Neutral to earth fault affecting both RCDs

[Roadhousezamora]

I ran the test using the plug top on two different socket outlets on the ring main on the same side as the Rcd I was testing as well as from the extractor. Same 50v error ?

I wondered if it may be some capacitance from a knackered alarm battery which is a fused spur off the ring?

 

[timhoward]

I ran the test using the plug top on two different socket outlets on the ring main on the same side as the Rcd I was testing as well as from the extractor. Same 50v error ?

Ooh hang on. We might be barking up wrong tree.
Do you remember if it was ">50v"?
What tester is is?

 

[Roadhousezamora]

Ooh hang on. We might be barking up wrong tree.
Do you remember if it was ">50v"?
What tester is is?

It's a Dilog 9110 (can't afford anything premium right now lol) it's ok but prone to being temperamental.

 

[ELECNEWT]

I ran the test using the plug top on two different socket outlets on the ring main on the same side as the Rcd I was testing as well as from the extractor. Same 50v error ?

Test instruments meeting the requirements of BS EN 61557 must perform a pre-check when testing to ensure that the earth potential does not rise to a dangerous level during the test. This will apply to earth loop impedance testing and RCD testing since these tests involve producing a current on the earth. If the test current is expected to produce a dangerous voltage, the tester should inhibit the test and show a warning, such as >50 V.

 

[timhoward]

It's a Dilog 9110 (can't afford anything premium right now lol) it's ok but prone to being temperamental.

Nowt wrong with that.
This is where the devil is in the detail:

(On some other makes that message can mean something else.)
For the little time it takes, I'd see if it's now fine with the dodgy socket circuit's neutrals removed.
If still a problem it's IR testing each circuit again I'm afraid, start with L+N together to CPC.

 

[Aaron b]

Are you testing the RCD at the board or on a circuit? No experience with meggers (and we don't install RCD's often for utilities) but I would have assumed that the test had failed because the voltage on the earth conductor was at 50V. I assume the cable resistance was too high to achieve 250mA at <50V. Was there a '>' or a '<' in front of the warning?

I don't understand how a neutral earth fault would cause an RCD not to trip. The only reason I can think of for the fault on the socket circuit not to trip would be if it was sticky from lack of use (or a DC leakage issue).

If the cable isn't accessible and you cant install a new one, could you disconnect the faulty cable and derate it to 20A?

 

[Aaron b]

Sorry, I see the guy's above beat me to it, I don't know why I assumed a merger either... ?

 

[Roadhousezamora]

I was reading someones explanation and what I took from it was that the amount of current lost from the neutral was less than 30mA and therefore not triggering the RCD... Didn't fully understand how that happens though.

Yeah I may well have split the ring into two radials, there is a spare way in the board as well.

I have been testing on the circuit and not at the Rcd itself with no load so will try this when I'm next there.

If the Rcd responds at the device itself I will re add the loads but remove the neutral earth fault and see if they continue to operate.

I really appreciate this, being new my confidence when it comes to this sort of fault finding isn't great but I'm keen to make sure I get to the bottom of it, for myself as much as the client.

 

[pc1966]

What was the Zs at the test points used where the RCD failed? (Using the no-trip option, obviously!)

Seeing >50V warning to me sounds like the earth/CPC is exceptionally poor the test current is pushing it up above 50V.

 

[pc1966]

It's a Dilog 9110 (can't afford anything premium right now lol) it's ok but prone to being temperamental.

Have you tried a new set of probes?

I have the 9118 and it is fine (though I question the no-trip accuracy in some low Zs cases) and so far not seen the 50V warning. Probes are "consumables" as eventually the flexing breaks the conductors or the plugs get weak and don't establish as good connection.