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