Consumer unit change

[paulthe spark]

Only qualified a year or so About to do a cu change for my neighbour, never done a cu change before in an occupied property but i thought it would be good experience for me although ive obviously fitted boards before. Anyway was just wondering if theres anything you more experienced guys think i should be aware of when doing a cu change? Ive read about a borrowed neutral on these forums, can someone please explain to me what a borrowed neutral is? also should i do an Insulation resistance test first before i start to detect any wiring problems before i become responsible? any help much appreciated
cheers lads

 

[Darkwood]

A borrowed neutral is say for example you wire a light on the lighting circuit using live earth but instead of using the lighting circuit neutral you wire it to a socket joint box hence you have borrowed the neutral from another circuit.
This is extremely dangerous because if you turned the sockets off and diconnected the neutrals the may become live as you have another circuit from a different mcb using them.

 

[saveloy]

Borrowed neutral is most common on lighting circuits that are seperate ie where you have an upstairs circuit seperate from a downstairs circuit. What happens is, when there is 2 way lighting on the upstairs/downstairs landing lights ( switched from both upstairs and downstairs ) power is brought from say the downstairs via 3 core to the upstairs landing light but the neutral is "borrowed " from the upstairs lighting. Not unheard of to have borrowed neutrals on any circuit though.

I personally would do my dead tests first, before any other work as you can then point out any probs to the customer, and any extras that may be required. If you do the board change first, you stand a chance of them turning round and saying you caused the problems when you changed the board.

Also check the main and equipotential bonding is up to scratch as you may have to upgrade.

Edited to say: Agree with Darkwood.
Dangerous because if you isolate one cicuit you can still get a belt off the neutral as it still acting as neutral for the other circuit

 

[ajelec10]

Always do your checks like ring continuity and insulation resistance before or as you go so you know that if you get to the end and find a broken ring you didn' cause it
Regards
AJ

 

[paulthe spark]

ok i understand what a borrowed neutral is now but can you explain why you wouldn't just use the neutral as normal. Like what kind of situation would you have to borrow a neutral?

Also i hear todays more modern consumer units are a lot more sensitive to faults and so on as say older fuse boxes. So may find after the swap faults exsist that wern't previously detected. How true is this? also will i have to tell him to buy a 17th edition board? we are still fitting the wylex 16th edition boards at work

 

[Darkwood]

ok i understand what a borrowed neutral is now but can you explain why you wouldn't just use the neutral as normal. Like what kind of situation would you have to borrow a neutral?

DIY enthusiasts do it because they dont understand the dangers it creates and electricians do it too but more through inexeperience or rougeness and as saveloy mentioned it is more common to mixed up neutrals on lighting circuits when the pass through a common swiching point like the landing switch for the downstairs light and upstairs lighting.
However its done or whatever reason its done for it posses a big danger to us sparkies if you dont keep your wits about you.

 

[saveloy]

One reason is because years ago lighting was on one circuit so wasn't a problem. When circuits are split into upstairs and downstairs it becomes a problem. Another reason is because they haven't used cable with enough cores. I've even seen the cpc core being used as a neutral to get over the problem. All boils down to bad workmanship really.

As for the board it should be fitted to 17th edition standards. When you say 17th edition board do you mean dual RCD? Any board can be used and fitted to 17th edition with the correct use of RCBOs or RCD to protect the relavant circuits. You have to organise the board in order to minimse disruption in the event of tripping by either nuisance or fault.

 

[Spudnik]

And another one is when a neutral is borrowed from say, a socket with the live taken from the switch feed, to use for wall lights etc.

 

[SirKit Breaker]

The biggest problem i come across in my area with CU swaps is where the lighting is wired in singles, with the lives looped in the switches usually with one from downstairs lights into one fuse and upstairs into another, but then only having one neutral back at the board cos they have looped the entire circuit at the pendants using only one neutral.

................Howard

 

[paulthe spark]

thanks for all your replies fellas

 

[piz]

Hello
I have had shocks from these in the past so important to test 1st
also test continuity between neutral conductors you should only have it on a ring circuit
if you get a reading on the lighting circuits you will know you have problems if they are radial

 

[johnnyb]

Do all the tests you can before you begin mate, and make sure you have identified all the circuits, and mark them up, some circuits you have may be doubled up in the same fuse or breaker, it may be that some are radials but in a 30a fuse and you may not of allowed a 16a mcb in your new con/unit, and don,t forget to put the downstairs lights on the same rcd as the upstairs ring main,
Make sure you gas and water are 10mm etc.
Have a go and make a good job Paul.

 

[chocolate]

Hi there,

When you tear the cover off the board you're about to do, so some looking first. Count the number of reds ( or browns ), and count the number of blacks ( or blues ), and count the number of G&Ys. Hopefully you have equal numbers of each. If not, be suspicious.

Once you've disconnected everything, ripped the board from the wall, and all you have left are wires, do some simple insulation resistance tests - between all the neutrals to ensure they are all separate from each other, and between all lives ( for the same reason ). For cpcs, you should do the same and find that each circuit's cpc isn't linked to another circuit cpc.

If you find you get a reading between any two neutrals, then they've been linked somewhere in the property. You should find where, and fix it or disconnect it. Tell the client.

Enjoy the board change.

 

[piz]

Good Info

 

[Steve1412]

Have never swapped a cu without some drama...normally you have a rogue circuit which is powered up but no one knows what it controls...so you then have to start ripping floorboards up to find its been taped up somewhere..or theres the odd circuit where the wires are half an inch short so you have to washing line it straight across your other nealty dressed cables...hope urs is the exception