"Derek Potter" ll wrote
in message ...
On Sat, 14 Jan 2006 15:44:06 GMT, "Dan Hollands"
wrote:


"Derek Potter" ll wrote
in message ...
On Fri, 13 Jan 2006 13:47:07 GMT, "Dan Hollands"
wrote:


"Derek Potter" ll
wrote
in message ...
I need some information about the current approach to safety of new
equipment with regard to fire hazards created by a fault. I appreciate
there are generic standards covering the ejection of molten metal and
so on, but I am wondering about the application of the "single
component failure" concept in situations where a failure could
overload a semiconductor with the possible, though unlikely, result
that it ignites or ignites an adjacent part. I'm not asking about
*techniques* to avoid hazard, I'm asking about what is legally
required. Trick question - I'm not asking for legal advice, just
information concerning best current practice. I'm posting from the UK
but I suspect the regs will be substantially the same in all of Europe
and the US.

My particular concern is a small transistor driving an external alarm.
The power supply has a fuse but as it feeds several circuits, it
doesn't provide much protection for the external alarm circuit. I am
considering an active current limit in the supply but I still have
some reservations as to whether this meets the letter of the law. For
example, one scenario involves two events as follows:

1 The current limit fails spontaneously, but as this is not
monitored, the defect remains undetected, waiting for the second event
to happen...
2 Someone fiddles with the external wiring and causes a short.

The result is that the driver overheats, catches fire and there is
hell to pay. Now, it is perfectly true that this involves two
independent "failures" so at first sight would meet the "single
component failure" criterion. However, I suspect that a fault that is
never detected (and just lies there waiting for a chance to create a
hazard) may not count. Likewise, a fault that could be caused by Uncle
Fred with his screwdriver is hardly a spontaneous component failure.
So overall, would such a system meet the "due care" criterion?

I have severe doubts as to whether much equipment is designed with
this degree of concern but it would be goot to be ahead of the field -
without incurring too much cost.

Also, if this isn't the best newsgroup could someone point me in the
right direction? Most electronics groups seem to be full of people
selling stuff.

TIA.

Derek

If you are in the UK, you are under EU requirements and you should be
studying the appropriate IEC specs for the type of equipment you are
designing. Just as a personal opinion - if there is an output that when
shorted will cause a fire, then there should be some type of protection
for
the output.

Indeed so, but my question goes a bit further as I already have
protection in the shape of a foldback regulator. It's failure of this
protection that I'm asking about. This is unlikely to cause an
immediately hazardous condition, but, obviously, if the protection
fails, the circuit is then left susceptible to any other fault. In
this case it could be a fairly rare external event. The failure of the
protection device may not be detected without yet more circuitry to
monitor the foldback operation of the regulator! Am I being too fussy?
Do most commercial and consumer devices go this far?

Generally equipment design for general use is only concerned with a single
failure criteria. In my experience adding more circuitry increases the
complexity to the point the failure and problems are more likely. The
problem with all redundant circuits is the need to test them to insure
that
all of the redundant circuits are working. Statistical methods may be used
to determine how often the redundant circuits must be checked to achieve a
certain confidance level the system will operate properly when required.
In
your case I would see no need for extra circuitry

Execeptions are things like Safety Shutdown systems, Intrinsically Safe
Equipment to insures that sparks or hot spots don't trigger an explosion
in
explosive atmospheres and control systems in nuclear power plants.

Agreed completely and I tend to think, like you, that having a little
foldback regulator to guard against the occasional shorted load is
probably enough. The question hinges on what comprises a "single
component failure" since an external short in unprotected wiring
accessible to "Uncle Fred" is not exactly a component failure.
Likewise failure of the current limit doesn't create a fault in itself
but, as it's not going to be monitored, this doesn't quite settle the
matter - it leaves the system in a vulnerable state to an external
short.

I suppose, in a nutshell, the question comes down to whether
protection circuits are relevant to "due care" if an undetectable
failure in the protection leaves the system just as vulnerable as if
the protection were not there.

On another tack, I may work around this by fitting a fire-resistant
sleeve over the transistor. It can burn as much as it likes then, but
I was hoping to avoid the trouble.


Derek

It is almost impossible to make a prduct completely fail proof. All you can
do by adding more components is decrease the likely hood of a problem. If
the accidental shorting of external connections can cause a fire then it is
prudent to add protection such as your current limit circuit. That is all
you need to do. A problem will only occur if 2 unlikely events occur. If you
added another current limit circuit a problem would only occur if 3 unlikely
events occured. That is above and beyond what is required for normal use
products.

Dan