Multiple Bonding Points on TT System

[chloebear]

Hi All,
I have a domestic TT installation to do on a (Very) Large house that has 5 Consumer units, due to the nature of the installation having to be done in phases it looks like it will have to stay with the majority of the boards. Anyway the question is this :- as it is such a large house would it be acceptable to have different earth spikes feeding the consumer units? i.e. probably 3 earth rods - Rod 'A' feeding two Cu's, Rod 'B' feeding 1Cu & Rod 'C' feeding two Cu's with the main bonding being taken from one of the rods?
Is this acceptable or wil it causeissues with the earth potentials?

 

[electric-ian]

Hi Chloebear,

I might be wrong but assuming that all consumer units are supplied from a single source, you should put an MET there. Presumably you have a main db feeding the sub-boards so run an earth from the MET when you run in the sub-mains.

 

[chloebear]

Sorry, should have clarified earlier - there are two seperate feeds into the house to be brought in from a seperate PME supply. The house will be a TT system, which brings up the point of multiple earth rods - so efectivly there would be 2 MET locations.
Should the equipotential bonding to the main services only come from one MET or could it be bonded at multiple points?

 

[Mr Mark Sparks]

Sorry, should have clarified earlier - there are two seperate feeds into the house to be brought in from a seperate PME supply. The house will be a TT system, which brings up the point of multiple earth rods - so efectivly there would be 2 MET locations.
Should the equipotential bonding to the main services only come from one MET or could it be bonded at multiple points?

if its a pme/tncs supply why will the house be tt?

 

[chloebear]

The supply originally comes in to a small factory unit on the site via 3 phase - one phase has been dedicated to the house supply which is fed U/G via a 25mm SWA to the house aprox 20metres away - hence Factory is PME and house is TT. But because of a lot of historic changes to the house there has been at some time a further supply to the house hived off one of the other phases and brought in at the other end of the house to feed one of the building extensions - so 2 feeds in each at different ends of the house.

 

[Mr Mark Sparks]

http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

have a look at this chart to see if you need to tt or if you can use the swa as cpc
it has to be equal or larger than the main earth in the factory do you know what size it is?

 

[andyb]

What cable is feeding these sub mains?
Is there a cpc in the cable?

 

[chloebear]

Thanks for the table - very useful.
Not sure what the type the existing cable feeding from the intake is, but may replace anyway. If so it would be replaced with 3 core, so including cpc, if so - does this mean that the house could become part of the PME from the factory - I was always unde the asumption that Pme was not to be exported.

 

[Mr Mark Sparks]

my understanding is that you can carry over and use a cpc as long as it is at least as big as the main earthing conductor at the origin
this can include swa if big enough
then you will be able to bond as normal
and house will be at the same potential throughout

 

[chloebear]

That would be good.

 

[andyb]

my understanding is that you can carry over and use a cpc as long as it is at least as big as the main earthing conductor at the origin
this can include swa if big enough
then you will be able to bond as normal
and house will be at the same potential throughout

It's not the main earth it's the main bonding that has to be the same size.

 

[Mr Mark Sparks]

It's not the main earth it's the main bonding that has to be the same size.

so the cpc you take over has to be the same size as the main bonding in the origin building is that what you are saying??

can anyone else clarify these points?

 

[andyb]

All exposed conductive parts of an installation have to be bonded to the main earth terminal.
So feeding another building on a pme system then any services or exposed metal has to bonded back to the met with the correct size cable, 10mm minimum depending on the origional neutral.
That's why it's sometimes easier to make it a tt system, then you only have to bond back to the earth terminal of the out building.

Hope this helps.

 

[Mr Mark Sparks]

All exposed conductive parts of an installation have to be bonded to the main earth terminal.
So feeding another building on a pme system then any services or exposed metal has to bonded back to the met with the correct size cable, 10mm minimum depending on the origional neutral.
That's why it's sometimes easier to make it a tt system, then you only have to bond back to the earth terminal of the out building.

Hope this helps.

that is why i thought if you did what i said it was ok because you are providing an earth path back equal to main earth

not saying you are wrong just what i was led to belive

 

[andyb]

that is why i thought if you did what i said it was ok because you are providing an earth path back equal to main earth

not saying you are wrong just what i was led to belive

Yes you're right, if the cpc is the same size as the main bonding conductors (providing the main bonding conductors are the right size) then you can use this as a combined cpc/bonding conductor.