Hi,
we have a TOP255GN flyback SMPS design for 100-265vac and 24vout, 24wout.

It is failing our 4kv/2s hipot test.
This is the hipot tester that we use...
https://national-instrument-hire.co.uk/megger-ft4-flash-tester

The flyback is totally surrounded in an earthed metal enclosure.
Many of the components are near to the enclosure walls.

Also, there's an insulated earth wire that runs from earth terminal on the mains connector, and touches mains L and N solder pads, and then "worms" on and touches Vout pos and neg pads....i just wonder if this approx 3mm thick earth wire is flashing through its insulation?

So we are looking for the sparks, and have it on 4kv rms (50Hz) whilst we do this...i just wonder at what point we have to stop because the 4kV will surely eventually damage many components.?

We have an X2 cap right next to a metal earth enclosure wall.....and that was flashing through to this enclosure (we saw the sparks)....and then the earthed enclosure also goes round to the secondary side, where there are 35V electro caps up against it....so maybe there is a multi-path flashover happening here?

We put tape over the wall near the x2 cap, and we still hear fizzling. The trouble is we have to part-disassemble it to look for sparks (so we can see inside), and then its not the real situation.

We also have a 24V secondary side via just 0.7mm away from the earthed enclosure wall, where the wall comes down to the PCB.
The primary HVDC bus electro cap has its circumference coming to within 1.5mm of the earthed enclosure.

We also have a Y2 cap from primary HVDC+ to Earth.
We also have a Y1 cap across the flyback transformer (HVDC+ to secondary ground)

We also have a VOM617A-4T opto which is only rated 3.75kv rms which does secondary to primary feedback. (and we are testing to 4kV rms)
The contractor just says that "it will be OK"....its not that far off in voltage 3.75kv to 4kv.

We are testing from L+N to output at 4kv rms.
We are testing from L+N to earth with 2kV rms.

Sometimes we see flashover but it doesnt fail on the hipot tester failure indicator..but sometimes it does fail.

The flyback transformer is rated 4kV/5mA/2Seconds....but we have been slamming it with continuous 4kV for long periods as we look for the sparks...do you believe now that our transformer is now totally ruined in terms of its isolation capability?
If a flyback transformer is rated for 4kV/5mA/2 seconds, then what happens to it if it is exposed to 4kV for 60 seconds?

Its a flyback that got crammed into an existent enclosure.

Also, if a product fizzles, and even sparks on a 4kv rms hipot test, but does not trip the "fail" inducator, and the discharge current meter doesnt visibly go up, then is that a fail or pass?
...When we get fizzling, (but no failure indication) whats then happening is that we just repeatedly do  4kv rms application to the product, to see if it eventually fails or not....but then surely any product is eventually going to fail if 4kV is repeatedly applied?