Delonghi Oil storage heater

I have a heater which has been unused for a while. it is now tripping the circuit breakers. I have PAT tested it at work and it fails, unless the side panel is removed.

When removed it passes the test and the oil begins to heat up.

With continuous monitoring of the PAT tester i replace the side panel and it again fails the test. Looks like a grounding issue when the two sides are touching.

Should there be a grounding strap between the side panel and the main unit? Why would this now fail having sat for a period?

In what way does it fail the PAT test?  I assume insulation resistance?  (it would not trip a circuit due to a poor earth connection)

Sounds to me like a wire is getting pinched, or a screw is piercing a cable as you tighten it etc.

The side panel is not going down with any force or being screwed down.

When the guiding lugs are put into the relevant position the pat test fails again (will get more info on that for you tomorrow). These lugs and the holes have the paint worn, so it's is metal to metal.

Unsure how to add a photo on this forum.

Have you insulation tested between lugs and live, lugs and neutral?