AFDD for main supply?

spynage · Dec 24, 2022

What would of stopped this happening?

Job I did this morning.

SWA supply for outbuilding had sliced basic insulation at the gland due to poor wire stripping. Another outgoing SWA feed for an external GYM was actually back feeding water into the consumer unit as it was uphill and had some head pressure of about 4 metres on the cable. The gym had a rotary switch with some plastic 20mm conduit directly above the SWA coming in so the SWA was simply funnelling water downhill into the main outbuilding C/U!

Anyway water dripped directly onto sliced conductors where it arced and roasted the insulation. Main 80a fuse didn't blow
though.

Ah. Also. The armour wasn't terminated at the metal consumer unit. It had been terminated outside and the earth bought in with a separate fly lead. The Gland for the 2 SWA conductors was plastic so no earth fault occured at the gland when the water caused the arcing. The cable WAS RCD protected.

Wondering if there is such a thing as an AFDD master fuse or switch? What's the maximum AMPerage of these things?

Cheers

Spynage

Shanep92 · Jan 11, 2023

The armouring of swa only needs to be earthed at one end, so long as there is a path back to the MET. although it’s very rare you’ll see that, and also a bit “rough”

I’m pretty sure that an RCD usually wouldn’t trip on a live-neutral short as there’s usually no imbalance in the loads of the 2 conductors, and the rcd is (usually) only looking for imbalances of more than 30ma… in practice, chances are that cable would smoulder away all day long.

Another thing to note is that it might not even be on an rcd, especially with it being a sub-main installed in armoured cable. The earthing arrangement of the installation can also play a massive role when it comes to RCD operation.

Main fuse definitely won’t have blown if the breaker supplying the cable didn’t.

If you had a master AFDD at the source, then a fault like that would have wiped the power to the complete installation. Which most likely wouldn’t be ideal.

You can protect individual circuits/submains using AFDD’s, for instance, if you had an AFDD at the starting end of the submain in question, you’d of lost power to the DB that the submain itself feeds, and you wouldn’t be able to regain that power until either the fault is rectified or the AFDD is removed. A lot of firms now produce AFDD/RCBO’s, but they’re still very expensive.

AFDD for main supply?

spynage

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