Adding RCD Protection to This Consumer Unit

[EFLImpudence]

That wouldn't be overload - and I did say 'if compliant'.

 

[ericmark]

Surely the MCBs in the garage are the overload protection for the cable - if compliant.

I seem to remember a limit of 3 meters, so to be compliant the overload device needs to be within 3 meters of the DNO head and unlikely to be damaged think it was 434.2.1 but there has been some argument about that, as it would mean an unfused spur has a maximum length of 3 meters.

So with a 60 amp DNO fuse you need 10 mm² and with a 100 amp 16 mm² and it would be unusual to wire a garage with such a heavy cable, in the main we see 6 mm² and the current it can take does vary as with all cables depending on installation method but clipped direct could be up to 62 amp with LSZH but unlikely clipped direct over whole length.

I remember a housing estate where they used 2.5 mm² up to an adaptable box, then 4 mm² SWA cable out to the garage, seems they had 4 mm² left over from another job, so one has to be careful.

However I have in the past been approached at work for advice, and given it with best intentions, then visited and realised there was something missing in the telling, so I am wary of saying simply that's OK go ahead.

 

[EFLImpudence]

I seem to remember a limit of 3 meters, so to be compliant the overload device needs to be within 3 meters of the DNO head and unlikely to be damaged think it was 434.2.1 but there has been some argument about that, as it would mean an unfused spur has a maximum length of 3 meters.

That is a DNO rule with little regard to electrical principles.

You frequently refer to the 3m. limit regulation for spurs which you misread. It only applies where fault protection is not met, in which case fire protection is also required.

So with a 60 amp DNO fuse you need 10 mm² and with a 100 amp 16 mm²

Yes.

and it would be unusual to wire a garage with such a heavy cable, in the main we see 6 mm² and the current it can take does vary as with all cables depending on installation method but clipped direct could be up to 62 amp with LSZH but unlikely clipped direct over whole length.

So, would 6mm² be protected against overload by, as in the OP, 42A of MCBs at the garage end? Obviously assuming everything else is compliant.

If, as the DNO state, you must additionally fuse after 3m. - but you may use a fuse of the same rating as the DNO fuse (60A or 100A) - what happens about the 3m. of cable from cutout to additional fuse - and how is the remainder of the run better protected?

Plus, how do the DNO train their fuse to ignore overload and fault current so that only your additional one will blow?

 

[ericmark]

That is a DNO rule with little regard to electrical principles.

The DNO does not need to comply with BS 7671, I did put up the regulation number. I personally consider it to be where we has a bus bar chamber with many boxes coming off it, where it may have a 500 amp feed, and clearly impossible to have a cable rated 500 amp between the bus bar and the unit being fed, more industrial than domestic, however BS 7671 is the rule book be it industrial, commercial, or domestic.

It is down to the designer to consider if the cable is likely to be damaged, and if damaged likely to cause a danger if not protected in some way. And in the main an unfused spur is unlikely to cause a problem when if damaged the likely result is a short circuit so as long as the loop impedance is less than 1.44Ω it will likely trip a B32 MCB in around 0.01 seconds.

The most dangerous supply cable is likely the 110 volt leads used in the construction trade using a fuse to the supply of the transformer so a 3 kVA transformer can supply 54 amp line to earth without causing a 13 amp fuse to blow, causing a very real fire risk.

However I have watched people walking on hot coals on the TV, does not mean I want to do it, and having an over load on the supply side of the cable is really the way we should go.

 

[EFLImpudence]

The DNO does not need to comply with BS 7671,

No, they just make up rules.

I did put up the regulation number.

Which you always misread. It has nothing to do with the subject at hand.

You have to read this one first:

433.2.2 The device protecting a conductor against overload may be installed along the run of that conductor
if the part of the run between the point where a change occurs (in cross-sectional area, method of installation, type
of cable or conductor, or in environmental conditions) and the position of the protective device has neither branch
circuits nor outlets for connection of current-using equipment and fulfils at least one of the following conditions:

(i) It is protected against fault current in accordance with the requirements stated in Section 434

(ii) Its length does not exceed 3 m, it is installed in such a manner as to reduce the risk of fault to a minimum,
and it is installed in such a manner as to reduce to a minimum the risk of fire or danger to persons (see also
Regulation 434.2.1).

 

[JohnW2]

If, as the DNO state, you must additionally fuse after 3m. - but you may use a fuse of the same rating as the DNO fuse (60A or 100A) - what happens about the 3m. of cable from cutout to additional fuse - and how is the remainder of the run better protected? Plus, how do the DNO train their fuse to ignore overload and fault current so that only your additional one will blow?

Quite so !

Furthermore, the DNOs 'jurisdiction' seems to end at (just before) the meter, so they presumably can't impose any rules in relation to the outgoing tails from the meter (i.e. from meter to CU/whatever), can they?

Kind Regards, John