Seperate test sheet for garage consumer unit on a TT system

[Tauruselec]

Hi Guys, hopefully someone will know the answer. Testing an install on a (domestic) PME system I have a garage consumer unit supplied from one of the RCBO's
I have installed an earth rod to supply the earth and not connected the CPC or armouring to the garage consumer unit end.
My question is when it comes down to test sheets I need to take a Ze at the garage board but on the NAPIT test sheets if I add a other DB to the installation it does not give me an option for another Ze.
I could obviously do another test sheet but it's another payment to master the copy etc and seems pointless.
It's asked if the dB is connected to the origin of the supply which technically its not so a Zdb test could be carried out but then the earthing arrangement would be different so I would need somewhere to note that down surely? Appreciate any help in advanced.

Cheers

 

[DPG]

Ze is only at source of supply, ie. intake.

 

[Tauruselec]

Ze is only at source of supply, ie. intake.

Yeah I get that, so when I take a Zs reading on a circuit rrom the garage board I will be getting a much higher reading due to the fact that the earth path/arrangement is not the same as the rest of the installation and or at origin
This will effect the permissable zs readings to ensure RCD fault protection so unless it's noted somewhere that the reading was taken using a tt arrangement surely someone could assume that the PME earthing was used and the readings are not satisfactory and deem the circuits unsafe.....

 

[Pretty Mouth]

This will effect the permissable zs readings to ensure RCD fault protection so unless it's noted somewhere that the reading was taken using a tt arrangement surely someone could assume that the PME earthing was used and the readings are not satisfactory and deem the circuits unsafe.....

All the garage circuits are protected by the upstream (presumably 30mA) RCBO in the main house. The max permitted Zs for all of the circuits downstream of this device is 1667ohms, regardless of earthing type. Note that value in the max permitted Zs column for each circuit on the garage board schedule of circuit details.

 

[Tauruselec]

All the garage circuits are protected by the upstream (presumably 30mA) RCBO in the main house. The max permitted Zs for all of the circuits downstream of this device is 1667ohms, regardless of earthing type. Note that value in the max permitted Zs column for each circuit on the garage board schedule of circuit details.

 

[Tauruselec]

Ahhh that makes sense I didn't think about changing the max zs section.
The garage circuits have their own rcd. As they are not connected to the cpc/armouring that runs in the swa then they surely the rcd would only operate on the garage board under fault conditions via the TT system.

The cpc in the swa is only adding additional protection to the cable supplying the garage consumer unit.

So knowing that I can enter the max zs in the test sheet as 1667 or 1533 I still feel there should be somewhere for another Ze??

Maybe I'm looking into it too much but seems odd to me

 

[Pretty Mouth]

The garage circuits have their own rcd. As they are not connected to the cpc/armouring that runs in the swa then they surely the rcd would only operate on the garage board under fault conditions via the TT system.

No, the RCBO at the house will see an imbalance for any earth fault downstream of it, doesn't matter what earthing system the CPC is connected to. Either the RCBO at the house, or the RCD at the garage, or both, will trip under a downstream fault to earth, it's pot luck.

So knowing that I can enter the max zs in the test sheet as 1667 or 1533 I still feel there should be somewhere for another Ze??

There is a cell for Zdb in the generic circuit details sheet. 1533? Where did you get that number from?

 

[timhoward]

I'm really curious why the distribution circuit isn't on an MCB. The cable meets impact protection rules. It feels as though 99% of the hard work has been done to make this a good design.

I still feel there should be somewhere for another Ze??

The term Ze is only relevant at the origin of the distribution. Everything after that is either a distribution circuit or a final circuit.

 

[Flanders]

The way I see it you take you zs of the supply to the garage between the swa and the line . This reading should be low enough to disconnect the supply in the event of a fault on the supply cable. As you have used an Rcbo this should easily be achieved , although as tim says I'm Not sure why you have used an rcd to protect the circuit an mcb should be ok if you have used an swa. I would then of fitted the rcd in the garage. .also does the garage need to be a TT system?

 

[Tauruselec]

The way I see it you take you zs of the supply to the garage between the swa and the line . This reading should be low enough to disconnect the supply in the event of a fault on the supply cable. As you have used an Rcbo this should easily be achieved , although as tim says I'm Not sure why you have used an rcd to protect the circuit an mcb should be ok if you have used an swa. I would then of fitted the rcd in the garage. .also does the garage need to be a TT system?

New domestic install everything is required or recommended to be protected with an RCBO. They are cheap enough so why wouldn't you?

Also it's stated that a seperate earth would need to be run to any exposed metal parts of the garage and taken back to the origin in the house if no earth rod is provided within a meter of the garage...
I thought Any outbuildings more than 1meter of the dwelling need a separate earth to comply with current regs?

 

[timhoward]

New domestic install everything is required or recommended to be protected with an RCBO

Agree for final circuits. But not for distribution circuits where impact protection is met by cable type and disconnection time can be achieved though ordinary prospective fault current. You really want to avoid more than one RCD if at all possible.

Also it's stated that a seperate earth would need to be run to any exposed metal parts of the garage and taken back to the origin in the house if no earth rod is provided within a meter of the garage...

In there is anything extraneous to bond at the garage then making sure the CPC of the distribution circuit is adequate to support it is my usual approach (taking into consideration the earthing system at the origin when sizing the bonding). Did you run 2 or 3 core SWA?

I thought Any outbuildings more than 1meter of the dwelling need a separate earth to comply with current regs?

Sorry but I'm aware of this or heard of it before. Do you have a reg number?

Don't get me wrong, it sounds like everything complies, but I just wanted to at least pique your interest that there may be ways to slightly improve the next one.

 

[westward10]

It is not correct that all circuits in a dwelling require additional RCD protection.

 

[DPG]

New domestic install everything is required or recommended to be protected with an RCBO. They are cheap enough so why wouldn't you?

Also it's stated that a seperate earth would need to be run to any exposed metal parts of the garage and taken back to the origin in the house if no earth rod is provided within a meter of the garage...
I thought Any outbuildings more than 1meter of the dwelling need a separate earth to comply with current regs?

I really don't think you will find a reg to confirm your last statement. Perhaps that has come from hearsay rather than an official source?

 

[Tauruselec]

No I appreciate the advice. As you said about it being a distribution circuit i see what you mean now, I didn't think about that.

And you know what I cannot find a reg number for it. I was something that I heard about when the 18th came out as I thought it was abit of a pain but assumed that

It is not correct that all circuits in a dwelling require additional RCD protection.

Agreed. But in the majority of circumstances they would be required. Unless we are chasing deeper than 50mm or adding thick steel sheets to cover cables.
To be honest I know eitherway I'm covered with an RCBO so I'd rather use one than not as the cost is negligible nowadays

 

[Tauruselec]

I really don't think you will find a reg to confirm your last statement. Perhaps that has come from hearsay rather than an official source?

Yes I think your correct. I spoke to someone earlier who does this alot and they said they think it was mentioned or talked about but seemed as if they went back on the idea and it was never made a requirement and that using the cpc from the house supply would still be adequate and comply.

More than likely hearsay then.

Cheers for the replies tho!