How to attach large house wiring to PCB?

[Active8]

It was installed by a professional licensed electrician - and has been
working for nearly 20 years now - so that is not quite a worry.

Things short.

I already asked about proper trace design - so this is not a worry. The
relay I'm using is rated for double the current that it will experience
- so that also isn't a worry.

Shorts cause traces and wires to heat up. You've never seen a TV
power board burnt or a transistor flame out. I have.

Umm, why?

It won't hurt bucause it just won't. Why let the other pole of a
relay go to waste?

I don't even need to respond to this.

But the ambulance and fire department might.

Right. Those are exactly my objectives.

Then re-read the response from that other poster which is directly
below this sentence, please.

 

[Active8]

On 10 Jan 2004 17:32:52 -0800, [email protected] said...

There's a reason engineering-types are often thought of as pessimists:
We always have to say "What if".
It's our job to consider worst-case conditions.

There's even an engineer's code of ethics I read somewhere that
says that in a few different ways.

 

[Tim Shoppa]

I already asked about proper trace design - so this is not a worry. The
relay I'm using is rated for double the current that it will experience
- so that also isn't a worry.

The problem isn't the current you expect the relay and traces to experience,
but the ones that they might experience under an external fault condition
(e.g. your load going dead-short). If they are being supplied by a 15 amp
breaker (most common household size), your board better be able to withstand
30 amps for a few seconds and substantially higher currents for a few
tenths of a second.

And mounting these devices in NEMA-rated metal boxes is a necessity. To
say that it will never catch fire is being hopelessly optimistic. Realistically
you have to have some way for it to catch fire and vaporize without burning
down the house.

Tim.

 

[Fred Bloggs]

The problem isn't the current you expect the relay and traces to experience,
but the ones that they might experience under an external fault condition
(e.g. your load going dead-short). If they are being supplied by a 15 amp
breaker (most common household size), your board better be able to withstand
30 amps for a few seconds and substantially higher currents for a few
tenths of a second.

And mounting these devices in NEMA-rated metal boxes is a necessity. To
say that it will never catch fire is being hopelessly optimistic. Realistically
you have to have some way for it to catch fire and vaporize without burning
down the house.

Tim.

It is not that simple- the board traces must withstand the maximum fault
current- thousands of amps- for the duration of the circuit interrupter
trip time or else they will become arcing fuses - creating an
intermittent connection to the load which is itself a fire hazard.

 

[JeffM]

- because the other wire

It won't hurt bucause it just won't.
Why let the other pole of a relay go to waste?
Active8

because it can be kept as a spare
when the 1st set of contacts pit/wear
(assuming that you have spent money
on a unit with more contacts than you need).

 

[Michael]

It is not that simple- the board traces must withstand the maximum
fault current- thousands of amps- for the duration of the circuit
interrupter trip time or else they will become arcing fuses - creating
an intermittent connection to the load which is itself a fire hazard.

wouldn't the circuit breaker trip if there was a short? Surely it wouldn't
catch fire in the half second before it tripped... Or would it? As I said -
I truly have no experience with such things. Or should I put a quick
blowing fuse on the board itself?

Michael

 

[Ken Smith]

Surely it wouldn't (Don't call me Surely :>)
catch fire in the half second before it tripped.

One microsecond is more than enough time to turn a PCB trace into a plasma
fire if the external impedance is low enough.