Sub-Main Testing

[The Builder]

Question for Bright Spark - other Electrical Gurus may be available ;o)

A country house with an adjoining building that is being converted as an annexe.
The electrician who re-wired the house a couple of years ago, has provided a sub-main from the main house CU which is a 10mm t&e from a 50A MCB.

I intend to provide a standard CU installed in the annexe.

My question is ….... when I make the usual tests on my circuits. Do I take into account the impedance of the 10mm supply cable. I'm thinking mainly of the EFLI tests.

[buildersmate]

Mine is a mere opinion - you need his Brightness for the definitive answer.

Since the purpose of EFLI testing is to ensure that the over-current protection devices trip within a minimum (specified) time (i.e the earth loop impedence is low enough to ramp the overcurrent up quickly), and also since the testing regime says one has to test at the mains entry supply point, at EACH CU, at each socket and the proportion of the lighting circuits, its clear what testing has to be done - the annexe has to be tested in its own right - then deal with the 10mm T&E.

In the annexe, testing will be straight-forward, so I guess your question doesn't relate to that - and the maximum impedance permitted is measured back to the annexe CU.

I guess your question relates to the protection afforded by the 50A MCB at the house - whose purpose is surely only to protect from a cable fault between the 2 CUs. My guess is that you measure the EFLI between the 2 CUs - and relate it back to the maximum impedance allowed for a 50A circuit. In which case it is the impedance of the cable between the two CUS (and its connections) that you are measuring - and you don't take it into account - since it is that very impedance that would prevent the 50A breaker tripping in adequate time under a fault condition (if too high an impedance).

[BRIGHT SPARK]

As you only have one supply then you only have one ze for the system.The efli for the final circuits of the annex will consist of the r1 +r2 in the final circuits and the swa....+ the ze
Is the swa protected only by the mcb or is there an rcd in circuit?

[BRIGHT SPARK]

Just to clarify................the annex db will have its own loop value..call this zdb2 and all the loop values up stream from that will need to comply with the disconnection times of the installed protection in db2.

[The Builder]

Oh dear. Looks like there was some right carnage on this thread last night . "answers removed" must have either been advertising........ or, more likely, Nobby the electrician coming over from the dark side to "say hello" ;o)

No matter. Thanks to you both, BS and BM, for your help.
You're both telling me what I need to know.

Testing back to the annexe CU is straightforward. You've confirmed what I thought... that the impedance of the cable between the CUs is a part of the overall Ze.
It's not even an SWA. It's a normal T&E routed several metres through the house. Only a 4mm CPC as well.

I can see that a low enough Zs for the annexe is maybe going to be difficult to achieve after factoring in the 10mm cable.
It sounds to me as though I may need to fit an earth rod at the annexe CU position rather than "exporting" the PME from the house.

[The Builder]

Yes, BS, the house DB has full RCD protection.

[BRIGHT SPARK]

TB......The loop reading for the 10mm will be included in the results on the EICR for the house db.The zdb for the annex db will be included on the annex EICR.You should disconnect the cpc for the supply to db2 at db2 , treat the annex as a TT....and fit an 's type' 100mA to protect the annex supply...this will avoid any downstream tripping of the house db if an earth fault occurs on the annex db.....(you can only use a 100mA if no part of the 10mm is within 50mm etc).
Dont be concerned with high loop readings ...because if the circuits are protected with a 30mA rcd then the max efli will be 1666 ohms...ish

[The Builder]

BS, thank you for that. Much appreciated. I thought I may be worrying unduly over high loop readings. The max efli of 1666 will ensure a max touch voltage of 50v.

Thanks again for your help BS and BM

[factor-fiction]

I'm glad you got some good answers in between the nonsense, The Builder. Yes, you were right about the 'visitor' who was going by the name of theflorist, but fortunately someone pulled the plug on him fairly quickly

[The Builder]

Thanks FF. I can just imagine the line Nobby was taking. He can't seem to accept that, if I'm not sure of something ......... I ask.
Buildersmate and Bright Spark are extremely helpful to all of us :o)