Voltage drop and ZS - Some thoughts

[Silly Sausage]

Pedantic I know...!

BS1362 fuse.
Zs @ 0.4s, 3A = 16.4, 13A = 2.42 ohms.

 

[Guitarist]

Pedantic I know...!

BS1362 fuse.
Zs @ 0.4s, 3A = 16.4, 13A = 2.42 ohms.

Where are those figures from Archy?

 

[pencilpusher]

Where are those figures from Archy?

I have seen these in the BGB with the other max ZS values.

 

[Silly Sausage]

Found them here...
http://www.shepleyelectricalservices.com/Max ZS Table.pdf

edit...

John, below, has just pointed out where they are in the Big Book!


I shouldn't have changed my picture earlier!

 

[johnboy6083]

sory archy, i meant 1362,.

Those figures are from table 41.2.

 

[johnboy6083]

Found them here...
http://www.shepleyelectricalservices.com/Max%20ZS%20Table.pdf

BRB only gives 5sec Z esses for 1362s.

if you look on the page beofre mate, they are there at the bottom.

Im having to look them now for an EICR im writing up

 

[pencilpusher]

Yep Table 41.2 BGB Page 55 gives 0.4 times for BS 1362 fuses.

 

[Guitarist]

Haha. Ty guys. Just looked in my BGB and I've even highlighted that table. I must be getting old...

 

[GaryM]

Look upon the supply to the FCU as a distribution circuit with protection from a B32A MCB and the final circuit from the FCU with protection from a BS1362 fuse.
You could add a second schedule of test results just for the final circuit from the FCU, treating the FCU as a distribution point.

Volt drop is measured from the origin of the installation.

 

[pencilpusher]

Look upon the supply to the FCU as a distribution circuit with protection from a B32A MCB and the final circuit from the FCU with protection from a BS1362 fuse.
You could add a second schedule of test results just for the final circuit from the FCU, treating the FCU as a distribution point.

Volt drop is measured from the origin of the installation.

Yes understood but a minor works would also meet the regs because it is not "a new circuit" in itself. Horses for courses I suppose!

 

[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?