RCD and replacement fails to operate

[John Bailey]

On completion of some recent minor work , I carried out the usual testing and noticed the incoming 100A 30mA rcd on the Consumer unit was failing to trip at X1 and X5 at both 0 & 180 degrees Further investigation revealed that the test button on the device failed to trip it as well . All well and good so far faulty RCD me thinks . How often do these devices actually get tested by the end user (despite the quarterly test label ) Replaced it . Checked again and although test button now trips, it still fails to pass RCD ramp or time tests ? Just in case the replacement unit was faulty ( however unlikely ) , I had another unit on the van changed that out with the same result .
Earth loop impedance testing on both on the ring final and incoming Ze, very low reading less than 1 Ohm bearing in mind this is a TT installation ( does seem exceptionally low,) I have checked all round the incoming supply head no sign any PME link I may have missed ????

Any ideas fellow sparks

 

[Toneyz]

Bet you the fridge is giving you false test reading turn all mcbs off and test again as above do not test rcd with load connect

I bet soon as you test without loads such as the fridge it will test ok I learnt the hard way too.

 

[Taylortwocities]

Bet you the fridge is giving you false test reading turn all mcbs off and test again as above do not test rcd with load connect

Turning off the MCBs will not remove the neutral connection! You need to remove the neutral link from the RCD as well.

 

[Rpa07]

Is this problem because it's a TT or because as Tel says 'a neutral earth fault' within the installation. I'm interested now.

 

[Toneyz]

If there was a N-E fault as soon as load turned on the RCD would trip if I am not wrong

 

[Rpa07]

True unless that anomaly of N/E relationship that has been mentioned in past that will cause RCD not to trip in normal operation.

 

[PEG]

A suspected faulty RCD,should not be condemned,until it is tested in isolation,that is,removed,from any and all other possible,additional faults. That way,you are only testing one component,and can diagnose accurately (EM issues aside)

 

[Des 56]

You should test the earth loop impedance of the rod rather than a ZS
It may be that it does not exist or is exceptionally high
Couple that with a neutral-earth fault and the Rcd may yet see no imbalance

 

[Ian1981]

Turning off the MCBs will not remove the neutral connection! You need to remove the neutral link from the RCD as well.

It won't isolate the neutral but I think he's just saying remove any loads on the outgoing circuits first and test the rcd without anything drawing current.
With a Ra so low apparently during his earth electrode test the rcd should trip on a neutral earth fault on a energised circuit.
An insulation resistance test is possibly the next step after rcd is tested again with loads disconnected.
Wonder if the earthing conductor was disconnected from the MET during the electrode test because that Ra is very low

 

[mhar]

There is no such test as a 5 x trip current on a 100mA rcd (only applies to 30mA) so it may be possible your meter 'knows' this and is not doing anything.
What meter are you using? A couple of months ago I went to test 6 mem 30mA rcbos all sitting side by side in a board, none would trip even when I set my meter to ramp test on a 300mA rcd. I was testing at the terminals. However when I tested from the circuits, they all behaved perfectly, contrary to what I was expecting. This was using a Metrel Mi3101

 

[Ian1981]

There is no such test as a 5 x trip current on a 100mA rcd (only applies to 30mA) so it may be possible your meter 'knows' this and is not doing anything.
What meter are you using? A couple of months ago I went to test 6 mem 30mA rcbos all sitting side by side in a board, none would trip even when I set my meter to ramp test on a 300mA rcd. I was testing at the terminals. However when I tested from the circuits, they all behaved perfectly, contrary to what I was expecting. This was using a Metrel Mi3101

I've had this on mem 2000 when using the pods to covert the mcb to rcbo.
Won't work with the wire connected into the terminal, needed to get my long thin prob into the terminal with circuit removed

 

[saspm]

It will be interesting to see the outcome! I've had similar things happen with suppression capacitors being the culprit.

 

[telectrix]

There is no such test as a 5 x trip current on a 100mA rcd (only applies to 30mA) so it may be possible your meter 'knows' this and is not doing anything.
What meter are you using? A couple of months ago I went to test 6 mem 30mA rcbos all sitting side by side in a board, none would trip even when I set my meter to ramp test on a 300mA rcd. I was testing at the terminals. However when I tested from the circuits, they all behaved perfectly, contrary to what I was expecting. This was using a Metrel Mi3101

this is common with some RCBOs; a short length of 2 core flex in series with your test leads will overcome it.

 

[Octopus]

Wonder if the op has done any IR tests.....

 

[Risteard]

If there was a N-E fault as soon as load turned on the RCD would trip if I am not wrong

Not always.

 

[Midwest]

I've experienced a non tripping RCD (including test button) on a TN-C-S install, with an N-E fault on one circuit, even had similar when doing monthly calibration on my MFT, having left the washing machine connected.

I know someone explained the reasoning before, but would the same issue be replicated on TT supply?