12-02-2015, 10:08 AM
Thanks again - I mentioned in a previous post that there is no exported earth - its a TT arrangement, as you surmised.
When I said "100A cable", I meant that when the cable was installed, the contractors rated it for 100A over that distance in case I expanded the workshop's power budget - it's a beast of a cable, and is terminated in the workshop's cellar, which is damp, hence the issue with failing RCDs. I'm not sure offhand of the mm2 spec.
You are also correct in that the CU in the cellar is using an RCD, not an RCBO - this was installed in 2005 or before (can't remember exactly), so that would explain the configuration which was prior to 17th Ed. The existing CU is not IP-rated.
My understand of the regs is that with a TT earth, you need RCD protection, and that a split CU as previously discussed would be OK - I hear what you say, so I would need RCDs on both sides of the CU, but how to protect the CU overall at the workshop end? I'm perfectly happy with per-circuit MCBs... why not use RCBOs? Is there a better/simpler/safer way?
FWIW, I've ordered (and will read an absorb!) "The Electrician's Guide to the 17th Edition of the IEE Wiring Regulations BS 7671:2011 and Part P of the Building Regulations, 3rd ed"
Regarding the general issue of long cables, the feed to the house is via a buried old 3-phase cable, containing lots of waxed paper etc., probably dating from the 1950s or before and running perhaps a quarter of a mile under fields. The impedance of the cable is very high - prior to the rebuilt of the property, the old Bakelite utility termination & fuses for the cable arced over (probably damp & carbon tracking) and started burning like a Roman Candle in the middle of the night, incinerating the switchboard above and burning down the garage attached to the house - the substation fuse didn't go (according the the utility provider) due to the length of the cable. Not much we can do about that!
The fire was one of the catalysts to rebuild the property...
When I said "100A cable", I meant that when the cable was installed, the contractors rated it for 100A over that distance in case I expanded the workshop's power budget - it's a beast of a cable, and is terminated in the workshop's cellar, which is damp, hence the issue with failing RCDs. I'm not sure offhand of the mm2 spec.
You are also correct in that the CU in the cellar is using an RCD, not an RCBO - this was installed in 2005 or before (can't remember exactly), so that would explain the configuration which was prior to 17th Ed. The existing CU is not IP-rated.
My understand of the regs is that with a TT earth, you need RCD protection, and that a split CU as previously discussed would be OK - I hear what you say, so I would need RCDs on both sides of the CU, but how to protect the CU overall at the workshop end? I'm perfectly happy with per-circuit MCBs... why not use RCBOs? Is there a better/simpler/safer way?
FWIW, I've ordered (and will read an absorb!) "The Electrician's Guide to the 17th Edition of the IEE Wiring Regulations BS 7671:2011 and Part P of the Building Regulations, 3rd ed"
Regarding the general issue of long cables, the feed to the house is via a buried old 3-phase cable, containing lots of waxed paper etc., probably dating from the 1950s or before and running perhaps a quarter of a mile under fields. The impedance of the cable is very high - prior to the rebuilt of the property, the old Bakelite utility termination & fuses for the cable arced over (probably damp & carbon tracking) and started burning like a Roman Candle in the middle of the night, incinerating the switchboard above and burning down the garage attached to the house - the substation fuse didn't go (according the the utility provider) due to the length of the cable. Not much we can do about that!
The fire was one of the catalysts to rebuild the property...
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