Does an RCBo or RCD cover the requirements

[Des 56]

Wasn't really a question, more of an observation that DurhamSparky said a L-E fault would blow the OCPD quickly as the fault current would be great. This isn't completely true on a circuit with a high Zs as the fault current would be lower, hence the reliance on an RCD.

I dont fully agree with the wording there JUD
Surely the earth fault curent would be low,but the Short circuit current is usually no different to a Tn system for bumping out the ocpd

The overcurrent on a Tn is more important Zs wise than it is on a TT
The RCD and RCBO s electronics dont like high fault currents apparantly and are not as reliable as on a TT system
The Tn system with an RCD is not as reliable for the function of the RCd as a TT system
OR so I was told

 

[JUD]

I dont fully agree with the wording there JUD
Surely the earth fault curent would be low,but the Short circuit current is usually no different to a Tn system for bumping out the ocpd

If you read my post again, you'll notice that I said a L-E fault (line to earth) on a circuit with a high Zs (Earth fault loop impedance) would not blow the OCPD in the required time but an RCD would.

No mention of short circuit (L-N) which should still have a high enough fault current to blow the OCPD.

 

[pevvers]

No mention of short circuit (L-N) which should still have a high enough fault current to blow the OCPD.

I would agree if the Zs was just over the limits, but not so much if it was ridiculously high, as long as the circuit is T&E.

As the neutral is larger than the CPC, obviously the impedance would be lower on the neutral-line loop. But if it was a ridiculously high Zs, i.e. a couple of Ohms over, then I would be measuring the line-neutral loop to make sure that it is low enough to produce enough fault current to trip the device in the required time....