Reverse polarity and a microwave!!

[FAA Greenie]

Good afternoon Gents,

I was called out to a fault today where the guys microwave had been tripping the 32A CB for a few days and had now packed in altogether. It's a built in model and was connected via a switched FCU that had been wired up with L-N reversed.

I have advised the guy to contact the people who fitted the kitchen (only 7 or 8 months ago) to come and take a look at it. I have isolated the FCU etc.

So the question is, would modern microwaves have built in protection agains this or would the fault just slowly damage the unit until it failed?

 

[grantr37]

A microwave should not be wired in reverse, this may well damage the appliance......the tripping of the circuit breaker after 20 seconds suggests to me that after switching on, the Large capacitor (2.1KV 0.7uF or thereabouts) on the primary stage of the High voltage DC supply circuit (which powers the Magnetron unit) is saturating in reverse and being discharged to ground (drained via the High Voltage Diode connected to the microwave Case/earth) as opposed to the way in which it normally drains/discharges (across a high watt loading resistor to dissipate charge/energy) which will cause your tripping issue.....correcting the wiring should fix this problem, although the damage might have already been done to the frequency generator stage of the circuit board in the microwave and the low voltage part as well...
There are no real SELV (separated/safety Extra Low Voltage) parts inside most microwaves, there is a "HOT" section on the PCB (Printed Circuit Board) and a Low voltage section (with a small transformer) on the same PCB separated by the air gap only (removed copper cladding/copper plate that has been etched/dissolved away from the main sheet of the circuit board at the factory) possibly with a thin floppy plastic film cover clipped onto the track/solder (back) side of the circuit board to prevent shorting from foreign objects…
There is a Neutral Rail on this circuit board which will have been reversed/swapped to become Live,there could be track lifting or dry solder joints, but most likely not...
If the Microwave is completely dead ( no lights) then you could open it and check the fuse in the fuse holder on the PCB (beside the small transformer) which will be about 250mA, it might have blown, causing a dead control board…. other than that the other components on the circuit board will be too much hassle to replace/rework…
If you open the Microwave, avoid touching the Capacitor in any way as it may still hold a big amount of charge that might not have been removed, the resistors for that have been known to fail before with age or overload…

DO NOT power on the microwave with the case open, I would not rely on the wave guide to direct the microwave radiation properly so you could be exposed if the machine is powered on when dismantled…and that would be very dangerous…

Check the fuse on the control PCB…..if it has blown it can be replaced, if not then the PCB is fried (components such as the PLC/micro controller chip that runs the machine and small capacitors will be dead) replacing the chip would require one with an identical PLC program burned into it, which you won’t have…

 

[grantr37]

Most of the Weight of a Microwave oven is the High voltage (step up)Transformer that powers the High Voltage DC Magnetron via a Bridge Rectifier....that weighs about 10KG on its own...

 

[grantr37]

almost forgot to add, in addition to the danger from microwave radiation, that big chunky Transformer in there has a secondary output of anything up to 4KV, its dangerous just like the transformer in the back of an old style cathode ray tube TV.....big bangbang bobo banga bang ....


I would have added this earlier if my browser hadn't frozen...

 

[tbs1983]

On the reverse polarity subject ive noticed some manufacturers have the L-N one way round on a twin skt and the opposite on a single socket. No excuses for sparky tho should have checked first.

 

[i=p/u]

sure mine a we russel hobs starts when you open the door never mind closing it.... and cant find rceipt , bummer

delonghi even

 

[FAA Greenie]

Cheers Grant, i did try the unit again with the correct polarity with no luck, though i didn't open the back up, the customer had bought a new one when i returned. Well worth bearing in mind should i come across another failed unit though.

Thanks,

Chris

 

[Engineer54]

Have the homeowners put a claim in to the DNO, ...It was they're problem/fault that caused the W/Machine and Micro to fry wasn't it??

 

[FAA Greenie]

I did ask if they had, they were told that it was up to them to claim on their house insurance if they wanted to. Sounds like a cop out to me but i have to confess ignorance on these matters....

 

[Engineer54]

Well, they should at least contact their insurance company and give them the fact's, they could well prise the money out of them. Insurance companies are always doing deals between each other, as to liability and whatever!!

 

[rmdes]

Reverse polarity can damage some equipment that use sensitive electronics, not sure about micros though, I would get the dodgy sparks back and make them pay for it anyway for a ****e installation