RCD Nuisance Trips - Which Test?

[nitro23456]

Been reading some bits and bobs on RCD nuisance trips.

Can someone explain to me which type of testing is for what and why you would do one over the other or both

1. An earth clamp meter..... turn all circuits off at CU, clamp main earth at CU, take readings for each circuit in turn, points you in right direction of which circuit to investigate further.

2. Insulation resistance test...... turn all circuits off, test insulation resistance at 500v for each, points you in right direction of which circuit.

Am I missing something? Why would you do both or why would you choose one over the other?

 

[flameport]

Depends if it's the fixed wiring or appliances causing the problem.

Insulation resistance will locate problems with the wiring, but is useless if the problem is in an appliance, or is due to a large number of items with a tiny residual current each (such as those buffoons who put an entire room full of desktop PCs onto a single RCD and then wondered why it tripped all the time.)

 

[nitro23456]

Thank you.

So, earth leak clamp to find circuit(s) and then insulation test to rule out a fixed wiring problem in that circuit?

 

[EFLImpudence]

clamp main earth at CU, take readings for each circuit in turn,

No, you clamp both the Line and Neutral conductors - that's what the RCD measures.

points you in right direction of which circuit to investigate further.

It might not.

 

[nitro23456]

Ok, so clamp line and neutral going to RCD because the RCD works by detecting differences between the two.

Assuming this helps identify the circuit, the insulation resistance test would then help determine if the problem is with the wiring or not and if not itl likely be an appliance on said circuit?

So a different approach could be the earth clamp meter as above and then go round unplugging appliances on said circuit(s) and remeasuring with the clamp? If no difference, check wiring.

 

[donrkebab]

The first test you should do is an rcd test, could save a lot of time if its only a faulty rcd.

 

[bernardgreen]

Clamping on the Earth conductor or CPC may show no current if the fault is to Earth and not the CPC

Measuring the Insulation Resistance is essential in determining where the fault is as the fault can still cause the RCD to trip even if the MCB for the faulty circuit is switched OFF

 

[EFLImpudence]

The first test you should do is an rcd test, could save a lot of time if its only a faulty rcd.

Yes, and that should be done with the outgoing conductors disconnected so you are only measuring the RCD itself.

 

[EFLImpudence]

I think it is just that Earth Leakage clamp meters are relatively recent.

IR testing is rather laborious - having to disconnect wires.

With the clamp meter on the main tails you get a live readout of what is happening, by switching on and off various appliances and circuits, and can deduce what is happening overall - although, not necessarily where; see Bernard's diagram.


Obviously, intermittent faults are not detectable with IR testing as they only happen when something unknown is done, e.g. stepping on a floorboard.
So, if you think you know what that 'unknown' thing might be then the clamp meter could verify it.

 

[nitro23456]

Thanks everyone.

One last question... why is IR carried out at 500v and not 250 or 1000? And as such, presumably all appliances downstream need unplugging completely?

 

[JohnW2]

One last question... why is IR carried out at 500v and not 250 or 1000?

250V could fail to detect marginal situations of insulation which was breaking down at voltages "around 250V". 500V is high enough to avoid that issue, but I personally see no reeal reason why one shouldn't use 1000V (other than that "500v is adequate").

....And as such, presumably all appliances downstream need unplugging completely?

Yep (or, agt least, isolated with a double-pole switch, if there is one), and anything hardwired needs to be somehow disconnected - and that includes things like dimmers and any other things with electronics in them (which, apart from potentially mucking up the test, might well be killed by the test if an L-N IR test is undertaken!).

Kind Regards, John

 

[Adam_151]

and that includes things like dimmers and any other things with electronics in them (which, apart from potentially mucking up the test, might well be killed by the test if an L-N IR test is undertaken!).

Though for the purposes of finding a fault taking out on RCD, you wouldn't seek to do the test between L and N. Strap them together, turn switches on and test to earth. I would remove computers, TVs, boilers etc from circuit, but wouldn't worry about dimmers, LED lighting etc.

Dont do 500v test unless its clear on 250v first, actually check it on continuity first before selecting IR, some testers even have a K-ohms continuity range which is good for a pre-indication before doing IR

 

[JohnW2]

Though for the purposes of finding a fault taking out on RCD, you wouldn't seek to do the test between L and N. Strap them together, turn switches on and test to earth.

True - at least in the first instance, that is what one would do.

I would remove computers, TVs, boilers etc from circuit, but wouldn't worry about dimmers, LED lighting etc.

I would take out of circuit anything which had (or might have) a connection to CPC (or any other 'earth'), since there would be potential for that to affect even (L+N)-E IR tests (particularly if it were faulty). I agree that probably doesn't include dimmers.

Dont do 500v test unless its clear on 250v first, actually check it on continuity first before selecting IR, some testers even have a K-ohms continuity range which is good for a pre-indication before doing IR

I would have said that decent IR testing kit is usually intelligent enough for that not to really be necessary. My MFT automatically adjusts the IR test voltage (and displays it) so as to limit current to 1mA, no matter how low the resistance of the circuit.

What I am inclined to add is that I would have thought that an IR problem in the fixed wiring would be a very unusual/unlikely cause of occasional 'nuisance RCD trips' - and that, in ohe rare occasions it did happen, IR testing would quite probably not help, since one would probably not undertake the testing at a time of manifestation of an intermittent IR problem!

Kind Regards, John

 

[ericmark]

My PAT tester shows leakage current and insulation resistance and does insulation resistance first, one can only measure leakage current if it's not enough to trip the RCD. My clamp-on does not have a low enough scale to measure leakage current.

Filters can leak current to earth, and 500 volt is enough above the normal 360 volt peak to show faults, without being enough to damage items when between line + neutral to earth, but will high light components which may be on the edge of breaking down. Remember insulation resistance done as DC when leakage current is AC so insulation resistance does not show capacitive leakage.

 

[JohnW2]

Filters can leak current to earth, and 500 volt is enough above the normal 360 volt peak to show faults, without being enough to damage items when between line + neutral to earth, but will high light components which may be on the edge of breaking down.

One would hope that capacitors 'rated' for 230-250V AC would probably tolerate 500V DC without being damaged, but I'm not sure that one could necessarily rely on that. I personally would try to take any such capacitors out of circuit before undertaking a ≥500V IR test.

Kind Regards, John