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Posted By Topic: Insulation resistance test - Sensitive equipment

Paul
Apr 02 2013 22:32

When carrying out an insulation resistance test on circuits with sensitive equipment (dimmers etc), is it the 500V DC that causes the damage or the small current?

Thanks,
Paul
   

Apprentice
Apr 03 2013 00:12

It's the voltage. The current at 500v is less than the normal operating current of the device.

V=IR, so the higher the voltage the lesser the current, and the lesser the voltage the higher the current.
   

Just another Sparky
Apr 03 2013 04:00

I would say it's both one causes the other i.e Voltage causes current, in the case of dimmers I'm guessing they're full of capacitors which are two plates separated by a thin dielectric, Put enough voltage across them and current will arc across the plates possibly damaging the insulated layer and bye bye dimmer.

Apprentice you may want to revise ohms law that explanation does not seem correct perhaps you're thinking more along the lines of Power P=VI for a constant output. My poly tutor years ago said to me I couldn't become an electrician if I didn't know Ohms law LoL.

Sorry in advance for using any social media language on this forum :-)



   

Paul
Apr 03 2013 08:23

Thanks for the replies.

I guess my real question is, is it safe to carry out an IR test between L - E & N - E with dimmers, electronics etc connected? I can understand why you can't test between L - N (potential difference of 500v across the sensitive load) but when testing between L - E say, the circuit is incomplete and no current will flow (providing there are no faults). However, the 500v will be applied. Will this voltage with zero current cause any damage?

Thanks
   

Ron Proffit
Apr 03 2013 09:41

Paul,
My suggestion is that if you use a clip large enough to go across both the neutral & earth pins of the plug, you have shorted the sensitive equipment and can put the 500v across all live conductors to earth.
Ron P
   

Apprentice
Apr 03 2013 19:28

Just another sparky.

Yea you're right I muddled up my formulas.

P=VI is the one I meant where the current and voltage are inversely proportional. So my answers whole reasoning is kind of screwed now. That's why I'm Apprentice and not Sparky yet.
   

Bob
Apr 03 2013 21:22

its the current flow at high voltage
bridge out P & N when testing to earth

never had a problem with it

allowed to use 250 volts test on equipment with sensitve electronics and earth leakage test is another alternative
   

Just another Sparky
Apr 05 2013 01:55

Yes testing across all live conductors to earth is what I have been doing with sensitive loads and have never had a problem.

Apprentice I was not trying to discredit you in any way, making mistakes is all part of learning. Good on you for taking interest in the fundamental principals.


   

RobertNZ
Apr 05 2013 06:50

With Phase and Neutral shorted together, I will then test P&N to Earth. Removes the risk of releasing the smoke.
   

pluto
Apr 06 2013 00:18

What did they teach you during your apprenticeship?

Capacitors can be damaged by 500 Volt d.c. test voltage, but the most likely cause is damaging the semiconductors (Thyristors,SCR's or Triacs) in the dimmer control element.

Just look at the semiconductor data sheets the maximum voltage, in the non-conducting mode, they withstand is likely to be less than 500 Volts d.c. in a cheap dimmer.
   

Just another Sparky
Apr 06 2013 05:10

Didn't learn a lot on electronics in my apprenticeship mainly regs (8 years ago touch wood) so must admit I'm not to sharp on it but you're right if modern dimmers are built using semiconductor devices then these can be the first to fail. Seem to be replacing alot of capacitors when working at the Poly, Students seem to find a way of damaging them.


   

Just another Sparky
Apr 06 2013 05:10

Didn't learn a lot on electronics in my apprenticeship mainly regs (8 years ago touch wood) so must admit I'm not to sharp on it but you're right if modern dimmers are built using semiconductor devices then these can be the first to fail. Seem to be replacing a lot of capacitors when working at the Poly, Students seem to find a way of damaging them.