Sub main OCPD

[ferg]

I have seen rather frequently SWA sub mains taken from Henleys protected only by DNO fuse and a stand alone RCD.

I have seen it so often now that I am beginning to question my own knowledge.

My understanding is that the sub main should have it's own OCPD depending on the cable it is protecting.

Is there any possible justification or reason that would make this compliant?

The only possible justification I can think of is that if the Zs at the sub board is still low enough to take out the main fuse under short circuit fault conditions, the likely load on the sub board is lower than the CCC of the cable and that the RCD would operate if an earth fault occurred on the cable at a level that would not cause the main fuse to operate.

Have I missed something here or am I right in thinking this set up is non compliant.

 

[davesparks]

Gotta say I cant recall coming across this ever but would agree that if I did I would see it as non compliant.
I suppose playing devils advocate if the distribution circuit went from the intake to a remote building as a 100a supply it could be argued that it would be fairly pointless having two 100a OCPD's in series about a foot from each other!

Yes two 100A fuses in series is pointless, but it is the way it has to be done with the current regulations.
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Could I just ask about potential issues with extending meter tails when moving a CU? Say if the cu was originally 3 metres away from the DNO fuse and has been moved to say 15 metres from the cut out fuse?

The tails should not be longer than 3 metres, so you install a distribution circuit to feed to CU.

 

[barninks]

The tails should not be longer than 3 metres, so you install a distribution circuit to feed to CU.

Yes fine, that is what I would normally do - but just trying to pick peoples brains on here and see what views there seem to be on the subject

 

[davesparks]

Yes fine, that is what I would normally do - but just trying to pick peoples brains on here and see what views there seem to be on the subject

What are you getting at here?
The rules are quite clear on the subject.

Are you trying to ask if it's acceptable to ignore the rules and just connect 15m long tails to the incoming supply?

 

[barninks]

What are you getting at here?
The rules are quite clear on the subject.

Are you trying to ask if it's acceptable to ignore the rules and just connect 15m long tails to the incoming supply?

Sorry, didn't meant to cause confusion - just was interested to see different peoples views on the protection offered by swa

 

[Andy78]

Sorry, didn't meant to cause confusion - just was interested to see different peoples views on the protection offered by swa

That's literally the confusing part.
When people ask "What does everyone usually do ?" or "What's everyone's different views on....." It makes it seem as if there aren't already regulations that cover this situation.

 

[barninks]

I am sorry

 

[davesparks]

Sorry, didn't meant to cause confusion - just was interested to see different peoples views on the protection offered by swa

SWA offers protection by connecting any metal object which penetrates it to earth (by the armour) before it makes contact with any live conductor, this causes the OCPD to operate.
It also offers protection by having a relatively tough outer jacket and a pretty string general construction thanks to the steel armour.
It also has protection against UV by having a black sheath

 

[radiohead]

Yes two 100A fuses in series is pointless, but it is the way it has to be done with the current regulations.

I realise that.

 

[Simon47]

As it happens, I have a submain in the house here - except it's T&E. From a brief chat I had with the vendor's ex's father, it sounds like it did have an MCB, but he changed it when he was told that the T&E needed RCD protection.
Thing is, it says 63A on the front, so it's OK isn't it ?
It'll get sorted at some point, but for the time being I know there's a 63A fuse in the cutout.