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).

Of course if the likelyhood of the contacts sticking can be made vanishingly
small, two relays in series is just a silly idea <g>.  I will look into some
snubbing circuits.  I distrust MOV's because they fail short circuit,
rendering the circuit useless. At some stage they always seem to end up
being removed from a 'broken' appliance to render it working again (for a
while, anyway).

Any more comments?

Thanks
 Roland

----- Original Message -----
From: Chris Eddy <ceddy@NB.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, March 29, 2000 9:56 PM
Subject: Re: Relay failure modes [OT]


> Roland;
>
> Both failures can, of course, occur.  Both can be reduced to bare
minimums,
> though.
>
> Coils are designed to run within a specific range of voltages.  If you are
using
> an unregulated supply, though, you can easily vary well beyond the votlage
of
> the coil.  Temperature rise will getcha on the coil, much faster than old
age.
>
> Contacts are not designed to handle large inductive loads without help.
If you
> are breaking big currents, strongly consider carefullly designed (read
> experiment) snubber or MOV circuits, and if you are using DC, use diodes
and or
> TVS diodes to protect the contacts.  If you keep contact overvoltage down
to a
> minimum, you will dramatically extend the life of the relay.  If the
breakdown
> voltage goes high enough across the opening contacts, you will start in an
arc
> over condition and if it persists to a higher votlage/current across the
gap, it
> becomes a plasma glow arc.  It is this second stage of breakdown that will
> simply nuke your contacts in no time.
>
> Poor man's sanity check.. Place an AM radio next to the relay, tuned to a
> non-station, and listen when the relay changes state.  Try different
loads, IE
> resistors and inductors (another relay coil is grand).  Let us know what
kind of
> music you hear.
>
> Chris Eddy
> Pioneer Microsystems, Inc.
>
> Roland Andrag wrote:
>
> > Hello everyone,
> >
> > I would like to hear some comments on PCB relay (24 V) failure modes:
i.e.
>