Insulation Resistance Conflicting Answers!

[whazza22]

Getting really frustrated now, as I have posted a thread about testing insulation resistance on a 3 phase motor and central heating system.

I have had people saying that you test parts of a central heating system such as boiler, valves, stats etc on 250v. And test 500v up to the spur.

But then I have had people saying that you never test on 250v and that you should connect live and neutral together and test on 500v.

Also, am I correct in saying that when testing insulation resistance on a 3 phase motor, you take out the supply and feed from the starter (leaving motor connect) and connect them in a connector blocks, then test on 500v?

Can I have correct methods/answers please

 

[deafboy]

i would say their all correct, but officially you should use test method 2 which is live and nuetral together to earth just checked on site guide page 83 refers to both methods.
for the 3phase motor thats correct, you could also test up to starter then test load separate

 

[whazza22]

Cheers, do you connect live and neutral together in a connector block at consumer unit or do you do it at the spur?

Then would you just do one test at 500v between Neutral/live to earth?

 

[deafboy]

i usually dissconect live and nuetral and put both together in test lead croc clip but connector block is the safer option

 

[deafboy]

and yes providing there is not a significantly low reading just the one test and enter the same reading in each ins box on that circuit of test sheet

 

[Des 56]

Read well the reply in your other post by Dichroic,there lies what should and needs to be carried out

A word of caution with regard to testing at 1000v needlessly,some cables Micc possibly could be damaged by this test
There should be no need for anyone to test at 250 volt prior to the 500 volt test,because test 1 or 2 will have been chosen and the procedures followed,so no damage could occur

 

[dichroic]

I have scanned this straight out of Guidance note 3. Have a read and it may help you to understand.

Regards

Dichroic

 

[whazza22]

Thanks all, i understand now

Just one question, why do you connect live and neutral together?

 

[dichroic]

By connecting live and neutral together the test current is passing between line and earth and neutral and earth. What it is not doing is passing between live and neutral so it is impossible to put 500v potential across a sensitive electronic circuit.

Regards

Dichroic

 

[deafboy]

If live and neutral are both connected to the same 500v flylead then there is no difference in potential ie no voltage across the load. If a fault is found using test method 2 you then have to use test method1 to determine the exact fault (250v if load still connected)

 

[whazza22]

Is this same for central heating system?

 

[ringer]

By connecting live and neutral together the test current is passing between line and earth and neutral and earth. What it is not doing is passing between live and neutral so it is impossible to put 500v potential across a sensitive electronic circuit.

Regards

Dichroic

So what if there is an insulation fault between live and neutral? Combining the two and testing them against earth is not going to show up such a fault. I always thought combining live and neutral was only to be done if testing separately will cause undue inconvenience or damage. Combining L+N might be quicker and easier, but it doesn't make it right in all circumstances.

footnote: I am not a fully qualified electrician and will stand to be corrected if wrong.

 

[Des 56]

Edited my post and managed to delete it, oh just as well then

 

[ringer]

Still don't see how this can prove phase insulation is good, if fault lies between phase and neutral but not between phase and earth.

 

[Des 56]

I searched for this reply ,that was given to a simillar question in the past on another forum
The contributor gave these reasons why he did not carry out live-neutral insulation testing on a pir

I have copied his post because it seems relevant to the discussion, and seems to make a lot of points worth thinking about




Quote
Some history - the materials used to construct cable insulation and conductor insulation generally used to be very susceptible to water ingress. This lowered their effectiveness as insulators and could result in complete failure due to tracking faults in the insulation.

In addition, the materials used had a definite life and would deteriorate over time.

Insulation resistance tests could provide early warning of these problems and, in some circumstances this might be followed by some remedial actions such as drying affected cables.

The cables were so bad that even changes in the weather could cause insulation resistance values to change (up or down). In fact, Megger used to supply a little result book that had a place to record the weather along with the IR readings.

Now in addition to this it was a matter of law (yes of statute - Electricity Supply Regulations) that an installation should not leak more than one ten thousandth part of the maximum current demand to earth (about 1 mA for a 100A supply).

The equipment attached to the installation did not generally have any deliberate connection between phase and earth (or neutral and earth) so any reading indicating a low resistance path would probably be a fault.

Now to today -
Well supply line filters put pay to the 'one ten thousandth part' bit and that had to be dropped.

Next we have modern cable and conductor insulation. This is far less likely to suffer from water ingress and, apart from that caused by heat damage, deterioration is rare.

So we have a different situation - the value of the tests in contributing to the assessment of the installation has been somewhat reduced, partly because the tests can be subject to false positives caused by filters and the like.

Now I am not suggesting that they are not necessary, but I am saying view them in light of how effective they now are.