microwave oven broken

[Tim W]

Stephen wibbled on Friday 30 October 2009 15:34

2-5kV stored in a big capacitor with enough energy to kill you... It's a manageble risk, if you know it's there.

[Stephen]

I think another reply said that Panasonic ones were dearer than others. Whether there is a good reason or whether I am paying for the name I don't know. Each repairman wants to look at the machine and charge for their time doing so, so it is impossible to shop around for the best quote without paying a fortune in fees.

[Peter Parry]

No he is not correct, what he is exhibiting is the ignorance borne of slavishly and unthinking following "elfinsafety" and an inability to carry out a risk assessment.

They do, extraordinarily infrequently. Unstressed resistors are the most reliable component after a length of wire.

Using the MIL-HDBK-217F model for a composition resistor at its rated power (drain resistors actually operate below this) you are going to see a failure rate of about 200 failures per 10^9 hours operation. That is as close to zero as makes no difference.

For someone looking at a microwave once or twice in a lifetime the chances of encountering a failed drain resistor on an oven which powers up are minimal, far less than the risk of being killed driving to buy another one. Leaving it off for 24 hours will in any case allow the capacitor to self discharge through internal leakage and other components.

[Stephen]

On Fri, 30 Oct 2009 22:26:48 +0000, The Natural Philosopher wrote: [Bob Minchin wrote:]

I'm not Drivel. I am young and in my first house and new to DIY and hence I need, and ask, for a lot of help. I'm flattered that Bob has looked all my posts up on google. I'm sorry if Bob thinks I ask a lot of silly questions but how can I learn if I don't ask? I'm sure once upon a time Bob was a learner too. If there weren't any questions, this group would not exist. I did make very clear that I had no intention of removing the cover but it's nice to know what is underneath so that I can recognise when a repairman is talking nonsense and trying to rip me off. Sorry if I am being oversensitive!

[NT]

If I relied on this dense advice I'd be long dead.

1. discharge resistors do fail
2. components without such resistors get substituted during mfr now and then
3. ditto during repair
4. there are at times individuals involved in all stages of manufature and repair who couldnt care less about safety resistors

NT

[NT]

Dump operators dont charge any more for panasonics. If your repair people want to charge you for a quote AND charge you =A370 for a magnetron, you're looking in the wrong place. The old model of the local repair shop that always fits new parts is just not a workable business model any more. Try someone that works from home or a small workshop and routinely fits used parts. Sometimes you'l find shops that work like this in relatively run down areas. Get a free quote or walk.

NT

[Andy Dingley]

I had one of those fail when I was a student. It was impractical to have the drain resistor permanently connected, so it was on a spring contact that only engaged when the big power went off (and there were case & room interlocks too).

However the resistor, a big wirewound, failed by overheating one day, just from the energy stored in the capacitor bank. _Big_ capacitor.

[Peter Parry]

Indeed, every few hundred years.

Do they? Can you name a manufacturer who does this?

As it is rather difficult to buy such a capacitor without an integral drain resistor that seems somewhat improbable.

That's why they are built into the capacitor casing rather than fitted externally.

As for your rather hysterical claim that "If I relied on this dense advice I'd be long dead." the number of people killed by domestic microwave ovens (in either use or repair) in the UK since 1980 is, according to the HSE, zero.

[NT]

no mfr does it on an ongoing basis. Substandard batches of goods are hardly newsworthy though.

Its not hysterical, its a statement of fact.

I didnt know they collected stats on it

NT

[Peter Parry]

No, merely uninformed opinion.

What do you think you pay them for?

[NT]

Since I've had nice sparks from such things despite them sitting longer than your 1 day, how can this be 'opinion'?

Your notion that resistors almost never fail is naive. In real life they're subject to conditions that do cause failures at times, they're not kept within all the conditions of the data sheet tests. Vibration, voltage overloads, physical shock, mechanical stress, these thing happen irl, and failures follow.

That doesnt even address the question of how they collect stats on microwave deaths, and whether it covers all cases.

NT

[Peter Parry]

I haven't come across too many domestic microwave ovens operating in hostile environments (unless of course you count children as a hostile environment) but that is why MIL-HDBK-217F is so much more useful than data sheets as it models such excursions from the norm. Using the ground mobile model - the microwave in a cross country vehicle used cross country - the reliability does, as you say, drop. The MTBF goes down to merely 4,000,000 hours

They gather information on all industrial and commercial accidents. Until about two years ago they were also funded to collect domestic accident information from fire, hospital and ambulance service reports. While it is always possible for things to slip through it remains a fact that screwdrivers have killed more people in accidents than microwave ovens.

[NT]

Do you think all chinese manufacturers adhere to such standards at all times?

More importantly, we have 65 million people in the UK and IIRC around

20 million households. So say around 20 million domestic microwaves. Each one of those is plugged in 8760 hours per year. Now, how many discharge resistor failures would you expect to see per year in the UK in domestic microwaves using your figures?

1. Could that be because so few people are unwise eough to repair them without discharging the cap
2. This kind of foolishness would be found outside of insustry rather than in it.
3. Amulance and fire brigade aren't always called for a dead person.

NT

[Peter Parry]

It isn't a manufacturing standard but a method of predicting failure.

Now I know for sure you don't understand probability and risk. Almost every week in the UK someone wins a multi-million pound prize in the lottery. Almost every week in the UK more than one person dies, often unexpectedly, while going to buy a ticket to enable them to win that prize.

Should we ban the National Lottery on the grounds it kills so many people?

Assume your figure of 20million microwaves is correct. The hours on figure of 8760 is completely irrelevant as the capacitor is only energised when the oven is cooking, a averaged figure of about 100 hours is probably more realistic but let's be real pessimists and say each and every oven operates for 1 hour a day so about 400 hours per year. A total of 8,000,000,000 per year

In the UK it is quite likely a dozen or so microwave ovens have faulty drain resistors. Let's be real pessimist though and assume 100. That gives a one in 200,000 chance of your oven having this particular fault.

Let's further make the extraordinarily improbable assumption that working on such an oven will invariably be fatal so that 1 in 200,000 is your chance of death.

Compare that with the chances of you dying in a road accident of about

1 in 17,000 or the chances of death in pregnancy of about 1 in 8,200. I wonder how many husbands mention to their wives that pregnancy is nearly 25 times more lethal than repairing a microwave oven without precautions?

The reason the death rate from microwave oven repairs is zero is because the chances of one happening are very remote.

Nope.

You obviously have little experience of industry.

I'm not aware of any instance where people lying dead on the floor by the side of a smoking microwave have simply been placed in a relatives car and delivered to the undertaker for burial but any sudden and unexpected death requires a coroners inquest. The results of inquests also went to the HSE.

[Owain]

Probably not many as even though firtling about inside an electrical appliance is interesting and enjoyable, most men find trying to make babies even more so.

Owain

[NT]

every such assessment is based on certain specs or characteristics of the component under assessment. Why you assume all manufactured leak resistors conform to it I dont know. It does seem rather optimistic.

8 billion over 4 million =3D 2,000. I forget the other figure you had, but it still assumes all drain Rs are to spec, which for chinese mfred goods is a bit optimistic.

but youre still making assumptions I simply dont believe are valid. And the conclusions you reach just dont tally with real life experience.

sigh

And I'm sure that all polish, african etc workers here follow all laws to the letter. And involve the authorities when its the last thing they need.

You're trying to asses this, but just making too many assumptions.

And even if we went with your figure, its still dumb to not put a screwdriver across the terminals first. 4 seconds of trivial action to avoid the risk of death.

NT