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I work with temporary electrical installations to construction sites at the moment and have a query related to supply arrangements.

The supply earthing sysytem is a TT type. The main cut out fuses I've been told are 200A and the supply tails are 70mm in a three phase arrangement.

These 70mm are terminated to the supply side of a 60947-2 protective device that acts as the main switch for the temporary electrical installation. RCD protection is provided.

Also terminated to the supply side of the 60947-2 main switch are 16mm single core insulated/sheathed cables that are supplying the previous distribution board. On the DB is car park lighting and a barrier. Everything described is within a brick enclosure.

These 16mm single core cables connect into a 125A 4 pole 60947-3. From the 4 pole 60947-3 via 10mm conductors into a 63A 60898 3 pole protective device. Out of the 63A 60898 comes 10mm conductors from a swa that supplies a DB with only 2 circuits on. The outgoing 10mm neutral terminates into the outgoingside of the 4 pole 60947-3, bypassing the 3 pole 63A 60898.

For me there are a number of issues here that the office I work with don't agree with.

1. The 16mm are protected by the 200A fuse in the cut out should be 70mm? The office seem to think they can use backward protection from the 63A 60898. I disagree and think that these should also be 70mm.

2. The 10mm cables coming out of 4 pole 60947-3 are still protected by the 200A cut out fuse.

3. The 10mm neutral will not disconnect as it bypasses the 3 pole 63A 60898.

My office think that the 16mm are protected by the 63A 60898 protective device as its within 3m of the supply cutout and as this disconnects there will be no more current flowing. I disagree as the cut out fuses are 200A and therefore the 16mm cables need to be 70mm too as are protected by the cut out fuse and the 63A has nothing to do with the situation.

Has anyone heard of backwards protection within 3m?

Am I wrong in what I'm saying?
 
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Charlie_

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Arms
By backward protection I think what they are saying is that due to the 63a fuse downstream then the current on the 16mm is limited by that fuse
 
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  • #3
Thats what I understand by the term 'backwards protection' as well.
 

Ian1981

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Never heard of ‘backwards protection ‘ do you mean back up protection? where coordination with an downstream ocpd is coordinated with the upstream ocpd so allowing the downstream ocpd to have a lower Ka rating than the fault current as the upstream device will disconnect protecting the downstream device?

Your 16mm cables will have fault protection via the upfront rcd being a TT arrangement they may not require overload protection, depending on the final load in which they serve.
The rcd should disconnect all live conductors in a TT arrangement thus you should have isolation of the neutral as required
 
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  • #6
Thanks gents, I can see that regulation 433.2.2 permits this setup. Would have been nice if my office had just said look at this reg. It's one thing someone telling you it's okay. Better to read it in black and white.

What about the neutral bypassing the 63A 60898 though? My thoughts are that this should be a 4 pole 60898 so to ensure the neutral is disconnected too?
 

Ian1981

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Thanks gents, I can see that regulation 433.2.2 permits this setup. Would have been nice if my office had just said look at this reg. It's one thing someone telling you it's okay. Better to read it in black and white.

What about the neutral bypassing the 63A 60898 though? My thoughts are that this should be a 4 pole 60898 so to ensure the neutral is disconnected too?
Does the main upfront isolator/switch/rcd not isolate the neutral?
 
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  • #9
Never heard of ‘backwards protection ‘ do you mean back up protection? where coordination with an downstream ocpd is coordinated with the upstream ocpd so allowing the downstream ocpd to have a lower Ka rating than the fault current as the upstream device will disconnect protecting the downstream device?

Your 16mm cables will have fault protection via the upfront rcd being a TT arrangement they may not require overload protection, depending on the final load in which they serve.
The rcd should disconnect all live conductors in a TT arrangement thus you should have isolation of the neutral as required
Thanks for the reply Ian. No I don't mean back up protection. I was attempting to refer to the requirements in reg 433.2.2. I have herd of back up protection and it's good to learn how to refer to what you mentioned.
 
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  • #11
Ian no it doesn't.

The 16mm tails have been terminated onto the supply side of the RCD + 60947-2 arrangement. There is no RCD protection provided to the 16mm tails.

As the 16mm have been terminated with the 70mm supply tails from cut out fuse. Another RCD will have to be installed with a 4 pole device to isolate neutral as well as line conductors.
 
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