RCD tripping

[sellers]

Customer calls me back after I had rewired up and down sockets. The rcd trips when his bathroom underfloor heating (he installed) is left on for a while or straight away when thats on and the hoover is put on.

Go round, switch spur off, try hoover, works fine.

Measured the sensor resistance got 11ohms all ok. IR on heating matt. 250v. >99 L-E. 0 N-E. 31 L-N

Measured N-E on ohmmeter and got 103ohms

would you say the matt is damaged? and there is a short between N-E

Note MCB wasnt tripping Just RCD protecting that side of the cu was. Disconnected the matt and hoover would work fine no tripping.

 

[NozSpark]

Either that or a strand of the earth sheath is touching the N terminal/wire in the controller.

and don't IR L-N on a heating mat! they are connected!!

 

[NozSpark]

Come to think of it if L-E and N-E are different then it does sound like the kit is toast!

 

[Andy™]

sounds like a neutral-earth fault in the heating mat

 

[sellers]

Thats what I assumed, why would putting the hoover on make it trip straight away though?

 

[kme]

Hoover has a bit of leakage perhaps?

 

[Andy™]

Thats what I assumed, why would putting the hoover on make it trip straight away though?

extra current flowing on neutral, means more leakage to earth at point of fault. its probably not just the hoover either - any high powered load will do similar thing.

slightly related, had a similar RCD fault a while back. split phase TT supply. 100mA main RCD feeding TPN board (nothing connected to L3). from this, most of the place, and 2x submain - kitchen and basement. as soon as fridges etc were turned on in kitchen (fed from kitchen board), the main incomer would trip. but most of this board was 30mA RCBO's. none tripped. couldnt find any fault anywhere here. fault was a N-E on something plugged into basement DB. but the RCBO that fed that submain never tripped either. this was through the night, so only real load was in the kitchen. current finding its way through n-e fault and tripping main incomer. because the submain was fed by RCBO, when when turned off, it didnt isolate fault from main DB. some head scratching at that job