Connecting domestic sub-main to REC cutout

[wobbly2]

Been retired for a number of years and not kept up with the industry hence I'm well rusty, so apologies for that.

I recently moved home and found that the sub-main cable (10mm x 3 core SWA) feeding the garage/outbuilding is connected to the electricity boards meter & service fuse (cutout) via 63 amp 30 mA RCD.

Therefore, the sub-main cable and its downstream MCB board rely on the REC fuse for overload & short circuit protection. This was never permitted and I just want to check that is still the case.

 

[mattg4321]

Correct, that’s not right. Some numpty obviously thought the RCCB was an RCBO and provided overload protection.

Swap it for a switchfuse, assuming TN system and no RCD required on the sub main. Otherwise fit one as well as the RCD.

 

[pc1966]

As above, short answer is no.
Long answer is it is permitted if the DNO fuse is suitable and the DNO agrees to its use. In practice it is easier to fit a fused-switch.

You don't say what earthing system the house has (TT or TN) or what the DNO supply fuse rating is, but the smallest typically seen is 60A and that is getting a bit big for fault protection for 10mm cable for any moderate length.

Also you ideally want selectivity between the fuse feeding the sub-main and the house fuse so if someone accidentally puts a pick-axe in to the sub-main you don't lose the house supply as well. Typically you need at least a 1.6:1 ratio between similar BS88 fuses to achieve that, so if the house is on 60A then 32A for the sub-main feed, or 80A & 50A, etc.

If the sub-main fuse is as low as 32A then you will have negligible selectivity with any MCB in the garage CU, even if you limit the socket MCB to 20A. But it would be fine with faults down-stream of 13A plugs due to the fuses in them.

If your house main fuse is 80A or 100A and so permits a 45A sub-main fuse, and the end of sub-main Zs is less than 0.76 ohms (on-site guide Table B4) to meet 5s disconnection with such a fuse then you will get modest selectivity with a 32A B-curve MCB, but even there feeding the garage sockets off 20A makes more sense as they are unlikely to need anything like that total current.

 

[wobbly2]

I deliberately didn't give too much info in the OP as long posts often seem to put people off reading/answering.

Quick rant:
Why do "they" keep changing the terminology - REC becomes DNO, discrimination becomes selectivity - confuses us old gits.

It's likely the installation was done by the last but one property owner - a chippy with appalling DIY skills and a brilliance at bodging, either that or he had a succession of really bad tradespeople in; bath, plumbing, roof, windows, doors and that’s in 4 weeks.

Back to the job at hand.
Earthing system: TNS with a lead sheathed service cable.
Ze: ???
Zs: ???
DNO fuse/Cut-out: 100 amp BS1361.
Sub-main enclosure: Contactum M4ENC - 4 way DIN rail metal clad with 63amp 30mA RCD (Contactum B6303/2).
Sub-main to garage: Estimated 18 metres of 10mm BS6724 SWA 3 core with one core used as earth and armour glanded into steel enclosures at both ends.
Garage DB: 4 way MCB board fitted with 100 amp main switch, 1 x 20 amp type B, 1 x 6 amp type B and 2 spares (Contactum M4N).
Main C/U: Square D Quickline??? Q01-8 (Stab-in type MCB connection to bus-bar).

I would like to have been able to supply the garage sub-main from the main house MCB C/U and regain some much needed wall space (by removing the existing sub-main DB) but I can't find a suitable MCB for that C/U to give selectivity with the garage MCB's so I either ignore discrimination and put up with the inconvenience or give up wall space.

Many years ago, I did some research on feeding MCB consumer units from specialised feeder MCB's (not MCCB's) that provided discrimination with standard MCB's but they were rather rare and were very expensive (I think Merlin Gerin did some). Have such devices become more readily available and cost effective? If so, can someone give me some suggestions/links?

As an alternative to fitting a new switch fuse I wondered about clipping a DIN rail mounting HRC fuse carrier/holder in the RCD enclosure (the type that protrudes through the enclosure front cover like an MCB does, however the only ones I can find are for 10 x 38mm fuses up to 32 amp (various makes) which, I don’t think will be suitable; especially if I later need more than 20amp in the garage/shed.

Wylex used to do the NSC range of HRC fuse carriers/holders up to 45 amp BS1361 which, I think were DIN rail mounted but I can’t recall if they would retro-fit into another makes enclosure and protrude through its front cover. Anyone know if they will? I can get an NSC45 carrier/holder for about £5 which is a lot cheaper than £60 to £70+ for a new Wylex DSF60M. Man-o-man have prices gone up!!!

If I have to go the switch fuse route does anyone know of any units other than the Wylex DSF??M range. The old Wylex 106 metal clad units no longer seem available even on eBay but I guess they will no longer comply as they have a plastic front?

Is there any reason why I should not move the RCD to the load/garage end of the sub-main so I don't have to access the house in order to reset it. I don't think the sub-main itself requires RCD protection as the earthing system isn't TT, therefore as suggested, it should only require a 5 second disconnection time. Anyone correct me on this?

 

[pc1966]

TN-S Ze could be up to 0.8 ohm (I think) and still considered in-spec so measuring it would be an important first step. If too high for OCPD disconnection then it would need an RCD for the sub-main, which is a pain. You don't need an RCD with SWA for any other reason as it is safe if penetrated by a nail, etc, it will take out the OCPD.

It could be a delay-RCD and all garage circuits on RCBO (or local RCD) so little change of false trip, but still an expense and space-hog you could do without.

Fuse holders to fit CUs are like hen's teeth. Schneider make some but it is unknown to me if they fit any of the matching Schneider CU's bus-bars. For example:

DF101 | Schneider Electric Rail Mount Fuse Holder for 10 x 38mm Fuse, 1P | RS

uk.rs-online.com

DF141 | Schneider Electric 50A Rail Mount Fuse Holder for 14 x 51mm Fuse, 1P, 690V ac | RS

uk.rs-online.com

DF221 | Schneider Electric Fuse Switch Disconnector, 1 Pole, 125A Fuse Current | RS

uk.rs-online.com

Depending on how strong you need the selectivity to be you can just feed it off a MCB, but few tell you what to expect. The Hager commercial catalogue (I happen to have to hand) lists MCB-MCB selectivity in tables and you might be able to get some sane creative solution that way.

For example 100A BS88 fuse to 40A MCBs are all coming in selective below around 5kA fault and probably your PSSC is way below that, so no risk of main fuse blowing.

Then a 40A C-curve to 20A B-curve is coming in selective below about 0.85kA and it is possible you won't be seeing that at the end of your sub-main, probably not at the appliance end of the flex for sure. A 40A D-curve is a bit better, but you would need to know the end of sub-main Zs as it might be high to meet 5s (don't have OSG table to hand for value).

 

[wobbly2]

I'll have to try and dig out my test equipment but its not been used for 10 plus years and was fairly old then. Its currently 20 foot up in storage with no immediate access so it may take a while and then may be no good. The current situation has been in place for at least the last 13 years

I've downloaded the latest Hagar Residential and Commercial catalogues and having not looked at any tables and curves for the same 10 plus years they show me how rusty I've become. Use it or loose it comes to mind!

The Schneider DF141 and DF221 look promising. Just prior to reading your post I had found similar by FERRAZ SHAWMUT MERSEN their CMS141 and CMS 221, and was trying to establish if they would be suitable for use here in the UK. Comparing the Schneider and Ferraz units they look almost identical with the only visible difference being the Schneider units are black and don't appear have a lock on the fuse tray whereas the Ferraz units do with cost being similar.