UK Rcd tripping, 77v on earth bar

[Icarus]

So to start off with i’ll tell you i’m a young electrician, 23, I am qualified but obviously will always be learning.

Was carrying out an eicr early last week. During the eicr I went to disconnect what I thought were the earths for the (switched off) kitchen ring, as they were in the corresponding terminal. I felt a slight tingle and thought that can’t be right. It turned out that it was the earths for one of the lighting circuits and the smoke alarms and there was 77v between the disconnected lighting cpc and earth bar and 20ish volts between the disconnected smoke alarm cpc and earth bar. Surely this can’t be right? The insulation resistance values are all good. I also done a bit of experimenting with disconnecting the main earth with the power on (please don’t crucify me) and with all cpc’s connected there is again 77v between the earth and neutral bars, and earth bar and main earth. The disconned main earth to neutral bar shows continuity (tn-c-s). Is this an issue or is it just my bad practice of not switching the full board off that’s made me notice? Should also say I tried the same with the shower cpc, same thing was happening but at a lower voltage (still increasing as circuits added though)

Another thing is the voltage appears to gradually increases as you turn each circuit on (the lighting circuit is the last circuit). I figured this this could be earth leakage through appliances etc? With the main earth connected and all cpcs connected everythings as it should be voltage and continuity wise.

Anyway the rcd was failing on x1 and also ramp test (>33ma according to my meter) but passing on x5. So I’ve changed the rcd and new one is testing out fine. However, the tenant has phoned up over the weekend and said the rcd is tripping intermittently. Just looking for ideas really, before I head back tomorrow morning. Do you think these are related issues? Is there maybe an earth and neutral reversed somewhere? Or and earth used as a neutral or vice versa?

 

[UKMeterman]

Do not energize an instalation if the earths are not connected.

 

[SJD]

Any disconnected earth can have a voltage on it if the relevant circuit is still energized - caused by things like surge protection devices & filtering, capacitors, leakage due to damp, even just the capacitance between cores in a cable. How much current it can actually deliver depends on the cause.

When carrying out an EICR, you'll often find the earths (or Neutrals) are not in the correct terminals at a consumer unit, e.g. the cable was a bit short, an MCB position was swapped but not the earth (or N), or someone just couldn't be bothered.

So if you ever disconnect what you think is the E or N for a circuit, while there are still circuits powered up, you have to assume it could be live! And even if it is the correct E or N, it could still be "borrowed" and in use by another circuit.

Re the tripping RCD, did you complete a set of IR tests for the circuits?

 

[Icarus]

Appreciate the feedback

Any disconnected earth can have a voltage on it if the relevant circuit is still energized - caused by things like surge protection devices & filtering, capacitors, leakage due to damp, even just the capacitance between cores in a cable. How much current it can actually deliver depends on the cause.

When carrying out an EICR, you'll often find the earths (or Neutrals) are not in the correct terminals at a consumer unit, e.g. the cable was a bit short, an MCB position was swapped but not the earth (or N), or someone just couldn't be bothered.

So if you ever disconnect what you think is the E or N for a circuit, while there are still circuits powered up, you have to assume it could be live! And even if it is the correct E or N, it could still be "borrowed" and in use by another circuit.

Re the tripping RCD, did you complete a set of IR tests for the circuits?

appreciate the feedback, was definitely taught how to do it the right way but was never explained to me why. Will be taking more care in future. In this case the circuit had been moved previously from non rcd side to rcd so should’ve known better really.

The IR results were all quite high (90meg+). But the weather has been pretty wild this weekend so I am thinking perhaps this could be a factor as there are x 2 outside lights. So will be re-checking them.

Would a high resistance neutral be a reasonable idea? as I didn’t physically do any neutral to earth testing other than visually and the tug test and then again not at every accessory

 

[LastManOnline]

With the main earth connected and all cpcs connected everythings as it should be voltage and continuity wise.

Correct. And it's likely from your description that all is in fact in order. You are getting readings currently that are confusing for you, which is understandable. To "unconfuse" the situation bear in mind that what you are there to actually test is the "fixed wiring". In principle this means all appliances, lights, smoke alarms etc need to be decoupled from the fixed wiring (not always easy to do). It's these accessories that are almost certainly responsible for the "bizarre" readings on the CPC, s. These quickly become "unbizarre" when you find out from the appliance manufacture that they are using the earth for intentional leakage currents, which will explain the voltage readings. So you will learn from these experiences (like the rest of us) that you can very easily be duped in actually trying to fix something that is, nt actually broken

Anyway the rcd was failing on x1 and also ramp test (>33ma according to my meter) but passing on x5. So I’ve changed the rcd and new one is testing out fine. However, the tenant has phoned up over the weekend and said the rcd is tripping intermittently.

It may be that the new rcd is now detecting leakage currents which the older one was, nt due to been ineffective

. Just looking for ideas really, before I head back tomorrow morning. Do you think these are related issues.

No I Don, t. I think your first problem (voltage on CPC, s) is not a problem

? Is there maybe an earth and neutral reversed somewhere? Or and earth used as a neutral or vice versa?

Earth and neutral reverse, should, nt cause an rcd to trip. Its an imbalce between the 2 that causes it. An earth used as a neutral would certainly cause an rcd to trip. But that would not cause intermittent tripping, it would cause constant tripping