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Sherlock Ohms

Malfunctioning Fridge Causes Meltdown

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AnandY
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Gold
How can a microswitch collapse?
AnandY   9/29/2014 8:13:05 AM
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"A nurse had opened the lefthand refrigerator door to get baby formula. Later, she smelled smoke, saw black smoke pouring up the stairwell across the artwork, and called the fire brigade. Our report suggested that the microswitch internals collapsed when the nurse closed the door, short circuiting the terminals; the insulation started to smolder and, although it was fireproof, created enough smoke and soot to do the damage."

My question was that if the slamming of the door caused microswitch internals to collapse, why weren't there mitigational properties installed within the fridge, maybe sensors, that would have detected the failed terminal and adjusted electrical surges to reduce the damage done? If we are talking about high tech refridgerators, why not talk about high tech security as well?

naperlou
User Rank
Blogger
Re: How can a microswitch collapse?
naperlou   9/29/2014 11:22:23 AM
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That is a good question.  It would be interesting to know how "high tech" the refrigator was.  Was it a "standard" refrigator?  It was being used to store baby formula in some kind of medical facility.  Do they use standard refrigators?

tekochip
User Rank
Platinum
Re: How can a microswitch collapse?
tekochip   9/29/2014 3:29:52 PM
I wonder if the switch contacts or the faston connections were a little resistive.  I've seen appliances start fires from a bad crimp, and I even had a switch in my aircraft do the same thing while I was flying.  That was an attention getter.

a.saji
User Rank
Silver
Re: How can a microswitch collapse?
a.saji   9/29/2014 11:15:56 PM
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@tekcochip: Aren't there any other alternatives left for it ? 

Soldering Gunslinger
User Rank
Iron
Re: How can a microswitch collapse?
Soldering Gunslinger   9/30/2014 11:37:46 AM
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Oscar Wilde is quoted as saying that Brittain and the United States are two countries separated by a common language.

When we Yanks refer to a "Nurse", it generally means a medical practisioner.  On the other side of the pond, it can mean somebody who takes care of young children, so yes a Nurse might be in a home, specially one where expensive artwork may hang on the walls.

 

I remain,

 

The Old Soldering Gunslinger

William K.
User Rank
Platinum
Re: How can a microswitch collapse?
William K.   9/30/2014 8:22:30 PM
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I have used and abused quite a few microswitches over the years, and replaced a few broken ones as well. One thing is that the internals don't "collapse." When the switch fails due to a breaking leaf element it may close or not close, and it may possibly arc inside. One thing to remember is that in most of the UK the standard line voltage is 220 volts, rather than the 120 we are accustomed to here in the USA. The higher voltage may make arcing a bit worse, but actual Microswitch brand switches seem to be made of a material that would char and stink but not burn. Replacements of a much cheaper kind may be made of materials that are not so very fire resistant.

Given the standard materials that most refrigerators are made from it is also hard to imagine there being enough mass to burn that would produce enough smoke to do any damage.

Awile
User Rank
Iron
Re: How can a microswitch collapse?
Awile   9/30/2014 9:53:21 AM
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That failure seems like a one-in-a-million failure mode.  If the design team had considered mitigating for it, I suspect they would have beefed up the switch and terminals, rather than installing sensors.  

If the current didn't trip a breaker, what would be sensed?  What would be the response?  Would the risk of a false shutdown of the refrigerator create higher overall loss than suppression of smoking wires?

These kinds of questions keep us engineers scratching our  heads.

a.saji
User Rank
Silver
Re: How can a microswitch collapse?
a.saji   9/30/2014 11:10:57 AM
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@Awile: Yes if you consider closely, the questions are reasonable. You should have some sort of a lead point towards it. 

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