Meter tail max lengths?

[EFLImpudence]

Why do you say 'not allowed' - because of 314.1?

Are you querying 'not allowed' itself as opposed to no one would do it?

Talking about new installs - yes and even more so 314.2.

If a single RCD in a consumer unit is not now allowed then surely the same goes for placing one upstream.

If so, how do you feel about a fuse protecting long tails

Unavoidable.

(or an up-front RCD in a TT installation)?

Would not be fitted now, surely.

 

[JohnW2]

Why do you say 'not allowed' - because of 314.1?

Are you querying 'not allowed' itself as opposed to no one would do it?

Yes.

Talking about new installs - yes and even more so 314.2. ... If a single RCD in a consumer unit is not now allowed then surely the same goes for placing one upstream.

I would agree, but we've recently seen PBoD questionning whether 314 actual go as far as 'forbidding' a single RCD.

If so, how do you feel about a fuse protecting long tails

Unavoidable.

True - but if one has already got such a (single, installation-wide) fuse ('unavoidably' if the tails are long), would it make things much worse to also have an RCD as well (so that the tails could be buried)?

(or an up-front RCD in a TT installation)?

Would not be fitted now, surely.

I've got them, and probably would have to even if it were a new (TT) install. Although all the final circuits are individually protected by RCDs/RCBOs, I have some long distribution circuits - and the fuses in the switch-fuses obviously do not provide for the required disconnection times for L-E faults in the distribution circuits - so how else could I have achieved those required disconnection times without up-front (Type S) RCDs?

Kind Regards, John

 

[EFLImpudence]

Are you querying 'not allowed' itself as opposed to no one would do it?

Yes.

Whilst the regulations may not specifically state -
11: Though shalt not have an installation covered by one RCD.
as you have said there are several instances which could be read as such and if it can be avoided with SWA then it should be.

True - but if one has already got such a (single, installation-wide) fuse ('unavoidably' if the tails are long), would it make things much worse to also have an RCD as well (so that the tails could be buried)?

Yes, a single bulb 'popping' in the servants' toilet would take out everything.

(or an up-front RCD in a TT installation)?

Not a 30mA.

I've got them, and probably would have to even if it were a new (TT) install. Although all the final circuits are individually protected by RCDs/RCBOs, I have some long distribution circuits - and the fuses in the switch-fuses obviously do not provide for the required disconnection times for L-E faults in the distribution circuits - so how else could I have achieved those required disconnection times without up-front (Type S) RCDs?

Not a 30mA.

 

[JohnW2]

Whilst the regulations may not specifically state -
11: Though shalt not have an installation covered by one RCD.
as you have said there are several instances which could be read as such and if it can be avoided with SWA then it should be.

As you know, that's essentially my view.

I've got them, and probably would have to even if it were a new (TT) install. Although all the final circuits are individually protected by RCDs/RCBOs, I have some long distribution circuits - and the fuses in the switch-fuses obviously do not provide for the required disconnection times for L-E faults in the distribution circuits - so how else could I have achieved those required disconnection times without up-front (Type S) RCDs?

Not a 30mA.

It's true that, per historical practices, mine are 100 mA ones, but they probably could just as easily be 30 mA. The difference between 30mA and 100mA RCDs is not particularly useful in terms of 'discrimination' at the best of times, and totally irrelevant in terms of discrimination when the upstream one is time-delayed. I'm not even sure how the 'historical practice' of using 100mA ones actually arose. (and, no, my distribution circuits are not 'buried!).

Kind Regards, John

 

[EFLImpudence]

Aren't the up-front RCDs all S type?

These, as you know, cannot be used to protect (new) buried tails.

I suspect historically the '3x' figure was used but not necessarily relevant with RCDs, depending on the fault.


Plus, of course, it would be particularly silly with a 'top of the range' all RCBO board to have an up-front 30mA device.

 

[JohnW2]

Aren't the up-front RCDs all S type?

AFAICS, historically they've all been 100mA Type S - but I'm not sure I understand how any of that arose. As I said, once it's Type S, there's no point in having >30mA in the hope that it will produce some sort of discrimination (which it probably wouldn't, anyway). Furthermore, AFAIAA, these 'historical practices' for TT installations arose before there were (m)any downstream RCDs, so even the time-delay (and certainly not the 100mA) wasn't needed at that time. However, the moment one has downstream RCDs/RCBOs, one needs a TYpe S, but I don't really see why it needs to be 100mA, rather than 30mA.

These, as you know, cannot be used to protect (new) buried tails.

Sure, even if 30mA, a Type S RCD is no good for protection of persons. As soon as there are any downstream RCDs/RCBOs (which, these days, there usually will be), one obviously needs any upstream RCD to be time-delayed - so, as you say, in that situation the upstream one would not be adequate to protect buried tails (becasue it was Type S).

Plus, of course, it would be particularly silly with a 'top of the range' all RCBO board to have an up-front 30mA device.

I don't think that it would be any more silly with a 30mA than with a 100mA one, but I agree that it would be silly to have a regular (i.e. not time-delayed) RCD up-front. However, as I've said, if there are long distribution circuits/tails to be protected against L-E faults in a TT installation there would, AFAICS (and as with my installation), be a need for a (30mA or 100mA) time-delayed RCD up-front, even with your all-RCBO CU. However, as per the next paragraph, your all-RCBO CU might well be non-compliant with 314 if there had to be any sort of up-front RCD, unless all those RCBOs were DP!! ....

.... As previously discussed, there is a potential problem (and I confess I have two or three examples) if one has an up-front RCD (to protect tails/distribution cables) and any SP RCBOs protecting final circuits - since a N-E fault on a final circuit can then take out the whole installation (well, one whole phase, in my case) - arguably in violation of 314.

FWIW, for the first decade or so I was in my present house, with no RCDs in any of the CUs, the entire TT installation was protected by an up-front 30mA 'regular' (not Type S) RCD for each phase, the previous 4-pole VOELCB having been ripped out soon after I first walked through the door

Kind Regards, John

 

[plugwash]

However, the moment one has downstream RCDs/RCBOs, one needs a TYpe S, but I don't really see why it needs to be 100mA, rather than 30mA.

Because of variability. AIUI the regs require that a RCD hold in at half it's rated trip current, and trip at it's rated trip current and do not say at what point exactly in that range it goes over the edge.

So if you have a time delay but the same nominal trip current and the upstream RCD happens to have a lower actual trip current than the downstream RCD then a low current fault*, particulally a gradually increasing one such as something getting gradually more damp could trip the upstream RCD without tripping the downstream one.



* in the general sense of the term fault.

 

[JohnW2]

Because of variability. AIUI the regs require that a RCD hold in at half it's rated trip current, and trip at it's rated trip current and do not say at what point exactly in that range it goes over the edge. So if you have a time delay but the same nominal trip current and the upstream RCD happens to have a lower actual trip current than the downstream RCD then a low current fault*, particulally a gradually increasing one such as something getting gradually more damp could trip the upstream RCD without tripping the downstream one.
* in the general sense of the term fault.

Yes, I suppose that may well be the explanation, now. What you say is obviously theoretically true, but only because of "...a low current fault*, particulally a gradually increasing one such as something getting gradually more damp...". Most faults are going to be >30mA, and not 'gradually increasing', so they should take out the non-time-delayed one first.

However, as I said, although I may be wrong, I thought that the 100mA 'convention' for up-front RCDs arose before there were (m)any downstream RCDs (i.e. in CUs), such that discriminaton was not an issue. Mind you, I must be at least partially wrong - because, as I said, before I had RCDs in CUs, I had up-front 30mA RCDs.

Whatever, given that there will now nearly always be downstream RCDs, such that the up-front one has to be time-delayed, as EFLI has said, a time-delayed RCD would not be acceptable for providing the required protection for buried tails (even if it were 30mA), even though it's OK for protecting the tails against L-E faults (BS7671 faults).

Kind Regards, John

 

[drgl]

Surely this isnt actually a supply move though?

Hes just moving the CU...

Cutout and meter would be staying exactly where they are, therefore its not a DNO job at all?

The only tricky bit would be that your "not allowed" to put your own equipment in that box, but need a switchfuse for tails over 3m... I suspect you might well get away with putting a switchfuse in there though.

Also, why cant the tails be buried? Surely thats fairly typical in say a block of flats?

My mates flat has the cutout and meter in a box down at the entrance, then SWA running from there vanishing off into the wall/floor, then appearing again at the consumer unit.

The new CU location would be about 4m away from the current meter location. So can SWA cable be used from the meter to the new CU location? I should have a sparky coming out in the next week or so to quote on rewiring the whole lot so I'll see what they say.

 

[JohnW2]

The new CU location would be about 4m away from the current meter location. So can SWA cable be used from the meter to the new CU location? I should have a sparky coming out in the next week or so to quote on rewiring the whole lot so I'll see what they say.

It doesn't necessarily have to be SWA (your electrician can advise/decide) but, as has been said, if it's 4m from meter to CU, there will have to be a switchfuse at the meter end.

Kind Regards, John

 

[drgl]

Thanks John. Does that have any down side? eg, if a trip goes in the CU does the whole lot go off? New CU's are split load which is a good idea IMO as a whole house is unlikely to be plunged into total darkness, however would a switchfuse change that scenario?

 

[JohnW2]

Thanks John. Does that have any down side? eg, if a trip goes in the CU does the whole lot go off? New CU's are split load which is a good idea IMO as a whole house is unlikely to be plunged into total darkness, however would a switchfuse change that scenario?

If the fuse in the switchfuse were to blow, then that would obvioulsy 'take out' your whole house. However, not only is the switch fuse unavoidable if you have >3m from meter to CU, but the fuse is most unlikely to blow (something else in your CU would trip before that happened) - probably no more likely than the DNO's service fuse blowing.

Kind Regards, John

 

[drgl]

Thanks again John, very helpful. I'm sure our old (new!) house had what you describe! IIRC the live & neutral tails from the meter went into this switch before then going off to the consumer unit. Wish i'd taken a picture of it now to clarify!

 

[sparkwright]

Based on experience as well as a recent post, consider fitting another kiosk outside to house your switch fuse.

As far as I know, tails are allowed in brick cavities, at least they were. Definitely the safest place for them IMO. Or in a boxing, perhaps in the corner of a room.

Would have thought a study to be a good room for the consumer unit anyway, bearing in mind you are likely to have shelves and cupboards, which could hide the consumer unit.

Seems hardly worth the effort.