Fused isolator for 3 phase

[Fernicarry]

I'm having my cable head moved by SPEN and expect they're going to want an isolator and/or fuse since the consumer unit will be more than 3m away.  

 

Its a 3 phase supply but with only one phase metered at the moment. I currently have a fused isolator like one of these https://www.fusebox.shop/products/fusebox-fms080-80a-main-switch-fused.  As part of the move I want to get everything ready for 3-phase for the glorious future of EV's and HP's.  The new position is quite tight for space so I'm looking for a compact solution.   Hoping to frame out the space this weekend if I can get a good idea of how much overall room I will need. 

 

I have a Wylex REC4 to provide the isolation part and if that's all they want then, great, job done.  For the fused part I'm thinking of these options:

 

- a REC3 with 3x 100A MCB's (as well as the REC4) -- but current breaking capacity of MCB's isn't as great as fuses, on the other hand they probably trip way faster so that gives discrimination with the fuse in the cable head, if there is a fault that an MCB can't clear then the cable head fuse will deal with it

- modular fuse carriers like Lawson MS221 -- these are 2 modules wide so I'd need a compact 6 module enclosure for all 3 phases but I could live with only 2 available so potentially another REC4 enclosure would do

 

Any other ideas?

 

[Thorfun]

I'm not an electrician and don't know for sure if they'd allow separate fuses and isolator. I bought a 3-phased fused isolator. not cheap and not small either!

 

 

https://www.cef.co.uk/catalogue/products/4920299-100a-tp-n-metal-fused-switch

 

we only have a single phase meter and running the house at the moment but I didn't want to have to upgrade later so just put the 3P fused isolator in from the start. when/if we move to a 3P meter it'll just be a case of connecting the tails to the fused isolator.

[Fernicarry]

I can imagine that wasn't cheap, anything for 3P seems to be silly money.  But I'd be struggling for space to fit a switch box like that.  Its an old house so I don't have a purpose built space for the cut out.   Its currently in an upstairs bedroom in a fairly narrow boxed in cupboard and SPEN have agreed that they'll honour the existing dimensions in the new location at the front door.  I'm making the new space quite high so that components can be stacked vertically if needed.  Cut-out roughly half way up, go up to the meter, back down to the isolator alongside the cut-out and then out the bottom.

 

Since I'm currently wired for single phase I think what I'll do is put a main switch and a single 100A MCB in a small metal enclosure and call that my "consumer unit".   That way the 3m rule doesn't kick in.  There should be enough space for a 4 or 6 way unit below the cut-out.  I can revisit when I'm ready to start using the other 2 phases.

[Mattg4321]

You don’t want an MCB protecting a distribution circuit/sub main as you’ll never achieve selectivity between that and the final circuit. A fault on one of the final circuits can cause the MCB at origin of distribution circuit to operate. Very annoying and poor design. 
 

The only way to do it properly is with a switch fuse isolator. Whether that be TPN or SP. 
 

Edit, wrong link!!

Edited December 3, 2023 by Mattg4321

[Fernicarry]

Thanks.  I suppose it depends on the PFC of the final circuit and the curve of the MCB.

 

Isn't there also the same selectivity problem between the fuse in the cut-out and the one on my side?  Or do they have different characteristics and if so what type should I be looking for on my side if I go with the option of fuse carriers in an enclosure?

[FuerteStu]

The prices you are seeing are probably (like most wholesalers) way higher than they would actually charge. 

 

Try calling them up and saying 'what's the best price you can give me'. 

 

I've seen them cut the price in half before for one off sales, even lower for regular customers. 

[FuerteStu]

22 minutes ago, Fernicarry said:

Thanks.  I suppose it depends on the PFC of the final circuit and the curve of the MCB.

 

Isn't there also the same selectivity problem between the fuse in the cut-out and the one on my side?  Or do they have different characteristics and if so what type should I be looking for on my side if I go with the option of fuse carriers in an enclosure?

Fuses are much more tolerant to surges, the mcb will always trip first. 

Problem is, you shouldn't have to worry about going out there to reset the breaker. You'll be better off getting a fused Isolator. If the worst ever happens, yours will go first before the DNO