E-Stop in motor controls

[petersonra]

Well, _presuming_ that contact sticking is a random failure, and that failure of either contact set is an independent event, the chance of _both_ contacts sticking is the _square_ of the change that either contact will stick.

However it is not clear that these are two independent events; both contacts will age at the same rate, and are powered by the same system, and are in pretty much the same environment.

Still I'd expect a substantial improvement in reliability by using a dual contact arrangement.

-Jon

These are force guided contacts. The contacts are tied together mechanically, so the chance of one NO contact being open and another NO contact on the same contactor being closed is close to nil.

 

[winnie]

These are force guided contacts. The contacts are tied together mechanically, so the chance of one NO contact being open and another NO contact on the same contactor being closed is close to nil.

Does this mean that if one contact sticks for any reason, then both contacts are likely to be held closed?

That would seem to _double_ the chance of a contact sticking!

Am I missing something?

-Jon

 

[petersonra]

Does this mean that if one contact sticks for any reason, then both contacts are likely to be held closed?

That would seem to _double_ the chance of a contact sticking!

Am I missing something?

-Jon

That is exactly what it means. However, because you actually have two seperate contactors with their contacts in series, you have a redundant means of shutting off power. The chance of both relays or contactors failing in the same way simultaneously is very low.

And, it is self checking on reset, so if it finds a fault (like a stuck contact) it won't turn on so you have to fix it.

 

[wireman]

Force guided contacts basically means that the contacts can't stick. They will break apart (thereby internally destroying the switch) but will not stick.

This is not the exact wording but it describes the action.

 

[dbaird]

Control reliability of machine control systems or devices can be
achieved by the use of, but not limited to, one or both of the
following:
* The use of two or more dissimilar components, modules, devices or
systems, with the proper operation of each being verified (monitored) by
the other(s) to ensure the performance of the safety function(s).
* The use of two or more identical components, modules, devices or
systems, with the proper operation of each being verified (monitored) by
the other(s) to ensure the performance of the safety function(s).
These methods require that the safeguarding device, its interface to the
control system (or directly to the actuator control) and actuator
control meet the above requirements.

In addition to the benefits already discussed, the safety relay will detect both an open and/or shorted circuit on the safety input channels and inhibit the outputs until the problem is fixed.