Fail-safe 1

[asimpson]

How are domestic electric heaters(+1kW) protected in the event of failure of primary contol device and made to fail safe? Nuisance switching shouldn't be an issue since this is a last line of defence.

I am concerned a NC thermoswitch may fail to open or the main power relay fail to open.

I have seen NO thermo-switches in an electric oven which short out power and blow mains fuse or RCD. I am concerned this would blow fuses in the domestic circuit that may be needed by other devices.

Anybody got knowledge of best practices?


Many thanks

 

[itsmoked]

Standard is an approved thermal switch or fuse. They need to either blow permanently or trip and require a manual reset. It's that simple and they're safe precisely because they are that simple.

Virtually ALL domestic electric heaters are protected with a permanent thermal fuse. Look harder it's there or you're looking a something homemade or rinky-dink.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Re reading your confusing post yet again. You start asking about domestic heaters and switch to ovens.

I have never seen any product of the type you're referring to that had a crow-bar for over-temp protection. THAT is not 'best practices'.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[controlsdude]

When I did heat treating furnaces, the overtemp device was a FM approved device, that would trigger the contactor opening. The TC or thermoswitch was an input to this overtemp not directly connected to a contactor like your describing. So do you have this device? If not think your not safe on that system.

 

[asimpson]

Hi itsmoked.

I was referring to an electric oven as something I have seen in a product. In any case electric ovens and heaters are similar in principle and subject the the same overtemp failure problems.

 

[VEBill]

All my baseboard electric heaters have over-temp cut out switches built in. They're not for failure of the control system because the heaters are designed for 100% duty cycle (e.g. January 2014, LOL). They're in case of air blockage.

In Canada, electric ovens are supposed to be on their own circuit, but I can't imagine anyone using a short circuit as the overtemp cut-out. An open circuit protection makes so much more sense.

Also, many ovens are "self cleaning" via extraordinary temperatures, (800F?), so any over temp limit would have to be above that.

Most appliances have thermal fuses wired in series, sometimes intentionally on the neutral return (not sure why...).

 

[asimpson]

My concern with open circuit devices is that they may not be intrinsically safe. If they fail to open electricity is still flowing. I suppose it is a question of reliability. If a NC thermal switch had been carrying a current larger than its rating due to some other fault it may become welded shut and unable to open.

The short out method I mentioned does rely on fuse blowing however fuses are reliable when correctly used. Of course I could be incorrect in my assessment of how the system worked and it was a long time ago.

My experience is with industrial equipment where safety relays with multiple redundancy are used. A domestic system needs to be more cost effective.

Thermal fuses seem to be way to go.

 

[itsmoked]

Yep, there is probably nothing more reliable than a rigorously tested thermal fuse applied within its design limits.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Crowbarring of the mains just sounds plain dangerous. What if the breaker doesn't open like it's supposed to, and instead of a couple hundred dollars damage, the wall wiring catches fire and burns down the house? So, are you going to simply trade the risk of the thermal fuse from opening to the breaker not opening, which has a lower probability, probably, but with a gigantic potential cost? What if opening the breaker results in removing power from a refrigerator, or alarm system?

Hazard analysis should dictate a design that causes the least potential damage. If there is that much concern about the thermal fuse opening, one could possibly put two of them in series.

TTFN
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[asimpson]

Protection I have in mind is for failure causing potentionally fatal consequences like causing fire or electrocution.

I appreciate safety should be designed into systems to avoid risk however there is always the posibility of failure leading to injury or fatality.

Does earthing electrical products not work this way by allowing stray mains power to short out to earth rather than leave potentionally fatal power exposed?

 

[zeusfaber]

Both types of device have two main failure modes: Failure to provide the protective action when they ought to be doing it, and going into protection mode when there's no need.

Because both types of device are used as fallback safety devices, rather than as a single standalone control system, the "failure to protect" failure, while not "failure into a safe state" doesn't itself create an unsafe condition until the primary control fails. The design uses redundancy to achieve safety rather than relying on fail-safe components.

For the series thermal cutout, the "unwanted protection" failure is a fail-safe condition. Without having to depend on anything else, your heater goes off. This is a nuisance, but nothing is going to catch fire.

For the parallel thermal crowbar, the "unwanted protection" failure generates an unsafe condition (an overcurrent)and you are then dependent on the correct operation of other safety devices to stop bad things happening. This is not a fail-safe approach.

A.

 

[stevenal]

The concern is that a thermal fuse fails to open per its design? I agree this would be a rare situation. If it remains a concern, put two of different manufacturers/ratings in series.