Only if the disconnector is rated to open under the  current that may be flowing when at the time - not always clear, as a switch disconnector is generally not rated for breaking against a heavy fault - does it have arc chutes for example ?

However, if the power will be removed in the case of a short circuit fault by something else, the upstream fuse or a breaker  suitable for the PSSC, then you may argue that the switch disconnector is protected by that, and it is acceptable for the switch disconnector to be sacrificed then, as the back-up device does the ADS.

Generally, to have the earth fault detection operate a proper breaker is vastly preferred.

Mike

Due to a change in earthing arrangement, I need to add overall earth leakage protection to a 400 A LV distribution board. The board has an incoming switch disconnector, with overcurrent protection provided by fuses upstream.

What's providing ADS to the submain cable? I would have though it more conventional to have the "RCD" element at the start of each circuit - that also avoids any difficulties in making the incoming cables double/reinforced insulation before the incomer device.

   - Andy.

Cheers for your reply Andy - The supply to the board is directly from a DNO cut-out, protected with fuses, via AWA singles. We were expecting TN-C-S and have ended up with TT, hence the need to add the earth leakage protection.

AWA & TT? What's the armour connected to?

   - Andy.

It's connected to the earth provided by the local earth electrode.

eek. A fault there makes the building alive and never trips.
M.

Because the AWA is upstream of the proposed earth leakage protection, but the loop impedance is too high to operate the fuses, you mean?

quite so - how good is your electrode at popping a 400A fuse or whatever - I'd be very surprised if it is that low a resistance - the electrode at the substation won't be much less than an ohm to Terra Firma, even if your electrode is. ;-)  If the round trip loop really is that low you do not need the earth fault trip ;-)

Put the EFR current transformer over the equivalent of the incoming tails if you can - I appreciate it may not be that sort of set-up.

Mike.