Insulation resistance with vulnerable equipment unable to be removed

[Questforknowledge]

Good evening

Was wondering when testing for.insulation resistance and there is vulnerable equipment in circuit but unable to disconnect, I understand that you combine the line and neutral conductors and test to earth. I am just wondering if it's ok and safe to do this at 500v as there is no potential difference between the two I assume. Guidance i have read said you can do this and reduce the test voltage to 250v but If 500v is safe why would you lower the test voltage

Look forward to an answer
And t.g.i friday

 

[telectrix]

as you describe L short to N. test to E@ 250V, then if unsure test @ 500V.

 

[Questforknowledge]

as you describe L short to N. test to E@ 250V, then if unsure test @ 500V.

Did you mean 250v first then 500v?

 

[westward10]

Only time I use 250v is when equipment incorporates surge protection filters.

 

[Wilko]

Hi - yes do 250V first if there are concerns. It is less than the peak V seen 50 times per second so can’t damage anything.

 

[Lucien Nunes]

the peak V seen 50 times per second

Only if you have a 25Hz supply... on 50Hz it's 100 times per second.

I agree that 500V L+N > E is usually perfectly OK for normal equipment and like Westward only usually switch down to 250V where surge protection would conduct and result in an incorrect reading. Any device designed to operate on 230V mains (which, at its upper limit has a peak voltage of 357V), that would be damaged by a brief exposure to 500V DC to earth through a significant source resistance, is already suspect IMO.

In many appliances, the devices most subject to damage by excessive voltage to earth are suppression capacitors placed L-E and N-E at the inlet. These are usually class-Y self-healing types that are designed to fail O/C, unlilke conventional film/foil capacitors that typically fail S/C or leaky. If a transient voltage spike on the mains breaches their insulation, the affected area seals itself off, reducing the capacitance but averting an earth leakage or shock hazard. Within 230V equipment, the lowest suitable rating of class-Y capacitor is Y2, designed for operation up to 300V AC (at +10%, Vpk = 467V). Therefore 500V DC is not far outside normal rated operating conditions.

Testing L-N is a different matter and can cause all sorts of unpredictable effects as the IR tester is effectively trying to charge reservoir caps, break through controlled rectifiers etc. There's no point contemplating whether this would cause damage as the reading itself is of no use.

 

[Wilko]

Only if you have a 25Hz supply... on 50Hz it's 100 times per second

I know I shouldn’t ... But isn’t the test voltage DC so whatever polarity a 50 Hz AC wave will exceed it 50 times per second?

 

[littlespark]

If it’s vulnerable equipment, are you allowed to turn it off to perform an IR test?
And if it’s turned off, what’s stopping you disconnecting?.

most periodic tests are now done L+N connected. Too many electronics in circuit to be bothered disconnecting every smart switch, LED lamp and smoke detectors.

 

[Lucien Nunes]

a 50 Hz AC wave will exceed it 50 times per second

We can reasonably assume that positive peaks and negative peaks have the same effect on the insulation, so we are only interested in the magnitude, not the polarity, of the instantaneous voltage. And a 50Hz waveform has 100 peaks of magnitude sqrt(2) x Vrms per second.

 

[Questforknowledge]

Hi - yes do 250V first if there are concerns. It is less than the peak V seen 50 times per second so can’t damage anything.

Thank you for replying much appreciated