Earths at outbuildings...

[Engineer54]

Just took the time to do a couple of quick adiabatic calc's using k=154 on these tables and well passed ( 120mm/150mm)

 

[Chr!s]

I don't know why we keep harping on this 90C aspect for cables, it's highly unlikely that you'll ever see a 70C cable working at anywhere near that tempreture, let alone a 90C cable.

Totally agree, just not sure as to why specific K values haven't been used?

 

[Chr!s]

Just took the time to do a couple of quick adiabatic calc's using k=154 on these tables and well passed ( 120mm/150mm)

But what level of fault current did you use for the adiabatic, im not disagreeing with the fact the cable is protected, just the tables are not correct.

As long as I2T is less than K2S2 then there's no issue, also taking into account the magnetic effect of the armouring.

It was just the case id worked out some values and cross referenced them with the tables and noticed a difference, then realized they had used K values for PVC. Reality is as you say using the adiabatic they will be ok.

Regards Chris

 

[ICE]

I was under the impression if it was a PME head you was not supposed to extend the earth to the out building but just use a 2 core and stake the out building with an electrode, this was told to me by my 2391 tutor when I was on my course. Is this right or wrong, either way can you point me to the reg in the BRB that states it.

Cheers

ICE

 

[Deleted member 26818]

No we can't point out the Reg. in the BRB, as there isn't one.

 

[Deleted member 9648]

I was under the impression if it was a PME head you was not supposed to extend the earth to the out building but just use a 2 core and stake the out building with an electrode, this was told to me by my 2391 tutor when I was on my course. Is this right or wrong, either way can you point me to the reg in the BRB that states it.

Cheers

ICE

A 2391 tutor who believes in mythology?....the mind boggles.:uhoh2:

 

[Boberto]

Well what ICE was saying was what I was getting at in a way, except it the install is a TT system at origin do you run a 3 core to the outbuilding carrying a CPC as one of the conductors or do you run a 2 core and put an electrode in at the out building connecting to the sub mains there?

 

[Engineer54]

Well what ICE was saying was what I was getting at in a way, except it the install is a TT system at origin do you run a 3 core to the outbuilding carrying a CPC as one of the conductors or do you run a 2 core and put an electrode in at the out building connecting to the sub mains there?

Why would you want to isolate a TT system earthing system?? If anything you would want to connect the 2 rod positions together giving you a theoretical better/lower Ra level for the installation as a whole!!

 

[Boberto]

Agreed Engineer. If the reading on the rod at the sub building is low. In this case 18 ohms. Would I be right in saying that no regs would be broken if a 2 core cable is used to save on cost?

 

[Engineer54]

Agreed Engineer. If the reading on the rod at the sub building is low. In this case 18 ohms. Would I be right in saying that no regs would be broken if a 2 core cable is used to save on cost?

No Reg's will be broken, but will in my opinion be a false economy!! The cost of a 3 core cable is not going to be that much more than a 2 core...