Roland, and others;

If you want to take the relay contact protection task even further, I found a
great book titled "noise reduction techniques in electronic systems" by Henry
Ott.  It is at borders for $90.

There is a section on relay contact protection, which details the exact failure
modes, the different circuit protection topologies, and even design equations to
determine the right component values, like R and C in a snubber.  It is a
treasure on this subject!

And on the failure protection for your system, I would strongly suggest using a
single relay, and adding an activated crobar across the AC line to snap the
breaker if the unit gets out of hand.  It could be a second relay or even a
triac.  If you like, you can even hard wire the protection, such as (if limit
switch AND AC being output FOR so much time, ACTIVATE crowbar).  Make sure the
unit lights up a light that tells your customer that the unit saved him $1000
today.  Call it the money light.  Then sell him a replacement board for $999.

Chris Eddy

Roland Andrag wrote:

> I have opened the relays and can actually observe a (very) small spark when
> the contacts open.  The relay drives a 7 VA 220 V contactor, and is rated to
> 7 A.
>
> The reason that I asked the question is that in the project I'm working on
> it can be fairly catastrophic if the relay fails to open and the contactor
> stays in ($1000's of damage can result).  The likelyhood of this happening