Fail-safe
Fail-safe
(OP)
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
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





RE: Fail-safe
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
RE: Fail-safe
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
RE: Fail-safe
RE: Fail-safe
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.
RE: Fail-safe
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...).
RE: Fail-safe
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.
RE: Fail-safe
Keith Cress
kcress - http://www.flaminsystems.com
RE: Fail-safe
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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RE: Fail-safe
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?
RE: Fail-safe
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.
RE: Fail-safe