Insulation Testing 2/3

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Apache

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Now we have the earth loop testing sorted out I'm going to ask about the insulation test. I know there is another thread on this but my questions are far more basic! :|

I don't understand what insulation the test is measuring. Is it the insulation of the cabling in the system? Is it the insulation between the live and neutral conductors in the cable.

It seems to begin with a continuity test and then passing increasing voltages upto 1000V through live and neutral? What is it we are measuring? It seems everything has to be unplugged on the circuit in question.

What would anomalous readings suggest? Damaged cable?

Thanks in advance for your help

:D

 

The Godfather

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Insulation tests are carried out at 500V

Between Live and CPC, Neutral & CPC and L/N to CPC.

They are performed to test the integrity of the installed cable, that is to make sure it is not damaged in a way that it could conduct/arc between the conductors.

That simple..

Quote 'What would anomalous readings suggest? Damaged cable?'

Yes (and or further investigation (connection fault for example))..

Don

 

Apache

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Mr Godfather,

how does this arrangement work? You help me with my question do I now do something for you? I don't want to get on the wrong side of you, my girlfriend wouldn't like a horses head in the bed.

I await your instruction

 

seajayess

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Low readings could also mean damp conditions or to just make it more confusing, I have been testing a factory PIR, couldn't switch off supply as some circuits couldn't be switched off. Consumer Unit had around 10 lighting circuits some with emergency lights. (example) circuit 1 MCB switched off confirmed dead, live Neutral and CPC removed R1+R2 taken 1.75 ohms insulation resistance L/CPC 0.056 ohms N/CPC 0.067 ohms L/N omitted do to to many lights to remove.

System wired in SWA and singles in Conduit to a switch bank PME connection.

Found out that when it was installed, at a switch bank they had only taken 1 Neutral and had used it for the neon indicators on the emergency lights on 2 different circuits, so the Neutral was connecting the Live to the Neutral through the second circuit through the PME connection back to the CPC giving all the low readings, took a while to trace but once I found it removed the Neutral at neon retested circuits L/CPC 675 M ohms N/CPC 700 M ohms hope this makes sense CJS :)

 

Apache

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Low readings could also mean damp conditions or to just make it more confusing, I have been testing a factory PIR, couldn't switch off supply as some circuits couldn't be switched off. Consumer Unit had around 10 lighting circuits some with emergency lights. (example) circuit 1 MCB switched off confirmed dead, live Neutral and CPC removed R1+R2 taken 1.75 ohms insulation resistance L/CPC 0.056 ohms N/CPC 0.067 ohms L/N omitted do to to many lights to remove.System wired in SWA and singles in Conduit to a switch bank PME connection.

Found out that when it was installed, at a switch bank they had only taken 1 Neutral and had used it for the neon indicators on the emergency lights on 2 different circuits, so the Neutral was connecting the Live to the Neutral through the second circuit through the PME connection back to the CPC giving all the low readings, took a while to trace but once I found it removed the Neutral at neon retested circuits L/CPC 675 M ohms N/CPC 700 M ohms hope this makes sense CJS :)
If you borrow a neutral from another circuit surly that's not safe? There could be current from the other circuit and you could get a shock even if isolated?

 

SPECIAL LOCATION

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Just to add my twopence worth on this one.

In simple terms..

The Insulation resistance test is to verify the cables ability to "Contain" the voltage that is travelling down it.

Every manufacture constructs their cable with insulation that should be suitable not to break down under maximum voltage peaks present during normal operation...

e.g. on a single phase domestic installation the alternating voltage Will be peaking 230v either side of the 0v (zero/earth) ref point.

because a 230v AC is a potential max peak to peak voltage is up to 500v we do insulation resistance test at 500v DC..

We test between each of the conductors

L - E

N - E

L - N

Brand new undamaged cable will have an extremely high reading off the top of the max scale of the meter..

As a cable ages, the insulation resistance values will gradually reduce over time.. older installation should have a value exceeding 2megOhms.

The tests will identify things such as... Cable damaged during installation work,

e.g. sheathing damaged,

cable squashed / pinched,

nail / screw penetrating cable

Items than can cause errors or inconsistencies with readings:-

Dimmer switches,

Fluorescent tube light fittings,

Neon indicators,

12v transformers,

Timer clock,

fridge/freezer pump motors,

Extractor fan timer circuits,

PIR sensors,

Boiler circuits,

Capacitance effect along long cable runs,

Damp or water ingress in an accessory/J-box

Or any other appliances that are permanently connected.

Testing 500v Dc across some items of equipment can damage any items still connected.. e.g. electronic timer/control equipment.

Where it is unknown if anything is still connected it is beneficial to do a test at 250V first across L - N, this will highlight any potential problems!!

On an existing installation were it is not feasible to fully disconnect all appliances.. test can be carried out with L & N joined.. i.e. (L&N) - E.

Note..

The continuity test is carried out first because if all of the conductors don't go the whole length of the circuit..

Then you aren't going to prove much with your Ins Res test!

 

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Trailer Boy - Electrician.
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If you borrow a neutral from another circuit surly that's not safe? There could be current from the other circuit and you could get a shock even if isolated?
As you say..

borrowed neutral is a No-No.. ;)

and it can cause c0ck-ups on your Ins res readings!!! :(

 

The Godfather

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Mr Godfather, how does this arrangement work? You help me with my question do I now do something for you? I don't want to get on the wrong side of you, my girlfriend wouldn't like a horses head in the bed.

I await your instruction
The Don is amused.

The Godfather

 

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Trailer Boy - Electrician.
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Mr Godfather, how does this arrangement work? You help me with my question do I now do something for you? I don't want to get on the wrong side of you, my girlfriend wouldn't like a horses head in the bed.

I await your instruction
:eek:

The Don is amused.The Godfather
:) :D :^O:^O:^O:^O

Obviously apache is a very wise person who knows not to stand on the wrong side of the Don.! :) ;) :D :D:x:x

 
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