[chbot] Insulation Tester

[Mark Atherton]

Hi Richard,

I built an HV insulation tester many years ago which plugs into a 
VARIAC. The output network is such that output current is limited to 
50uA at 5kVDC so can test reverse breakdown of such rectifiers as 
1N4007s without damaging them. The only thing that might kill you is if 
you left the unit charging up an HV cap over a few minutes, then 
discharged the cap across your heart.

The modern insulation used on enamelled copper wire (even the thin 
stuff) seem to tolerate 5kV OK. I have also tried the lowest voltage 
cable I could find (CAT5) and strapped a twisted pair across the unit at 
full smoke without incident.

For appliance testing, the HV test methods you outline seem a good 
start. Add an inspection step to check integrity of double-insulated 
wiring, and that the earth wire can carry 10A with less than 1V drop and 
I think you may be getting pretty close.

If you are not interested in appliance testing, then what are you trying 
to do ?

https://electronics.stackexchange.com/questions/448368/barrier-and-slot-within-reduced-area-on-a-pcb 
looks interesting

And, yes, I think moisture and dust are high up there on causes for HV 
breakdown. Nothing like a nice damp pile of dust on a PCB to ruin your 
day. And of course HV can electrostatically attract dust; good reason to 
put HV electronics in an dust sealed box.

The world starts to get a bit odd at 1500VDC (the onset of EHT), but I 
suspect you may only be interested in good-old-mains.

Many people on this list are way more knowledgable than me on this 
fascinating topic, so I hope one will pop up and correct all of my errors.

Regards,

Mark



On 24/01/2021 9:49 PM, Richard Jones wrote:
> Here is a link to the insulation tester that I bought for $38.61US: 
> https://www.aliexpress.com/item/32888285436.html  The goal being not to 
> repeat destruction of my test setup by faulty PSU construction.
> 
> For a PSU / Wall Wart test I short live and neutral together and perform 
> a 1000V test between Live+Neutral and Earth (if there is one) and 
> between Live+Neutral and 0v output.
> 
> So far I've not detected any leakage on everything that I've tested, and 
> I've not blown up any electronics either.
> 
> When I measured the tester output voltage it came up short of 1000V, 
> which I put down to the input resistance of my voltmeter in series with 
> the unspecified output resistance of the tester.
> 
> I could bring the tester along to the next meeting if anyone is 
> interested, while keeping in mind that it is a potentially dangerous 
> device (pun intended :-)
> 
> I'm no expert on insulation testing other than having designed Telco 
> line interfaces years ago, so any comments on the test approach most 
> welcome.
> 
> For telco line interfaces we used to consider 5mm PCB gaps between 
> copper tracks adequate insulation. These days I find that JLCPCB are 
> quite happy to route air gaps between mains and low voltage connections 
> for no extra charge. So I design 1.5mm air gaps as well on a pcb routing 
> layer. I view this as a much safer approach to give reasonable isolation 
> even when things get damp and/or dirty.
> 
> Richard Jones