start again from scratch and be methodical, I don't possibly see how the metering can be causing voltage swings, there's no way it could be that high a burden so I'd completely forget the metering for now, I think the low IR results it's giving are just a red herring.