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?

 

[telectrix]

can't see reverse polarity damaging the micro.

 

[Silly Sausage]

What about the fuse in the FCU?

 

[markc123]

Did it trip whilst he was using the microwave? Or was it just tripping randomly? if it's been tripping whilst hes in the middle of heating something up, that can't of done any good to the elements inside

 

[SirKit Breaker]

Most appliances are no longer polarity sensitive, if i recall last time i opened a microwave up, the 230v just goes to a transformer, and all the innards are SELV or whatever. My main concern would be that with reverse polarity at the outlet, then the microwave plug top fuse is effectively in the neutral. This recent install was obviously very well inspected and tested prior to being put into service NOT.

Cheers............Howard

 

[telectrix]

reverse polarity apart, i suspect the microwave itself to be at fault. but obviously, if the kitchen fitters are to be called back, i wouldn't correct the polarity issue till they,ve seen it and had their noses rubbed in it. good style.

 

[FAA Greenie]

Hi Guys, the fuse is fine and there is power to the microwave lead, i corrected the polarity via a junction box at the back with still no operation of the microwave. I'm happy that the unit is goosed but was just wonering if the wiring error would have caused the fault. The CB was tripping about 20 seconds after the microwave was switched on. I have left the wiring as i found it.

Also when i asked if he had recieved a certificate and a notification after the install he just looked at me blankly!!

 

[markc123]

Was the reverse polarity not just at the FCU? :s why a junction box :s

 

[Silly Sausage]

Am I being thick or missing something here...
According to the MCB curve, it would trip @ 20secs with a current of about 120A, and the (??A) fuse is OK?

 

[Wonderwall]

I once went to a domestic property due to electrics playing up upon inspection I found reverse polarity at the main db. It corrected itself once the microwave was unplugged. So perhaps this is the same problem? And not the kitchen fitters at fault

 

[gnselectricals]

I once went to a domestic property due to electrics playing up upon inspection I found reverse polarity at the main db. It corrected itself once the microwave was unplugged. So perhaps this is the same problem? And not the kitchen fitters at fault

That sounds like a tt with a L-E fault and a lack of/ faulty RCD to me.

 

[FAA Greenie]

No idea at all why the 13a fuse stayed in tact, perhaps it's just spiking something. I inspected the rear of the FCU, definately L-N in the wrong holes, clear as day. All other sockets in the ring main test fine for polarity, the system is a TNC-S. The ring is complete for all LNE and i did a LN-E insulation test which was absolutely fine.

 

[Wonderwall]

That sounds like a tt with a L-E fault and a lack of/ faulty RCD to me.

No. It was a pme supply

 

[FAA Greenie]

Hi Guys, just thought i would put this one to bed. I went back last week to correct the faults, turns out that they didn't want the original guys back because of 'other issues'. However, turns out, having spoken to the lady of the the house that a week prior they had had some sort of surge that had also knocked out the washing mashine. The DNO had come out and fixed something or other down at the sub station.

I would put good money on this damaging the microwave at the same time. Though i still have no idea why the 13a fuse in the FCU stayed in tact whilst the 32a CB tripped instead....

 

[Des 56]

Problem No 1
External suppliers open circuit neutralt caused 400 volt to be applied to appliances,knacked washer resulting and perhaps caused a Neutral earth fault in microwave

With an appliance having reverse polarity and appliance fuse in line conductor,breaker will go and not affect the plug top fuse

 

[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