Low insulation resistance reading

[An Buachaillín Bán]

Hello everyone,

Hope you're all well.

I'm carrying out an overall/global insulation resistance test on a newly wired 3 phase installation we did recently. I joined all 3 phase together to neutral and tested from the busbar to earth.

I'm getting a reading of 1.33MOhms, and here in Ireland, we require a minimum of 2Mohms. I had disconnected the surge protection device previous to that. It is all RCBOs for lighting and sockets. A couple of 3 phase mcbs feeding coffee machine and oven but no appliances plugged in.

When I turned off all the mcbs and rcbos and tested busbar to earth again, I'm still getting the same reading of 1.33Mohms. Any idea what could be causing this? Is it something in circuit? Because I do have many lamps, leds etc in circuit. Is it best to disconnect all of these, which would be a monumental pain in the hole, or should the 3 phases together to neutral not negate that?

Thanks all.

 

[DPG]

Have you tried testing them separately rather than joined together, to see which one is the low one? If it's a new install then the IR seems very low.

 

[westward10]

Find the culprit or culprits remove from the equation do another global test if okay look into the disconnected culprit/s.

 

[An Buachaillín Bán]

Have you tried testing them separately rather than joined together, to see which one is the low one? If it's a new install then the IR seems very low.

I didn't actually. I will join each phase to neutral and test to earth tomorrow. I will let you know how that goes. Thanks

 

[pc1966]

Did you try at 250V and 500V? If they differ by a lot then you still have a SPD to E somewhere. It could be a permanently wired bit of equipment like a boiler, etc, where you don't have (or did not find) an all-pole isolator for it.

Switching off the SP/TP MCBs might isolate some circuits and help find dodgy cable or accessory, but maybe not - the L might be energised via a load (like a lamp) from the N-E voltage you apply. To really find a culprit you need to disconnect the related N each time and test it plus the related MCB-isolated L

 

[davesparks]

Do the RCBOs have functional earth connections? If so you may be testing through the electronics inside the RCBO.

 

[An Buachaillín Bán]

Do the RCBOs have functional earth connections? If so you may be testing through the electronics inside the RCBO.

Find the culprit or culprits remove from the equation do another global test if okay look into the disconnected culprit/s.

Have you tried testing them separately rather than joined together, to see which one is the low one? If it's a new install then the IR seems very low.

Do the RCBOs have functional earth connections? If so you may be testing through the electronics inside the RCBO.

Did you try at 250V and 500V? If they differ by a lot then you still have a SPD to E somewhere. It could be a permanently wired bit of equipment like a boiler, etc, where you don't have (or did not find) an all-pole isolator for it.

Switching off the SP/TP MCBs might isolate some circuits and help find dodgy cable or accessory, but maybe not - the L might be energised via a load (like a lamp) from the N-E voltage you apply. To really find a culprit you need to disconnect the related N each time and test it plus the related MCB-isolated L

Hi all.

Thanks for taking the time to help and respond to me.

I sorted the issue. A 3 phase oven circuit was causing the low IR reading. The oven cable was tied into my 3 phase isolator, but I did have the isolator turned off. I'm assuming the element was throwing off my reading?
Or as pc1966 said that the L might have been energised from voltage put through on N+E?

I must add that the guys supplying all the kitchen equipment did say to me a week ago to power up that oven first and check is it working as it is second hand. I'm beginning to question if the oven could be faulty too.

 

[timhoward]

The oven cable was tied into my 3 phase isolator, but I did have the isolator turned off. I'm assuming the element was throwing off my reading?

If the isolator is only isolating the phases (as many rotary isolators do) then reading N-E right through to a damp or faulty element would pull down your global IR reading.

Especially if it’s a 2nd hand oven that may have been sitting around for months.

Well done for finding it.

 

[nicebutdim]

If the isolator is only isolating the phases (as many rotary isolators do) then reading N-E right through to a damp or faulty element would pull down your global IR reading.

Especially if it’s a 2nd hand oven that may have been sitting around for months.

Well done for finding it.

I could be wrong, but have a funny feeling that regulations in IE require that 3 pole isolation is used and neutral shouldn't be broken. Perhaps this only applies to the main isolator or perhaps it doesn't apply at all...

 

[timhoward]

I could be wrong, but have a funny feeling that regulations in IE require that 3 pole isolation is used and neutral shouldn't be broken. Perhaps this only applies to the main isolator or perhaps it doesn't apply at all...

I want to create a new code for that:
PI : Potentially Interesting

 

[nicebutdim]

I want to create a new code for that:
PI : Potentially Interesting

Pretty certain I only ever see 3 pole main switches with removable neutral link. Maybe OP or @Risteard can confirm or make me look foolish

 

[mainline]

Pretty certain I only ever see 3 pole main switches with removable neutral link. Maybe OP or @Risteard can confirm or make me look foolish

You need a special switch if you want to switch 3 P and N - one where the neutral contact is set back so it closes early and breaks late, relative to the 3 phases.

 

[nicebutdim]

You need a special switch if you want to switch 3 P and N - one where the neutral contact is set back so it closes early and breaks late, relative to the 3 phases.

4 pole main switches are fairly common here, but the point was more about the fact we're free to use them, whereas I believe they can not be used in IE.

 

[mainline]

4 pole main switches are fairly common here, but the point was more about the fact we're free to use them, whereas I believe they can not be used in IE.

What about if its TT earthing ?

 

[nicebutdim]

What about if its TT earthing ?

I guess that might be different and something I hadn't considered.

 

[Risteard]

What about if its TT earthing ?

TT (Direct) Earthing is essentially non-existant within the State, bar perhaps a very few relics. TN-C-S (Neutralised) applies to very nearly 100% of electrical installations.

 

[MDJ]

I could be wrong, but have a funny feeling that regulations in IE require that 3 pole isolation is used and neutral shouldn't be broken. Perhaps this only applies to the main isolator or perhaps it doesn't apply at all...

should never break a neutral via a switch , if it hangs you can end up with 400 volts on a 230 volt circuit which will cause havoc.

 

[mainline]

should never break a neutral via a switch , if it hangs you can end up with 400 volts on a 230 volt circuit which will cause havoc.

On a TT you need to isolate all conductors including the neutral, hence the switch where the neutral contact is set back so it closes early and breaks late, relative to the 3 phases.

 

[nicebutdim]

should never break a neutral via a switch , if it hangs you can end up with 400 volts on a 230 volt circuit which will cause havoc.

I'd agree where functional switching is concerned, but isolators are manufactured for a specific purpose and to specific standards - there should be no danger of any live conductor 'hanging'.

 

[mainline]

TT (Direct) Earthing is essentially non-existant within the State, bar perhaps a very few relics. TN-C-S (Neutralised) applies to very nearly 100% of electrical installations.

No caravan parks, swimming pools, stables etc ? That would use TT earthing ?.

 

[davesparks]

4 pole main switches are fairly common here, but the point was more about the fact we're free to use them, whereas I believe they can not be used in IE.

I wouldn't say free to use, you would only switch the neutral when absolutely necessary.
Switching the neutral introduces a greater risk of losing the neutral to the load whilst the phases are still connected thus damaging the load.

 

[davesparks]

I'd agree where functional switching is concerned, but isolators are manufactured for a specific purpose and to specific standards - there should be no danger of any live conductor 'hanging'.

There is always a non-zero chance of a fault happening.

And why would you need to switch the neutral on a TN supply?

 

[nicebutdim]

There is always a non-zero chance of a fault happening.

And why would you need to switch the neutral on a TN supply?

That's part of the reason why I'm raising this point. I'm under the impression that it isn't permitted in IE, where the vast majority of supplies are TNC-S, but I see plenty of 4 pole isolation on TN supplies in the UK.

I have questions to ask which expand on this and don't wish to hi-jack the OP's thread more than has already happened. It's not a subject I've seen under much discussion on the forum, so have started a thread and would welcome any input.

Isolation of TN supplies.

I haven't seen a lot of discussion on this subject and have questions pertaining to isolation where TN earthing is present and any peculiarites which may exist between TN-S and TNC-S. I see mostly 3 pole main switches on TN earthing systems, but do see quite a few instances where all live...

www.electriciansforums.net

 

[davesparks]

That's part of the reason why I'm raising this point. I'm under the impression that it isn't permitted in IE, where the vast majority of supplies are TNC-S, but I see plenty of 4 pole isolation on TN supplies in the UK.

I have noticed a trend towards installing 4 pole main switches where until a few years ago 3 pole would always have been preferred.

 

[MDJ]

On a TT you need to isolate all conductors including the neutral, hence the switch where the neutral contact is set back so it closes early and breaks late, relative to the 3 phases.

I may have been mistaken regarding TT I will look into that, thank you for the valued reply, but in general I stick to my post, a good topic for us sparks to discuss for sure.