sub main

[ElectricIANJ]

Hi guys im doing a job that needs a sub main to a outbuilding was going to use 16swa but not sure what to do at origin with conecting in to a consumer unit size wise any ways round it or will it fitt ok

 

[telectrix]

best way is to split the tails before the house CU with henley blocks, then into a 63A D/P fused isolator, then your SWA tails to the outbuilding. you could use a spare way on the CU with a 63A MCB. depends on whether you are going to exoprt the earth or TT the outbuilding. why 16mm though. big load?

 

[ElectricIANJ]

not realy just being safe,over safe i spose, i thought you had to TT now with earth on cable only from house on Rcd supply

 

[telectrix]

no need to rcd SWA cable. RCD can go in outbuilding. you can export the earth .

 

[i=p/u]

if no services?? you can export, correct me if im wrong, i can take a 'eeee awwww!!!!

 

[Chr!s]

if no services?? you can export, correct me if im wrong, i can take a 'eeee awwww!!!!

A lot will depend on whether where your exporting will be within an equipotential zone

 

[ElectricIANJ]

now thats somyhing ive not come across, what classes as equipotential zone, not sure im understanding what your saying.

 

[PC Electrics]

Good question Ian. I've asked it before and not had a satisfactory answer.
The term "equipotential zone" does not appear in the definitions in BS7671 (unless I'm blind).
So, what is it and what defines its boundaries?

OP: it's okay to be cautious, but if you're jumping from 10mm to 16mm just on the basis of caution (rather than sound engineering) are you adding unnecessary cost to the installation?

 

[telectrix]

as i understand it. the equipotential zone is the house. if you are exporting thr earth from the house MET to the outbuilding, you are then making the outbuilding a part of the equipotential zone and so, if you have extraneous metal parts in the outhuse, these must be bonded back to the MET.

 

[ElectricIANJ]

ok so its like any submain but as its detatched i export the earth then bond up as norm in the out building

 

[telectrix]

yes, but you would take the earth back to the house MET, preferably using a core of your SWA .

 

[ElectricIANJ]

i would have used 3core to take earth to outbuilding is that what you meant

 

[i=p/u]

yes if has no services ie water ??????

 

[malcolmsanford]

now thats somyhing ive not come across, what classes as equipotential zone, not sure im understanding what your saying.

Good question Ian. I've asked it before and not had a satisfactory answer.
The term "equipotential zone" does not appear in the definitions in BS7671 (unless I'm blind).
So, what is it and what defines its boundaries?

OP: it's okay to be cautious, but if you're jumping from 10mm to 16mm just on the basis of caution (rather than sound engineering) are you adding unnecessary cost to the installation?

It is what it says on the tin really, it is a zone where all the metalwork is of an equal potential, as Tel says in your home by bonding the services to the main MET you have created a equipotential zone where all that metal work is of an equal potential. This is done by the fact that your MET is connected to the mass of earth either by an earth electrode in a TT system or the DNO provided earth, as they both utilise the earth tapping back at the Transformer. So by bonding the pipework to the MET you are making the potential the same.

As a outbuilding is beyond your equipotential zone then if there are services within the outbuilding you have to create a seperate zone as in a TT system, or if your exporting the earth then you extending the house zone and therefore those services will need to be bonded either with a seprate bonding conductor or a combined bonding/cpc conductor back to the MET at the house.

 

[i=p/u]

so can it be 1 or the other long as zs is in compliance

 

[Chr!s]

Good question Ian. I've asked it before and not had a satisfactory answer.
The term "equipotential zone" does not appear in the definitions in BS7671 (unless I'm blind).
So, what is it and what defines its boundaries?

Agreed, its not mentioned as such, GN5 touches on it a little.

The way i look at it is a zone of equal potential under fault condition.The two main paths of current for electric shock are hand to foot and hand to hand.

Take a single room, Extraneous coper pipe, next to an exposed conductive part. Now if theres a fault on the exposed conductive part there will be a difference in potential between the two, so we bond the pipe, this reduces the touch voltage of hand to hand shock.But what if the floor introduces earth potential, then we must consider hand to foot shock.

When creating an equipotential zone we ensure that simultaneously accessible parts are either at the same potential or make them so they dont introduce potential. Most floors in houses dont introduce earth potential, so the risk considered is hand to hand.

Thats not to say some floors dont introduce earth potential, many tiled or concrete floors may.

So if we export the pme to an outbuilding can we create an equipotential zone?

Is the floor at earth potential?

Are there exposed and extraneous parts?

If we have a broken pen current may flow through the exposed and extraneous parts, now if the floor introduces no potential then theres no risk of hand to foot shock, the risk is hand to hand, but if we bond all exposed and extraneous back to the met then we will have similar potential between the two, so reducing the risk of hand to hand shock.


HTH

 

[Chr!s]

I dont forget, Potential isn't always necessarily earth potential,

 

[poheZ]

how about terminating it in a gland and connecting it to an mcb!

 

[malcolmsanford]

is this deja vu

 

[IQ Electrical]

Hi guys im doing a job that needs a sub main to a outbuilding was going to use 16swa but not sure what to do at origin with conecting in to a consumer unit size wise any ways round it or will it fitt ok

You might want to look at the consumer unit size and construction first, is it metal clad or pvc?

Is the 16mm cable size to allow for a large load or volt drop on a long run?

Check the rating of the consumer unit and indeed, the supply itself before adding large loads.