What test to carry out before consumer unit change?

[pc1966]

Thank you for this very in depth answer!

I did some fault finding to try find a nuisance trip of the house which I will be changing the CU. I’ve narrowed it down to fridge freezer, fish tank filter or pump as I put them in an extension lead for the time being on a socket fed by the RCD that wasn’t tripping and it tripped same day.

Having a current clamp meter that goes down to 1mA or so is useful here to find out if it is an accumulation of AC leakage from capacitance (so you see around 10mA-ish all the time) or a fault (usually near zero but something occasionally spike enough to trip it).

This is good value for that job (but like most affordable ones, AC-only):

https://cpc.farnell.com/multicomp-pro/mp780050/mini-leakage-current-tester/dp/IN08364

Basically you clamp L&N of each circuit to see what is leaking. Clamping the CPC can give bizarre high readings due to parallel return paths in TN-C-S cases.

It is currently a RCD split board installed by Mr bodger. I’ve recommend they get it redone and changed to RCBO board. They also use so many extensions. Every socket has a fully occupied extension lead minimum 4 plugs. Anyways I did IR test with everything unplugged and it was clear.

Probably too much on one RCD.

More sockets are always handy if there is space to fit them. Or at the very least having good quality extension blocks for the likes of TV/AV/Hi-Fi and IT stuff.

Olson Electronics (London based company) make really good metal cased ones, but at a price. We have some dating from 1991 still in good working condition!

Will need to do R1 + R2 tests tho.

I did end to end and like you mentioned about the split ring being fed by 2x 32a MCB that’s exactly what I found. I also found another ring which had been split but this ring had one of its LNE leg just connector blocked floating inside the CU.

OK, you know that well!

Can I just check the Ze or does Zs need to be carried out too before the CU change?

Ze (i.e. supply earth tested with installation off and it separated) is most fundamental as it is the only one you ought to rely upon for fault disconnection:

• If TT it should be under 200 ohms, and definitely find and inspect the rod(s) it to see if in sound condition.
• For TN supplies you would normally see under 0.8 ohm (TN-S) or 0.35 ohm (TN-C-S) but that is not guaranteed. Those TN values are used in the OSG for guidance on circuit size, etc.

Measuring Zs at the DB to get max PFC, and measuring max PSCC, is easier and the higher of the two tells you if the OCPD can safely disconnect, etc. But with modern domestic CU you are very unlikely to ever see an issue as they can disconnect to 6kA at least. Old rewirable can be a risk though, at 1kA or 2kA typical max PFC.

But checking Zs is not telling you Ze is good - it might be relying on service pipe bonding that can change externally and without warning due to repairs when metal pipes are replaced with plastic.

When finding out if there are borrowed neutrals, I’m guessing I would check continuity between neutral and other neutral conductors at the board. For example disconnect 1 neutral and test between that conductor and the neutral bars?

Typically you need to IR one circuit to another, and with two-way switches in all permutations, as typically it is L of one circuit returning via N of another so only in some positions is it connected, and it depends on the lamp (which could be LED or CFL) being driven in to conduction which might need tens of volts at least.

I think the ROI regs demand that circuit-circuit isolation is measured, basically this sort of test.

 

[ppelec100]

Sorry for not quoting your message properly I’m not too familiar with how to use this on mobile.

That’s quite a cheap current clamp meter, thanks for linking me that will have to buy it.

Yeah I thought it would be too much on one RCD too, thats why I thought I’d suggest them the RCBO board.

Okay thanks for the info on the Ze Zs. The intake is a TN-S as it’s earth of the armoured outer sheath. Hopefully the Ze is good as they haven’t even got any bonding to water or gas. Will have to install these before I even start on the CU stuff.

So for the borrowed neutral I would need to IR test L from one circuit to N of other circuits let’s say upstairs lighting L and downstairs lighting N? If there is a lamp in and the switches are operated on off on off and I get a reading it will give me a low reading as it’s passing through L and passing through the lamp back through N?

You lost me when you mentioned I would need tens of volts at least?

In theory wouldn’t the continuity method between one circuits neutral and another circuits neutral give me an idea too if there is a borrowed neutral as well as the IR test?

Thank you

 

[Grant1987]

I do end to end on all rings
I do IR tests where possible
and R1 R2 at easy positions

Ze
Zs
RCD trip times

personally don't bother with figure 8

This is exactly the best way to think about testing just simplify it without overcomplicating matters. Providing you know the max permissible values or know where to find them then it’s all straight forward

 

[littlespark]

Reading #7, this could be an absolute mare of a job.
You’re going to find all sorts.

Depending on how bodgy it’s been, and age of propert, expect incomplete rings, no cpc between downlights, no earth link to metal accessories, attic jointboxes without lids….

 

[timhoward]

So for the borrowed neutral I would need to IR test L from one circuit to N of other circuits let’s say upstairs lighting L and downstairs lighting N? If there is a lamp in and the switches are operated on off on off and I get a reading it will give me a low reading as it’s passing through L and passing through the lamp back through N?

I actually wouldn’t do this, I’d stick to L+N to E for IR tests. That is basically pat testing the entire house at once and it isn’t uncommon to have to go around and unplug things to find the item that is on its way out / the last one I did was an oven on a plug top.
Any other IR test permutation or between circuits risks damage to connected equipment.

In theory wouldn’t the continuity method between one circuits neutral and another circuits neutral give me an idea too if there is a borrowed neutral as well as the IR test

Yes in my view continuity is enough.
You are basically after big picture tests to give you the general idea before a CU change.

 

[pc1966]

I should have said that any IR testing that is not L+N to E should be done at 250V in case there is a low power load on the circuit.

L+N to E is (or should be!) safe to do at 500V as any components rated for such use, such as class Y capacitors, are designed to survive impulses to several kV without failing so you have little risk of L-E fault being triggered (and a dangerous voltage if CPC open due to another fault).

The exception to that are SPD, as they are designed to conduct above around 400V to limit surges, so they typically appear as below around 0.5M at 500V, but above around 10M at 250V.

Any other IR test where you go from L to N in any permutation (same circuit, or checking for borrowed neutral between two circuits) should be done at 250V to protect low power electronics.

Basically your 230V AC supply could be as high as 253V, and that peaks at sqrt(2)*253 = 358V so electronics are OK to that voltage. But your IR tester can go to 500V (or a bit more) but only at 1mA or so current. So any significant load will never see 500V, but a low power LED lamp, USB charger, cooker clock, etc, that takes less than 0.5W on standby will get charged to 500V+ and probably damaged by the IR test.

 

[pc1966]

Yeah I thought it would be too much on one RCD too, thats why I thought I’d suggest them the RCBO board.

Always go RCBO if at all possible. Far less trouble later on and cost difference now is only around £100 in most cases.

Okay thanks for the info on the Ze Zs. The intake is a TN-S as it’s earth of the armoured outer sheath. Hopefully the Ze is good as they haven’t even got any bonding to water or gas. Will have to install these before I even start on the CU stuff.

Sounds like you have quite a poor installation to start with!

Yes, bonding is important from a shock safety point of view, and on TN-C-S cases also a fire risk as you might have an open-PEN fault trying to divert the neighbourhood's neutral current via some boiler's 0.75mm CPC if no 10mm service pipe bonding!

So for the borrowed neutral I would need to IR test L from one circuit to N of other circuits let’s say upstairs lighting L and downstairs lighting N? If there is a lamp in and the switches are operated on off on off and I get a reading it will give me a low reading as it’s passing through L and passing through the lamp back through N?

Basically yes. The typical case for a borrowed neutral is the live of circuit #1 feeds the 2-way switch on floor #1, and in turn to the 2-way on floor #2, and then back via the nearest neutral (circuit #2) and not the neutral associated with the supply (#1).

It is very dangerous as someone might isolate circuit #2 but then someone switches the light on and its neutral becomes live due to the cross-linking.

If you fit all RCBO they will find it fast!

You lost me when you mentioned I would need tens of volts at least?

If you have LED or CFL lamps they won't show any conduction at the volt or so used for continuity testing (or resistance testing via a multimeter).

But as above, testing should be done at 250V to avoid damaging a light or dimmer.

In theory wouldn’t the continuity method between one circuits neutral and another circuits neutral give me an idea too if there is a borrowed neutral as well as the IR test?

No, as in most cases the cross-linking is via some load like a lamp. While you could get the borrowed neutral case appearing in other circuits, and you might get two circuit's neutrals linked by mistake, the most common reason was someone not having 3&E cable for two-way lights on a building with lights split as upper floor on one circuit and lower floor on another, etc.

 

[ppelec100]

Reading #7, this could be an absolute mare of a job.
You’re going to find all sorts.

Depending on how bodgy it’s been, and age of propert, expect incomplete rings, no cpc between downlights, no earth link to metal accessories, attic jointboxes without lids….

I’ve been at the house to fault find and thankfully as part of the fault finding I carried out IR test so I guess I wont have to do that again. I found 2 broken rings and rectified the problem. All sockets are plastic and all lighting are rose ceiling except for 3 rooms but they’re just fluorescent fittings or some sort of spotlight fitting, hopefully with cpc’s. No down lights. Will have to do R1 + R2 to check. There were a couple minor visual things I noticed which are quick fixes.

 

[ppelec100]

I actually wouldn’t do this, I’d stick to L+N to E for IR tests. That is basically pat testing the entire house at once and it isn’t uncommon to have to go around and unplug things to find the item that is on its way out / the last one I did was an oven on a plug top.
Any other IR test permutation or between circuits risks damage to connected equipment.


Yes in my view continuity is enough.
You are basically after big picture tests to give you the general idea before a CU change.

Thank you for your input.

 

[ppelec100]

I should have said that any IR testing that is not L+N to E should be done at 250V in case there is a low power load on the circuit.

L+N to E is (or should be!) safe to do at 500V as any components rated for such use, such as class Y capacitors, are designed to survive impulses to several kV without failing so you have little risk of L-E fault being triggered (and a dangerous voltage if CPC open due to another fault).

The exception to that are SPD, as they are designed to conduct above around 400V to limit surges, so they typically appear as below around 0.5M at 500V, but above around 10M at 250V.

Any other IR test where you go from L to N in any permutation (same circuit, or checking for borrowed neutral between two circuits) should be done at 250V to protect low power electronics.

Basically your 230V AC supply could be as high as 253V, and that peaks at sqrt(2)*253 = 358V so electronics are OK to that voltage. But your IR tester can go to 500V (or a bit more) but only at 1mA or so current. So any significant load will never see 500V, but a low power LED lamp, USB charger, cooker clock, etc, that takes less than 0.5W on standby will get charged to 500V+ and probably damaged by the IR test.

Thank you, that last part really helped me to understand the voltage the equipment can handle being IR tested at.