Fire hazard question

I wonder if anyone can help me? :)

I am trying to find out what the current "best practice" would be apropos fire hazard protection in a particular situation as follows:

The unit is a (small) fire alarm, it has a 12V external alarm line. The curreent level there is normally 120mA but because it is external, there is a significant risk of someone interfering with the wiring and shorting it out. The 12V is essentially unlimited current but the circuit is protected against gross faults by a 1A fuse. The external alarm driver is a TO92l MOSFET, Ron ~2 ohms, which will obviously rise as it heats up, so I can envisage it taking a couple of amps for several seconds before the MOSFET literally burns out. Whilst most small semiconductor burn-outs take place harmlessly in the air above the PCB, it is obviously a fire hazard, albeit a remote one. To provide some protection my colleagues have suggested a semiconductor current limit.

My question is what we should be aiming to do - it is common enough for people to talk of "single component failure" i.e. good practice requires that a hazard will not be caused by any single component failure. Unfortunately I believe there are strings attached: IIRC, generic European and American standards specify that potentially hazardous faults should be monitored or discoverable through routine testing. In addition I believe they only count if they are spontaneous internal component failures, not external conditions caused bu human interference or bad installation.

The problem is thus that the current limit could fail spontaneously - a single component failure - and this not be detected. So we are back to square one, with the system vulnerable if someone causes an external short. This causes an internal burn-out which may be a slight fire hazard. All in all, the frequency of such fires occuring is probably incalculably small, but I think the system could escape the "single component failure" criterion if this is applied strictly.

This query has appeared in alt.electronics. Please note that I am not asking for circuit suggestions as I can design circuits in my sleep (and frequently do). I have had a string of circuit suggestions ranging from relying on a PCB trace as a fuse, through to using a PTC thermistor as well as the current limit, all of which are unacceptable for various reasons. Clearly such additional protection would be "belt and braces" but, in any case, such specific circuit suggestions only reflect one individual's ingenuity, they do not elucidate the safety design *critera* which, in the worst possible scenario could be invoked in court under the heading of "due care"... I just want to know what regulations, recognised best practice, and general standards (US and Europe) have to say about this kind of situation: where multiple faults are needed to cause a hazard but the faults may not meet the criteria of being

TIA - remember, no circuit suggestions, thank you.

Some regulations can be quite restrictive and at the same time a bit ambigous. to get round a similar problem of a single point failure going unnoticed and subsequent failures/short circuit cuasing something to fry and emit smoke flames etc, the solution was to make the component non flamable/contain the flame/smoke. a simple resistor designed to fail open circuit without emiting any smoke was used aka fusable resistor. some of these just had a plastic sleave covering an ordinary resistor.

Colin =^.^=

....

Well, I don't know if you'll disqualify this as a "circuit suggestion", but why not run a constant-current driver instead of constant-voltage. It's inherently short-proof, but you don't say what kind of logic you're using in the overall system (i.e., a contact closure to 12V? An open a la broken foil?) How you're sensing that "external alarm line" makes a lot of difference.

Or, just current-limit the 12V supply, and report when the current limit is exceeded.

Good Luck! Rich

Sorry, the external alarm line is an output alarm like a sounder/lamp etc, not an input. It is not monitored. the relevant BS to this particular application does not require it.

A constant current driver is, in effect, what my current limiter becomes under overload. It's failures in the constant current driver that I'm worried about. As I said, it wouldn't cause an immediate hazard but it would leave the system vulnerable and as it's not monitored this could mean it can't be counted as protection.

Obviously testing the current limit would require a periodic check whether automatic or manual.

With your current limited supply, you can't report when the current is exceeded as it won't be until the external fault occurs. By that time a failure of the current limit could have occurred and not been noticed.