Supply Side Overload Protection

[benstoneham]

Anyone know what a typical OP device rating is (appreciate there may not be one)..

I also like to know what people consider best practice is for the following

Context is a re-wire where it would be better to service the property using 2 CUs.

Notionally 35MM SWA has been selected for feed between CUs (internal), which at 154A (clipped direct, 3 core) is obviously more than ample. Run is around 25M by the way.

Question is then what to run between the Isolation switch and Service Blocks (which will be 200A).

Allowing for known/ likely load diversity, 35MM cable should also be OK in practice, but obviously if the upstream OP is left at 100A (default supply breakers in the CUs), then the principle that the cable rating should exceed the upstream OP rating is not met.

Ging to 50MM cable would be one option.. but given that it twice typical tails size, it does seem excessive..

One possibility I suppose is to fit an inline breaker before each CU - 63A perhaps would be sensible.

Not having come across this before, I wondered what others might do.

 

[benstoneham]

Just to clarify - in my original question I am talking about DNO supply side, not the CU!!

 

[Adam_151]

I'm having difficultly visualising what you are intending to do, perhaps a drawing would help?

 

[benstoneham]

Sure, this is what I was hinking of:

 

[benstoneham]

Yikes.. sorry, I didn't realise it was that wide!! Also, ignore 63A Type C in L/H CU.

 

[JohnD]

What is the design load after diversity?

There must be a reason why you don't want a switchfuse at the meter.

What is this reason?

 

[benstoneham]

No reason re. Switchfuse, just hadn't considered it (although I have some theory background including 2382-10, my practical background in domestic is very limited as day-to-day I am in a Datacentre environment, so anything related tends to be in this context).

Haven't go my spreadsheet with me tonight, so I can't tell you off hand what the total design load was....safe to say it is way lower than the 200A though (this is domestic 5 bedroom with 7 rings, 8 radial lighting,4 radial Fixed app and a radial cooker). At a guess - perhaps 75A total? I can confirm tomorrow.

As I said in my original post, on that basis, 35m ought to be ample. But I'd like to know what best practice is - perhaps as you say John, a Switchfuse is the obvious solution.

 

[JohnD]

also:

what is the rating of the company fuse for this phase?

And you are using a Memera AP (plastic) and a metal adaptablr box for the SWA - if you use a steel-enclosure Memera AM (metal) (more expensive) you won't need the SWA box.

the Memera 2000 range are fairly roomy inside (The AD are IMO a bit tight in larger sizes). not sure if a 19-way is ex-stock, but the double-deckers are quite big.

I don't see the point of the ME RCD if you have RCBOs - an MET would be more suitable. But if you go double-decker you can have DP switch and RCBOs in the top, and MCBs and HE RCD in the bottom

 

[benstoneham]

Company fuse? Apologies for obvious ignorance, but which do you mean by this? The Switchfuse on the install side o f the meter? I don't know is the answer as I hand't considered it until you mentioned it.. obvious though it is now, I had been thinking in terms of the OP being further downstream prior to the meter offsite from this property.

Re. Memera kit - thanks for the advice; its stuff like this that you just don't know unless you are doing it day to day.. Memera Kit is good by all accounts I understand, but obviously a Metalclad would make much more sense. You are righ that the 19way isn't ex stock and since I drew that up, I had decided on a 14 which is.

Re. the 100Ma ME RCD - yes, that had occured to me also and I had thought that I would go with just the DP switch versions and put the fridge on a non RCD (appropriately labeled) radial.

Having said that, I couldn't see a Time-delayed 100mA version in anything other than the split boards, which would be better perhaps, but I'll check with them tomorrow.

 

[JohnD]

"Company fuse" is Main Fuse or Supplier's Fuse - on top of the incoming supply head, before the meter. Might be 100A, might be 80A or even 60A. The label on the outside of the fuseholder will be the maximum, inside it might be lower. Electricity Co.s often put a 60A fuse in a 100A holder for a domestic installation (that's what I've got) and will only upgrade it if it blows.

you can buy a 100MET as a "spare part"

 

[benstoneham]

Thanks John, yes that was the Fuse I was thinking of.. so for arguments safe, were this a 100A with a design load of 75A all should be well(because this would go before the 35mm capacity was exceed).

Re. the Memera MET - got a price on it this morning (£97.01 - ouch!), so once again thanks for the tip.

 

[JohnD]

If you put your email addres in your profile, you might find one cheaper.



p.s. this is what I mean by a double decker. There is a main switch (can be an RCD if yiou want) which controls the whole thing and feeds an upper rail, and a lower rail which can have its own subsidiary DP switch or RCD if you want.


http://i113.photobucket.com/albums/n228/JohnD_UK/POL_0124-1.jpg
http://i113.photobucket.com/albums/n228/JohnD_UK/POL_0125.jpg
http://i113.photobucket.com/albums/n228/JohnD_UK/POL_0113.jpg

 

[benstoneham]

Just updated my profile.

I got the quotes from Rexel Senate and CEF, both were very similar (around 20%) so I took it that they were reasonably competitive (I have trade accounts with them, but don't put much through).