Have an double outbuilding, which has it's own feed via armored cable to a small consumer unit ... which amongst other things will provide a single 6A lighting cct. and a 16A radial cct. Not wired out yet - now time to do it :-)
I had initially thought of putting all the wiring within the building in conduit/ducting ....
I have had a re-think ... all sockets & switches will be surface mounted, and I can still proceed with conduit for the vertical drops to switches & sockets, to provide basic low impact mechanical protection.
Is there a need for the rest of wiring to be 'enclosed' or protected by using Hi-Tuff or similar.
If there isn't I could wire out with T&E and only protect the vertical drops, and have it clipped to trusses in between drops and runs back to CU. (open ceiling to trussed roof )
This is a non-habited building (in official Building Regs terms), but I do need it to be compliant with Regs as will require NICEIC cert.
Surface wiring with some bits given extra protection in conduit or trunking is perfectly acceptable for environments where there is a low risk of mechanical damage.
It can, in the sense that it has adequate contact separate to count as an isolator. Whether you want an RCD as the main incomer switch is a different matter.
Yes. This would be common practice on 16th edition TT systems where a
100mA trip time delayed RCD would be used in the place of a main switch on the consumer unit.
Thanks John ... outbuilding is a TT system .... so RCD with 2 mcbs will provide all my needs ... I'll connect up to a non RCD protected feed at house end, and only earth amour wires at house end. Both house & outbuilding have their own earth rods.
Yup sounds ok - if you use a RCD as the main switch in the outbuilding, then obviously a RCD trip will lose lighting on a trip - but that may well be ok for your application.
Yup, use an insulated CU at the far end.
Is the house TT as well? If so, then the submain will need RCD protection at the head end as well in order to clear fault arising from cable damage or the outbuilding suffering some disaster.
I have a similar arrangement with a split load CU dedicated to outside circuits in the house. That is setup 16th edition style with a 100mA trip time delayed RCD as the incomer, and a 30mA trip RCD protected section feeding outdoor sockets, and lighting etc. The garage submain is fed from the 100mA side. In the garage there is a split load CU with a normal main switch - with internal lighting on that part, and then a
30mA RCD with the garage and workshop sockets, plus external lights on it. So the submain to the garage has RCD protection, however its independent of the 30mA protection for the sockets in there.
House is currently TT ... at some poiunt they should put me on PME. I can certainly feed from RCD at house ... and RCD at outbuilding.
Do either of these 'have' to be 100mA time delayed ? ........ I ask, as a standard 32A 30mA RCD is around =A319 whereas the 100mA time delayed jumps to 100A capacity and =A378
Opt A ... I use a 32A mcb out of split load CU in house which has 30mA RCD protection .,...... at outbuilding fit RCD & 1x16 and 1 x 6a mcbs
Opt B if I have to use 100mA delayed ... install additional enclousre with 100mA RCD (100A) and a 32A MCB and use this to feed cable to outbuilding. At outbuilding fit 100mA RCD (100A) & 1x16 and 1 x 6a mcbs
If I use opt A is 30ma standard RCD ok at outbuilding ? .... or does it have to be 100mA time delayed.
the head end as well in order to clear fault arising from
While you are TT, without RCD protection on the head of the submain, you may find you have no reliable way of clearing an earth fault on it[1].
That depends ;-)
If you want cascaded RCDs, then the upstream one must be time delayed - otherwise there is no reliable way of ensuring which will trip first should a fault occur (assuming the fault is over the trip threshold of both devices)
So with the classic 16th style setup for a TT system, you would swap the incomer switch for a 100mA Type S RCD. That would protect what would otherwise have been the "unprotected" side of a split load CU, and then a normal 30mA trip RCD would cover a subset of the ways used for socket circuits.
In the past I had a TT setup with two CUs - one for sockets etc with a
30mA trip RCD, and a separate CU, with a standard 100mA trip RCD for lighting etc. Since they were not cascaded there was no need for an expensive type S.
In these days of the 17th edition its a bit different. Many CUs have multiple RCD protected sections, and these can be wired to that they are not cascaded. That eliminates the requirement for type S (aka Time Delayed) RCDs in most cases.
Submains however are still hard to deal with without a time delay RCD at the head *if* you want to keep the lights on in the outbuilding on a trip.
Assuming the house end of the cable is on the RCD side, there is no point in having further RCDs downstream in this case. You would just have more devices to reset on a trip!
If you have a 100mA type S at the head end, then you don't need another at the remote end. Just 30mA RCD(s) where you need them. So a garage for example - surface wired so no need for RCDs to protect hidden cables - the lights could be on a main switch side of the garage CU (since they still have fire protection etc from the head end 100mA RCD), and then a
30mA section for the sockets etc.
30mA or lower is required any place you want the RCD to protect you from shock. Higher trip thresholds are only for protecting the installation from the effects of earth faults. So for shock protection you need 30mA or lower. If you were installing a socket primarily for use in the garden, you may argue that 10mA protection would be better.
Yup...
Opt C: bog standard 30mA RCD at head end - no further RCDs in the outbuilding. A £15 non maintained emergency light to keep the lights on if you get a trip.
[1] Imagin a spade through the cable shorting live to earth. If you have
10 ohms of earth electrode resistance, then you maximum fault current will only be 230/10 = 23A, or not enough to trip the MCB so the power would stay on indefinitely!
For full "independent" protection, the underground cable and outbuilding, an RCD at the CU connected to the SWA underground cable and a simple DP isolator with two mcbs in the garage. The whole lot is then protected with no influence from other circuits. The only point is that if the RCD trips you need to go to the main house to reset, instead of keeping it all self-contained inside the outbuilding.
Cascading RCDs (one on CU and one in garage) means the RCD at the CU just protects the SWA cable.
Option C is the cheap an cheerful option. Perfectly legal but with some minor drawbacks.
It is an ideal soloution to an outbuilding that is going to be used for storing junk and powering the odd powertool. If the head end is sharing a
30mA RCD with other circuits and the outbuilding contains a fridge/freezer then a sperate RCD or RCBO at the head end would be a better option.
Quick question Rick - have you got non-RCDed circuits in your house? You mentioned a splitload CU, these tend to be used with TN systems not TT systems.
Indeed - with an emergency light if loss of lights on a trip is a problem. The only other downside, is if you do get a trip, you need to go to the head end to reset it.
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