Voltage drop and ZS - Some thoughts

[GaryM]

Yes understood but a minor works would also meet the regs because it is not "a new circuit"

Yes it would if it's an extension to an existing circuit, as it is in your case.
I guess I was more thinking from a new installation POV.

 

[Guitarist]

So using an FCU it can be an extension to an existing circuit, as well as a new circuit added to an existing distribution circuit.

 

[GaryM]

So using an FCU it can be an extension to an existing circuit, as well as a new circuit added to an existing distribution circuit.

How does a FCU differ to a 1 way CU? Both have DP isolation, both have an OPD and max Zs values of BS1362 fuses are printed in BS7671 so why can't we add a separate schedule of test results for the outgoing side of a FCU?

DB reference - FCU/1
Location - Kitchen
Zs at DB - 0.38ohms
Ipf - 0.61kA
blah blah blah!

If the maximum Zs of the load side of the FCU is less than that allowed by a B32 MCB then you may not need to go to that length but if it ends up being higher due to the length of the circuit then by treating the load side individually you can show that the max measured (or calculated) Zs is within limits for the chosen BS1362 fuse.

Just thinking out of the box

 

[malcolmsanford]

How does a FCU differ to a 1 way CU? Both have DP isolation, both have an OPD and max Zs values of BS1362 fuses are printed in BS7671 so why can't we add a separate schedule of test results for the outgoing side of a FCU?

DB reference - FCU/1
Location - Kitchen
Zs at DB - 0.38ohms
Ipf - 0.61kA
blah blah blah!

If the maximum Zs of the load side of the FCU is less than that allowed by a B32 MCB then you may not need to go to that length but if it ends up being higher due to the length of the circuit then by treating the load side individually you can show that the max measured (or calculated) Zs is within limits for the chosen BS1362 fuse.

Just thinking out of the box

Your going down a very dangerous road here and I would suggest you read the definitions for what is a distribution circuit is and what is a final circuit, and why the disconnection times are up to 5secs for distribution and 0.4 seconds for final (TN).

Then you may realize that why you can not class the spur as a distribution

 

[GaryM]

Malcolm, it was more tongue in cheek about comparing a FCU to a consumer unit but reading the definition of a distribution board a FCU does seem to have similar, if not the same attributes.
My main point was a separate schedule of test results could be documented for the load side of the FCU, nothing at all wrong with that, especially if the measured Zs at the end of the circuit falls outside the limits of the protective device for the supply side of the FCU. In the OP's case the supply to the FCU is from an existing ring final with a disconnection time of 0.4s. I would also document a disconnection time of 0.4s if the supply to the FCU was also a radial.

 

[brman]

Malcolm, it was more tongue in cheek about comparing a FCU to a consumer unit but reading the definition of a distribution board a FCU does seem to have similar, if not the same attributes.
My main point was a separate schedule of test results could be documented for the load side of the FCU, nothing at all wrong with that, especially if the measured Zs at the end of the circuit falls outside the limits of the protective device for the supply side of the FCU. In the OP's case the supply to the FCU is from an existing ring final with a disconnection time of 0.4s. I would also document a disconnection time of 0.4s if the supply to the FCU was also a radial.

I agree, I firmly believe an FCU is a DB and can be treated as such. Of course that doesn't mean the supplying ring can be treated as a distribution circuit. At least I hope so, I've did a cert last week when I did exactly this, putting the FCU+radial on a separate test schedule! As it happens, on this job the max Zs met the requirements of the main board but I would have been happy if it only did that for the FCU.

 

[malcolmsanford]

So what we are saying guys is that a BS 1363 FCU can be classed as a distribution board, in as far as that in respects of it having a fuse, a switch, neutral and earth connection away we go.

But we won't go as far as saying that the RFC that is feeding this FCU is a distribution circuit, which it can never be if you read the definitions of both distribution/final circuits, because we know that is wrong.

 

[GaryM]

I never said that but did clarify where I was going with my thought process in my previous post.

 

[malcolmsanford]

So lads where are we going to place the cheesecloth when we carry out the 16Ka test on this spur unit as is required for all distribution boards to BS EN 60439 UK Annex ZA

Gary I know your post was tongue in cheek but i'm afraid you have now got people agreeing with you that a BS 1363 accessory is going to afford you the same standards as a Distribution Board in a domestic situation as per BS EN 60439 UK Annex ZA.

As I said mate it is a dangerous road

 

[brman]

So lads where are we going to place the cheesecloth when we carry out the 16Ka test on this spur unit as is required for all distribution boards to BS EN 60439 UK Annex ZA

Gary I know your post was tongue in cheek but i'm afraid you have now got people agreeing with you that a BS 1363 accessory is going to afford you the same standards as a Distribution Board in a domestic situation as per BS EN 60439 UK Annex ZA.

As I said mate it is a dangerous road

Malcolm, you are going beyond my knowledge here as I have never read 60439. So you are saying there are ways (eg the 16Ka test) that a FCU will never pass the full requirement for a DB? That I can well believe.

But are you saying that that also means we cannot use the FCU fuse to verify max Zs?

I guess what I am getting at is that I can understand it could be wrong (or misleading) to call a FCU a DB but, using the principle that Max Zs is to ensure trip times if a fault occurs, it is still ok to use the FCU to determine max Zs requirements.
Am I still on dodgy ground?

EDIT: having read this again I can see that if the Zs is really low then the FCU could flash over, so you would still require the protection of the main DB (with its 16kA capability etc) but in this scenario that would be ok?

 

[GaryM]

I'm pretty sure a BS1362 fuse is rated to break 6kA, more than likely same as the B32 MCB supplying the FCU.
You'd need a Zs of 0.038ohms at the incoming side of the FCU to see fault currents that high anyway which is pretty unlikely!

Where a 60439-3 (and 5486-12) CU comes into it's own is with a type 2 1361:1971 fuse rated at no more than 100A - with this combination the short-circuit rating of the protective device in the consumer unit may be taken to be 16kA. However, it's extremely unlikely to see anything near that value unless you like at the side of a substation.

 

[malcolmsanford]

It is extremely unlikely that you really need 25/16mm tails, in a domestic situation how many of us have seen a nice set of 4/6mm tails or I should say 7/036 or 7/044 jogging along nicely for 40 odd years without any sign of burning.

And I'm sure that even if you doubled a Zs value on a protection device it will be extremely unlikely not to trip within it's parameters.

And it is extremely unlikely that someone will get fatally electrocuted in their home........................