Switch Fuse Enclosures

[Aragorn84]

I'm planning on replacing the cable to an outbuilding with SWA. Its currently installed with some T&E shonkily buried in the gravel with no protection and an oversize breaker, and clearly needs to go. Weathers getting less crap now, so want to get the details sorted so i can get the parts ordered.

Its currently connected to an MCB in a old secondary CU that will be disappearing shortly so the plan is to install a switchfuse in its place.

There are various switch fuse options, but not too happy with them. The metal clad ones seem to be hugely oversize for some reason, and the 800KMF is nice and compact but has no provision for terminating SWA into it given its plastic construction, meaning the addition of an adaptable box or similar, which then ruins the compactness and aesthetics.

I would also like to build in provision for another outbuilding supply, for a garage i will be building later in the year, and having two huge switch fuses mounted side by side will look a bit crap.

What i'm wondering, is there any reason i cant use a "normal" Metalclad CU enclosure, bring the tails into it and use a two pole isolator and a din rail mounted fuse holder (or even an MCB?) to protect the SWA?

As an example; this enclosure with suitable parts installed:

http://www.ebay.co.uk/itm/321335958690

A second isolator and second fuse could then be installed into this enclosure with minimal effort, when it comes time to run the supply to the garage.

Thoughts?

 

[Taylortwocities]

Sure, that would work. Isolator and MCB would be fine. Fuse if you like.

But re

A second isolator and second fuse could then be installed into this enclosure with minimal effort, when it comes time to run the supply to the garage.

Thoughts?

Normally, if you have one supply, there is only one isolator (aka main switch) to totally isolate the DB. Separate fuses/MCBs to isolate the outgoing lives.
If you really want to, you could fit MCBs that break line and neutral but that is not usual unless the supply require that.
Examples http://www.schneider-electric.co.uk...protection/miniature-circuit-breaker/c60.page

 

[mfarrow]

Fuse holders by the better known brands are pretty rare these days. Wylex, Hager and Schneider are the ones I can think of, with Wylex having the best fuse availability owing to their spread across the Standard Range as well.

I did something similar and needed forked busbar for the fuse holder.

View media item 65328 View media item 65330

 

[Aragorn84]

Thanks for the replies.

Are your suggesting the board itself MUST have an integral single point of isolation? Or just that it typically does and this would be a bit unusual?

My thinking is that i would want to maintain DP isolation just like you'd achieve with a pair of switchfuses, they'd just be sharing a box.

There will be a main isolator upstream to isolate the whole installation.


RE the fuse holders, yes after looking this afternoon i've realised they do seem pretty rare, especially if you want more than 30A. The existing outbuilding is probably fine on 30/32A but i'd have preferred to run 40, and for the garage i'd like as much power out there as possible. MCB's might work, but there seems to be issues with discrimination and it seems to be suggested that fuses are better suited?

 

[JohnW2]

Are your suggesting the board itself MUST have an integral single point of isolation? Or just that it typically does and this would be a bit unusual? ... My thinking is that i would want to maintain DP isolation just like you'd achieve with a pair of switchfuses, they'd just be sharing a box. ...

If you were concerned about that, and your CU had the space, I suppose you could have three DP isolators - one for the whole 'board' which, in turn, fed one for each of the circuits.

Kind Regards, John

 

[mfarrow]

Wylex fuses go up to 45A, any higher you need a fuse switch.

Just go for 2-way standard range and fill it with what you like. For some reason there's better availability of standard range cartridge holders than there are din rail mount.

 

[Aragorn84]

John: I'm not personally concerned, it was more a question of if the regs actually required it.


The standard range stuff looks like a relic from the 1940's, and while i dont want to be advocating form over function, theres some aesthetic properties to be taken into account.

The schneider enclosure i linked would match the main CU, which i felt would make it look somewhat neat.

 

[JohnW2]

John: I'm not personally concerned, it was more a question of if the regs actually required it.

Although it's often discussed, I can't think of any reg which specifically requires it - the only reg I can think of is the one which requires a single point of isolation for the whole installation - for a high proportion of domestic installations, that can obviously be the main switch in the (only) CU.

However, I suppose there is a general 'expectation' that any CU. or anything which looks like a CU, will have single overall 'main switch' - so to have a third one such as I suggested would avoid any potential (and, I suppose, theoretically potentially dangerous) 'confusion'.

The standard range stuff looks like a relic from the 1940's, and while i dont want to be advocating form over function, theres some aesthetic properties to be taken into account

I agree, up to a point. Do I take it that these bits of distribution equipment are to be 'on view', rather than hidden away in a cupboard, enclosure or rarely-used room?

Kind Regards, John

 

[Aragorn84]

They're on the wall in what i guess you could call the front porch or cloakroom. Not really on view as such, but i guess they are visible to anyone coming into the house. I just prefer for things to look "right" At the moment its a horrendus mashup of old clart and when sorting it out it makes sense (to me anyway!) to get it looking tidy.


I can get a 40A fuse holder in Wylex as mentioned above, which would do for this outbuilding:

http://www.tlc-direct.co.uk/Products/WYNSC40.html

Unfortunately the biggest they do is a 45A, and i would like to have had a bit more for the garage, just-in-case.

However, i guess nearer the time i can decide if 45A will be sufficient or if i need to go for a bigger MCB for the second submain.

 

[mfarrow]

The 45A fuse is to BS 1361 (BS 88-3), which will allow a more sizable overload than it's BS 88-2 cousins (like the 40A). I wouldn't be concerned if your largest garage breaker is going to be a B32.

In fact if you look at the BGB appendices, you'll see 45A BS 88-3 equivalent to a 50A BS88-2 in disconnection times, and it will hold 80A for 1/2 hour.

 

[JohnW2]

The 45A fuse is to BS 1361 (BS 88-3), which will allow a more sizable overload than it's BS 88-2 cousins (like the 40A). I wouldn't be concerned if your largest garage breaker is going to be a B32. In fact if you look at the BGB appendices, you'll see 45A BS 88-3 equivalent to a 50A BS88-2 in disconnection times, and it will hold 80A for 1/2 hour.

That's all very well, but one really shouldn't (not the least because it would be non-compliant) design a circuit to run under overload conditions, even if the OPDs would 'allow it',should one?

The OP seemed to be suggesting that he would prefer the supply OPD to be >45A because "would like to have had a bit more for the garage, just-in-case". That presumably means that he is contemplating a situation in which the demand (i.e. 'design current') in the garage might rise above 45A (which, assuming proper design) would imply more than one 32A circuit there.

... or am I missing something?

Kind Regards, John

 

[mfarrow]

John, no you're not missing anything, but I don't think a garage will need any more than 32A on a final circuit. It could be wrong of course, but let's wait for the OP to return.

It terms of 45A versus 50A, I was talking discrimination and short term overload with respect to power tools startup currents etc.

Diversity will allow you to have quite a bit in there, and the 80A was saying that it could be achieved without causing issue. 4x 3kW heaters using a standard diversity calculation will still allow 14A left over for lighting and an appliance.

 

[Aragorn84]

Spot on John.

It ofcourse seems unlikely, but i guess there could be a scenario where theres a couple KW of heating running as well as perhaps a lathe or milling machine or whatever. I dont have such equipment yet, just a compressor and welder, but its on the "would like" list, so when i'm building the garage i'll be sure to spec the power feed so that its large enough to cope with any potential future demands.

Ofcourse its entirely likely that 45A will be plenty, so i'm fairly happy with that solution, however i will try to specify it such that should i need more, i can swap the fuse out for something larger.

There are tables which specify the maximum Zs value for each type of fuse/MCB, and also tables which specify the resistance of a set length of cable. If i know the Ze value, is it permissible to add Ze to the cables resistance (R1+R2?) to get an idea of expected Zs? and how much extra should i add on to ensure its likely to meet Zs once installed?

 

[JohnW2]

John, no you're not missing anything, but I don't think a garage will need any more than 32A on a final circuit. It could be wrong of course, but let's wait for the OP to return.

I agree - but it was the OP, not me, who was concerned that 45A might not be enough for possible future requirements. Even though the OP is now talking about lathes, milling machines, compressors and welders, it would seem very unlikely that, in the context of a garage, more than one of those would be operating simultaneously - although his heating would obvioulsy be on top of that, and very probably running concurrently with one of those other loads.

As I think we're both saying, I think it comes down to making a sensible estimate of the likely maximum demand (taking into account diversity, if/where appropriate) and ensuring that the design current of the circuit (hence OPD ratings etc.) is at least as high as that estimated maximum demand.

Kind Regards, John

 

[JohnW2]

There are tables which specify the maximum Zs value for each type of fuse/MCB, and also tables which specify the resistance of a set length of cable. If i know the Ze value, is it permissible to add Ze to the cables resistance (R1+R2?) to get an idea of expected Zs? and how much extra should i add on to ensure its likely to meet Zs once installed?

You can certainly use that approach to "get an idea of the expected Zs". Don't forget that, if you are doing it by the book, as well as the Zs and current-carrying-capacity considerations you also have to make sure that the voltage drop at maximum design load is acceptable.

Kind Regards, John