Cu Change Main Earth HELP!

[mcl]

Hi

Am pricing for CU change for a single barn holiday cottage. Forms part of a group. All the meters are in one building at the end, approx 30m away from the CU at this barn. Seems like all ducted underground. The main fuse is 60a, and the Meter tails are 16mm so ok but the main earth (supply PME) is only 10mm2. I think it would be very difficult and costly to get 16mm upgraded main earth back to Western Power fuse.
Thought that another option is forget the PME main earth and TT locally (just other side wall with CU) to the CU?

What do you think?

Thanks MATT

 

[jeffelectric]

pme 0.35ohms if the ze is lower . it is good?

 

[telectrix]

I'd leave the 10mm in. Just note it on your cert.

 

[mcl]

Thanks for replies but I thought the main earth compliance was mandatory not something could note as non bs7671. I may be doing some bathroom wiring later in year so need to be sure all ok

 

[telectrix]

If you were installing the earthing conductor as new, then yes, 16mm om a pme is the way to go, but bear in mind that bs7671 is not statutory. If the 10mm satisfies the adiabatic equation, then i would note it as a departure.

 

[Deleted member 9648]

Hi

Am pricing for CU change for a single barn holiday cottage. Forms part of a group. All the meters are in one building at the end, approx 30m away from the CU at this barn. Seems like all ducted underground. The main fuse is 60a, and the Meter tails are 16mm so ok but the main earth (supply PME) is only 10mm2. I think it would be very difficult and costly to get 16mm upgraded main earth back to Western Power fuse.
Thought that another option is forget the PME main earth and TT locally (just other side wall with CU) to the CU?

What do you think?

Thanks MATT

The 60a supply to the barn is a distribution circuit,the main earth is from the PME head to the MET at the intake....you can therefore upgrade the main earth to 16mm if required to the MET and switchfuse...the 10mm you refer to is the earth for the 60a distribution circuit,not the main earth.

 

[dave274]

why would 10mm not be big enough? the adiabatic works out less than 10mm so its meets the requirements of table 54.7 and 543.1.4 or am I missing something?

 

[mcl]

The 60a supply to the barn is a distribution circuit,the main earth is from the PME head to the MET at the intake....you can therefore upgrade the main earth to 16mm if required to the MET and switchfuse...the 10mm you refer to is the earth for the 60a distribution circuit,not the main earth.

Hi Thanks for opinion but the the existing main earth comes direct from the connection at western power fuse. Each barn has its own meter and service cutout. Therefore dont think it is a disribution earth. Ther MET is at the CU.

 

[telectrix]

reading wirepullers post and looking at the situation ( without seeing it.) this might be appropriate. fit a MET at the head, 16mm from head to MET, the the existing 10mm earthing conductors from this new MET to each sub-main.

 

[mcl]

reading wirepullers post and looking at the situation ( without seeing it.) this might be appropriate. fit a MET at the head, 16mm from head to MET, the the existing 10mm earthing conductors from this new MET to each sub-main.

Thanks, this seems a good idea. What about bonds then. Cant remember reg but sure they had to be connected to MET. If the MET is then by the service head the bonds are at the CU?

 

[Geoffsd]

I think you are getting muddled.

543.1.1 says THE earthing conductor shall be sized according to 543.1.3 OR 543.1.4.


543.1.3 - use adiabatic equation.

543.1.4 - Where it is desired NOT to calculate the minimum csa use table 54.7 (which says the same size as the neutral up to 16mm²).


Bonding should be 10mm² for PME - Table 54.8 (or larger if DNO require).

 

[Tuttle]

There's also 542.3.1 which says the earthing conductor for PME shall meet the requirements of 544.1.1 for the cross sectional area of a main protective bonding conductor.

 

[dave274]

yes I was missing something, I was reading table 54.7 incorrectly thanks geoffsd you made it much clearer.

 

[mcl]

My thinking is that to be 100% compliant I could TT CU even though Ze obviusly higher but all RCD protected anyway and keep MET and bonds where they are?

 

[Engineer54]

Hi Thanks for opinion but the the existing main earth comes direct from the connection at western power fuse. Each barn has its own meter and service cutout. Therefore dont think it is a disribution earth. Ther MET is at the CU.

You don't say how many holiday dwelling were talking about, but at the main DNO intake there should be a series of Switch Fuses or a Ryefield panel that supplies these holiday dwellings. The main earth conductor off the PME head will need to comply with PME requirements to the MET in the intake building/room. (which may require a larger conductor size than 16mm, depending on the the actual size of the DNO supply) The supplies away from this building, to the respective dwellings, are in effect distribution circuits. The individual Dwellings MET's are actually EMT's, there can only be One MET and that is always at the origin of the supply.


Personally i would have run in 3 core 16mm SWA from the intake position to the dwelling units, but you have what you have and that's a 10mm Earthing conductor. So check if it meets all the criteria, if so, it should be fine.

 

[Engineer54]

My thinking is that to be 100% compliant I could TT CU even though Ze obviusly higher but all RCD protected anyway and keep MET and bonds where they are?

You're joking now arn't you?? ....Please say your joking!!!

 

[telectrix]

putting eng's post into plain english......... if it ain't broke, don't fix it. LOL. :75:

 

[mcl]

You don't say how many holiday dwelling were talking about, but at the main DNO intake there should be a series of Switch Fuses or a Ryefield panel that supplies these holiday dwellings. The main earth conductor off the PME head will need to comply with PME requirements to the MET in the intake building/room. (which may require a larger conductor size than 16mm, depending on the the actual size of the DNO supply) The supplies away from this building, to the respective dwellings, are in effect distribution circuits. The individual Dwellings MET's are actually EMT's, there can only be One MET and that is always at the origin of the supply.


Personally i would have run in 3 core 16mm SWA from the intake position to the dwelling units, but you have what you have and that's a 10mm Earthing conductor. So check if it meets all the criteria, if so, it should be fine.

Hi
Thanks for advice. There is 6 cottages, but all have seperate western power fuses and separate meters all in one building. The appears to be no MET here. The tails and Earth travel 30m to the CU. It appears to be the same with each of the others. Although there are 6 cottages together, they are all seperate supplies and all cottages owned by different people. Therefore I am sure the MET is at the CU? I know it sounds stupid to TT a supply when it is PME but if I the main eaarth needs to be 16mm and it cant be done without digging 30m then is it not an option?

 

[Clive P]

Hi,

The MET ( cable sized according to incoming supply ) should be at your intake position, ie where the meters are installed, and the 10mm cable with the 16mm tails is your supply to your CU at the Barn. At the Barn you would have installed your EMT. Just think it as being a normal install with long tails.

Regards.

 

[D Skelton]

Unless there is an extremely high or extremely low PFC, what on earth is wrong with a 10mm main earth on a 60A PME?!?

Required tail size is 16mm, therefore main bond will be 10mm. The main earth is sized according to the bonding conductor. Ergo sum... 10mm main earth.

I wouldn't entertain using any higher than 10mm for the main earth for any supply up to and including 100A regardless of its type (PFC permitting).