Talk To Me About Earthing Systems

[Onoff]

Not sure where I'm going with this but bear with me...

A neutral earth fault might cause an upstream RCD to trip. Say a nail driven through a cable. Or accidentally shorting N&E even with the fuse tucked in your pocket.

Will it trip with a TN-C-S supply? The leakage path goes back down the combined conductor doesn't it? OK there's potentially other conductive paths.

I'm half then thinking of a question on the lines of is tripping more likely on a TT than a TN-C-S?

(No, btw, I'm not asking this after just fitting a new, all RCBO board).

Struggling to get my head around it.

 

[dobbslc]

So are you saying the fault is before the RCD?

 

[Guineafowl]

N-E short finds parallel paths in the other circuits, TT and TNCS.

The Wardmeister:

 

[sparkysy]

What supply system are you connected to?

TN systems don't usually have an upfront RCD for protection as a TT system would as disconnection times can be met through the low impedance back to the transformer.

With all RCBO's then the fault should be cleared by them!

 

[Dave-sparks]

Will it trip with a TN-C-S supply? The leakage path goes back down the combined conductor doesn't it?

Yes it will trip, but it's not leakage. The tripping is caused by some of the normal neutral current of the installation being diverted via the N-E fault.

I'm half then thinking of a question on the lines of is tripping more likely on a TT than a TN-C-S?

It is far more likely on a TNCS than on a TT. With TT (assuming it truly is TT and hasn't picked up next doors TNCS connection via bonded pipes etc) the return path for the diverted neutral current is normally much higher impedance and so much less current flows.

 

[Onoff]

Yes it will trip, but it's not leakage. The tripping is caused by some of the normal neutral current of the installation being diverted via the N-E fault.

Blond moment tonight.

Leakage to earth will mean less current through the N coil in the RCD, cause an imbalance and trip.

Better earth path of course with TN-C-S.

 

[Onoff]

So are you saying the fault is before the RCD?

The install side.

 

[Onoff]

What supply system are you connected to?

TN systems don't usually have an upfront RCD for protection as a TT system would as disconnection times can be met through the low impedance back to the transformer.

With all RCBO's then the fault should be cleared by them!

Me? I'm TT. A 4' rod, earth plate then another rod.

 

[aesmith]

Will it trip with a TN-C-S supply? The leakage path goes back down the combined conductor doesn't it? OK there's potentially other conductive paths.

N and E aren't combined within your installation.

 

[DAPPH]

It's all due to Electrickery and Eddie Currants who play the AC/ DC games . .