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