Borrowed neutral discovered when changing CU and discussion on DIY CU change

[trev]

Re: Borrowed Neutral discovered after changing CU

Ah but come on P&S, he left a note about "multiple means of isolation" :clown:

 

[Dan the spark]

Re: Borrowed Neutral discovered after changing CU

Ze was 0.4

Didn't check the resistance on the existing ring mains, but equally I didn't make any change to them - ergo I should not have adversely affected the safety of them by changing a fuse for an MCB.

I did test the RCDs, they operate as expected.

What was the earthing arrangement?

 

[plugsandsparks]

Re: Borrowed Neutral discovered after changing CU

Takes a few seconds to test continuity of ring and matters alot - why bother with Ze and rcd tests and not check you have a ring - makes no sense

 

[rsmck]

Re: Borrowed Neutral discovered after changing CU

then left one neutral energised by two MCBs, nice for the next poor sod to get his hands spiked.
2/10 for effort

Oh go on, 4 at least ?

The neutral energised by two MCBs is VERY CLEARLY marked, surely if you see a note on a CU which states "WARNING: Multiple isolations required due to borrowed neutral in lounge" and states both the circuit numbers, and is directly below those MCBs, it would be noticed by anyone working on it.

I know it's less than ideal, but moving both lighting circuits onto one MCB would potentially risk exceeding the MCB rating, causing that to trip.

I was under the impression (having seen it before, albeit in a commercial building) that this was an acceptable practice.

If it's that big a cause for concern I could put them both onto the same MCB. I do actually value advice and constructive criticism given.

 

[electric will]

Re: Borrowed Neutral discovered after changing CU

Oh go on, 4 at least ?

The neutral energised by two MCBs is VERY CLEARLY marked, surely if you see a note on a CU which states "WARNING: Multiple isolations required due to borrowed neutral in lounge" and states both the circuit numbers, and is directly below those MCBs, it would be noticed by anyone working on it.

I know it's less than ideal, but moving both lighting circuits onto one MCB would potentially risk exceeding the MCB rating, causing that to trip.

I was under the impression (having seen it before, albeit in a commercial building) that this was an acceptable practice.

If it's that big a cause for concern I could put them both onto the same MCB. I do actually value advice and constructive criticism given.

so you see overloading a mcb as a no no but your happy to overload a neutral cable?

 

[rsmck]

Re: Borrowed Neutral discovered after changing CU

What was the earthing arrangement?

TN-S

(need to add more text as the forum won't let me post a 4 character reply)

 

[Dan the spark]

Re: Borrowed Neutral discovered after changing CU

Could you tell me what the earthing arrangement is?

Got it, sorry

 

[rsmck]

Re: Borrowed Neutral discovered after changing CU

so you see overloading a mcb as a no no but your happy to overload a neutral cable?

Only one downstairs room has a connection to that (upstairs circuit) neutral, the rest of the downstairs lights have the 'correct' neutral and no connection to the other (upstairs) circuit neutral, there is no risk of the neutral cable being overloaded solely by the addition of the lounge lighting.

 

[rsmck]

Re: Borrowed Neutral discovered after changing CU

Takes a few seconds to test continuity of ring and matters alot - why bother with Ze and rcd tests and not check you have a ring - makes no sense

Very fair point, and it would have shown the problems I later found myself spending time finding... won't make that mistake again, and will probably pop back and test just to put my own mind at ease if nothing else.

 

[Dan the spark]

Re: Borrowed Neutral discovered after changing CU

Only one room has a connection to that neutral, the rest of the downstairs lights have the 'right' neutral and no connection to the other (upstairs) circuit neutral, there is no risk of the neutral cable being overloaded by the addition of the lounge lighting - even with all the lights on upstairs.

Brilliant!!..... I'm done here, and so I'm guessing are most

 

[plugsandsparks]

Re: Borrowed Neutral discovered after changing CU

Oh go on, 4 at least ?

The neutral energised by two MCBs is VERY CLEARLY marked, surely if you see a note on a CU which states "WARNING: Multiple isolations required due to borrowed neutral in lounge" and states both the circuit numbers, and is directly below those MCBs, it would be noticed by anyone working on it.

I know it's less than ideal, but moving both lighting circuits onto one MCB would potentially risk exceeding the MCB rating, causing that to trip.

I was under the impression (having seen it before, albeit in a commercial building) that this was an acceptable practice.

If it's that big a cause for concern I could put them both onto the same MCB. I do actually value advice and constructive criticism given.

eeek, fire fire ! hope the neutrals OK about it,
About the note, nice gesture but TBH, notes are not noted for being noteworthy, i wouldn't do it, especially in any commercial or industrial, its just not good enough.

 

[rsmck]

Re: Borrowed Neutral discovered after changing CU

eeek, fire fire ! hope the neutrals OK about it,
About the note, nice gesture but TBH, notes are not noted for being noteworthy, i wouldn't do it, especially in any commercial or industrial, its just not good enough.

Noted, will move to the same MCB. Thanks.

 

[D Skelton]

Re: Borrowed Neutral discovered after changing CU

So Rsmck, you have rewired your last two homes, when designing the installations did you carry out an adiabatic equation to ensure that your main earthing conductor and main equipotential bonding conductors were correctly sized? What system were you on and what would have been the highest Ze reading acceptable for that type of supply? Did you ascertain whether supplementary bonding was needed or could be omitted? Were you able to differenciate between exposed and extranoius conductive parts? Which protective measure did you utilise? Did you determine the prospective fault current prior to selecting over current protective devices? Did you carry out the correct calculations to ascertain the current carrying capacity of each cable taking account of grouping, ambient temperature and thermal insulation? How did you ensure that your voltage drop on each circuit was lower than the maximum allowed for the type of circuit? Did you adhere to the building regulations with regards to the routing of each cable in structural walls and joists? Did you ensure that cables buried in walls were routed in prescribed zones? What were the reference methods used with regards to how the cables were mounted? How did you ensure that the selection and erection or wiring and accessories minimized the spread of fire throughout the installation?

When performing your dead, then live tests, did you carry them out in the order specified in BS 7671? How did you confirm continuity of main and supplementary bonding conductors, visually or by measurement? What would have been the highest acceptable resistance reading for these? Did you use test method one or two to confirm continiuity of circuit protective conductors? Where exactly in the installation were you required to determine the prospective fault current under both short circuit and earth fault conditions? What would have been the maximum Zs permitted on any of the ring final circuits on a B32 60898? Did electrical equipment installed in special locations comply with its specific requirements? How did you determine your maximum demand?

How were test results recorded, were they recorded on an electrical installation certificate as required by BS 7671?

Seriously, I could go on all night...

Why, when you freely admit to not being competent to complete even basic electrical work are you carrying out a consumer unit change? Let alone rewires?!?! The amount of callouts I've had to burning CUs coz the old man wanted to 'ave a go himself!

It seems to me that you've left a circuit borrowing another circuit's neutral on a seperate MCB?!?! You've also repaired a nicked cable so that the RCD would set but have you any thought as to why the fuse didn't blow beforehand when a live part was in direct conection to an exposed conductive part??? The words 'continuity or protective conductors' spring to mind, but you wouldn't know that would you. Do you have a missus and kids? Because if you do, please stop playing at something that you know nothing about! Replacing a socket and whacking up a new light fitting is one thing, but carrying out the sort of work you are talking about with a serious lack of the fundamental knowledge we as electricians are required to know is in my opinion idiotic! You endanger yourself and others!

Would you replace your own boiler???

A little knowledge is a dangerous thing.

I'm sorry if my post comes over as rude and aggressive, I would just really like you to see the error of your ways before you really hurt someone!

Rant over.

 

[rsmck]

Re: Borrowed Neutral discovered after changing CU

So Rsmck, you have rewired your last two homes...

I might regret replying to this, but here goes...

Did you carry out an adiabatic equation to ensure that your main earthing conductor and main equipotential bonding conductors were correctly sized?

No, by selection from table.​

Did you ascertain whether supplementary bonding was needed or could be omitted?

Yes​

Were you able to differenciate between exposed and extranoius conductive parts?

Yes​

Did you determine the prospective fault current prior to selecting over current protective devices?

Yes, although don't ask me to quote it now, there'll be a note somewhere.​

Did you carry out the correct calculations to ascertain the current carrying capacity of each cable taking account of grouping, ambient temperature and thermal insulation?

Yes, most are clipped direct, a few are within plasterboard walls, none are within or under any insulation, again this was all considered before deciding what cables to use.​

How did you ensure that your voltage drop on each circuit was lower than the maximum allowed for the type of circuit?

Again, this was worked out at the time. Exactly how isn't fresh in my mind, but it was something that was considered​

Did you adhere to the building regulations with regards to the routing of each cable in structural walls and joists?

Where possible - there were already some notches in joists in non-compliant positions from the original installation which were reused (to prevent the need to make more holes and potentially further weaken the joists)​

Did you ensure that cables buried in walls were routed in prescribed zones?

Yes​

When performing your dead, then live tests, did you carry them out in the order specified in BS 7671?

Yes​

How did you confirm continuity of main and supplementary bonding conductors, visually or by measurement?

The resistance was measured​

What would have been the highest acceptable resistance reading for these?

Again, this was something that was researched (and would be again if I was repeating it) but is not fresh in my mind as I don't do this every day.​

Did you use test method one or two to confirm continiuity of circuit protective conductors?

Test method 1​

What would have been the maximum Zs permitted on any of the ring final circuits on a B32 60898?

I have no ring final circuits.​

Did electrical equipment installed in special locations comply with its specific requirements?

There is no electrical equipment in special locations other than bathroom lighting, which does comply with the requirements (actually it's an SELV lighting system)​

How did you determine your maximum demand?

As per the guidance in the OSG, interestingly this was something that *was* actually submitted to a third party as I had the DNO replace our cut out fuse (as both my property and the neighbours was fed from the same 80A cut out)​

How were test results recorded, were they recorded on an electrical installation certificate as required by BS 7671?

Well that depends what you mean by 'certificate' - they take the same format, but I expect it would not be considered a valid certificate as it wasn't completed by an electrician. For the record I did ask the LABC *before* undertaking any of this work who confirmed that there was no requirement to notify them (although they did want a building warrant application and calcs etc for removing a wall)
​

- - - Updated - - -

Do you have a missus and kids? Because if you do, please stop playing at something that you know nothing about! Replacing a socket and whacking up a new light fitting is one thing, but carrying out the sort of work you are talking about with a serious lack of the fundamental knowledge we as electricians are required to know is in my opinion idiotic! You endanger yourself and others!

Nope I just endanger us, no kids. I do have a partner, and she understands it about as well as me (maybe not much by your estimation) but we are both quite happy with the results.

Would you replace your own boiler???

Personally, no, but some people will - and some will do it safely.

 

[rsmck]

Re: Borrowed Neutral discovered after changing CU

As an aside I should add that I did undertake a lot of research (and discussed it with actual electricians I know - albeit working commercially rather than domestically) before doing my own rewire (how did we sidetrack this thread that much ) and a year and a bit ago when it was done it was fresh in my mind.

Quizzing me now on cable regs, reference methods, and calculations is probably not the best, if I was ever to undertake another full rewire (be years before I move house again) I would do the same sort of research again - not least because all the regs will likely have changed

I do accept that in this case (changing a CU) I should have undertaken the same tests as I did at home (even though I wasn't in a position to change any of the extant wiring) as it would have saved a lot of time (and looking foolish )