Choosing a CU for TT systems?

The 100mA RCD needs to be time delayed.

If all circuits are RCD protected why is the 100ma RCD needed? It's only protecting the short amount of cable /within/ the cu that is effectively a continuation of the meter tails beyond the isolator.

AIUI two seperate 30ma protected CUs fed from a henley would be acceptable.

Time-delayed = "selective" and may have an icon on the front with an S in a square:

non-selective:

Not all the circuits are RCD protected. The CU is divisible into 3 sections, the first of which has no RCD protection.

Plastic rather than metal clad I take it?

That sounds a bit cheap for a time delayed RCD to me...

If the 100mA RCD is a normal non time delayed one, then nope - you won't get discrimination on any earth faults that result from leakages exceeding both trip thresholds.

A way round it would be to retain the main switch and add a pair of RCBOs to the non RCD positions (or not use those positions)

Another point to bear in mind with TT is that isolation must be all-pole (double-pole in a single-phase installation). Hence normal MCBs and RCBOs with solid neutrals can't be considered to provide isolation for individual final circuits. Groups of circuits fed via 2-pole RCDs can be isolated using the relevant RCD, but to isolate circuits in the 'unprotected' section of a 17th ed. rig requires use of the main switch, which the designer might (or might not) consider fails to meet the 'division for convenience' requirement.

Another option is to use double-pole MCBs (and a board that will accept them), but they're not so readily available in the UK.

I'm wondering whether an option for Andy is just to retain the original main switch -> 2-RCD split (with no unprotected ways or RCBOs). Group sockets and anything else needing 30 mA protection on one side and put everything else on the 100 mA side (assuming surface wiring).

A further option, e.g. to keep lighting separate, is to take the incomer thro' a separate isolator then split the 'tails' to two separate CUs, perhaps a single-RCD one for lighting and a 2-RCD one for power.

Yes, as I understand this to be a requirement with a TT system. Incidentally, I've got a few feet of meter tail going into this (across the width of the cupboard under the stairs), what's adequate protection for this between the isolator and the CU?

Specifically, it's one of these (although from TLC locally)

That is I think the problem. I asked TLC for the delayed RCD incomer and was told I didn't need it. I think they're wrong, and despite the damned thing being =A350 quid more (How much?! Just for time delay?) I think I need to take it back and swap it. 50 mile round trip I don't often have the chance to do, something else I really don't need.

One rather wonders what good the stock 100mA double pole RCD is then... Is the need for discrimination by time as well as by current really that crucial?

In other penny-pinching news, MK's delayed RCD is 20 quid cheaper and also a rail mount. Anyone know if they're compatible?

I need to use these to supply two outgoing feeds to the workshop. There are devices in the workshop that are incompatible with 30mA RCDs: Specifically a kiln (high leakage current) and also large motors and welders with inrush currents that are a problem for domestic circuit breakers.

AFAIK, I can't get 100mA RCBOs and as it's TT I have to RCD protect everything, so I think I do need a 100mA RCD incomer.

As meter tails are insulated and sheathed, they don't need protecting unless you do a risk assessment and can identify a risk that says they specifically do. In which case it is usual to put them in 50 x 50 plastic trunking with open ends (no need to derate for temperature)

Dunno. This document:

You sure they sent you the right one, cos it sounds like you got the high-integrity one?

Yes. If there is a 100mA fault on the 30mA circuit, both will see a fault current worthy of tripping. If the 100mA RCD is not time-delayed, then either the 30 or the 100 or both will trip. If the 100 is time delayed, then only the 30 will trip.

Probably.

And yet another way around it would be to buy the MK time-delayed, put it in a separate enclosure and put it between the isolator and the CU.

As it's TT, you either need a 100mA time-delayed RCD protecting the whole installation, or every final circuit has to be protected by an RCD somehow. It would be cheaper to put RCBO's in your three-busbar consumer unit and keep the non-RCD main switch. If you need a 100mA protected circuit for the kiln, put an MCB in the consumer unit and an external

If you have a potential problem with 100mA of leakage, then you need to further sub-divide your installation using Henley blocks and put a 100mA time-delayed on each outgoing way on the Henley blocks.

I think I'm going to end up back with two CUs.

Put the switch incomer back into the 15 way Wylex and run it as a straight-up 17th Ed split load with two 30mA RCDs. Take the 100mA fast RCD back to TLC

Buy an extra MK 100mA delayed RCD and couple of MCBs in a simple 4-way housing. Connect up via the Henley block (This is already in, as I have old & new CUs in place during the switchover).

Workshop CUs have their existing switch incomers and 30mA RCBOs on the socket circuits, plain MCBs otherwise (surface conduit wiring, so I don't need to RCD it to protect the cabling).

Everything is protected, although the low-risk and the high-leakage circuits in the workshop are only on the RCD in the house.

Pour a new floor in the workshop to take heavy machinery Get the 3-phase workshop supply re-instated. This is really awkward and could cost too much, owing to a pole issue. I'd also need to remove the single phase feed, to avoid multi-phase in the same building issues (they're being awkward over that).

Yup, it reduces the chance of an earth fault inside the CU before the RCD(s) (which you probably can't clear)

If they are under 2m I don't recall there being any special requirements.

Not sure I can see much obvious from the descriptions either...

Yes, because you don't get any discrimination by current really (save a few mA of "window" that represents earth leakage small enough to trip one but not the other). Chances are most real faults or attempts to electrocute yourself will trip one or both in unpredictable combinations.

RCDs with trip thresholds higher than 30mA are there for infrastructure protection in cases where the overall ELI is too high to meet required disconnection times (or any disconnection at all for that matter). So under the 16th edition one could use a normal 100mA RCD on all non socket circuits so long as it was not upstream of a lower trip threshold device. Using two CUs was an easy way to do that. If however you wanted to do that in one CU, then you need the time delay to get discrimination. The 17th edition's penchant for 30mA protection makes them a little less useful than they were.

Well I have used MK type S RCDs in Hager and the "one before most recent style" Contactum CUs - and they fit fine. There is a fair chance they will fit yours.

If you wire those submains such that they don't require enhanced protection from a 30mA RCD[1] then that is not a problem.

Could you for example add an additional small CU dedicated to the outbuilding feeds, and put your 100mA "normal" RCD in that - might be cheaper than the type S RCD!

Yup either that or a separate CU.

Perhaps I should have read this before typing the last response! ;-)

I wonder why - it would just mean you has a second "copy" of one of the phases....

I'll have a look in my stores over the weekend. I had a couple of 100mA Wylex main switches knocking about. If one of them is time delayed then you can have it.

Allegedly it's because I might then bond the single phase live to its counterpart from the three phase supply, without noticing, then years later they re-phase the supplies on the external poles (two different poles too) without telling me, and then there's a 400V bang that neither party was expecting.

One way perhaps, but I don't fancy trying to haggle a return by post.

I think I'd still need the delayed 100mA. The workshops have two CUs with a mix of 30mA protection (mostly RCBOs), but not on the lights (protected cable) or kiln radial (round blue socket). However that still needs a bare minimum of protection from something, as it's TT, so it's either a single upstream 100mA with discrimination (as suggested), or lots more RCBOs all round.

I have found one in my stores. 100A 100mA WRMT 100/2 S.

So email me with your details and I'll get it into the post for you.

Cheers

I think I would be tempted to keep the main switch, use some RCBOs on the non protected side for some of the house circiuts (paid for buy not buying a second CU) use one of the 30mA RCDs for the rest of the house circuits and replace the other 30mA RCD with a 100mA one for the outbuildings

Cheers

I got an incorrect item from them once and they emailed a label (postage paid) to return the wrong item. The required item arrived next morning. Another time (low value item) they just sent replacements. Generally they are quite reliable and any problems easily sorted.

Or wire each outbuilding as seperate TT supplys with individual earth rods.

The only difference is the expensive CU has fixeds bus bars. Ie they are set at 4, 6 and 5 ways

Your CU is one of these

and allows you to put the no of RCD and non RCD ways as you see fit.

NOTE this is a work in progress picture as some of the MCBs are now RCBOs