Back fusing

[rich.250]

Hi Guys,

We have an installation we are about to carry out some extras on.
One of which is a small 3 phase DB adjacent to the main switch fuse in the remote DNO's building next to the transformer to install power to 3 caravan hook up points. (Max 20a per phase)

Some background info,
The supply is TN, 800amp Schneider ACB as incoming over current set to 550amps.

Ze from the switch fuse is 0.02

The switch fuse is a 630amp with 500A fuses fitted.

As everything is sealed and no busbar chamber is present for alterations I was thinking of installing tails 25mm ho7 insulated and sheathed singles from the load side of the switch fuse (to ensure we still have one point of isolation) through existing galv trunking to new DB which will be attached to it also.
In the DB install a 63a CB as incomer.
This obviously relies on back fusing as with a busbar system, but have never really done this without a chamber. I know the principal is the same but just wanted some thoughts.

Thanks in advance, Rich.


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[westward10]

You might want to check out Section 434 of BS7671.

 

[Leesparkykent]

Hi Guys,

We have an installation we are about to carry out some extras on.
One of which is a small 3 phase DB adjacent to the main switch fuse in the remote DNO's building next to the transformer to install power to 3 caravan hook up points. (Max 20a per phase)

Some background info,
The supply is TN, 800amp Schneider ACB as incoming over current set to 550amps.

Ze from the switch fuse is 0.02

The switch fuse is a 630amp with 500A fuses fitted.

As everything is sealed and no busbar chamber is present for alterations I was thinking of installing tails 25mm ho7 insulated and sheathed singles from the load side of the switch fuse (to ensure we still have one point of isolation) through existing galv trunking to new DB which will be attached to it also.
In the DB install a 63a CB as incomer.
This obviously relies on back fusing as with a busbar system, but have never really done this without a chamber. I know the principal is the same but just wanted some thoughts.

Thanks in advance, Rich.


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Sounds ok to me.

 

[westward10]

You need to check Section 434 and do some calculations as to whether those conductors are adequate.

 

[Leesparkykent]

25mm with a 63A circuit breaker will be adequate. No branchs and under 3m and adequately protected by the steel trunking...I can't see it being a problem.

 

[westward10]

Will those conductors withstand the possible FAULT current, nothing to do with the 63A device.

 

[telectrix]

Will those conductors withstand the possible FAULT current, nothing to do with the 63A device.

good point. rough head calc. i'd say 50mm- 70mm was required.

 

[Leesparkykent]

Will those conductors withstand the possible FAULT current, nothing to do with the 63A device.

The fault current from where?

 

[davesparks]

Will those conductors withstand the possible FAULT current, nothing to do with the 63A device.

But the 63A device will limit the fault current, assuming it is correctly selected

 

[rich.250]

I have just re done my calcs,
35mm is very close but on the right side.
Am I right with the following? Doubting myself now.

T =kxs/ 11500
(All the above squared)

So 13225 X 1225 = 16200625

/ 132250000

= 0.1225 sec

Information for fig 3a3c shows with 11,500 fault current the device will disconnect in less than 0.1 seconds

Am I missing something?


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[rich.250]

But the 63A device will limit the fault current, assuming it is correctly selected

I was thinking along the lines of a fault on the 2m cable inside the galv trunking.
However slim it may be.
Or am I over thinking this?


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[rich.250]

I would give likes, but.......... Lol


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[Leesparkykent]

I was thinking along the lines of a fault on the 2m cable inside the galv trunking.
However slim it may be.
Or am I over thinking this?




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Being in the metal trunking, using insulated and sheathed singles and keeping the length to a minimum you've installed it in such a manner as to reduce the risk of fault to a minimum by reinforcing the protection of the wiring against external influences.

 

[westward10]

I am lost for words that you consider the 63 device will solve any problems with regards to thermal damage of conductors should a fault occur between the point of connection and said protective device.

 

[Leesparkykent]

I am lost for words that you consider the 63 device will solve any problems with regards to thermal damage of conductors should a fault occur between the point of connection and said protective device.

It wont but that's not the intent of the regulation as its permissible to be installed in such a manner as long as certain criteria is met for example under 3 meters and protection against external influences.

 

[davesparks]

I am lost for words that you consider the 63 device will solve any problems with regards to thermal damage of conductors should a fault occur between the point of connection and said protective device.

I don't think anyone said it will.
The tails are to be enclosed in steel trunking and kept as short as possible so regulations wise they will be compliant with the 63A device as the ocpd

 

[ipf]

...as with a bus bar system, basically.

 

[rich.250]

Thanks for all the replies
I thought it would be as a busbar system.
The fact that I have proved it will disconnect quicker than the maximum time allowed by calculation doesn't mean that the cable will withstand said fault for 0.1seconds?
How would I calculate withstand current of the cable then?
Because that is how it was taught to me on my 2396 a few years ago.
Thanks again.


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[UKMeterman]

Hi,
I am not providing guidance, this link may or may not help

http://www.napitonline.com/downloads/CP 3 07 P 10-11 16th Cable Sizing 2 of 2.pdf

 

[Deleted member 26818]

Wierd.
Dwelling: Meter - Tails - CU, no problem, done everywhere.
Industrial: Meter - Tails - DB, suddenly there's a problem?