PAT test fail microscope

[RaceDiagnostics]

This is an unusual one, we got 5 microscopes into the workshop having failed the insulation resistance test when being PAT tested.

Our PAT tester in the lab is out of this week for calibration so had a look with a multimeter.

Now there is not much in the microscope, just a transformer and bulb. So what could go wrong.

First test was checking resistance across line and ground and neutral and ground. Both were showing ~ 8 mega ohms, but with the leads switched round they were showing a negative resistance, eh! So switched to voltage and found up to half a volt across the terminals.

That makes no sense as there is no power source in the microscope. Next I opened it up for a closer look and desoldered the iec socket.

Have a look.

Something very weird about that socket, looks like it could power a perpetual motion machine.

 

[reb78]

I wonder if there are rotating magnets in there. Perhaps it's a Curious Device.

 

[MetalMonkey]

I was wondering if there was a capacitor or mov or similar attached to the socket - however the part number does look to be just a straight iec socket...

 

[cheb]

I'd be checking the battery in the meter.

 

[jsf55]

Have you got an analogue machine ? it might be a phantom reading

 

[OJCar]

Have you got an analogue machine ? it might be a phantom reading

This!

 

[RaceDiagnostics]

Tried two different fluke meters with the same result. I'm calling it a miracle.

 

[ajlelectronics]

Tried two different fluke meters with the same result. I'm calling it a miracle.

High impedance meter picking up stray RF / magnetic fields from nearby equipment; Carbonised traces on the surface from heating or arcing causing an electrolytic reaction?

 

[RaceDiagnostics]

It's not the former as a new socket reads zero as expected sitting next to it.

The microscope is a 20 W load so unlikely to have ever overheated, I've cleaned the socket meticulously and it still does the same.

I've got 5 of them all doing the same thing, very weird age related deterioration if the plastic somehow interacting with the terminals.

Also just got my PAT tester back and its reading ~ 0.9mohm at 500v.

 

[Hitch]

Do you have a 250v /low power setting on your pat tester?

 

[Brad93]

You need a multimeter with LoZ

 

[Morrisman]

Internal corrosion, dissimilar metals causing a battery effect.

Or more likely just a miracle.

 

[Hotstuff]

Although 0.9 MOhm is a pass reading for a class 1 appliance, but you are wise to investigate further.

The appliance and IEC lead are usually tested as two separate appliances; the reason being that the lead could be unplugged and used elsewhere.

I expect you’ve already tried this but try testing just the IEC lead using your appliance tester, then plug in the disconnected male IEC socket onto the lead.

 

[RaceDiagnostics]

Yes, it's a marginal pass at 250v.

 

[Guineafowl]

Looks like it’s turned into a leaky capacitor - +R one way, -R the other, DC voltage present, low IR.

I can see that base getting pretty hot, especially if the bulb is left on overnight. I’d go for carbon tracking, maybe in small internal cracks due to age/heat/voltage spikes.

Some bugs like Pseudomonas will form a biofilm (hard to clean off) that might be partially conductive or act like capacitor plates. What sort of lab? Wash your hands!