On Sun, 2 Apr 2000 20:42:27 +0200, you wrote:

>Chris, thank you very much for your long reply (sorry for the delay in my
>answering).
>
>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
>and damage occuring is however fairly small.  If the probability of the coil
>burning is or can be made vanishingly small <g>, two relays in series could
>be used to just about rule out damage resulting from contacts sticking.
>However, maintenance costs go up sharply should coil failure be prominent,
>especially since servicing units involves travelling long distances to get
>to them.  Relay cost is not an issue (at about $1 for an Iskra 1803, it just
>doesn't feature agaist cost of other components).
I believe you can get relays, designed for safety cutout applications,
with a mechanism to reduce the risk of sticking by positively forcing
the contacts open - the phrase 'positively guided contact' rings a
vague bell.