Electrical installation in an outbuilding

[Nick]

{Split from this thread, as it's an interesting conversation in its own right}

This is all good stuff, thanks. 

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?

Because I'm on top of a hill and my workshop is 100 mtrs from the house on its own phase, I've also got a 100A lightning barrier unit and "cleaner" (a Telematic MA20 - should this be replaced? Its 15 years old...).

Suggestions welcome!

[ppppenguin]

"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.

[Nick]

I'm pretty sure of my mechanical skills and basic electrical capabilites (got a degree in EE and built a house or two, the current one being pretty high tech). But I'm far from over confident and am out of touch with the 2011 and subsequent changes.

The workshop has one of the three phases and is earthed back at the house and fused at 100A at the supply inlet. There is a subsequent fused 60A isolation switch post utility meter and then a 100A armoured cable underground to the workshop which has several earthing rods and a buried grid - that was designed and installed by specialists -the feed screen is indeed isolated at the workshop. Live and neutral go into a single consumer unit with a two pole rcbo followed by per-circuit mcbs...., i.e. if it trips, the workshop goes dark.

I like the idea of a split consumer unit, presumably with the lighting unit and alarms on say a 10A rcbo and the rest on a 50A one.... I have a mate who helps me who is a qualified electrician but he mainly does data centres, not domestic...

Note. One of the other phases is for the main house, the other for a cottage and another workshop in the grounds. All three phases are also used by the ground source heat pump in the house, but thats all...

[ppppenguin]

Nick, you are confusing RCBO and RCD. RCBO is an RCD and MCB in the same device.

Typical RCBO: http://www.screwfix.com/p/wylex-16a-30ma...rcbo/44169

Useful CU with twin RCDs and several MCBs. "High integrity" means that you can configure 1 or more MCBs without RCD protection. Normally used for alarm circuits and suchlike. If installation is TT (has its own earth rods, you say your workshop has this arrangement) then ALL circuits must be covered by an RCD. Most CUs like this can be configured very flexibly.I fitted one similar to this at a friends house last year: http://www.screwfix.com/p/bg-10-way-high...-rcd/72088

[Nick]

Nick, you are confusing RCBO and RCD. RCBO is an RCD and MCB in the same device.

Not confused at all - I was asking if you could protect each half of the unit with an rcbo, i.e. Combine the overload protect for each half with rcd protection, then use a seperate mcb for circuit.

In your scenario of using an rcd in each half, where is the fuse for the cu in the workshop, or are you relying on the one back at the house?

Thanks

[ppppenguin]

No, you can't use an RCBO like that, or at the very least it would be a rather non-standard arrangement. The CU itself doesn't need any further protection. Normally it's protected by the supply utility's fuse, in your case by the fuse at the sending end of the cable. The big cable is also protected by that fuse. Also if you use a 50A MCB or RCBO in the workshop CU there won't be adequate discrimination between that and the 60A fuse at the source of the cable. There also may not be adequate discrimination between the 50A RCBO and a 32A MCB for a ring final.

This sort of situation can arise in a domestic installation where a very powerful eelctric shower is fitted, needing a 50A MCB and the utility's fuse is only 60A. Ideally the utility's fuse shoul dbe uprated to 80A or 100A. In practice a blind eye is often turned.

Incidentally, you wrote "100A cable" in an earlier post. Cable has a size in sq mm. The current rating depends on various factors, mainly on method of installation. A long cable may need to be larger than required for current carrying to meet voltage drop and loop impedance criteria.

[ppppenguin]

"Live and neutral go into a single consumer unit with a two pole rcbo followed by per-circuit mcbs...., i.e. if it trips, the workshop goes dark."

I rather doubt it's a RCBO. Almost certainly an RCD. These are also marked with a current rating but that's purely a maximum current it's allowed to pass. There's no overcurrent or short circuit trip. This sort of arrangment, RCD as main switch followed by MCBs, was common until outlawed by the 17th ed regs. It was a low cost way of meeting 16th ed regs and was sometimes used in the days of 15th ed. I never liked it, preferring to protect those circuits that needed RCDs with individual RCDs or RCBOs.

[Nick]

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...

[ppppenguin]

With a TT system all circuits must be RCD protected. The CU I illustrated is typical of current domestic practice. It's the cheapest way of meeting 17th ed. The better way is a simple main switch with an RCBO for each circuit. The only downside is expense.

Not sure what to say about the damp conditions. I assume there's no water falling directly on the CU so IP rating isn't very important. The problem is damp atmosphere hence corrosion and tracking. A plastic bodied CU will be better under these conditions. I would certainly stand it off the wall by 0.5" or so. Some bits of plastic conduit would do for that. What I'm really worried about is how the MCBs etc will fare under damp conditions. Might be worth an enquiry to MK. My preferred solution would be to put just a main switch (or even a junction box) in the cellar and the CU in a dry location. A tiny bulb or small resistor, perhaps 2 or 3 watts dissipation, inside the CU would be an unconventional solution to damp. I don't know if it's a good idea.

Ancient paper insulated cable is actually very reliable unless it's been physically damaged. I've seen jointers working on stuff that's well over 50 years old and inside it looks as good as new.

[Nick]

Ancient paper insulated cable is actually very reliable unless it's been physically damaged. I've seen jointers working on stuff that's well over 50 years old and inside it looks as good as new.

When the Utility Emergency Response Team came - 2 hours after the fire brigade had called them - a team of navvies had dug down to the cable outside the house. It was raining. The hole was filling up with water. The cable was still on fire in the garage about 5 metres away...

The guy from the utility company just got into the hole with big wellies on and rubber gloves, and using a huge pair of ceramic-bladed shears, he cut the cable...

That stopped the source of the fire - the fire brigade then went in and extinguished the rest...

The guy in the hole then proceeded to splice new tail onto the cut cable whilst it was still live and in the rain - I was rather concerned about this but he said it was normal - one of the navvies kindly held a brolly over him whilst some of this was going on

He said that the 240VAC stuff is OK when its live and you know what you are doing, it's the 11KV and higher where it gets interesting... and it was simpler and quicker to do it live rather than have to mess with the substation...

They filled the splice with resin with a sand filler, and that was that... he did say that the main feed from the substation was fine...

I do use APC UPSs on critical infrastructure as the nominal mains can go up to 255VAC and stay there for long periods - it also drops to 220VAC and sometimes is all over the place... we get a fair few brown-outs, especially in the winter (sometimes 10+ a day), and the occasional black-out.

The joys of being out in the country...