Earthing A Supply To A Wooden Shed

[Metallica]

I am considering the options which may apply to the earthing arrangement for a supply to a wooden shed.
The property has a TN-C-S supply from which a 60A isolator feeds a 3-core 16mm2 swa sub main 13 metres to a detached garage. The property has a supplementary earth rod and the garage also has another supplementary earth rod. In the garage is a 50:50 split fully RCD protected board and an earth marshalling block to connect the cpc's. So the garage is an extension of the property equi-potential zone. I am proposing to take a 16A supply from the garage board to a wooden shed using a 35 metre run of 3-core 6mm2 swa cable which has already been installed underground. My question is primarily whether I should export the earth and extend the equi-potential zone again or opt for a TT system. In either case I propose to install an earth rod adjacent to the shed which if not used as a separate earth would be used as a supplementary earth.
My secondary questions are:
1). If the installation can be done as pme, can the c.s.a of the earth conductor and armour be combined to satisfy the 10 mm2 requirement for a bonding conductor?
2). Although earth leakage discrimination is not vital could this be achieved on a pme installation or is this a good reason to go for TT?
3). Do I have a choice? I have a preference for extending the equi-potential zone with multiple earth rods as I think this gives better protection when using power tools on leads which can reach from one building to another.
I am not an electrician which is why I am raising these questions. I will be completing the electrical design and installation in accordance with the current edition regulations in readiness for a Part P qualified electrician to inspect and test the work.

 

[Strima]

Is there anything in the shed that requires bonding back to the MET?

The supplied earthing arrangement stops at the suppliers head so technically you're not exporting anything. The problem with having additional earth electrodes in addition to the suppliers TNC-S arrangement is if the supply neutral is lost all the neutral currents will try and use your electrodes to get back to their point of generation, this includes any other effected properties depending upon the location of the fault.

My advice is to consult the electrician who will be 'signing this off' as he/she will be putting their name to it.

 

[Metallica]

Thanks for the reply. There will be nothing in the shed which requires bonding to the MET. Yes, I will be consulting the electrician but I like to get an understanding first in order to be able to talk sense.

 

[westward10]

You are not an electrician but you intend to design and do the installation in accordance to current regulations for another individual to test and inspect. If you are doing this according to current Regulations why are you asking advise on here because I assume if you are designing it you are well versed with current Regulations.

 

[UKPN]

I am considering the options which may apply to the earthing arrangement for a supply to a wooden shed.
The property has a TN-C-S supply from which a 60A isolator feeds a 3-core 16mm2 swa sub main 13 metres to a detached garage. The property has a supplementary earth rod and the garage also has another supplementary earth rod. In the garage is a 50:50 split fully RCD protected board and an earth marshalling block to connect the cpc's. So the garage is an extension of the property equi-potential zone. I am proposing to take a 16A supply from the garage board to a wooden shed using a 35 metre run of 3-core 6mm2 swa cable which has already been installed underground. My question is primarily whether I should export the earth and extend the equi-potential zone again or opt for a TT system. In either case I propose to install an earth rod adjacent to the shed which if not used as a separate earth would be used as a supplementary earth.
My secondary questions are:?
1). If the installation can be done as pme, can the c.s.a of the earth conductor and armour be combined to satisfy the 10 mm2 requirement for a bonding conductor?
2). Although earth leakage discrimination is not vital could this be achieved on a pme installation or is this a good reason to go for TT?
3). Do I have a choice? I have a preference for extending the equi-potential zone with multiple earth rods as I think this gives better protection when using power tools on leads which can reach from one building to another.
I am not an electrician which is why I am raising these questions. I will be completing the electrical design and installation in accordance with the current edition regulations in readiness for a Part P qualified electrician to inspect and test the work.

You have no choice. You need to lose all those rods, they are inviting a hazard. Your best course is to feed the shed with an RCD and a separate rod for the shed only.

Regards, UKPN

 

[Metallica]

I posed these questions on here because I have found nothing in the 17th or 18th Editions which preclude extending the equi-potential zone to include the wooden shed and would have liked the benefit of other opinions on this interpretation before concluding that TT earthing is the only option. With regard to the use of supplementary earth rods there is very little mention in the regs although Section 5.1 of Guidance Note 8 does show an example. Whether supplementary earth rods are hazardous or beneficial seems to be subjective. Installation of supplementary rods can reduce touch voltages and were proposed to be a requirement for inclusion in the current regs but were removed after the consultation stage for reasons of burden on property owners and contractors. This implies to me that the danger of rods carrying neutral currents in the event of a loss of the supply neutral was considered by the IET to be outweighed by the benefits. My thoughts are that in PME installations without supplementary earth rods, the neutral currents would find their way back to the point of generation via metal service pipes which could be more hazardous.

 

[UKPN]

I posed these questions on here because I have found nothing in the 17th or 18th Editions which preclude extending the equi-potential zone to include the wooden shed and would have liked the benefit of other opinions on this interpretation before concluding that TT earthing is the only option. With regard to the use of supplementary earth rods there is very little mention in the regs although Section 5.1 of Guidance Note 8 does show an example. Whether supplementary earth rods are hazardous or beneficial seems to be subjective. Installation of supplementary rods can reduce touch voltages and were proposed to be a requirement for inclusion in the current regs but were removed after the consultation stage for reasons of burden on property owners and contractors. This implies to me that the danger of rods carrying neutral currents in the event of a loss of the supply neutral was considered by the IET to be outweighed by the benefits. My thoughts are that in PME installations without supplementary earth rods, the neutral currents would find their way back to the point of generation via metal service pipes which could be more hazardous.

Burden on owners and contractors? It was always a no no, the IET should have known that. The next farce will be "foundation earthing" No benefit at all, impossible in obvious scenarios, and-----burden on owners and contractors!

Regards, UKPN

 

[radiohead]

Its a shed FFS.

 

[telectrix]

Its a shed FFS.

but if OPis in london a shed could fetch £500/week rent.