Switch Fuse Enclosures

[Aragorn84]

Is there any reason for various sources providing wildly differing figures for cable resistance?

For instance this claims to be an exceprt from 17th:

http://www.batt.co.uk/upload/files/4d4a_copy1.pdf

And using that a 10mm cable has a resistance of 4.4mOhm per meter

this:

http://www.tlc-direct.co.uk/Figures/Tab5.5.htm

claims 1.83mOhm

as does Table 4.1 here:

http://onlinelibrary.wiley.com/doi/10.1002/9780470998373.app4/pdf



Perhaps 4D4B, because its talking about voltage drop, incorporates the resistance for R1 + Rn together which is fine, however surely you'd expect that to be twice the 1.83 value mentioned in the other tables, when it appears to be about 20% higher than this?

 

[JohnW2]

Is there any reason for various sources providing wildly differing figures for cable resistance?

Yes, there is a reason - you appear to be comparing chalk with cheese! The Table 4D4B figures relate to an operating temperature of 70° C, whereas the TLC and other figures you quote relate to an operating temperature of 20° C. As the footnote tothe TLC table says:

Note that to allow for the increase in resistance with increased temperature under fault conditions the values of {Table 5.5} must be multiplied by 1.2 for p.v.c. insulated cables (see {Table 8.7})

... which of course almost precisely explains:

...however surely you'd expect that to be twice the 1.83 value mentioned in the other tables, when it appears to be about 20% higher than this?

Kind Regards, John

 

[mfarrow]

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.

That would be 39A with 3kW heaters, and assuming you had a pretty hefty industrial lathe/milling machine such as this:

... going up to 42A if you had a fairly powerful MIG welder in there.

The only thing I would consider an issue would be water heaters, if you want to put a little washup area in there, in which case you will be needing to abandon Wylex in favour of someone like Schneider. They do some very nice looking SP and DP fuse cartridges rated up to 125A (google SBI fuse holder). You'll need to get a factor to order them in for you.

 

[Aragorn84]

Those SBI ones look ideal, match the board and appear to be cheaper than the wylex ones too:

http://www.more-control-shop.co.uk/mgn15707-p-323147.html

There appears to be two versions, one taking a 14x51 fuse and going up to 40A and one taking a 22x58 fuse going up to 100A

Is there any info on wether these fuses have a general name/type so i can find their specs for Zs etc? They dont appear to state BS88 or BS1391 etc?

 

[Aragorn84]

So some trawling suggests that BS 88-2 is the same as IEC 60269-2 gG type fuses?

So something like this:

http://uk.farnell.com/cooper-bussmann/c14g40/fuse-40a-14x51-500v/dp/1701362

Would be the equivalent to a 40A BS88-2?

 

[mfarrow]

So some trawling suggests that BS 88-2 is the same as IEC 60269-2 gG type fuses?

The text for BS HD 60269-2:2013/BS 88-2:2013 has been extracted from
IEC 60269-2:2013 and is identical to the text for sections E, G, and I. However,
wherever a reference is made to IEC 60269-1 in the text this should be taken
as a reference to BS EN 60269-1 (BS 88-1).

They appear to be.

 

[Aragorn84]

Bringing this back up.

I've been thinking about the two isolators and connecting them up.

Is it acceptable to link across the input terminals of the two isolators, with the tails going into the first isolator then a link across to the second? It feels slightly "wrong" but i cant find anything that suggests it IS wrong. I also notice that some boards use a pair of tri-rated cables between the main switch and the pair of RCD's, and neutral bars so i suspect its probably ok?

Does this link need to be the same size as the tails (25mm) or can i use say 10mm on the grounds that its a 40A outgoing way?

Would i be better using Tri-rated, or normal solid/stranded "singles"?

 

[JohnW2]

Is it acceptable to link across the input terminals of the two isolators, with the tails going into the first isolator then a link across to the second? It feels slightly "wrong" but i cant find anything that suggests it IS wrong. I also notice that some boards use a pair of tri-rated cables between the main switch and the pair of RCD's, and neutral bars so i suspect its probably ok?

As you say, it's no different from the internal connections one sees in 'split-load' or multi-RCD CUs.

Does this link need to be the same size as the tails (25mm) or can i use say 10mm on the grounds that its a 40A outgoing way?

The cable obviously has to be adequate to carry the intended design current of the circuit AND also must be large enough to be adequately protected by whatever over-current protective device is protecting it. This obviously applies as much to the main cables going into and out of the isolators as it does to the link between the two of them.

Kind Regards, John

 

[Aragorn84]

Ok, so they need to be big enough to be covered by the service fuse?

That seems odd, because the link cables you see inside wylex boards for instance are nowhere near as big as the tails...

 

[Aragorn84]

Or maybe you can say its protected by overload from the upstream fuse?

 

[JohnW2]

Ok, so they need to be big enough to be covered by the service fuse? That seems odd, because the link cables you see inside wylex boards for instance are nowhere near as big as the tails...

Or maybe you can say its protected by overload from the upstream fuse?

On the face of it, it's difficult to see how one could justify the link cables within a CU not being large enough to be adequately protected by the cutout fuse - even if one invoked the upstream over-current protective devices (which would probably result in some discussion!), in the case of a CU, the total current rating of MCBs supplied by one RCD (i.e. the total current that could theoretically flow through one RCD without anything tripping) could easily exceed that of the service fuse!

Kind Regards, John

 

[Aragorn84]

And yet, some CU's have 63A RCD's, and could be installed in an installation with a 100A service fuse...

 

[mfarrow]

On the face of it, it's difficult to see how one could justify the link cables within a CU not being large enough to be adequately protected by the cutout fuse

433.2.2 allows you to reduce the cable size up to 3m before the overcurrent protective device.

I would hazard a guess that the argument for the RCD size to be OK is because of the nature of the load not likely to carry overload.

 

[JohnW2]

On the face of it, it's difficult to see how one could justify the link cables within a CU not being large enough to be adequately protected by the cutout fuse

433.2.2 allows you to reduce the cable size up to 3m before the overcurrent protective device.

It does, indeed - but, as I said, the OPDs downstream of one RCD in a CU could well allow a total current through the RCD (hence cables) at least as great as the rating of the cutout fuse. A cable not adequately protected by the cutout fuse could therefore quite possibly not be adequately protected by downstream OPDs either,

I would hazard a guess that the argument for the RCD size to be OK is because of the nature of the load not likely to carry overload.

Probably very true in reality, but it's probably really a bit naughty to have a RCD rated at less that the total design currents of the circuits it protects. Let's face it, a couple of 32A sockets circuits could theoretically both be ('quite legally'!!) be fully loaded, before one started adding on other circuits, so a 63A RCD (pretty common in CUs) could easily be overloaded.

Kind Regards, John

 

[JohnW2]

And yet, some CU's have 63A RCD's, and could be installed in an installation with a 100A service fuse...

Indeed - see my response to mfarrow's post.

Kind Regards, John