Safe solution of having a cu 10 meters away from meter

[jas0n]

Hi haven't made a post in a long time but this issue has got me thinking. This is the 1st time I have had the consumer unit quite a distance from the meter. Needed some advice in making sure I am making the right decision.

currently I have been working in a house that did have three cu's, two running the original part of the house and another which had powered a extension which was done over the past 10 years. The cu powering the extension is fine and does not need attention. The other two I have cut off and also removed all the wiring on the original part of the house as it was a complete joke by having jb's all over the place and taped up connector blocks and also wires that had been attacked by a mouse infestation at one stage. Due to this I have reworded the original part of the house but run all cables to the extension to the cu that is left.

i will be putting a new cu under the other to power the original house. 

My my plan was to have an additional meter box outside next to the original that will contain a 100a fused switch which will have 25mm swa feeding the new and existing cu.

Does this sound about right to another electrician who has been in the same situation as I am now.

thanks

jason

 

[ProDave]

Exactly what I have, but with an 80A fuse in a KMF Switch Fuse.
 

 

[kerching]

Sounds fairly standard as PD just said

if using a KMF style have you given any thought as to how you are going to get the 25mm2 into it?

just reminding

 

[jas0n]

Was thinking to have a big enough trunking that will be between the swa and switch, therefore have a gland on the swa attached to trunking and cable to go through another drilled hole in trunking to the switch. 

 

[Zeespark]

Why would you need trunking when using SWA?

Andy Guinness

 

[Andy™]

KMF cant take SWA directly

 

[jas0n]

That's why I'd use some chunky trunking to contain the cable or I would end up using a metal box to take the swa and then glands to take cables singly to kmf. 

Just thought if trunking was butted up to the kmf it would look neater

 

[kerching]

That's why I'd use some chunky trunking to contain the cable or I would end up using a metal box to take the swa and then glands to take cables singly to kmf. 

Just thought if trunking was butted up to the kmf it would look neater

If metal box then all cores will need to go through ONE hole

is exposed then cores will need to be double covered

does the inner covering of SWA count as a cable 'sheath'?...just wondering , serious question

 

[apprentice87]

I would put the KMF [or whatever] in the original meter box. I know officially this is not allowed, but i have not had any trouble, and i got loads in mine!! IF they ever create about having stuff in THEIR meter box, i will tell them to get courgetteed and to PROVE that it is in their box. I can go and buy just the same box myself from the same makers..

I do like how it is "their" box. Now tear the door off. Suddenly it is YOUR box.......

john..

 

[steptoe]

I have a SWA in my place glanded into a metal adaptable box with ISCOs butted tight to it, a slotted hole in top to ISCOs, then to an MCB and TD 100mA,

I'm sure a pic on here somewhere. 

There was some contention about the earth being separate, but IMHO its fine, and my assessor agrees as the whole metal box is earthed anyway. 

 

[apprentice87]

What was the contention about the earth being separate??

A cpc is SUPPOSED to pass through the same hole as the live conductors in a ferromagnetic enclosure, but mine does not, and nobody has ever noticed......

john...

 

[Rob_the_rich]

What was the contention about the earth being separate??

A cpc is SUPPOSED to pass through the same hole as the live conductors in a ferromagnetic enclosure, but mine does not, and nobody has ever noticed......

john...

if the cpc went through a separate hole and there was enough current flowing from the live conductors to the cpc to create eddy currents large enough to start heating the box, surely the ocpd would operate?

 

[Rob.]

if the cpc went through a separate hole and there was enough current flowing from the live conductors to the cpc to create eddy currents large enough to start heating the box, surely the ocpd would operate?

In a domestic application, most probably. In an industrial environment not necessarily. 

However in a domestic application if you have a L>E or N>E big enough to start causing heating via eddy currents. The CPC in the circuit within the house would be glowing red, an most likely would have caught fire. 

It's good practice to stick them all through the same hole, I can't remember the reg off hand but I'm sure it read something on the line of 'install to mitigate the problem of eddy currents' An in a domestic installation with a small supply, you're not going to get an issue.

 

[Rob_the_rich]

521.5.1 it does seem to suggest that all conductors, including cpc should enter by the same hole, although an additional cpc may enter via a different hole.

 

[apprentice87]

So far as i know [and i am not an electrician and this is only from memory] it is because in the event of a fault, the fault currents can be VERY high. This causes large "electromagnetic forces" akin to mechanical forces if you like, that can tear wiring asunder. This apparently, is obviated if all the conductors pass through the same hole...

john...

 

[Rob.]

So far as i know [and i am not an electrician and this is only from memory] it is because in the event of a fault, the fault currents can be VERY high. This causes large "electromagnetic forces" akin to mechanical forces if you like, that can tear wiring asunder. This apparently, is obviated if all the conductors pass through the same hole...

john...

This effect still doesn't really apply in a domestic property. 

I think what you are thinking of here, is single conductors tied to a cable tray for instance. 

Under a large fault they will repel each other with a substantial amount of force. @Tony S has done a good piece about it in knowledge base section, under a heading of transformer tails. 

 

[apprentice87]

Hi Rob,

Yes, i agree entirely with what you say. No idea what sort of fault currents would be needed for this sort of thing to happen, but very large ones i would think!!!

john..

 

[Rob.]

If you feel at a loose end, you can work it out with this haha.

Ft = (0.17 x Ip²)/S

Where
Ft = Maximum Force per unit length of cable (N)
Ip= Peak short circuit current (KA).
S = Centre to centre distance between neighboring conductors (m)

 

[Rob_the_rich]

If you feel at a loose end, you can work it out with this haha.

Ft = (0.17 x Ip²)/S

Where
Ft = Maximum Force per unit length of cable (N)
Ip= Peak short circuit current (KA).
S = Centre to centre distance between neighboring conductors (m)

I will bite. I did a spreadsheet, attached. Apparently, at 240V with a Ze of 0.01 and a distance of 3mm between conductors, the force is the same as nearly 41 American men standing on top of each other (https://en.wikipedia.org/wiki/Newton_(unit)). Can this be right?

Edit; oops I did separation between cables as opposed to centre to centre. Putting in 10mm stil gives 12 americans. 15mm gives 8 americans.

View attachment Force.xlsx

 

[Rob.]

Yes, but that force is spread uniformly over the entire cable length, from origin to source of fault.