Discuss TT Distribution Best Practice in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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owenskinner

Hi all.

New to this place and have a question about TT distribution.

New 80A TT Supply being put in.
Have to send it 45m to a new Barn Conversion but am unsure best practice.
I am going to put a time delayed 100mA RCD by the Main fuse and then get a consumer unit with 30mA standard type RCD at the Barn Conversion.

Is there any need to protect it for overload and short circuit? As I understand it anything more than 3 Metres from the DNO fuse then it needs to be fused. If so is a High current (80A) MCB sufficient or should an HRC be used?

I was thinking of using a double pole 80A 100mA RCBO, but no one seems to make them (RCBO) with a higher current rating than 40A (Or do they????).

This led me down the path of Wylex 4 Way Metal Clad Panel (Compare Wylex ESM8 Wylex Enclosure Kent Products), 100A 100mA time delayed RCD and then Doube Pole 80A MCB (High Current MCB Double Pole 80A - High Current MCB Double Pole 80 Amp - High Current MCBs Double Pole - High Current MCBs 63 Amp To 125 Amp - Expert Electrical).

Am I being belt and braces or is this the best way to go?

Any neater, solutions more than welcome.
 
the cable to the barn needs OCPD . 80A switch fuse is the norm, but nothing wrong with an 80A MCB.
 
Thanks Telectrix. Is 'OCPD' Over Current Protective Device? Not used to the abbreviations..
 
You are doing a barn conversion 45m from an existing supply and taking 80A off that supply. Are you sure that the existing will take the additional load?
 
It is a new supply for the Barn Conversion so has no other load. I did recommend to the client to have the new supply put in straight to the Barn but he didn't want his driveway (mud and a few cobble stones) dug up! I have done the cable calcs and it came up with 35mm so I am putting in 2 Core 35mm SWA buried 600mm down.
 
Surely the distribution SWA cable needs RCD protection also. 100mA time delayed RCD at the beginning for discrimination, 30mA at the Barn end also.
 
I thought under the ElectricitySafety, Quality and Continuity Regulations (ESQCR) 2002 all new supplies had to have an earth provided?
 
I thought under the ElectricitySafety, Quality and Continuity Regulations (ESQCR) 2002 all new supplies had to have an earth provided?

No, the DNO is only obliged to maintain the upkeep of an existing earth, they are not required to provide one if one does not already exist.

EDIT: Obviously if they are supplying a PME then that's a different matter.
 
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I agree with the fact that if the supply does not have an existing earth provided then it is up to the used of the supply. But somewhere along the line I have picked up that new supplies are different that is why DNO are putting PME in
 
Yes, I'm puzzled too. Surely there's no need to RCD protect a 35mm SWA distribution circuit properly buried 600mm under the ground?

Just a normal switchfuse will do the job. Personally I'd try to get one with a big box to make directly terminating that 35mm easier! Is it single phase only?

BTW, is there no chance of getting PME?
 
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No it doesn't require RCD protection, if the Ra of the earth electrode is sufficiently low enough to allow the OCPD to operate.
However, as that is probably not going to be the case, then an RCD is the only option to achieve the required disconnection times (1s for TT distribution circuits).
 
You are doing a barn conversion 45m from an existing supply and taking 80A off that supply. Are you sure that the existing will take the additional load?

It is a new supply for the Barn Conversion so has no other load. I did recommend to the client to have the new supply put in straight to the Barn but he didn't want his driveway (mud and a few cobble stones) dug up! I have done the cable calcs and it came up with 35mm so I am putting in 2 Core 35mm SWA buried 600mm down.
 
Re: TY Distribution Best Practice

No it doesn't require RCD protection, if the Ra of the earth electrode is sufficiently low enough to allow the OCPD to operate.
However, as that is probably not going to be the case, then an RCD is the only option to achieve the required disconnection times (1s for TT distribution circuits).

Thanks spin , I was starting to think i was losing the plot and had been doing it wrong all this time !
 
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