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 0.2s. The result is 1.68mm ( 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

 

[Davisss]

Just checked the regs book and i think the disconnection time should be 1s on the second D.B. This makes the earth size 3.77mm.

 

[SimpleSimon]

Both ideas are do able. Treating the 2nd DB on an MCB would be like supplying a shed. So first question you would need to check is that the equipatential zone would need to be extended to incorporate this.

Secondly is is how to achieve discrimination between the boards. You could look at changing all circuits on DB1 to RCBOs therefore protecting all circuits on DB2 by the circuits RCBO.

 

[Darkwood]

I assume if you lose the rcd on the sub you are allowed and its not actually protecting the T&E to meet regulations.

Its always best to use the supplied earthing whenever possible even if it does mean a bit of deco', I assume you didn't install the original so just say it as it is... TT-ing is a last resort IMHO when you are provided with a good DNO earth. TT systems can be unreliable by their nature so recommend corrective measures to ensure earthing is the best it can be.....

as for the design of this install it is very poor indeed!

 

[Engineer54]

For the life of me, why would anyone ditch a sound DNO TN-S earth supply for a TT earth??
As for you claiming this is ''brilliant'', what the hell are you thinking of!!

If i were you, i'd connect the supplied TN-S earthing to this outbuilding. As already been stated the rest of the installation design for this outbuilding is **** poor to say the least!!

 

[Davisss]

Thanks for the input guys. Much appreciated. I haven't measured the EFLI at the first D.B with the supplied earth. The maximum permitted Zs for a 50A 1361 fuse is somewhere between 0.96 Ohms (45A) and 0.70 Ohms (60A). I can't seem to find any info for a 50A fuse to hand. If i add the resistance of the 10mm T+E sub-main it could be very near to the mark. And then i have to consider the other circuits fed from that. I have done a couple of equations based on possible Zs and the supply earth (4mm) just isn't anywhere large enough.
Yes, i agree, it is a very poor design that i am testing. I am simply trying to find the best solution for the client!

Simon, there is no worry on the equipotential zone side of things as the only utility is water and it's in plastic pipe.

Darkwood, no i didn't install this. Very poor design to say the least!

 

[Davisss]

Just an update to those interested. Spoke to the NICEIC man on the blower and he agreed with both of my proposals. And he confirmed that the cpc of the 10mm T+E feed is indeed not large enough!

Kind Regards,
Dave

 

[Bobster]

He agreed with substituting a TN-S system with a TT one?

 

[davesparks]

What exactly is the problem that you are trying to fix at this place?

At post #2 you have stated that the earth to The second DB needs to be 3.77mm (presumable after calculation) and you actually have a 4mm earth.

 

[telectrix]

Just an update to those interested. Spoke to the NICEIC man on the blower and he agreed with both of my proposals. And he confirmed that the cpc of the 10mm T+E feed is indeed not large enough!

Kind Regards,
Dave

i spoke to the man from niceic. he said the earth was flat and the sun orbited the earth.

 

[GEOFF S]

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 0.2s. The result is 1.68mm ( 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

If you are useing the RCD for earth fault curent, wheres the problem with 4mm, whats the fault current?

Cheers