Single pole use of DP FCU


Postby van » Wed Jun 10, 2009 9:45 pm

Hi all. Was hopeful those knowledgeable re: current Wiring Regs may be able to clarify this Q...

Had an electrician wire up a double socket & lights in our garden a while back. Demanded & received certification & all works no problem.
However, an accident has cracked one of the 2 FCU`s feeding the new circuit from the ring in the dining room, so being fairly competent with basic electrics & it being a straight swap, i bought a replacement DP FCU.
However, after dismantling the unit i was surprised to find only one of the poles connected - no neutral at the FCU.
Being curious i had a look & made sense of what had been done, but was still surprised that it was permissable for only one pole to be connected.

Would appreciate confirmation or otherwise that this is ok / good practice

Can supply the rest of the circuit details if required

Thanks in advance, Van
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Postby kbrownie » Thu Jun 11, 2009 10:54 am

Hi Van,
So you have a FCU with no neutral connection?
This can't be right it shouldn't work! unless the neutral has been borrowed from elsewhere which is a no no! As all circuit conductors should run together.
Could you describe the cable core colours and the configuration of terminals within FCU and the socket the FCU is being feed from?
KB
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Postby sparx » Thu Jun 11, 2009 3:48 pm

Hi Van,
If unswitched FCU then neutral is not switched just fuse in Live line.
As neutral has to go with live any way is it terminated in chock block or has leckie been smart & slit outer sheath of cable, cut live only at FCU and 'Lost' spare Neutral slack in back box?
If the latter then good on him since he has eliminated one unnecessary joint!
Presume he has fitted one FCU for socket & seperate one for lights?
seems a good job to me. As well as certs. you should have received a PartP register card from LABC to show he has registered job since outdoor work needs notifying,
regards Sparx
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Postby van » Thu Jun 11, 2009 11:07 pm

Thanks to both of you for taking the trouble to reply. Sorry about the duplicate posting - thought i`d done it wrong first time around. Doh!

Anyway, seems i`ll need to give you more info - stop me when you`ve enough.

The `circuit` starts at the back of a socket on the main ring circuit. From there it (a live, neutral & earth) goes to the first FCU (13 amp fuse)

Out of the back of this box there are 2 live wires + 2 earths + ONE neutral

One of the lives (& one earth) goes to the 2nd switched FCU which is next to it, but only 3 amps. This switch turns the garden lights on or off. This is the cracked one. & the one with no neutral connected, only 2 lives.
The switch live from this then joins the 3 wires from the 1st FCU & goes through the wall into a plastic ip56 box on the outside wall.

Inside the box he`s connected the 4 wires i mentioned to some 4 wire armoured cable. This goes into the ground, down the garden & is connected the other end into a similiar ip56 box next to where the outside sockets are.

in this box he`s connected the live from the 13 amp FCU to the outside socket
& the switch live from the 3 amp FCU to the lights.
Obviously the earths & neutrals from both the socket & lights are connected together in the box.

Phew! i think that`s everything! Hope i`ve made it easy to understand. Sorry if it`s a bit long
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Postby sparx » Sun Jun 14, 2009 8:02 pm

Hi Van,

Ok I get the picture, from original circuit is spur to 13A FCU for socket, live/line looped to second 5A FCU for light.
Effectively 2 Lines at different fuse ratings in same cable down garden with shared neutral & shared CPC.
Total load is on Neutral but is limited to 13A if second FCU is on fused side of first one, or 13+5A if second Fcu is off live side of 1st, one (which it should not be).
EAsy test - turn of power FCU - lights should also not work either.
If so no prob as total load on spur only can be 13A.
Put another way , lighting FCU should be fed fron downstream/fused/ outgoing term of power FCU.
hope that makes sense,
regards Sparx
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Postby van » Sun Jun 21, 2009 9:39 pm

Sorry Sparx - hadn`t realised you`d replied until just now. Thanks for taking the time & trouble.

Yes, you`ve described it pretty spot on. The feed from the ring does go thru the 1st FCU then to the 2nd - meaning the entire circuit is protected by the 13amp fuse so load can only ever be 13amps maximum.

As you have said, the neutral is shared by both circuits (i assume they are classed as seperate circuits or sub circuits?) I`ve been told that `borrowing` neutrals is bad practice. Is a `shared` neutral not the same by a different name?
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Postby sparx » Tue Jun 23, 2009 8:20 pm

Hi Van.
Not same really as in your case it is all fed from same line/neutral from one ring circuit.
'Borrowed Neutral' usually happens with 2 way switching on stairs when upstairs feeds 2 way switch circuit, later on some one wants to take a feed from downstairs switch to another light downstairs so rather than run another cable upstairs 'borrows a neutral from hall light say.
you now have 2 independant circuit linked together by the neutrals.
It never showed up until recently when using more than one RCD/RCBO, meant current out from one and back through another causing unbalance = so called 'nuisance tripping'
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