What test to carry out before consumer unit change?

[ppelec100]

What test should I carry out before changing a consumer unit to ensure that it is eligible for one?
I’m guessing R1 + R2 to make sure it has cpc at circuits. Say for lighting do I check every single light fitting or...?
IR test I’m assuming of course so there are no trips.

I’m not a testing qualified just installation. So this info will help me out for a CU coming up. I will be getting someone to test it and cert it afterwards.

Thanks for any help.

 

[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.

 

[ppelec100]

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

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!

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!


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.

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.

Yeah fusebox RCBO’s and boards are quite well made and cheap too.

I have seen the stopcock for the water pipe and seems like it’ll be a nightmare to get to. The gas is just on the other side of the consumer unit apparently fingers crossed.

Okay the borrowed neutral thing makes sense. So you know how I mentioned about the continuity method and you mentioned that I may still get continuity through led lamp... hypothetically if the lamps were removed or switches turned off then I did the continuity test between 1 neutral and another neutral it should be clear right?

 

[pc1966]

Yeah fusebox RCBO’s and boards are quite well made and cheap too.

They seem to be a favourite for any budget install here, as the likes of Hager or Schneider cost a lot more.

I have seen the stopcock for the water pipe and seems like it’ll be a nightmare to get to. The gas is just on the other side of the consumer unit apparently fingers crossed.

In selecting a position to bond the pipe you have to be practical as well. So as close to entry as sensible, and normally on the installation side of any meter/stopcock from an ownership point of view. Also it should be before any T-junction as they may not remain electrically sound.

Okay the borrowed neutral thing makes sense. So you know how I mentioned about the continuity method and you mentioned that I may still get continuity through led lamp... hypothetically if the lamps were removed or switches turned off then I did the continuity test between 1 neutral and another neutral it should be clear right?

Yes, if you establish the L route to a given lamp socket, and the N route, and they are different circuits then you have a borrowed neutral.

 

[ppelec100]

They seem to be a favourite for any budget install here, as the likes of Hager or Schneider cost a lot more.

In selecting a position to bond the pipe you have to be practical as well. So as close to entry as sensible, and normally on the installation side of any meter/stopcock from an ownership point of view. Also it should be before any T-junction as they may not remain electrically sound.

Yes, if you establish the L route to a given lamp socket, and the N route, and they are different circuits then you have a borrowed neutral.

If you found a t junction I’m guessing you would crocodile clips either side and check continuity to establish if it was electrically sound?

Thank you for your replies you’ve helped me massively!

 

[pc1966]

If you found a t junction I’m guessing you would crocodile clips either side and check continuity to establish if it was electrically sound?

It also depends if the type of joint is likely to remain conductive, and if it is some location that might get changed soon.

A soldered T is probably a sound electrical bet, also possibly a clamp brass style, but I'm not sure the metal push-on would remain a good conductor, and obviously the plastic sort will never be!

Really if you can get the cable to at least the incoming pipe before any joints it is a far better and safer bet. Sadly, and in spite of the warning labels, plumbers still cut off earth bonds and fail to consider that during any work.

 

[ppelec100]

It also depends if the type of joint is likely to remain conductive, and if it is some location that might get changed soon.

A soldered T is probably a sound electrical bet, also possibly a clamp brass style, but I'm not sure the metal push-on would remain a good conductor, and obviously the plastic sort will never be!

Really if you can get the cable to at least the incoming pipe before any joints it is a far better and safer bet. Sadly, and in spite of the warning labels, plumbers still cut off earth bonds and fail to consider that during any work.

Again thank you. Since we are on the topic of consumer unit change...

Wanted to ask when it comes to changing the tails what do you recommend?
Normally I would cut the seal but this house had a smart meter fitted and stupidly they didn’t fit a DP isolator.

Can I still cut the seal and pull the fuse or would it be better to cut the seal of the smart meter and pull the outgoing tails out so that when I put them back in I’m doing it live but only the outgoing meter terminals are live?

Would they know if the fuse was pulled as the meter will have no power? They shouldn’t know if I cut the meter seal and pull out the outgoing tails unless they got a tamper thing? Any resealing kits you could recommend?

 

[timhoward]

Wanted to ask when it comes to changing the tails what do you recommend?

The only recommendation most on here will feel comfortable making on a public forum is to phone up the DNO and request an isolator be fitted. Cost is often completely random, I've known free to £150, and I've also known them suddenly halve the price if I pretend to change my mind.
There's no shame in getting it done properly, it's a good investment having one and in some situations I do it.

Both removing a smart meters supply and opening the front cover will trigger the anti-tamper alert and they have been known to arrive rather quickly. So I certainly wouldn't advise either of the proposals you mention.

 

[ppelec100]

The only recommendation most on here will feel comfortable making on a public forum is to phone up the DNO and request an isolator be fitted. Cost is often completely random, I've known free to £150, and I've also known them suddenly halve the price if I pretend to change my mind.
There's no shame in getting it done properly, it's a good investment having one and in some situations I do it.

Both removing a smart meters supply and opening the front cover will trigger the anti-tamper alert and they have been known to arrive rather quickly. So I certainly wouldn't advise either of the proposals you mention.

Hmm they are a pain to come and fit one in and it’s rather off putting for the customer especially if it adds to the cost of me putting in a consumer unit. I know it is what it is and it’s my safety, just a hassle really.

I mean if I was to attempt to remove the tails from the existing board live using insulated 1000v gloves and Henley blocks, and I find the ends of the tails a bit messy and needing to be cut properly, are they able to be cut separately and stripped back whilst live? I probably won’t as I don’t like working live especially with tails but hypothetically they can can be cut without exploding?

 

[SparkyChick]

Hmm they are a pain to come and fit one in and it’s rather off putting for the customer especially if it adds to the cost of me putting in a consumer unit. I know it is what it is and it’s my safety, just a hassle really.

I mean if I was to attempt to remove the tails from the existing board live using insulated 1000v gloves and Henley blocks, and I find the ends of the tails a bit messy and needing to be cut properly, are they able to be cut separately and stripped back whilst live? I probably won’t as I don’t like working live especially with tails but hypothetically they can can be cut without exploding?

You should have the customer contact their energy supplier as many of them will fit isolators.

You should absolutely not consider changing the board with live tails, the amount of manipulation they may require makes this a fools errand at best and a one way ticket to the big scheme in the sky is most definitely on the cards. If you've never arc welded, I suggest you have a go and see what a low voltage 100A arc is like and then consider you'll be dealing with something that can (for a very short time at least) deliver maybe 1000A.

To call it stupid would be an understatement of epic proportions.

As it appears your questions about testing have been answered, I'm going to close this thread. Have the customer talk to their energy supplier about an isolator and if they won't do it, then get the DNO out to pull the fuse at the very least. I've found that they are often willing to pull the fuse and then go and sit in the van for the time it takes me to connect a pre-installed isolator to the meter... they check the connections for tightness, seal everything up and go on their merry way... one fuse pulling fee, isolator fitted and you can go about your day knowing you'll go home at the end of it.