Lowest acceptable IR

[alchemist]

Did some pre-upgrade checks on a customer's wiring today to see if they're ok for a new consumer unit.

The cooker and the immersion heater circuits had L-E and N-E IRs of about 0.17M ohms. The pump and heater element where still connected. The cooker when disconnected did not improve the result of the cooker circuit, so i'm guessing poor wiring.

The cable looked silver, I think tinned copper - so must be pretty old?

The other circuits have IRs above 3M ohms. I think they are probably due a rewire.

My question is, what is the lowest IR I can get away with without tripping an RCD?

By my reckoning, the lowest total resistance of live and neutral to earth can be 7k ohms or above before it will trip an RCD. Which I get by doing 230/0.03. 0.03 being 30 milliamps.

I don't have an earth leakage clamp meter unfortunately. I intend to make the cooker and immersion heater non RCD circuits, and the others on the RCD.

 

[maccarooni]

Al.

My question to you would be, would you reconnect these circuits to an new consumer unit. the minimum required IR reading per circuit is 1meg.As you have did these pre-upgrade checks, tell the customer the results.

Macca

 

[PC Electrics]

I don't have an earth leakage clamp meter unfortunately. I intend to make the cooker and immersion heater non RCD circuits, and the others on the RCD.

Unlikely to be possible - bet there's some buried cables in those circuits.

 

[alchemist]

Isn't this a case where the regs aren't retrospective. I believe that you must only add RCD protection for burred cables in walls where you are installing them.

Upgrading the CCU to add RCDs is only going to improve safety, i'm sure the general rule when doing electrical work and think it's stated in the regs somewhere is not to make things less safe as a result of changes.

Less than 1M ohm might be the limit in the regs, but that's why i've advised the customer they could do with a rewire, I don't think that means I can't upgrade the CCU, i'm not responsible for the cables that are already there, adding a CCU won't suddenly make the cables less safe.

 

[monkey5]

was there a neon in the cooker switch, that could give a low reading.

wayne

 

[alchemist]

was there a neon in the cooker switch, that could give a low reading.wayne

The cooker is actually wired to a surface mounted double socket under the worktop in a cupboard. The cooker has a plugtop. I tried the IR with the cooker unplugged, and checked inside the socket but nothing looked suspicious. The extractor hood is on the same circuit, via an FCU above the wall units. That was turned off when I tested L-E and the reading was still low.

I'll be putting a label on the socket under the worktop 'Cooker only' so I don't have to have it on an RCD. The owner never even new it was there till we looked for it.

 

[PC Electrics]

I'll be putting a label on the socket under the worktop 'Cooker only' so I don't have to have it on an RCD. The owner never even new it was there till we looked for it.

Oh dear!

If the circuit contains buried cables you must put it on an RCD otherwise it will not conform to 17th Ed'n.

Which scheme you in? ELECSA are doing 1 day seminars covering common issues like this. Check to see if your provider is doing the same.

 

[Doc Hudson]

Isn't this a case where the regs aren't retrospective. I believe that you must only add RCD protection for burred cables in walls where you are installing them.

You must ensure that all circuits you do alterations or additions to comply with the current regs, or note deviations to those regs. When change a CU you are altering the characteristics of every circuit due to the different protective devices used, e.g. a 30A 3036 protected circuit becomes a 32A 60898B protected circuit. So all amended circuits should be 1.0Meg or higher. I think your question is covered in this guide.

http://www.esc.org.uk/pdfs/business-and-community/electrical-industry/BPG6_09.pdf

Doc H.

 

[PC Electrics]

And RCD protection must be added where required to comply with regs.

ie: sockets and buried cables and supp bonding omission.

There aren't many circuits in a normal domestic which don't require an RCD.

 

[maccarooni]

Isn't this a case where the regs aren't retrospective. I believe that you must only add RCD protection for burred cables in walls where you are installing them.Upgrading the CCU to add RCDs is only going to improve safety, i'm sure the general rule when doing electrical work and think it's stated in the regs somewhere is not to make things less safe as a result of changes.

Less than 1M ohm might be the limit in the regs, but that's why i've advised the customer they could do with a rewire, I don't think that means I can't upgrade the CCU, i'm not responsible for the cables that are already there, adding a CCU won't suddenly make the cables less safe.

Sorry Al but you are responsible for the cables that are already there if they fall below the limits of acceptance. you will have to take responsibility for connecting these circuits to the new consumer unit.

without trying to upset you or anyone, if you were to connect a circuit that falls below par, and then theres a problem what happens then. Do you turn around and say well told the customer that the IR readings were 0.17 and its not my fault that the immersion won't work or the cooker won't work cos they had to be disconnected after they kept knocking out the rcd. I know you said that they were not going to be rcd'd but 99% of circuits in a domestic dwelling, when CU upgrade is carried out have to be.

Macca

 

[kme]

I have trouble believing I`m reading this, TBH.

If the O/P believes that non-retrospective regs allow him/her to ignore the BS in force at the time- get on with it ( which I think is what will happen anyway in this case.)

To give the O/P his due, he DID test the circuit, and post his results, which is more than some others do.

BUT: if our O/P is under this sad misapprehension, chances are that others are, too.

To spell it out for you all:

Once a circuit is de-energised, YOU become liable for verifying its safety & conformity, before re-energising.

Hope that helps

KME

 

[maccarooni]

A+1 KME.

you have such a way with words.

Macca

 

[ProDave]

I'm more concerned at a cooker just being plugged into a 13A socket. I'm surprised nobody has picked up on that.

Unless it's just a little table top Baby Belling or similar, then any proper full sized cooker is going to draw way way in excess of 13A if you turn everything on.

So I would be advising rewire the cooker circuit completely with a proper cooker switch and cooker connection plate.

As to the immersion heater, has he actually tested if the fault is the cable, or the heater element? Chances are the element has gone, and a simple replacement element will fix that problem. If it IS the element at fault, you most certainly don't want to ignore that one, replace it.

Two simple faults that IMO should be corrected and then the jobs done properly.

 

[Manator]

I have to agree with the other posters, the link Doc Hudson included in his post will help sift through what you should be doing. Simply recording a deviation would be a poor excuse for not doing what you know should be done.

I am in agreement fully with KME, and with the above post by Prodave, a double socket outlet as the outlet for a cooker circuit is not good unless it is a small inbuilt oven with a plug top outlet, or is just for the ignition of a gas cooker. The immersion heater will also have to have a thermal cutout thermostat fitting for the circuit to comply, this "guidance" was brought about after the tragic loss of a youngsters life, and any non thermal cut outs should be coded on any PIR undertaken.

IR tests can be lowered by various items most common are neons, other common instances are hidden socket outlets spured off circuits that are not intended to serve socket outlets, I have seen many of these and sometimes are a pain to find.

The minimum IR may be 1M ohm but even at 3M ohm I would be worried about the status of the fixed wiring. After saying that 3M ohm is above the minimum and would therefore be ok to reconnect if the client refuses your advise to rewire.

Any circuit below 1M ohm will need to be replaced.

 

[kme]

Many of the built-in single ovens are provided with a moulded plug, for connection to a socket outlet. If this is the case; and a double socket has been installed to facilitate connection of the ignition system of the (gas) hob, this becomes a moot point.

As far as immersion & cooker cct insulation readings; to expand on what my colleagues have said:

Separate the component parts of the circuit, to determine if the supply cable, switch or a connected load are causing your problem (if the oven is on a plugtop, and the extractor is on the same circuit - what else may be on that circuit, too?)

Another little gem for you: if you haven`t found the problem yet, you haven`t done enough testing.

KME

 

[monkey5]

just a thought when you tested cooker circuit did you remove the earth rom bar in cu or just croc clip on to fully loaded bar-might make a difference.

if you have and still no better reading id be doing as kme says and split circuit up.did a PIR the other day i had a garage ring main with low reading the other day but cable was clipped so could see no visable problems and was only a couple of years old, so split it down and turned out to be 2 of the socket outlets, changed them and reading greater than 299.tested between socket outlets to be definatly sure and they were down. took 10mins job done customer happy.

id disonnect at FCU fully and not just turn off either.

wayne

 

[Doc Hudson]

id disonnect at FCU fully and not just turn off either.wayne

Agreed, disconnecting is the only way to test a suspect circuit with IR testing IMHO. Can be to many parallel paths back otherwise.

Doc H.

 

[Riggy]

Another little gem for you: if you haven`t found the problem yet, you haven`t done enough testing.

KME

Brilliant quote!

This should be our mantra for all testing questions from now on.

 

[alchemist]

Thanks for all the replies.

A couple of points of clarification. I'm not going to just go ahead and do it. I thought, incorrectly that it would be permissible to do the CCU upgrade prior to rewiring because it will lead to an improvement in safety, as long as I could be sure there would be no nuisance RCD tripping.

The cooker circuit I am sure is the cabling, no loads were connected when I did the last IR test. The immersion circuit L-E was tested with the FCU switch to the immersion off, so the test would not have included the element. The water pump was connected still though.

I did this provisional testing to see if the upgrade was permissible, and had made that clear to the customer. As i'm not getting paid to do the testing I kept it to the minimum necessary. If they are willing to accept the cost of more investigation and some rewiring i'll go back. If they want to leave things as they are, that's their call.

The cooker is a small built in unit, with a plugtop. It's got a gas hob.

Sounds like it's going to be for the customer to decide if they want to spend some money for me to do exhaustive testing and possibly rewire faulty circuits.

This all originally started because I put an extractor in the bathroom for the customer. I said it would need to be RCD protected. Gave them several options (this is all before the extractor was fitted), one of which was a CCU upgrade, which they were thinking about anyway so decided on. I put the extractor in thinking I would certify it with the CCU when RCDs were present. Now i've come to do the CCU and it looks like rewiring is coming into it.

With the benefit of hindsight, I should have tested for the CCU upgrade first before doing the extractor so I could have gone with adding RCD just on that circuit, now i'll need to add an RCD on the circuit with the extractor if they don't want the rewiring, which means i've wasted time testing circuits I didn't need to.

No harms been done, other than wasting my time.

Sorry and one question I forgot to answer.. yes I disconnected all conductors to keep them separate when testing. Originally I did a global IR and found it low so went on to do circuit by circuit.

 

[Zeespark]

Alc, you could have just wired a small consumer unit via a Henley block, and installed an RCBO onto the upstairs lighting circuit only. That way you would not have had to worry about the old consumer unit. This in time could have meant that you could have advised your customer to have the consumer unit replaced in time.

Andy