Patting a microwave oven

[mark641]

I bought a leakage detector which says "must not exceed 5 mw/sq cm "
Please: how far away should you be holding the meter?

One oven failed. The PAT meter says "Riso > 20 Mohm Ileak = 0.84 ma "
How is this possible? It seems to violate Ohm's Law. (This is just a Q
out of interest, I will in any case condemn the oven).

 

[Lucien Nunes]

The leakage is predominantly capacitive. The insulation resistance may be perfect as indicated by the DC insulation test, yet the capacitance from line to earth of any interference filters etc will leak a current approximately 230/2.pi.f.C when powered from AC mains (or proportionately less when subjected to a substitute leakage test).

Why will you condemn the oven? What is wrong with it?

 

[static zap]

Checking visually for paint damage , on black door face and flat cabinet surface (and mechanical state of door hinge) ..will usually give clues .. Why leaks happen.
-- If calibration of detector is to be trusted !

 

[EricMark]

I remember ovens failing on a random basis, and re-testing they passed, turned out to be mobile phone, you need to turn off mobile phone before testing any ovens.

 

[mark641]

Thank you very much who replied. Quotes are cut:

Why will you condemn the oven?

Because the meter says "fail" & I dont have the foggiest idea how to repair the oven.

Checking visually ...
-- calibration of detector ...

I always do inspect, the regulations are perfectly clear on this. I have sometimes found
visible faults on something with a green sticker. Meter was calibrated in July. I have no
reason to suspect that they did it wrong.

... mobile phone...

Weird!!! It almost makes me believe the fairy-tale that you must not use a phone
during take-off or landing as it might disrupt the aircraft's equipment. But I will try...

 

[DPG]

Thank you very much who replied. Quotes are cut:



Because the meter says "fail" & I dont have the foggiest idea how to repair the oven.



I always do inspect, the regulations are perfectly clear on this. I have sometimes found
visible faults on something with a green sticker. Meter was calibrated in July. I have no
reason to suspect that they did it wrong.



Weird!!! It almost makes me believe the fairy-tale that you must not use a phone
during take-off or landing as it might disrupt the aircraft's equipment. But I will try...

Not weird really - mobile phones and microwave ovens use similar frequency waves.

 

[Lucien Nunes]

Because the meter says "fail" & I dont have the foggiest idea how to repair the oven.

But it also said 0.84mA, which is a pass for class I non-handheld. What is your pass threshold set to and why?

 

[static zap]

Does leakage meter have instructions with it , about always operating microwaves with a "Suitable load" .
... Ie 125ml of cold water ( replaced cool, to keep from boiling).
Higher readings will occur if microwave is used improperly !
( Drying something till it has no water -can produce carbon char ! )
( A few sources say , modern magnetrons can take this -

but I always suspect it's sales talk ,​

that may damage "lesser models" or need a bulb change after !)​

 

[davesparks]

Because the meter says "fail" & I dont have the foggiest idea how to repair the oven.

The meter is only saying fail because the result is outside of the limits you have set it up with, and that limit which you have set appears to be incorrect.

 

[static zap]

microwaves with a "Suitable load"

apologies totally on wrong wave-length with other reply..
Not obeying ohms law -
Instead of thinking exotic semi-conducting goo , just think
capacitor as it charges up with DC so actually is more like trying to test a battery for that charges then starts discharging .. readings all over the shop !
(revisited it will now have some residual charge !)

 

[mark641]

Thank you for further replies. The meter's pass/fail levels are below: they were set not
by me but at the factory; and there is nothing in the manual to say I can change them.
[automerge]1571954586[/automerge]
Apologies, I sent an incomplete message:

Class I / II /cord
Earth 0.2ohm / --- / 0.2ohm*
Riso 1Mohm / 2Mohm / 2Mohm
Ileak 0.75mA / 0.25mA/ ---

* with allowance for long cord. Sorry also that my attempt at a table is a mess.

When leak-testing the oven the manual did say to put in a bowl of water.

 

[ronsimpson]

I thought the 1l of water was for power testing.

The manufacturers Sharp, Samsung and Panasonic all quote 275ml, which I suspect is related to the IEC705 standard.

BS EN 60335-2-25 specifies that the maximum permitted
leakage with a load of 275 ml is 50 W/m2
(equivalent to 5mW/cm2 ) at a distance of 5 cm from the oven.

In practice, modern microwaves just don't leak and any leakage ought to be investigated. Except old Litton / Menumaster machines whose doors rot away and seem inherently leaky anyway