death by computer

[buzz]

How is the interlock on a typical microwave oven implemented?

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Just curious.

 

[Sal Brisindi]

Buzz,
If I were you I would get rid of that microwave oven, not worth the risk.

Sal Brisindi
www.tuberadios.com

buzz wrote:

How is the interlock on a typical microwave oven implemented?

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Just curious.

 

[Winston]

Sal Brisindi wrote:

Buzz,
If I were you I would get rid of that microwave oven, not worth the risk.

Sal Brisindi
www.tuberadios.com

buzz wrote:


How is the interlock on a typical microwave oven implemented?

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Just curious.

Thanks for posting that, Buzz. Safety stuff is important.

Sal makes a good point, but forgot to mention the simple adjustments to
make to the oven's software, first.

1) Unplug the power cord.
2) Snip the cord off where it exits the oven.
3) Place the oven on a tarp in the back yard.
4) Don your face shield, earplugs and gloves.
5) Carefully remove the door. (You may use an angle grinder or torch)
6) Cut the door into two pieces of approximately equal mass.
(You may use a fire axe.)
7) Gently tap the exterior in several places using a long handled five
pound maul. (Overhead swings, with as much force as you can muster.)
You may stop tapping when the cubic area occupied by the oven is reduced
to 70% of its original value.
8) Retrieve the various pieces of oven that attempt to exit the area.
If you have tapped the exterior properly, exterior parts will be found
as much as twenty feet away.
9) Toss one of the door pieces in the trash, along with the power cord.
10) Over the next few weeks, place two random pieces of oven per week into
the trash until it is all gone.

This will put the proper values into those registers and cause the
oven to stop endangering you and your visitors.


--Winston

 

[Ken Taylor]

"Winston" <Winston@BigBrother.net> wrote in message
news:8dycncjz7ffCEQvd3czS-w@speakeasy.net...

Sal Brisindi wrote:
Buzz,
If I were you I would get rid of that microwave oven, not worth the
risk.

Sal Brisindi
www.tuberadios.com

buzz wrote:


How is the interlock on a typical microwave oven implemented?

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Just curious.


Thanks for posting that, Buzz. Safety stuff is important.

Sal makes a good point, but forgot to mention the simple adjustments to
make to the oven's software, first.

1) Unplug the power cord.
2) Snip the cord off where it exits the oven.
3) Place the oven on a tarp in the back yard.
4) Don your face shield, earplugs and gloves.
5) Carefully remove the door. (You may use an angle grinder or torch)
6) Cut the door into two pieces of approximately equal mass.
(You may use a fire axe.)
7) Gently tap the exterior in several places using a long handled five
pound maul. (Overhead swings, with as much force as you can muster.)
You may stop tapping when the cubic area occupied by the oven is
reduced
to 70% of its original value.
8) Retrieve the various pieces of oven that attempt to exit the area.
If you have tapped the exterior properly, exterior parts will be found
as much as twenty feet away.
9) Toss one of the door pieces in the trash, along with the power cord.
10) Over the next few weeks, place two random pieces of oven per week into
the trash until it is all gone.

This will put the proper values into those registers and cause the
oven to stop endangering you and your visitors.


--Winston

While both responses are correct, the manufacturer *needs* to know that this

can happen. That is **really** bad design. It should indeed be a dumb-ass
interlock so that this can't happen. I'm appalled - what brand is it? I
don't ever want to see one in my vicinity!

Ken

 

[Dan Mills]

Ken Taylor wrote:

<Of microwave oven with (buggy) software controlled door interlock>.

While both responses are correct, the manufacturer *needs* to know that
this can happen. That is **really** bad design. It should indeed be a
dumb-ass interlock so that this can't happen. I'm appalled - what brand is
it? I don't ever want to see one in my vicinity!

I thought the standard way to do this was with 2 microswitches, one to cut
the power when the door was opened and the other to short circuit the
primary of the transformer under the same conditions (obviously mounted to
ensure the correct sequence of operation).....

Why the hell didn't whoever made the silly thing do what every other
microwave manufacturer has been doing for years (I know, a small signal
switch and use the same power switching device used for the duty cycle
control is cheaper!).

BTW: the correct fix for this unit surely involves thermite?

Regards, Dan.
--
And on the evening of the first day, the lord said.... LX1, Go!
And there was light.
The email address *IS* valid, do not remove the spamblock.

 

[Ken Taylor]

"Dan Mills" <dmills@spamblock.demon.co.uk> wrote in message
news:c76am3$dsn$1$8302bc10@news.demon.co.uk...

Ken Taylor wrote:

Of microwave oven with (buggy) software controlled door interlock>.

While both responses are correct, the manufacturer *needs* to know that
this can happen. That is **really** bad design. It should indeed be a
dumb-ass interlock so that this can't happen. I'm appalled - what brand
is
it? I don't ever want to see one in my vicinity!

I thought the standard way to do this was with 2 microswitches, one to cut
the power when the door was opened and the other to short circuit the
primary of the transformer under the same conditions (obviously mounted to
ensure the correct sequence of operation).....

Why the hell didn't whoever made the silly thing do what every other
microwave manufacturer has been doing for years (I know, a small signal
switch and use the same power switching device used for the duty cycle
control is cheaper!).

BTW: the correct fix for this unit surely involves thermite?

Regards, Dan.
--
Agreed, but that only solves the immediate problem. What about all the other

units out there?

Ken

 

[John Crighton]

On Mon, 03 May 2004 09:05:05 -0400, buzz <buzz@tranquility.moon>
wrote:

How is the interlock on a typical microwave oven implemented?

Usually, there are two micro switches behind the door that disconnect
the supply to the big power transformer when the door is opened.

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Just curious.


I am curious too, why don't you open the unit up and

tell us what you find. There might be ant shit, dead
cock roaches or who knows what fouling the micro
switches or fouling the control printed circuit board.

We won't know until you take a look and report back.

I saw posts about smashing the thing up.
Learn from the Greens. Repair, Re-use, Re-cycle.
Investigate the problem, confirm with your eyes that there
are no door safety switches then come to a decision.

If there are no safety door switches, well, you are a
techo type, so fit one or two.

Regards,
John Crighton
Sydney

 

[James Meyer]

On Mon, 03 May 2004 09:05:05 -0400, buzz <buzz@tranquility.moon> posted this:

How is the interlock on a typical microwave oven implemented?

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Just curious.

Whatever you do, keep that oven in its present state. Do not open it.
Do not try to fix it. Do not throw it away.

That oven is a gold mine! You should be able to extort thousands of
dollars from the manufacturer. If your hand begins to hurt from the experience,
that might be worth hundreds of thousands.

Jim

 

[Ken Taylor]

"John Crighton" <john_c@tpg.com.au> wrote in message
news:4096c69d.4258762@News.individual.net...

On Mon, 03 May 2004 09:05:05 -0400, buzz <buzz@tranquility.moon
wrote:

How is the interlock on a typical microwave oven implemented?

Usually, there are two micro switches behind the door that disconnect
the supply to the big power transformer when the door is opened.


Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Just curious.


I am curious too, why don't you open the unit up and
tell us what you find. There might be ant shit, dead
cock roaches or who knows what fouling the micro
switches or fouling the control printed circuit board.

We won't know until you take a look and report back.

I saw posts about smashing the thing up.
Learn from the Greens. Repair, Re-use, Re-cycle.
Investigate the problem, confirm with your eyes that there
are no door safety switches then come to a decision.

If there are no safety door switches, well, you are a
techo type, so fit one or two.

Regards,
John Crighton
Sydney

Except that it is a dangerous design if this can happen, and there are more

out there. The source needs to be attacked.

Ken

 

[Andrew VK3BFA]

buzz <buzz@tranquility.moon> wrote in message news:<40964482$0$3059$61fed72c@news.rcn.com>...

How is the interlock on a typical microwave oven implemented?

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Just curious.

Hi Ben,
I think you should submit this oven to the Guiness book of records -
at least 3 mechanical uswitches switches have reversed their function
in order for this to happen. Absolutely amazing that you can get this
sort of fault........

73 de VK3BFA Andrew

 

[Tim Auton]

john_c@tpg.com.au (John Crighton) wrote:
[snip]

I am curious too, why don't you open the unit up and
tell us what you find.

Nooooooo. Don't touch it, certainly don't open it. Your legal case
would be blow out of the water if you open it up. The people who made
that microwave need to be sued to ensure they realise cutting corners
on safety is not cost effective.

Of course, now you've reset the microcontroller (or whatever happened
when you turned it off) you case is probably already blown anyway.


Tim
--
Love is a travelator.

 

[N. Thornton]

buzz <buzz@tranquility.moon> wrote in message news:<40964482$0$3059$61fed72c@news.rcn.com>...

How is the interlock on a typical microwave oven implemented?

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Just curious.

its a joke thread, the way interlocks are set up doesnt allow them to do that

regards, NT

 

[Tim Shoppa]

buzz <buzz@tranquility.moon> wrote in message news:<40964482$0$3059$61fed72c@news.rcn.com>...

How is the interlock on a typical microwave oven implemented?

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Every microwave I've ever worked with had two interlock switches:

1. A switch that closes when the door is closed, providing power to the
magnetron.

2. A switch that closes when the door is open, shorting out power and
blowing the fuse if switch #1 is closed at the same time.

Now, interlock switches do fail (in fact they do rather often), but
switch #2 is designed to prefer fail closed (blowing the fuse) rather than
open. But they aren't railroad-grade vital relays, just consumer-grade
switches.

Tim.

 

[Ken Taylor]

"Tim Shoppa" <shoppa@trailing-edge.com> wrote in message
news:bec993c8.0405031636.1f2b9d23@posting.google.com...

buzz <buzz@tranquility.moon> wrote in message
news:<40964482$0$3059$61fed72c@news.rcn.com>...
How is the interlock on a typical microwave oven implemented?

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

I unplugged the oven for about 10 seconds, but the problem persisted;
whenever the door was opened, the oven would start cooking. After the
oven sat unplugged for a week, the problem went away and the oven seems
OK. The whole time, the light worked properly: off when door closed and
not cooking, on when cooking.

I would have thought there would be a "dumb" interlock based on a door
switch, perhaps backed up by software also. But this appears to be a
software-only interlock.

Every microwave I've ever worked with had two interlock switches:

1. A switch that closes when the door is closed, providing power to the
magnetron.

2. A switch that closes when the door is open, shorting out power and
blowing the fuse if switch #1 is closed at the same time.

Now, interlock switches do fail (in fact they do rather often), but
switch #2 is designed to prefer fail closed (blowing the fuse) rather than
open. But they aren't railroad-grade vital relays, just consumer-grade
switches.

Tim.

The OP's description seems consistent with the switches being used as inputs
to a uP, which then decides whether to turn things on or not. If correct
this is not good, for the very reasons the OP has mentioned.

Ken

 

[Spehro Pefhany]

On Tue, 4 May 2004 15:08:24 +1200, the renowned "Ken Taylor"
<ken123@xtra.co.nz> wrote:

The OP's description seems consistent with the switches being used as inputs
to a uP, which then decides whether to turn things on or not. If correct
this is not good, for the very reasons the OP has mentioned.

Ken

Such a thing is not allowed. The two switch plus fuse "monitor" as
someone else described makes microwave ovens very safe (unless
deliberately tampered with).

BTW, the standard safety interlock on injection molding machines
involves three totally independent safeties- one electrical, one
mechanical and one hydraulic.

<sniff> I smell troll.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Ben Bradley]

On Tue, 04 May 2004 01:04:49 +0100, Tim Auton
<tim.auton@uton.[groupSexWithoutTheY]> wrote:

john_c@tpg.com.au (John Crighton) wrote:
[snip]
I am curious too, why don't you open the unit up and
tell us what you find.

Nooooooo. Don't touch it, certainly don't open it. Your legal case
would be blow out of the water if you open it up. The people who made
that microwave need to be sued to ensure they realise cutting corners
on safety is not cost effective.

DO NOT give it back to the manufacturer for them to 'check out.' It
would be in their best (short-term) interest to have the evidence 'get
lost' in a warehouse somewhere, and to deny a problem.

PLEASE POST the brand name, model number and serial number here on
Usenet. If you can get more documentation on this, especially if it
can be reproduced (in a controlled environment where you don't have to
heat up your hand or anything), you can write this up and put it on a
webpage, and post (email to the appropriate address) a short note to
comp.risks with a short note and URL to the full story. There are
surely a lot of other forums which would be interested in the story.

Google (Web AND Usenet) for that model and microwave ovens in
general, to see if anyone else has had a problem like this.

Of course, now you've reset the microcontroller (or whatever happened
when you turned it off) you case is probably already blown anyway.

If it happened once, it may be reproducible. It's worth it (to
other microwave users) to find out.

I'm apalled that the safety interlock would go through a
microcontroller or even electronic logic gates when it's easy enough
to design it with the path going through electromechanical components
that fail to 'safe' modes so the magnetron can never, ever be powered
with the door open.

Reading the rest of the thread, someone said 'a joke thread'
meaning this was not a post about a real situation, but rather a
fabrication meant to get responses (a 'troll' post). Occam's Razor
tells me this is most likely a troll. I've taken apart a few microwave
ovens (magnetrons have pretty good ceramic magnets, though hard disk
drives have much better ones), and while I haven't totally traced out
the wiring, I suspect the interlocks are all electromechanical (and in
series with the relay that the microcontroller uses to turn on the
magnetron), so that the inverted-operation of the door cannot possibly
happen.
If that's not true of this model microwave (if it IS possible for
it to fail this way), then the company will eventually go out of
business, and the OP needs to post the brand name (and others need ro
verify this failure mode) to protect both the product owners and the
company shareholders.

Tim

-----
http://mindspring.com/~benbradley

 

[David Lesher]

buzz <buzz@tranquility.moon> writes:

How is the interlock on a typical microwave oven implemented?

Recently, an 8-year-old oven became homicidal and would turn ON when the
door was opened and OFF when the door was closed. At the time, the oven
had just finished a cooking cycle and turned itself off; when the door
was opened it started up again. My hand (near the open door) got warm,
and I could tell from the loud hum that the oven was in cook mode (it
wasn't just a fan running). Pressing the "clear" button had no effect.

Wow.

Every one I have worked on had THREE switches. As I recall:

a) On the latch itself; if the door is not latched, no
juice to the maggie or the timer.

b) A second in series; it's run on the door position.

c) A third that is NC on the door -- if a & b fail closed
with the door open, it dead-shorts the primary power to
the maggie.

All are HARDWIRED in series/parallel with power sources; i.e.
they don't change status bits on an embedded controller..

Usually a is combined with the lamp switch, but it may not be.

You seem to be in the US; I suggest you get in touch with the
Comsumer Product Safety Commission ASAP.




--
A host is a host from coast to coast.................wb8foz@nrk.com
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

 

[N. Thornton]

Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote in message news:<va3e905v3fpbe5jv4cvms0chefn4h4uavr@4ax.com>...

On Tue, 4 May 2004 15:08:24 +1200, the renowned "Ken Taylor"
ken123@xtra.co.nz> wrote:

The OP's description seems consistent with the switches being used as inputs
to a uP, which then decides whether to turn things on or not. If correct
this is not good, for the very reasons the OP has mentioned.

Such a thing is not allowed. The two switch plus fuse "monitor" as
someone else described makes microwave ovens very safe (unless
deliberately tampered with).

BTW, the standard safety interlock on injection molding machines
involves three totally independent safeties- one electrical, one
mechanical and one hydraulic.

sniff> I smell troll.

Best regards,
Spehro Pefhany

Yeah, I'm surprised how many suckers there are in this thread. Not the
usual s.e.d. standard I'm afraid.

Regards, NT

PS all nukes here have more than just 2 interlock switches, except for
very old ones (70s and earlier), which can have anything from 0 to 2.
The very very few that have 0 are truly dangerous, and were recalled
and retrofitted quickly, the ones with 1 are a danger and the ones
with 2 are a definite risk.

I did get one once that cooked with the door open, it had 1 interlock
switch that only operated once the door was several inches open, and
anyone who saw the thing was immediately scared of it, and rightly
enough. Probably 1960s or very early 70s.

The usual arrangement is a series switch in both L and N feeds to the
main transformer, plus a shorting switch which is in series with a few
ohms power resistor. These are operated by 2 separate machanical
pieces to avoid kids getting round them with a fork. Add a fuse and
its all done.

Software interlocking is not allowed for obvious reasons, and I dont
think any company large enough to manufacture nukes would really be
clueless enough to try it, then work out how to get past all the usual
requirements. Its not credible.

The switches are not ordinary consumer grade switches, they are rated
specifically for interlocks. Having said that though, things could be
quite different in the US.


Regards, NT (in Europe)

 

[Tim Shoppa]

Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote in message news:<va3e905v3fpbe5jv4cvms0chefn4h4uavr@4ax.com>...

Such a thing is not allowed. The two switch plus fuse "monitor" as
someone else described makes microwave ovens very safe (unless
deliberately tampered with).
[...]
sniff> I smell troll.

I agree that the ovens are safe, but they may have the appearance
of not being safe.

I myself have seen a failure mode similar to the original poster:
opening the door turns on the light and the fan, keypad and display
are on the fritz (lit up but not working right.) I did not hear
the noise of the transformer/magnetron inside getting power.

And I suspect that the OP's microwave didn't actually cook with the
door open either. He never said that he put a voltmeter on the magnetron
supply! With the fan blowing and the light bulb on, maybe the heat
he felt was just a warm breeze.

Tim.

 

[Spehro Pefhany]

On 4 May 2004 02:57:20 -0700, the renowned bigcat@meeow.co.uk (N.
Thornton) wrote:

The switches are not ordinary consumer grade switches, they are rated
specifically for interlocks. Having said that though, things could be
quite different in the US.

MANY years ago I was somewhat involved in a door test setup for CSA.
They functionally tested the doors on test ovens as part of the
approval process. No screwing around with inferring things from the
switch ratings or whatever- the door was actually opened and closed a
zillion times. I'm sure UL is similar.

Regards, NT (in Europe)

Best regards,
Spehro Pefhany
--
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