SIL2 motor interlocks

[nhcf]

We have an application which requires SIL2 motor interlocks - to prevent 2 pumps running simultaneously. My experience with SIL systems is limited to process instruments, not necessarily motor starters - is there such a thing as SIL-rated motor contactors/starters?

 

[jraef]

You can get "Safety Contactors" that have positively driven aux. contacts, which you can USE in system that can be evaluated to meet SIL2.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[nhcf]

Thanks Jraef -

Are the safety contactors you describe the starters? Or interposing relays in the control circuit - i'm trying to determine the bounds of the SIL system - whether it stops at the control circuit, or at the motor contactor. I am used to seeing SIL applied to valves/solenoids, but haven't really encountered starters.

 

[jraef]

A "starter" is a contactor + overload relay.

You can get a "Safety Contactor" in that the auxiliary contacts are "mirror contacts", meaning they are mechanically driven to their Open/Closed state by the same armature as the main power contacts and that the contact states of the Aux cannot change state without a corresponding change in the main power contacts, as specifically tested and listed by an acceptable Safety Testing agency ("self testing" is not allowed). This is necessary to provide the proper level of reliability needed when going for a SIL certification. The overload relay needs no such added certification because it is not the "operating device" per se. So you can MAKE a "safety" starter by using a Safety Contactor plus an overload relay.

SIL certification for power devices is a certification of the SYSTEM as a whole, but you cannot get SIL2 certification on a motor controller without having components used inside of it that have the proper certifications themselves, hence the need for a "Safety Contactor". You would ALSO need a Safety Relay in that system, along with any safety sensing devices associated with it.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden