"Jim Lesurf" wrote in message
...
In article , David Looser
wrote:


So I do know the limitations of the device.

Actually, no, you don't. You saw one and from your own admission judged it
simply on appearance and your general theories about the class of such
devices.

There's something odd going on here. :-) *You* can group all
electromechanical devices together and damn the lot of them, but if I try to
talk about a class of devices known as "thermal relays" you instantly try to
restrict the discussion to the particular unit that you used on the 600. I
wonder why that is? Just to make it clear, by "the device" I mean the device
known as a thermal relay, not specifically the particular one you chose to
use.

However I agree with you that such devices do tend to be far less reliable
than designing them out of the system.

Designing out any unecessary complexity improves reliability, and that
includes semiconductor devices. High-voltage semiconductors, including SSRs,
do not have a particularly good reliability record.


I don't think I missed any, though you seem to have missed the point
that making judgments on the reliability of electromagnetic relays
based on your experience of an *entirely different* device seems, at
best, hasty!

I agree. Hence my noting that you made no measurements on the specific
device I used as an example, and just judged it on the basis of appearance
and your own general opinions. :-)

You are so keen to keep coming back to my judgement of that specific device
aren't you? I have extensive experience of relays and their uses. Whilst
electromagnetic relays can be very reliable the thermal relay isn't (That's
thermal relays in general, not just your specific one!)

You may however have still missed the point that I have also experience
with conventional electrically operated relays in a range of applications.

And so do I!

So I'll stay with my views based on a mix of my experimental experience
and
having tried various alternatives. if you prefer electromechanical
switches
to solid state alternatives you are welcome to do so. :-)


That entirely depends on the application. For signal switching, particularly
low-level unwetted analogue audio/video I'd usually prefer electronic
switching. Loudspeaker relays in high-powered amps can be a problem too
because the contact material needs to be able to handle high currents yet
also work well with very low signal currents. But then SSRs are no use in
that application either.

But the particular virtues of the relay - very low on resistance,
negligable off-state leakage, ability to offer a wide range of contact types
and number, galvanic isolation between coil and contacts etc. can make it a
very attractive option in many applications.

At it's peak in the 1960s the Strowger-based PSTN network in the UK had in
excess of 10 million relays in use, and by then many of them were over 30
years old. Several hundred might well be involved in any one call. Unlike
selector mechanisms relays generally did not receive routine maintenance,
nor were they (or selectors for that matter) replaced as a standard repair
technique. Individual relays were wired into relay sets and replacing them
was a fiddly job and rarely done. When exchanges were retired after a
typical life of 40 years almost all of the original relays and selectors
would still be in use. Yet relays were rarely the cause of faults, selector
mechanisms, contact banks, selector jacks or indeed shelf wiring were in
fact the usual cause of faults in Strowger exchanges. Of course these were
mostly PO3000 type relays, bulky and expensive by modern standards, but
very reliable.

David.