A little assistance required regarding earth sizing.

[Davisss]

Hi guys,

I have been doing a bit of testing today and need a liitle input if possible. The installation is TN-S at source. There is a 50A switchfuse which feeds two distribution boards in a B & B outbuilding. The supply comes into the first D.B (in one of the B & B rooms) into the top of an upfront RCD (30mA) and then feeds the next D.B ( in the other B & B room). This also has an RCD (30mA) up front. The first D.B has been TT'd which is brilliant and has the supply earth segregated. However the earth from the second D.B has its earth from the first D.B earth bar instead of it's own earth stake.

As it stands, any RCD testing on the first D.B results in the second D.B's RCD tripping. Not ideal. But my main concern is the cpc that feeds the second D.B is an inadequate size. It is 4mm (10mm T+E). Ideally, they should have TT'd this D.B aswell. Unfortunately (as ever) it has all been plastered and finished long ago.

Potential soulutions:

1: TT the second D.B and segregate the incoming cpc feed. The downside is surface cabling to achieve this.

2: Could i get away with feeding the second D.B off the first D.B via an MCB? I would change the second D.B to a main switch up front. PFC for the second D.B is 0.434kA and measured Ze is 3.57 Ohms. I calculated the adiabatic equation using a disconnection time of 1s and a k factor of 115. The result is 3.77mm ( i think!) Would this be acceptable! Obviously, any earth fault in the second D.B would result in the RCD tripping both rooms out! Not ideal!


Any input would be greatly appreciated!

Thanks in anticipation,
Dave

 

[JohnW2]

PFC for the second D.B is 0.434kA and measured Ze is 3.57 Ohms.

That presumably isn't a true Ze, given that it's TTd - was it measured with some extraneous-c-ps bonded to it?

Kind Regards, John

 

[Davisss]

Hi John. That is indeed a true Ze. They have either got very lucky with one electrode or there may be multiple ones joined.

 

[ban-all-sheds]

Could i get away with feeding the second D.B off the first D.B via an MCB?

So that one guest can cut the power to the other?

 

[Davisss]

Yes BAN, not ideal at all. But that's what people get when they try to save themselves a few quid by doing a cheap job. They get comeback. Just spoke to the NICEIC man on the blower and he agreed with both proposals.
Now to tell the client!

 

[JohnW2]

PFC for the second D.B is 0.434kA and measured Ze is 3.57 Ohms I calculated the adiabatic equation using a disconnection time of 1s and a k factor of 115. The result is 3.77mm ( i think!) Would this be acceptable!

That presumably isn't a true Ze, given that it's TTd - was it measured with some extraneous-c-ps bonded to it?

Hi John. That is indeed a true Ze. They have either got very lucky with one electrode or there may be multiple ones joined.

That's pretty amazing - I think I would still be looking for 'parallel paths' to earth!

Whatever ... I presume that the PFC of 434A you mention is the PSSC, since even with a Ze as low as 3.57Ω, the PEFC at the DB would only be about 64.4A;. You appear to have undertaken your adiabatic calculation using the PSCC (434A), so that your answer (3.77&#937 relates to the minimum CSA of the L&E conductors under short-circuit conditions. If you repeated the calculation using the PEFC (~64.4A) you would get an answer of about 0.56mm² for the minimum CSA of CPC/earth conductor under L-CPC fault conditions. Also, returning to you original question ...

Could i get away with feeding the second D.B off the first D.B via an MCB? I would change the second D.B to a main switch up front.

Given the potential inconvenience it could create, I don't fully understand why you want to do that - is it simply to allow you to use smaller cable between the DBs (that cable otherwise only being protected by the 50A switch fuse)?

Kind Regards, John

 

[JohnW2]

Yes BAN, not ideal at all. But that's what people get when they try to save themselves a few quid by doing a cheap job. They get comeback. Just spoke to the NICEIC man on the blower and he agreed with both proposals.

I'm a little surprised that is regarded as acceptable. For a fault (or even malicious/accidental act) in one bit of guess accommodation to be able to kill the entire supply to another bit of guest accommodation (different guest) sounds perilously close to being non-compliant with one or more bits of 314! ... and all that seemingly for the sake of a bit of probably 10mm² cable.

Kind Regards, John

 

[Davisss]

Hi John,

Thanks for taking the time to reply. Yes, it really is a true EFLI. Mind you i have had lower!

Yes, i did take the PSCC as my figure for calculations as that was the higher reading.

I was entertaining options and feeding the second D.B off of the first D.B via an mcb would indeed allow me to use the cpc between boards. Not ideal, i know, but the whole installation isn't ideal. Just trying to make the best out of a bad situation.

I'm going to try and convince him to go for an earth electrode for the second D.B. More expensive but a better solution.

Regards,
Dave

 

[JohnW2]

Hi John, Thanks for taking the time to reply. Yes, it really is a true EFLI. Mind you i have had lower!

Fair enough, even though I remain surprised!

Yes, i did take the PSCC as my figure for calculations as that was the higher reading.

You don't need to do that to determine the minimum CSA for the CPC. You can simply use the PEFC which, as I said, is around 64Ω and leads to a minium CPC CSA of just 0.56mm²

I was entertaining options and feeding the second D.B off of the first D.B via an mcb would indeed allow me to use the cpc between boards. Not ideal, i know, but the whole installation isn't ideal. Just trying to make the best out of a bad situation.

I think I may be missing something. On the face of it, the 10mm² T+E between DBs would seem to be adequate both in terms of being protected by a 50A switch-fuse and also having an adequate CPC (in terms of adiabatic calc). Is the issue perhaps in relation to main bonding of extraneous-c-ps that have not already been main bonded to the first DB? How far apart are these DBs? At the 'very worst', you could presumably run an additional 10mm² G/Y single between them?

Kind Regards, John

 

[EFLImpudence]

The first D.B has been TT'd which is brilliant

Why?

However the earth from the second D.B has its earth from the first D.B earth bar instead of it's own earth stake.

Would it not be better to reinstate the TNS earthing?

As it stands, any RCD testing on the first D.B results in the second D.B's RCD tripping.

Should that happen? The other way round, yes.

But my main concern is the cpc that feeds the second D.B is an inadequate size. It is 4mm (10mm T+E).

Perfectly adequate.

Ideally, they should have TT'd this D.B aswell.

Why?

Potential soulutions:
1: TT the second D.B and segregate the incoming cpc feed. The downside is surface cabling to achieve this.

Why?

2: Could i get away with feeding the second D.B off the first D.B via an MCB?

Can it not be fed from the supply tails/cable?

PFC for the second D.B is 0.434kA

As above PSCC.

and measured Ze is 3.57 Ohms.

Technically not Ze.
Is this due to the sheath failing? or -
if not then even allowing for worst 0.8Ω actual Ze (at origin) that equates to a distance of over 400 metres.
Is that the situation?

calculated the adiabatic equation using a disconnection time of 1s and a k factor of 115. The result is 3.77mm ( i think!) Would this be acceptable! Obviously, any earth fault in the second D.B would result in the RCD tripping both rooms out! Not ideal!

As above - 0.56mm².

 

[JohnW2]

and measured Ze is 3.57 Ohms.

Technically not Ze. Is this due to the sheath failing? or - if not then even allowing for worst 0.8Ω actual Ze (at origin) that equates to a distance of over 400 metres. Is that the situation?

I believe the OP is talking about the Ze of the local TT earth. That's why I questioned, twice, it's low value, but the OP seems to think that it's the result of a 'very lucky' TT electrode and/or multiple TT electrodes. However, as I said, I still suspect parallel paths

FWIW, I agree with all your other points/questions.

Kind Regards, John

 

[EFLImpudence]

Ah. I misread and thought the second DB was earthed to the origin.

So, it is, in fact already TT'd and not so far.

Zn of 0.53 Ω

 

[JohnW2]

Ah. I misread and thought the second DB was earthed to the origin. So, it is, in fact already TT'd and not so far.

As I read it, the first of the DBs is locally TT'd and the second is (or is being proposed) sharing that TT earth via the CPC between DBs.

Zn of 0.53 Ω

If I understand correctly what you mean by "Zn", I think it should be half that figure - a PSCC of 434 A implies (at 230V) an L-N loop impedance of 0.53Ω, So the N, or the L, part of that will be 0.265Ω, won't it? However, I'm not sure what point you were making about "Zn".

Kind Regards, John

 

[EFLImpudence]

As I read it, the first of the DBs is locally TT'd and the second is (or is being proposed) sharing that TT earth via the CPC between DBs.

Yes I see that now. Further confusing was the OP proposing to TT the second board.

If I understand correctly what you mean by "Zn", I think it should be half that figure - a PSCC of 434 A implies (at 230V) an L-N loop impedance of 0.53Ω, So the N, or the L, part of that will be 0.265Ω, won't it? However, I'm not sure what point you were making about "Zn".

I use Zn as the symbol for the Neutral loop; is that not normal?

I was pointing out that the distance of the buildings cannot be that far.

 

[Davisss]

Hi guys.

Just to clear a couple of things up.
The over protective device for the 10mm T+E sub-main is a 50A 1361 fuse. When i phoned the helpful bloke at the NICEIC he said to use BS88-2 fuse to work out the prospective fault current as per table in the bible. Therefore using 220A as that figure, and working out the adiabatic equation you get the figure of 4.91mm. Hence why it has been TT'd (i presume).

The second D.B has been tapped off of the top of the first D.B's RCD and into its own RCD upfront. The earth has been segregated in the first D.B (as it's now a TT). The cpc for the ongoing feed to the second D.B has been terminated into the earth bar in the first D.B.

I have talked to the client and i am going to TT the second D.B and Obviously segregate the incoming earth in that board.

And, yes John, I can assure you that it really is a true EFLI!!
Will be interesting to see what figure o get when i bang the first rod in for the second D.B!

Job done. I'm going for a beer!

Beast Regards,
Dave