Main 185mm earthing query

[Alrebcon180]

Hi guys bit of knowledge needed..
Have a 2x185mm 3phase supply going to a panel board with 800amp main switch from a UK Power networks transformer about 15m away, also run with them a 185mm earth which goes to a earth bar next to the panel board where all the outgoing swa earth overlays are connected also. What size earth should go to the panel board a 185mm as that's the size of the incoming earth and theain switch is 800amps or can it be smaller like a 70mm as the outgoing earth's are all connected to the earth bar..
Some thoughts on this would be great.. Thanks

 

[Deleted member 26818]

You can use whatever CSA the adiabatic equation comes up with.

 

[Silly Sausage]

You can use whatever CSA the adiabatic equation comes up with.

It sounds like quite a meaty supply to be asking questions on the internet about!

 

[davesparks]

Assuming someone has calculated that a 185mm earth is required for the feeder then I would say it needs to be 185mm to the panel board.

 

[davesparks]

Plus on a job like that there is usually a long enough offcut to do the link to the panelboard anyway so why worry, just put it in.

 

[Deleted member 26818]

The design has probably used Table 54.7 when determining the CSA of the earthing conductor.
Two 185mm2 supplies with one 185mm2 earthing conductor.

 

[Nigel]

The adiabatic equation is not needed for line conductors over 35mm so as long as the earth cable is the same material as the line conductor the earth will need to be at least half the size of the line conductor feeding the panel.

 

[LeeH]

Plus on a job like that there is usually a long enough offcut to do the link to the panelboard anyway so why worry, just put it in.

Never seen any off cuts on my jobs...

 

[Deleted member 26818]

The adiabatic equation is not needed.
For line conductors over 35mm so as long as the earth cable is the same material as the line conductor the earth will need to be at least half the size of the line conductor feeding the panel.

There that’s better.