Is it acceptable to enter low IR on MWC

[FreeLec]

When doing minor works on a circuit, which may take a very short time to complete and it requires a MWC the IR test between L-N and E may be zero due to other electronic items.
You can end up spending 10 times longer looking for and disconnecting them just to get a good IR reading.

The company I work for has a policy of entering the low reading with an explanation of why and then arranging extra time at another time and charging to diagnose and rectify, even if it just means tracing electronic items, disconnecting and re-testing.

This is acceptable to the establishments they are working for.

However, it does not seem correct to enter what is effectively a fault on a certificate that is supposed to show the circuit as tested and okay, just as with an EIC. It's not a EICR so I would have thought this wrong.

However, what other way of doing it is there short of saying to the customer that before any work is done the circuit has to be tested as to fitness and any faults rectified but it's done the other way round with this lot ending up with a fault on the MWC. Only when the fault is cleared (or found that it is just electronic items) is the MWC changed.
The one with the fault is still given to the client until the updated one is done.

There are times, though, when no one has gone back to re-test as the client has given no further instruction, so the client has ended up with a MWC with an IR fault.

Any thoughts?

 

[FreeLec]

davesparks
Agreed. However, in reality I would never get anything done.
A spark I work with said he thinks it only refers to anything that has been added.
I agree with you, if a circuit has a fault by rights it should not have any modifications until the fault is rectified. I test for Zs as this is quick. If there is a problem I don't start work on it, report it, move on to the next thing.
However, IR can involve a lot longer process and I would never get anything done if I ended up fully testing the circuit with disconnecting every piece of electronic item that has functional earths etc, just to make sure the circuit IR is OK before I start anything.

Also, you may be modifying in a way that you are adding nothing but removing and re-routing.
Same issue.

Murdoch
NO! That's not what I said.
Re read the response I gave to you (no 9)
Read the last sentence.


Remember, I work for a company, not myself. I already said that I'm not comfortable with this and this is not the only work being done. Other work does not have these issues.

 

[FreeLec]

Something's wrong with this. I keep loosing the message I post.

 

[FreeLec]

Something's wrong with this. I keep loosing the message I post.

 

[Lucien Nunes]

I don't understand why you would think that the two comments would not tally

Because I design, build and test a wide variety of electronic equipment and find that very few devices in general usage today deliberately have such a low resistance L+N->E. Functional earthing yes, capacitive leakage current yes, all understood, but 10kΩ DC resistance? Typically the lowest I encounter for an SMPSU is around 7MΩ. We know about RCBOs but I didn't count them as connected equipment and you are in a position to disconnect them.

OK, I understand that you might be coming across a lot of faulty circuits and your original question still applies to those, but I would be keen to know what current-using devices, other than faulty ones, are complicating your testing to the extent that it requires callbacks.

 

[FreeLec]

Regarding circuits that have faults, until I am 100% certain that all equipment is disconnected I cannot say for certain that there is a fault in the circuit.
Many is the time that I have tested a ring or radial and have got low IR until I find something hidden out of the way or not expected to be on the circuit being tested. Unplug it and finally the reading will go up to >999. This can also include active RCD sockets.
Lighting can have something somewhere in the dropped ceiling that can take an age to find. Controllers for DALI, DSI, 1 to 10V etc.
As to exactly what the equipment is; I don't keep a note so I cannot tell you what exactly they are.
My intention now is to directly test various equipment disconnected from the supply, between L-N and E on the supply lead and see what type of equipment cause the problem.
When you are testing you don't take notice of exactly what the item is. You see something and disconnect it or unplug it.
I don't know how long this will take until I get a decent list of items but when I do I'll post it under an obvious title for you and others to see.
However, I don't understand why you think that there should not be an issue. You accept that RCBOs have that issue so why not other types of equipment?

Here are links to another post I put up. Look at the response from others who replied with an explanation showing that there is an issue. I was trying to understand what it is that causes this. I have a basic understanding of electronics but when it gets to deep I get lost in the explanation.

IET Forums - Electronic Equipment and IR testing

Electronic Equipment and IR testing. What component causes low reading to earth? | Screwfix Community Forum

The callbacks are agreed with the client in the sense that we have been told not to spend time doing something that has not been factored into the original quote. Account has not been taken of any problems. No one is being conned. The client wants it this way as there are sometimes limited time slots to get the work done and has to be tightly scheduled. Drives me mad. It's frustrating to work that way.

 

[Lucien Nunes]

Look at the response from others who replied with an explanation showing that there is an issue.

Some relevant comments there but also a lot of errors. I'm not picking this apart to be awkward, I was genuinely surprised by your original comment and it set me thinking about the possibilities. But I could not find any valid ones, and neither did these other threads. Let's look at the key points raised:

Capacitor charging current. Yes, true, but only for a few seconds unless your IR tester is faulty. Capacitors wired L+N->E are never very large in value, to limit AC leakage and because of the job they do (RF suppression for EMC). I often test equipment with quite high capacitance to earth and it only takes seconds for the reading to settle with a normal tester.

Capacitor leakage. I don't know how many capacitors these guys test but at a guess I've tested many tens of thousands in my life, of all kinds, sizes and ages. Any of the types of capacitor that are used L+N->E (normally class Y construction) that test under tens or hundreds of megohms, are faulty. But I've never found any that are not also physically destroyed, because they are designed specifically to have high integrity against developing leakage. Other kinds of capacitors do develop significant leakage, e.g. wax-impregnated papers in vintage radios over 60 years of age; if the radio works today most likely they have been changed already and they weren't usually used L+N->E anyway. 'Aluminium foil jobby' is not a kind of capacitor I recognise but if it means aluminium electrolytic, they do have high leakage but are never used L+N->E for a bunch of reasons.

Bleeder resistors. Exist, often across X-caps (L-N) which are typically much larger, but not commonly class Y capacitors. The numerical example given contains errors: The initial voltage of interest is Vpk not Vrms, 325V for 230V AC, 50/325=15% so 2RC<5s and R<=250MΩ. A resistor that high is expensive and potentially unstable, so practical designs use lower resistors on the rare occasion they are fitted at all. I mentioned 7MΩ - that's a favourite of Sony that one still sees occasionally, more often its >99MΩ. If we take your '0.01' reading as 20kΩ, as it could well be, it would still need 7000/20 = 350 pieces of Sony equipment of these specific models on the circuit to cause it.

Inductors and zener diodes. Since when were inductors ever connected L+N->E in appliances? Nonsense. Zener diodes / VDRs can be present as surge protection and I did mention these as a typical special case where rated for 120V mains only. But testing at 250V should not cause 230V-rated surge arrestors to conduct significantly, or else they would try to clamp the 325Vpk of normal mains. Therefore they will not badly affect the reading at 250V (They can ruin it at 500V though).

Filters. Well these are where the said class Y caps are usually located, usually with some inductors, often built into the IEC inlet etc as a power entry module. They are no more and no less than the components inside them. All reputable filter makers will permit insulation tests, many filters are tested to >2kV in the factory (although repeated, prolonged tests at this voltage will cause damage). Even the cheap nasty unknown brand ones survive 250V just fine, or they wouldn't last on the mains at all.

There are other errors in posts in those threads (e.g. confusing different types of capacitors and their functions) but they are not so relevant to my point. Which is, that ordinary non-faulty equipment that might be connected to ordinary circuits (I am not talking about multi-megawatt induction furnaces or electrode boilers!) will not under normal conditions present a DC resistance of tens of kilohms to a 250V IR test L+N->E. Unless you can find real-world examples.

E2A OK, OK, I've thought of one - old sheathed heating elements that have not been used recently. Leave a Baby Belling from the 80's plugged in but switched off only in the line, and you'll get a low reading due to moisture absorption in the mineral insulant. They used to fail PAT IR tests regularly, although not often at 10k . Run them for a few hours, at first the value falls as the moisture condenses at the cold tails, and then they will usually come up fine once it's evaporated again. But they are pretty easy to spot and unplug.

 

[FreeLec]

Thanks Lucian.

I'm glad I started this thread.
I really appreciate the info you've posted. So the info on the posts I directed to you is misleading. I would never have known.
I understand electronics to a point but some things go to deep to get a grasp of without an underlying understanding.

So from what you are saying, no equipment connected to a circuit, be it power or lighting should pass enough current at 250V so as to get a low IR between L-N and E.

One problem though.

It does.

Many's the time I have had to go and search for an item plugged in that when I eventually find it, the difference is the change from 0.00 or 0.01 to >999 Mohm.
Now I accept that a faulty item can cause this but this happens far too often. All those items causing this can't all be faulty.

Here is a comment from someone who replied to the same issue on another forum.
"Nearly every circuit I test has low IR, that is until I unplug the surge protection extension. Another useless invention causing problems".

There are some circuits that have N-E faults but that would not be the case if the reading changed when removing the item from the circuit. The fault would still be there.

Normally, under the pressure to get the job done quickly, I don't stop and think about what I've just taken out of circuit. Next time I will keep a record of what the item is and also do a test on it between L-N and E. That will identify what's going on.

You have really got me thinking now. Your information and what I experience on site are conflicting. Normally, my reaction would have been to say you must be wrong but you seem to know what you are talking about. You can usually tell the difference when someone is guessing or knows what they're talking about.

Like I say, I will post the results of what I find but this may take some time. Unfortunately, I don't have the time to check that out at the moment as I have to record what I find and move on to the next one.
Even when we are doing EICR it's the same thing. If you can't find what the problem is in 5 mins record the result and move on. At the moment I am starting at 06:00 to get the quite time in the morning to get as much done as possible. Normally I start at 08:00 so I still get in at 07:00 to avoid the traffic. I can get in at 06:00 and test a few circuits for a few hours each morning for a while and see what it is exactly that is causing the problem rather than just unplug everything.

Until now I was always sure that it is the circuitry in electronic equipment that is causing the problem but now you have got me wondering.

The thing about the MI insulated heating element. I had a coffee maker that I didn't use for a year. When I went to use it it started smoking from the element then went pop.
I called the manufacturer who explained that if it's not used for a while moisture can get into the end of the element. I was very surprised by this. That would mean from the time of manufacture all items containing MI elements must be used soon after and never put into storage for too long.
That seems a bit strange to me. Personally, I think he was fobbing me off and the item was faulty. All MI elements are totally sealed from what I can see, so moisture should not be able to get in unless there is a fault.

 

[Lucien Nunes]

Here is a comment from someone who replied to the same issue on another forum.
"Nearly every circuit I test has low IR, that is until I unplug the surge protection extension. Another useless invention causing problems".

I'm not ruling out that there might be some oddball surge protectors that come in on the low side at 250V. I would be interested to hear about them if you find them. My experience of commercially available units with type 3 MOVs is that they normally read >10MΩ and oftern >99MΩ at 250V between any pair of conductors. The clamping voltage is often around 400V so the results on a 500V test are unpredictable, often between 0.1 and 1 MΩ though. I have probably PATed 100 different types and brands and they normally pass fine at 250V, so I am wondering whether the poster of that comment is testing at the higher voltage? The most common reason we find for failure is that they rattle when shaken, due to physically disintegrating inside.

You have a point about active RCD sockets; USB-equipped sockets are another installed electronic nuisance, often substandard and non-compliant, impossible to isolate for general testing. My work doesn't normally involve general purpose socket-outlet circuits so thankfully rarely encounter them.

I agree about your coffee maker - I think he found a peg to hang his faulty appliance on. Sheathed elements used to be un-sealed and moisture did get in even when new, it was a known and significant problem for testing but it did not normally cause the elements to go bang. You could run them to dry out, if they were really low they might trip an RCD but this didn't indicate they were irredeemable. Back when PAT was a new thing and we were still trying to devise test regimes, I had dialogue with element makers trying to pitch the test criteria so that testers could pass damp-but-sound elements that otherwise would be considered failures. We got to know their foibles in more detail than we cared to!

Thanks again for expanding on this in detail.

 

[davesparks]

davesparks
Agreed. However, in reality I would never get anything done.
[

Then perhaps you should stand aside and let someone who is competent to do the job take your place.

 

[PEG]

Bit harsh Dave,Landrover swivel hubs leaking again? :smile5:

 

[davesparks]

Bit harsh Dave,Landrover swivel hubs leaking again? :smile5:

Nope, just the wonderful nhs has bolloxed up my prescription again

 

[Midwest]

OP, ain't making no comment on what you and Lucien have been talking about, but as regards RCBO's, can't see that they should be an issue. If the circuit your testing has been disconnected, they won't factor into the test?

Which end of the circuit are you conducting your tests from?

What sort of minor works are you doing for your employer?

 

[bigspark17]

Then perhaps you should stand aside and let someone who is competent to do the job take your place.

NOT NICE TO SEE COMMENTS LIKE THIS!!! As it says by moderators if your reply cant help the OP then dont reply, that is just rude to a person who obvious is concerned and reaching out for help over something he has concern for!

 

[FreeLec]

DAVESPARKS

I have just noticed your stupid and ignorant comment.

If I was not competent I would not know that something was not right.
I already said that I did not feel happy about the way I am being asked to work.
I am being told to work that way.
I am installing about 50 bulkhead emergency lights next to existing failed integrated fittings.
It is in a school where I have to drag tools, materials, rubble bags, vacuum cleaner, etc between 14 different buildings and back to the same ones as different rooms are being used at different times.
I have to get each one done as quickly as possible within the allocated time.
I have not been given time to disconnect any item of equipment not allowing an acceptable IR result.
I've been told to put a failed IR on the MWC with a comment about why the result is likely + further investigation required.

Maybe you should think it bit more and read the post properly before you make nasty comments.

I also see you thanked Murdoch for making the sarcastic comment of money making con.
If both you and he read the post properly you would both have seen that I said this is pre-arranged with the client. How can it be a money making con if the client has agreed that if something can't be done within an allocated time then a return visit has to be arranged and the initial charge is only for the first allocated time period. They have agreed this.

 

[FreeLec]

Midwest

I only mentioned RCBOs as an example of a device that causes a problem on IR to earth. (However, I have seen sparks test with both L and N still in terminals and then fail the circuit).
I test from the DB with both L and N disconnected from the MCB/RCBO/Neutral bar.

The minor works I have explained just above.
This problem I am having is only with this particular job. On other jobs I am doing one thing at a time and can properly test the circuit, but on this it's a nightmare. Working where there are schoolkids you have to take so many precautions and this adds more time. Then there are access problems. It once took me all day to do a 2 hour job there because I had to have access to 3 rooms and 2 corridors. When I could get into one I then couldn't get into the other. I needed to go from one to the other and back. Worked on occupied offices before but it's the first time I've had to work like this.
I'm going to have the same problem tomorrow where I am working in one room but the lighting circuit feeds another room that's being used. Not looking forward to it. I just hope it's nice an bright with the sun shining so they won't need lighting on.