UK When is an isolator not an isolator?

[clanky]

I need to add in another supply to a new workshop but im running out of space next to my dis board. I currently have two extra supplies (via tails to henley's) going to an outside barn and the utility part of my house. Looking at how these two supplies have been fitted (previous owner) the 'barn' supply is obviously the easiest option to copy due to size of the JB (80a RCD). But is this correct? The larger wylex box on the right also has a mechanical breaker on top of a cartridge fuse. I was always taught double isolation so to me the larger box is the right way despite taking up excess room. Or am i wrong and can i get away with the smaller box? ie remove the wylex unit and have 3 of the smaller RCD boxes? As ever TIA.

 

[oscar21]

The smaller unit probably hasn't got any over current protection, its more than likely just an 80A RCD as opposed to a RCBO (but maybe not) so the SWA cable isn't really protected. The one on the right hasn't got any RCD protection but has got overcurrent protection in the form of the fuse.

I would guess the outgoing SWA in both circuits probably doesn't need RCD protection at all (but it may do) but they both need fusing in some way.

If you are really struggling for space then you could replace everything there for a metal consumer unit with several MCB's to protect the outgoing SWA cables but they are much better on fuses, the ideal scenario is 3 of those wylex switch fuses.

*The SWA cables may need RCD protection if the incoming supply is a TT system (earth rod) or if they are supplying something critical that doesn't have RCD protection down stream.

*Fuses are better because even if the SWA is on a 50A MCB a fault on a 6A protected circuit downstream could still trip the 50A instead of/as well as the 6A where as a fuse probably wont blow as easy.

 

[westward10]

MCBs are not better than fuses you are almost certainly going to lose selectivity.
RCDs are used for fault, additional and fire protection. @oscar21 what would something critical be that needed RCD protection?

 

[littlespark]

just an observation..... no gromets in the top holes

 

[oscar21]

MCBs are not better than fuses you are almost certainly going to lose selectivity.
RCDs are used for fault, additional and fire protection. @oscar21 what would something critical be that needed RCD protection?

Thats why I said the SWA cables are better ON fuses, not THAN fuses.

By critical I mean sockets, hot tubs, things like that. If he replaces the RCD with a fuse because the SWA doesn't need it, whatever is on the end of it could be left unprotected. Safer the way it is now IMO.

 

[westward10]

Thats why I said the SWA cables are better ON fuses, not THAN fuses.

By critical I mean sockets, hot tubs, things like that. If he replaces the RCD with a fuse because the SWA doesn't need it, whatever is on the end of it could be left unprotected. Safer the way it is now IMO.

Apologies I misread your post.

 

[clanky]

The smaller unit probably hasn't got any over current protection, its more than likely just an 80A RCD as opposed to a RCBO (but maybe not) so the SWA cable isn't really protected. The one on the right hasn't got any RCD protection but has got overcurrent protection in the form of the fuse.

Thats correct. Its just a 100ma RCD on the left side.

I would guess the outgoing SWA in both circuits probably doesn't need RCD protection at all (but it may do) but they both need fusing in some way.

Both feeds go to a disboard with RCD incomers, so Im guessing you're correct (again) on them not needing RCD at this end?

*Fuses are better because even if the SWA is on a 50A MCB a fault on a 6A protected circuit downstream could still trip the 50A instead of/as well as the 6A where as a fuse probably wont blow as easy.

I'll be honest, this confused me. If the circuit is protected to 6A, it could still trip an upstream 50A mcb?

 

[clanky]

If you are really struggling for space then you could replace everything there for a metal consumer unit with several MCB's to protect the outgoing SWA cables but they are much better on fuses, the ideal scenario is 3 of those wylex switch fuses.

Thats what i suspected! Ive been looking around at different enclosures and it is possible to get slightly smaller ones but not by much! How about 3 of these, i suspect they might be a royal pain to wire up with 16mm tails but could it be a possible solution? (Earthing is TN-C-S according to the last EICR)

80A SP&N Fused Switch c/w 80A Fuse | Fusebox (FMS080)

 

[oscar21]

I'll be honest, this confused me. If the circuit is protected to 6A, it could still trip an upstream 50A mcb?

"B" type MCB's are designed to trip (on a short circuit, not overload) at between 3 and 5 times their rated current, so a 6A will trip at a max of 30A and a 50A will trip at a max of 250A. But a typical Zs of a 6A lighting circuit could be something like 0.7 ohms, which using ohms law on a 230V circuit would allow 328 amps to flow, more than enough to trip both a 6A and a 50A MCB, its just pot luck as to which one is quicker at tripping. Fuses have a different current curve and are more resiliant to short circuits.

 

[oscar21]

Thats what i suspected! Ive been looking around at different enclosures and it is possible to get slightly smaller ones but not by much! How about 3 of these, i suspect they might be a royal pain to wire up with 16mm tails but could it be a possible solution? (Earthing is TN-C-S according to the last EICR)

80A SP&N Fused Switch c/w 80A Fuse | Fusebox (FMS080)

Those KMF switch fuses arent really used for terminating SWA but you could put a metal adaptable box below each one and gland the SWA into those passing through into the KMf switch.

 

[westward10]

The only way I can think of getting three submains from that limited space is to replace what is there is a 60A triple-pole switch-fuse assuming that would fit. Only slight downside is that it does not switch the neutral which would be required for a domestic installation unless this is only required for consumer units, would have to have a look.

 

[clanky]

Food for thought. Thanks for all the input guys, im going to have a proper measure up and see what i can and cant fit. Numpty question though (go easy) Do RCD's protect upstream of them? ie ive just seen enclosures that have both the 80/100A cartridge fuses and an RCD isolator. But if the swa is feeding into a dis board which all ready has an RCD, will it detect a fault in the SWA mains feed in? or is it purely for the circuits downstream. Im just wondering if its better to have fault protection as well as an overload cartridge fuse for the submains SWA?

 

[westward10]

No the RCD only protects downstream. Tests need to be undertaken to see if the SWAs need an RCD for fault protection which can also be dictated by the earthing system. I would avoid an RCD wherever possible.

 

[clanky]

Ok, i have some direction on which way to proceed. Thanks for all the input guys, hopefully work will be completed soon. Cheers!

 

[Simon47]

If you are really struggling for space then you could replace everything there for a metal consumer unit with several MCB's to protect the outgoing SWA cables but they are much better on fuses, the ideal scenario is 3 of those wylex switch fuses.

It might need a bit of self assembly, but there are DIN rail fuse holders - so how about assembling 3 of those into a CU ? I imagine there aren't many (any ?) boards where that is an option ?

 

[sunray]

Those KMF switch fuses arent really used for terminating SWA but you could put a metal adaptable box below each one and gland the SWA into those passing through into the KMf switch.

I don't get why people add a superfluous metal box to terminate a SWA (unless the space is required, or to turn a corner etc) when there are so many correct ways to terminate SWA cables.

 

[oscar21]

I don't get why people add a superfluous metal box to terminate a SWA (unless the space is required, or to turn a corner etc) when there are so many correct ways to terminate SWA cables.

How would you terminate SWA into a KMF switch then?

 

[sunray]

How would you terminate SWA into a KMF switch then?

As you said they are not suitable.
I'd select a product more suited to the requirements of the job rather than botch something that isn't.

More to the point I can never workout why people do this sort of thing

 

[littlespark]

Only reason I could envisage would be an original plastic consumer unit, using a metal box to get the earth… then the consumer unit being changed later to metallic

 

[sunray]

Only reason I could envisage would be an original plastic consumer unit, using a metal box to get the earth… then the consumer unit being changed later to metallic

Why use a metal box to terminate the earth into a plastic enclosure?

In the Metal CU I posted the CU preceeded the metal box and the white plastic conduit fitting replaced the coupling (standing on top), supposedly to cure RCD tripping problems, note the voltage between CU and add on box.

 

[sunray]

If RCD is not required at this location use 3 DP B63 MCB (or as appropriate) in 3 encloseres.

Although I 100% agree with previous advice that fuse is more suitable, another possibility is use a CU enclosure to hold whatever devices are required, such as DP isolator, 60A fuse in holder & RCD as required for each circuit.
Add labels as appropriate about individual feeds etc.

 

[sunray]

If RCD is not required at this location use 3 DP B63 MCB (or as appropriate) in 3 encloseres.View attachment 122529

Although I 100% agree with previous advice that fuse is more suitable, another possibility is use a CU enclosure to hold whatever devices are required, such as DP isolator, 60A fuse in holder & RCD as required for each circuit.
Add labels as appropriate about individual feeds etc.

Sorry, didn't read OP correctly

 

[oscar21]

Why use a metal box to terminate the earth into a plastic enclosure?

In the Metal CU I posted the CU preceeded the metal box and the white plastic conduit fitting replaced the coupling (standing on top), supposedly to cure RCD tripping problems, note the voltage between CU and add on box. View attachment 122526

Dont see a problem with using a metal adaptable box to terminate SWA into something that isn't designed to take SWA. I did an EICR on a property the other day where it had a plastic CU that someone had taken a 10mm SWA garage feed off, they did exactly that, a 4" metal box under the CU, glanded the SWA into the box and passed the inner cores straight through into the MCB, everything properly earthed though.

They could have terminated the SWA straight into the plastic CU but would probably have snapped it off terminating the wires, I've seen loads damaged in this way over the years, or they could replace the whole board for a metal one at great expense but if it was done before metal boards were available, what then. Customer has a choice between a £500 new consumer unit or a £4.99 metal box, what would you choose.

 

[sunray]

Dont see a problem with using a metal adaptable box to terminate SWA into something that isn't designed to take SWA. I did an EICR on a property the other day where it had a plastic CU that someone had taken a 10mm SWA garage feed off, they did exactly that, a 4" metal box under the CU, glanded the SWA into the box and passed the inner cores straight through into the MCB, everything properly earthed though.

They could have terminated the SWA straight into the plastic CU but would probably have snapped it off terminating the wires, I've seen loads damaged in this way over the years,

Why should anything snap is it's done properly?

or they could replace the whole board for a metal one at great expense but if it was done before metal boards were available,

Metal boards have always been available

what then. Customer has a choice between a £500 new consumer unit or a £4.99 metal box, what would you choose.

good workmanship