Shunt Trip Breaker for Safety Shutdown

[DanHunter]

I was wondering if it is OK to use a shunt trip, 480VAC, breaker with a 24VDC control circuit as a safety shutdown in a SIL 3 safety circuit? Is one breaker sufficient, or will it be necessary to have two shunt trip breakers in series?

 

[Timbert]

It is essentially impossible to meet the requirements of SIL 3 with a single disconnecting means. To do so the device in question must have a Safe Failure Fraction of >99% (i.e. in this case, 99% of the failures of a shunt trip breaker would have to open)[IEC 62061 6.7.6.3].

I don't know of any SIL rated shunt trip breakers. The main reason is probably that shunt trip breakers unlike relays and contactors don't default to a safe state in the event of a loss of power to the control circuit. Think of it this way, if the wire to the shunt trip coil falls off you're screwed, you can't stop the machine; if the wire to a contactor coil falls off, you lose power and the machine comes to a stop, inconvenient but safe.

 

[DanHunter]

Thanks for your response. If a UPS supplied control circuit was used for the tripping function, would a single breaker be enough? I realize it is still not fail safe, but it is a better solution.

 

[Timbert]

Thanks for your response. If a UPS supplied control circuit was used for the tripping function, would a single breaker be enough? I realize it is still not fail safe, but it is a better solution.

Yes, it is a better solution, it is not a SIL 3 solution.

I will repeat what I said in my first reply: "It is essentially impossible to meet the requirements of SIL 3 with a single disconnecting means."

SIL 3 requirements are very demanding. A SIL 3 system should have a dangerous failure rate less than once per 10 million hours of operation (roughly 1140 years).

A SIL 3 solution also requires a detailed analysis of the hazards and behavior of the controls in the presence of a fault. It is not as simple as buying a SIL 3 rated component or building in redundancy.