Discrimination

[richsogecko]

Hi All,

My house is supplied via a PME cutout - Ze 0.07Ohm and the main CU is a 22W Split Hager Board with 2 unprotected ways and 3No banks of 4-6 circuits, each protected by a DP 80A/30mA RCCB

I've just run a supply to my shed - through the house in 6mm T&E to an external Wiska box where I've changed to 6mm 3C SWA for the onward supply to the shed.

At the Wiska box I haven't jointed the T&E CPC to either the SWA CPC or SWA Armour, which keeps the PME within the equipotential zone. The T&E CPC is terminated at the main DB so it protects the cable along it's length within the house.

The P/N of the SWA is terminated in a 2Way G/Board at the Shed which uses a 1.2m Earth Rod to complete it's TT supply. The SWA CPC & Armour are both terminated to the Garage Board MET, protecting the 'run of cable in the garden.

Ze for the Rod is 43Ohm (>21 but a lot less than 200 or even 1667Ohm)

The Incomer of the Garage Board is a 63A/30mA DP RCD and all the shed provides power to is :-

C1 - 16A MCB for Outdoor socket & Water Feature
C2 - 6A MCB for 7No 3W LED Spotlights.



The problem I've got isn't major but I'm finding when I perform an RCD test that the first device to trip is the RCD protecting the Shed Supply in the main house DB and this takes with it another 4+ circuits.

I'm not worried about nuisance tripping and realise it's just about sensitivity but wondered whether (and assuming the regs permit) I could achieve proper discrimination by fitting a Type S RCD in the main board upstream of the shed supply so in theory the Shed DB should trip first.

For clarity the main board is configureable enough that all household socket circuits could remain on non time delayed 30mA RCCBs protecting the 200/40ms tripping requirements.

Thanks in advance

Rich

 

[telectrix]

1.if the T/E in the house doesn't require RCD protection ( i.e. buried in walls<50mm deep ) then take it from a non-RCD way.

2. Why rely on a farty little rod. either use 2 screwed rods to go deeper, or use the PME earth , bearing in mind that any extraneous metal parts in the shed will require 10mm bonding back to the MET.

 

[richsogecko]

1.if the T/E in the house doesn't require RCD protection ( i.e. buried in walls<50mm deep ) then take it from a non-RCD way.

The T&E does require RCD protection - steel framed construction of whole house and all steel studwork

2. Why rely on a farty little rod. either use 2 screwed rods to go deeper, or use the PME earth , bearing in mind that any extraneous metal parts in the shed will require 10mm bonding back to the MET.

Doubling rod length could be an (awkward) option and definitely not as easy as fitting a Type S if that would work/be allowed
Bonding back to House MET is nigh on impossible and my understanding of Regs state you shouldn't export a PME?

Thanks

Rich

 

[i=p/u]

Maybe should of used swa right through and use unprotected way, I've seen many houses with 2 rcd inline and no problems unless a fault . Which is hardly ever... But reduce that annoyance by protecting the t&e cable

 

[telectrix]

OK, so ditch the RCD in the shed. fit RCBO in house CU so that a fault in the shed only takes out the shed.

 

[44ALLAN]

Bonding back to House MET is nigh on impossible and my understanding of Regs state you shouldn't export a PME?

10,9,8,7.....................lol
​

 

[telectrix]

exporting pme is actually extending the equipotential zone. and saying you should not do this is a myth. the only drawback is the bonding of ecp's.

 

[SPARTYKUS]

as Tel said can you run the t&e in a fashion so it doesnt need rcd prtection? Surface mount in trunking /conduit? Where does it run (exactly) now?

simply being told "do not export PME" is a mis-fact.

as stated it can be exported. i'll repeat that it CAN be exported.

Are there any services in the shed?

Have a search on this forum for articles there are plenty.

then you'll not need the earth rod. And your first line of defence will be an incredible Zs and threfore disconnection time! With additional protection from the RCD....

 

[richsogecko]

as Tel said can you run the t&e in a fashion so it doesnt need rcd prtection? Surface mount in trunking /conduit? Where does it run (exactly) now?

The T&E is run inside the stud and a full decoration has already happened so changing the installation method now isn't really viable/aesthetic

simply being told "do not export PME" is a mis-fact.

Thanks, much clearer now

as stated it can be exported. i'll repeat that it CAN be exported.

Useful info

Are there any services in the shed?

No services, just power and lights - does that mean I wouldn't need a separate 10mm bond back to the MET, because there's nothnig o bond

Have a search on this forum for articles there are plenty.

then you'll not need the earth rod. And your first line of defence will be an incredible Zs and threfore disconnection time! With additional protection from the RCD....

Assuming there's no needs to run a 10mm to the shed just for appearances sake then I think the easiest solution is :-

1) Reconfigure Main DB, moving circuit to an unprotected way (Thanks Telectrix, occurred to me too as I was mulling it over)
2) Substitute the current 20A MCB for a 20A RCBO
3) Link up the CPCs in the Wiska Box

Hey Presto! Instant 0.07Ohm Ze

Thanks for the constructive help

Rich

 

[Octopus]

Hi All,

My house is supplied via a PME cutout - Ze 0.07Ohm and the main CU is a 22W Split Hager Board with 2 unprotected ways and 3No banks of 4-6 circuits, each protected by a DP 80A/30mA RCCB

I've just run a supply to my shed - through the house in 6mm T&E to an external Wiska box where I've changed to 6mm 3C SWA for the onward supply to the shed.

At the Wiska box I haven't jointed the T&E CPC to either the SWA CPC or SWA Armour, which keeps the PME within the equipotential zone. The T&E CPC is terminated at the main DB so it protects the cable along it's length within the house.

The P/N of the SWA is terminated in a 2Way G/Board at the Shed which uses a 1.2m Earth Rod to complete it's TT supply. The SWA CPC & Armour are both terminated to the Garage Board MET, protecting the 'run of cable in the garden.

Ze for the Rod is 43Ohm (>21 but a lot less than 200 or even 1667Ohm)

The Incomer of the Garage Board is a 63A/30mA DP RCD and all the shed provides power to is :-

C1 - 16A MCB for Outdoor socket & Water Feature
C2 - 6A MCB for 7No 3W LED Spotlights.

View attachment 17210

The problem I've got isn't major but I'm finding when I perform an RCD test that the first device to trip is the RCD protecting the Shed Supply in the main house DB and this takes with it another 4+ circuits.

I'm not worried about nuisance tripping and realise it's just about sensitivity but wondered whether (and assuming the regs permit) I could achieve proper discrimination by fitting a Type S RCD in the main board upstream of the shed supply so in theory the Shed DB should trip first.

For clarity the main board is configureable enough that all household socket circuits could remain on non time delayed 30mA RCCBs protecting the 200/40ms tripping requirements.

Thanks in advance

Rich

Nice pictorial, what did you use to create it?

 

[richsogecko]

Powerpoint - nothing fancy just 'Shapes', 'Textboxes' and coloured lines

Thanks

Rich

 

[Octopus]

Powerpoint - nothing fancy just 'Shapes', 'Textboxes' and coloured lines

Thanks

Rich

Just time consuming then. lol

 

[Guest55]

and my understanding of Regs state you shouldn't export a PME?

Hes a witch ! get him lads !
;-)

 

[Octopus]

This is a really informative thread to the plethora of DIY'ers - shame its not in the arms

 

[whitestu]

bearing in mind that any extraneous metal parts in the shed will require 10mm bonding back to the MET

Not likely to be any in a shed or a garage for that matter.

 

[richsogecko]

Link doesn't work

 

[Engineer54]

Use your PME earth for your shed installation, and leave the rod connection to the EMT of the shed CU in place. It will simply act as another PME multiple earthing point to the service cut-out N-E connection....