Insulation Resistance . Please help I never get good reading

[studentspark]

Latest example,

Insulation resistance on kitchen sockets

Cooker and hob on own circuit (so not involved)


L+N connected together tested between earth at CU

Nothing plugged in (FACT)

Neon FCUs for appliances off

Neons must be on the load side so will not effect results . Is that correct?

Get IR of 0.02Momhs

IR test on old properties never pass, am I doing something wrong

help appreciated

 

[Paul_C]

Neons must be on the load side so will not effect results . Is that correct?

The neon is connected L-N anyway, so even if still in circuit it won't have any effect when you are measuring between L-N linked together and earth.

Neon FCUs for appliances off

Have you considered what happens if it's an FCU with a single-pole switch?

 

[Lectrician]

Never known an FCU with a single pole switch.....

I assume you are disconnecting the L & N from the consumer unit (the neutral must be out of the neutral bar and the lives fuse out or mcb off).

Double check there is nothing in circuit you are missing - I know you say FACT, but for sure? Did you wire the circuit?

 

[bernardgreen]

What voltage are you using in the test ? Should be 500 volts smoothed DC.

Any AC component in the test voltage will result in a current flowing in the capacitance between conductors and this will be seen a leaky insulation.

It maybe that the 500 volt invertor in your meter is no longer producing smoothed DC.

I have heard of an "electrician" testing IR with 230 volts AC and a milliamp meter without realising the effect of the capacitance between conductors.

 

[GarethW]

Neons must be on the load side so will not effect results . Is that correct?

That is correct, but if the FCU was wired incorrectly,
ie: Supply going to load side and the load going to the supply side,
then you would infact be measuring the resistance of the neon

 

[EFLImpudence]

L+N connected together tested between earth at CU

If you have everything unplugged and/or switched off, why have you not tried measuring between all conductors?

Nothing plugged in (FACT)

I'll bet there is.

 

[Paul_C]

Never known an FCU with a single pole switch.....

Some older units were S.P. switches.

That is correct, but if the FCU was wired incorrectly, ie: Supply going to load side and the load going to the supply side, then you would infact be measuring the resistance of the neon

Not when you have L-N linked together and are measuring between those two linked conductors and earth.

 

[studentspark]

Thanks for the replys

Lectrician you said

I assume you are disconnecting the L & N from the consumer unit (the neutral must be out of the neutral bar and the lives fuse out or mcb off).

Not sure what you mean there, if the fuse was off how would the circuit be tested?

So . No didn't do that

This is what I did.

connector lead to busbar and neutral, then put leads on busbar and earth
with all MCB's on.
got 0.02
So then I switched all MCBs off and tested one circuit at a time, switching on each MCB in turn and testing. As showed at college.

The kitchen returned the lowest reading of 0.02
The other circuits were poor about 3

I did not disconnect anything from the CU thinking that its joined anyway[/quote]

 

[JohnW2]

Whoops ...wrong thread. Apologies!

 

[Paul_C]

This is what I did.

connector lead to busbar and neutral, then put leads on busbar and earth
with all MCB's on.
got 0.02
So then I switched all MCBs off and tested one circuit at a time, switching on each MCB in turn and testing. As showed at college.

You're not going to be testing one circuit at a time with all the conductors still connected to the neutral bar. If there really is nothing left connected across L & N on each circuit, then opening the MCB for that circuit will remove the live conductor from the insulation test, but that's all.

And are there any RCD's in the panel which have a functional earth connection?

 

[flameport]

connector lead to busbar and neutral, then put leads on busbar and earth
with all MCB's on.
got 0.02

Testing the entire installation, so any single item connected anywhere in the building would show as a fault.
Could be a valid test - but only if you are going to search the whole house and unplug every appliance, remove all lamps, transformers, TV amps, built in appliances, smoke alarms, outside lights and dozens of other possible items.

So then I switched all MCBs off and tested one circuit at a time, switching on each MCB in turn and testing. As showed at college.

That process achieves nothing, as a fault or load between N&E anywhere in the installation will still be connected to what you are testing. Either you have misunderstood what the college was saying, or more likely, the persons running the course are fools.

I did not disconnect anything from the CU thinking that its joined anyway

Yes, they are all connected together, which is why the results you are getting are totally wrong every time.

The correct procedure is to remove the L&N wires from the protective device and neutral bar, and test between L&N, L&E, N&E.
Repeat for each and every circuit.

 

[Taylortwocities]

Think about it. You must completely disconnect the circuit under test.

Neutral is connected to earth.

For TNC-S it is connected at the incomer. For TN-S and TT it is connected at the supply transformer.

 

[JohnW2]

Think about it. You must completely disconnect the circuit under test.

With that I obviously agree, but ....

Neutral is connected to earth. For TNC-S it is connected at the incomer. For TN-S and TT it is connected at the supply transformer.

True, but presumably (hopefully!) he isolated the L busbar (hence also neutral feed to the N busbar(s)) from the supply before he started doing IR tests from the L busbar

Kind Regards, John.

 

[ban-all-sheds]

No, he didn't disconnect anything.

 

[Paul_C]

Well, we hope he disconnected the L & N busbars from the incoming supply by opening the main switch.