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

 

[Ian1981]

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

Are you sure it's not a 100mA rcd main switch?
Being a tt

 

[John Bailey]

Hi Ian that was quick. yes it was 30mA . I even increased the setting on the MFT to 100mA and 300mA in an effort to induce a tripping current

 

[Flanders]

Was all the load disconnected / turned off when you tested it?

 

[telectrix]

N-E fault, betcha.

 

[John Bailey]

No. there was the usual domestic background load on a few lights and heard the fridge compressor kick in

 

[Ian1981]

No. there was the usual domestic background load on a few lights and heard the fridge compressor kick in

Rcd should be tested with the outgoing loads disconnected
Test it at the db with mcbs open

 

[John Bailey]

OK will try that thanks

 

[Flanders]

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

 

[Ian1981]

N-E fault, betcha.

Perhaps in an un energised circuit

 

[John Bailey]

Thanks for the advice

 

[Ian1981]

Thanks for the advice

Let us know how you get on John.
Good luck with whatever it is

 

[John Bailey]

Cheers ,will post the outcome here .

 

[telectrix]

Perhaps in an un energised circuit

or an energised circuit with no load connected.

 

[Ian1981]

or an energised circuit with no load connected.

Yeah like an immersion heater used as back up hot water to a gas boiler perhaps

 

[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.