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?

 

[UKMeterman]

My link above explains the adiabatic equation, in other words what size of conductor is required so it doesn't melt before the fuse does. Another couple of points to watch out for:
1. What is the prospective fault current, calculate the likley force on the conductors in a fault condition and are they properly restrained.
2. Has the adiabatic equation applied to the protective earth conductors between the trunking and the DNO protective earth connection?

 

[westward10]

I can see your argument that you are going to employ Reg 434.2.1 but how are you going to reinforce the protection of the conductors against external influences.
I for one would not be connecting from the outgoing terminals of a 600A switch-fuse in such a manner.

 

[Deleted member 26818]

Is running the cables in trunking not sufficient?

 

[UKMeterman]

Is running the cables in trunking not sufficient?

The problem is getting the smaller conductors connected without compromising the 500A connections.

 

[westward10]

I am not sure the trunking is sufficient, how many sites have you been to where the lid is missing. 434.2.1 is not saying reinforced insulation which I understand but reinforcing the protection of the wiring. I honestly don't know but in my mind it would have to be permanent. I would opt for ensuring the conductors can withstand the fault protection in the first place as opposed to putting measures in place to prevent it. How would you put these measures in place within the dist board.
It has been mentioned likening this to connecting to a busbar but the same requirements apply, yes the conductors are protected from overload due to the latter protective device but it will not provide fault protection prior to it.
This is a personal view but I would be connecting nothing from that existing switch-fuse but I know you need a solution.

 

[davesparks]

Whether the trunking is sufficient or not will depend on the location.
If it's in a cupboard where a load of other junk is stored and damage may occur then it's very different to a dedicated intake room with restricted access and minimum chance of damage.

 

[westward10]

Do some calculations first and correctly size the conductors leaving the trunking scenario irrelevant.

 

[davesparks]

Do some calculations first and correctly size the conductors leaving the trunking scenario irrelevant.

For me it still raises the question of what fault is going to occur? I can't see the sense in guarding against a fault which cannot occur, much like providing overload protection for an electric shower, what's the point?

 

[westward10]

The 63A device provides overcurrent protection in normal circumstances. A fault can occur prior to this device which renders the device irrelevant to say a fault cannot occur is odd to say the least. This scenario has been compared to a busbar connection and my aim in these posts is to prevent the suggested installation. So you have a 500A rated supply, the closest busbar rating to this is 400A or 630A and from my experience of these busbars connections are very often factory made links from solid busbars,why to allow for fault current. These busbars systems are very often bespoke and to suggest the scenario of connecting a dist bd with 25.0 tails to it would receive a response of do you actually know What You Are Doing.
Walk away from it.

 

[rich.250]

Westward.
This is in a intake room adjacent to the site transformer around 100m from the industrial unit and has restricted access.
I have proved it will disconnect under the measure fault current by using 35mm insulated and sheathed cables to supply the new DB.
The existing trunking will be used which is bolted and linked with 185mm Earth.
This meets the adiabatic equation.
The cables will be run in galv trunking that is existing and as I myself carried out an EICR on this installation 6 months ago can assure you the trunking is sound, secure and meets IP4x as required.
As previously stated the cables will be 2m maximum.
Also I cannot see how another cable terminated with an adequately sized lug on a terminal will effect the existing circuit at all.
I'm confused with what you are trying to say is wrong here apart form your opinion.
Cheers


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[UKMeterman]

Go and look up the magnetic force on conductors when carrying your expected fault current

 

[westward10]

If your calculations say 35mm is adequate then that is fine, this was my point in the first place to do these calculations although your original thread stated 25mm.
Whether I would connect from the switch-fuse was a personal opinion as I realise you needed a solution to enable another supply.