SIL Rated Relay vs. Trip Relay For Circuit Breaker Applications 1

[ThePunisher]

Hi all,

Our client's medium voltage switchgear specifications indicated an ESD Relay which is SIL (Safety Integrity Level) rated and operates at 24 VDC with N.O. output contact (suitably rated at 125Vdc) wired to energize the trip coil which draws 3.5A at 125Vdc. The Process ESD system energizes the ESD relay which in turn closes its output contacts to energize the circuit breaker trip coil. The ESD relay is installed INSIDE the 13.8 kV switchgear.

The problem started when we discovered that the specified ESD relay's (which is SIL-rated)output contacts are not rated or suitable to make or break the circuit breaker trip coil as officially confirmed by the relay manufacturer.

As Electrical folks, we recommended two things:

1. Remove the ESD relays from the switchgear and install it inside an interposing relay panel. Install a standard switchgear rated trip relay, device 94T (ABB RXMH-2 or AREVA MVAJ type) whose coil will be operated by the ESD relay at 125VDC and whose output contacts will energize the circuit breaker trip coil at 125Vdc.

OR

2. Relace the ESD relay with a TRIP relay (ABB RXMH-2 or AREVA MVAJ type) operated at 24VDC and whose output contacts will energize the circuit breaker trip coil at 125Vdc.

My only dilemna is that our Instrument & Control Group (I&C) might require us that the TRIP relay should be SIL rated. The ABB RXMH-2 and AREVA MVAJ are note SIL rated and we could not find a standard TRIP relay that is SIL rated.

As Electrical practitioner, I should ensure that the relay that operates the circuit breaker has contacts that is rated for the purpose and duty. A standard TRIP relay is rated for such duty and I believe that any component that DIRECTLY control and operate the switchgear circuit breaker should comply with Electrical and switchgear manufacturing standards. Secondly, it is also my belief that SIL rating should looked at as an overall control system and should not be "prescriptive" but quantitative based on failure rates, reliability and availability of the ESD as a SYSTEM. This means, the TRIP relay which is operating the breaker and installed inside the switchgear MAY NOT necessarily be SIL rated per se. The interposing relay (ESD) or signal source that operates the TRIP relay can be SIL rated as they like but the battery limit should be drawn on the switchgear in terms of suitability and reliability to DIRECTLY operate the circuit breaker. Is there a trip relay (standard device 94) which is SIL rated in the market?

I am expecting a tight discussion on the above dilemna but I am hoping some subject matter experts in this forum can provide me their thoughts and suggestions especially if they have the same experience in the past.

I thank you in advance for any ideas that will be given.

 

[jraef]

Can you not use an interposing relay between the ESD relay and the trip coil? The SIL rating is a result of the functionality of the ESD relay, which would not change. The Interposing relay, maybe required to be Positive Drive, would just be a slave of the ESD relay.

"Will work for (the memory of) salami"

 

[LiteYear]

You are right that a SIL rating applies to a overall system, specifically a safety function, which extends from sensor, through logic, to actuator. The actuator is the device that activates the safe state. In the case of your ESD relay, it's the circuit breaker.

This means the relay *cannot* claim a SIL rating without doing one of two things: specifying in the product manual the complete system including the sensor, actuator and wiring, and specifying the safety integrity those components; or, claiming a percentage contribution to the SIL rating of the relay and requiring that the other components in the system do not push the total percentage above 100%. Either way, the point is that a SIL-rated product (a component) must include in the product manual or safety documentation, a specification on the intended usage. You either follow that specification or you *cannot* claim any SIL rating at all.

If the output contacts are not suitably rated (and the manufacturer must provide numbers for you to assess "suitability"), then you need an interposing relay (this is not unusual). You then need to use the safety documentation for the relay to assess whether the introduction of the interposing relay exceeds the failure rate contribution allowed for by components other than the relay. Some allowance will have already been made for the circuit breaker - you just need to ensure (and the manufacturer should advise) that the combination of the interposing relay and the circuit breaker does not exceed that allowance. Often, with force-guided relay contacts, this is not hard to achieve.

If you have been sold a SIL rated relay that does not provide this documentation, then you have been sold a lie. A component is only SIL rated if it provides specifications for the system in which is installed.

 

[ThePunisher]

Thanks for your inputs. By the way, I made an error by calling the relay ESD relay. The relay is actually a Process Shutdown Relar (PSR) which is part of a Process Shut-Down (PSD) system and is part of a specific Safety Instrumented Function (SIF). I believe the I&C Team included the circuit breaker in the SIF consideration (as an actuator) but I will confirm this.

The way I udnerstand what LiteYear posted is that the interconnecting system between the Logic Controller (in PSD) and the circuit breaker has an assigned % PFD (Probability Failure on Demand)budget and a small portion of the PFD may have been assigned from the logic controller to the circuit breaker (this is the Process Shutdown Relay and other conditioning device attached to it). Perhaps they might have assigned a SIL rating for this part of the system and having the PSR rated for the same assigned SIL rating provides an assurance of its suitability to the SIL assigned to this particular SIF.

Is my understanding correct?

 

[jghrist]

Normally, the circuit breaker trip circuit includes a 52a contact that opens the trip coil current. The ESD relay tripping contact should be maintained long enough that it is not the contact that interrupts trip coil current.

 

[LiteYear]

The interconnecting system between the Logic Controller (in PSD) and the circuit breaker has an assigned % PFD (Probability Failure on Demand)budget and a small portion of the PFD may have been assigned from the logic controller to the circuit breaker (this is the Process Shutdown Relay and other conditioning device attached to it).

I think that's accurate. It could in practice be a lot more complicated if there are redundant paths or diagnostic loops, but I think you've captured the core premise.

Perhaps they might have assigned a SIL rating for this part of the system and having the PSR rated for the same assigned SIL rating provides an assurance of its suitability to the SIL assigned to this particular SIF.

Yes! That's the key. You apply appropriate SIL capable components in order to achieve a SIL rated SIF. If the individual SIL ratings are not sufficient, then you need redundancy and voting systems to boost the overall SIL rating of the SIF.

 

[ThePunisher]

Thanks everyone for the good responses. I gathered the relay specification and requested for the relay PFD data or Reliability/Availability Data/Statement from the relay manufacturer. I discussed with our I&C/IT team and they will re-calculate the overall PFD of the particular SIF where our relay will be applied on.

Many Thanks and More Power!!!