11-02-2015, 05:21 PM
"As I'm about to (yet again) replace an old failed consumer unit in my workshop with a new, IP65-rated, one (hopefully to "stop the rot"), should I be just using an RCBO main switch (60A) and then per-circuit MCBs, or something different?"
TBH, if you need to ask this question you might consider getting a registered electrician to do the work and certify it. Since it's not domestic premises I think it's legal to do the work yourself. Not that I get too worked up about the legalities, I just want it safe.
This is a slightly complicated situation and I'm not 100% sure of all the answers but I would treat it as a supply to a separate premises. Hence the minimum would be a so-called "17th edition" CU. These have a main switch, 2x RCD plus MCBs. A single RCD incomer (not RCBO as you said) is not permitted under 17th ed. Reason being that a single earth leakage fault would put the whole place in darkness. The best and most expensive method is using a separate RCBO for each circuit.
There's also the question of what's porotecting the cable to the workshop. In a larger 3 phase installation the main switch will often be a 300mA or 500mA time delay RCD. If it's not time delay then there's no discrimination between it and ordinary RCDs downstream. Bit of a nuisance when a small fault cuts the entire 3 phase supply.
The earthing arrangements need careful consideration. Exporting the earth over that distance strikes me as a bit iffy. Easiest guaranteed safe method is to make the workshop TT with its own earth rod. The cable armour would need to be earthed at the sending end and carefull insulated at the workshop.
TBH, if you need to ask this question you might consider getting a registered electrician to do the work and certify it. Since it's not domestic premises I think it's legal to do the work yourself. Not that I get too worked up about the legalities, I just want it safe.
This is a slightly complicated situation and I'm not 100% sure of all the answers but I would treat it as a supply to a separate premises. Hence the minimum would be a so-called "17th edition" CU. These have a main switch, 2x RCD plus MCBs. A single RCD incomer (not RCBO as you said) is not permitted under 17th ed. Reason being that a single earth leakage fault would put the whole place in darkness. The best and most expensive method is using a separate RCBO for each circuit.
There's also the question of what's porotecting the cable to the workshop. In a larger 3 phase installation the main switch will often be a 300mA or 500mA time delay RCD. If it's not time delay then there's no discrimination between it and ordinary RCDs downstream. Bit of a nuisance when a small fault cuts the entire 3 phase supply.
The earthing arrangements need careful consideration. Exporting the earth over that distance strikes me as a bit iffy. Easiest guaranteed safe method is to make the workshop TT with its own earth rod. The cable armour would need to be earthed at the sending end and carefull insulated at the workshop.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
