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What clearance distance do you need for "1500VDC" isolation?

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Advanced Member level 5
Jun 13, 2021
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A customer has sent us a spec for a full bridge SMPS.
300W, 24VIN, 32VOUT, 225KHZ,

They request "1500VDC isolation"

Thats all they say,
They dont say whether it needs to be basic, supplementary, or reinforced isolation.
They also dont say what is the time it needs to withstand 1500vdc
They also dont say what is the maximum leakage current that can flow during the hipot test duration.
They also dont say what pollution degree, but given its to go in a toally sealed metal aluminium box, then i assume pollution degree is 1?

Isolation specs always just seem to say "1500VDC" and no more, as if it everything should be obvious about it.

As such, we will make it 1.5mm isolation creepage/clearance distance on the PCB...then we cant go wrong?

I found this on the web...
Table 3....

The customer has specified only a test voltage, that's no full specification of the isolation requirements. Isolation requirements will be defined depending on working voltage and possible transients, also enviromental conditions. It's particularly important if the isolation barrier is exposed to a continuous AC or DC voltage. Up to now, we only got a low voltage specification.

For the specified DC test, you can assume a test procedure and pass criterion as given in IEC 61010 (similar in many other standards)
The d.c. test voltage is raised uniformly from 0 V to the specified value within 5 s and held at that value for at least 1 min.

No flashover of CLEARANCES or breakdown of solid insulation shall occur during the test.

1.5mm clearance will be surely sufficient to pass the test.
For 1500VDC I would use at least 3.5mm on the surface of a pcb. Granted this is a little higher than some specs - but better safe and not sorry and not sued.
--- Updated ---

1500VDC can easily flash over 1.5mm in humid conditions where there has been a long term fine dust build up ( over several years )

also standard pcb with solder resist is actually a really poor material to resist such arcing. The glossy surface is a magnet to moisture from humidity.
Isolation breakdown in presence of pollution and tracking affine surfaces is caused by continous voltage exposure and usually not triggered by 1 min voltage test. This is reflected by increased creepage requirements depending on pollution degree and tracking index. You can't conclude creepage distances from a test voltage spec. Clearance and test voltage are based on the combination of working voltage and transient overvoltage but don't care for pollution degree. In so far there's a roundabout relation between clearance and test voltage but not to creepage.

Thus you need to ask for complete isolation spec.
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