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s_for_dragon
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Join Date: Oct 2008
Location: Maitland NSW Australia
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Relay Logic and SIL Level

12/04/2008 10:27 PM

Hi All,

I am a bit confused regarding SIS, SIF and SIL.

I understand that generally the logic solver part of a SIS is a PLC or DCS.

But what if all safety functions are controlled by relay logic only?

For example,

If a conveyor has an EStop button that cuts the power to the motor via a relay; Does the SIL level need to be calculated? Or is relay logic considered ultra reliable and not needing to be SIL assessed.

Regards,

RobW
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CSM Engineer
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#1

Re: Relay logic and SIL level

12/04/2008 11:15 PM

"Or is relay logic considered ultra reliable and not needing to be SIL assessed."

Absolutely not!!!

First, an E-stop is not generally intended to be a safety circuit, only a means to quickly bring equipment to rest; normally additional measures must be taken to allow operators to enter the hazardous zone. Second, relay-based safety circuits must be designed correctly and safety-rated components must be used. Also, a safety system controlled by PLC must use a safety-rated PLC with redundant processors and proper communications protocols. Of utmost importance is to design a system that will not allow the machine to restart while an operator is within the hazardous zone. Here is an example of a relay based safety circuit that allows one or two operators to approach a hazardous conveyor (more would require full LO/TO):

This circuit satisfies the requirements for SIL3 (Control Reliable). The MSR5T is a safety relay from Allen Bradley.

Hope this helps!

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JohnDG
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#2
In reply to #1

Re: Relay logic and SIL level

12/05/2008 1:55 AM

Hi, CSM Engineer - don't know if you're up to speed on A-B/Guardmaster stuff these days, but (as I found recently to my alarm!) the MSR5T has been phased out - the new recommended product is the MSR127RP/TP (using the TP (monitoring/auto reset) version for your cct). I've just bought 18 of them @ £107 ea. - for one system!!!

Cheers,

John

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CSM Engineer
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#3
In reply to #2

Re: Relay logic and SIL level

12/05/2008 10:20 AM

John,

Thanks for the update! This was a circuit from a few years ago that I had handy, so I'm not surprised that particular relay has been phased out. It's par for the course for AB to phase out a perfectly good product for a more expensive alternative! The AB stuff is really great, but they're sure proud of it! Good luck with your project!

Regards,

CSM

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Garthh
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#4
In reply to #1

Re: Relay logic and SIL level

12/06/2008 1:48 PM

Here is an example of a relay based safety circuit that allows one or two operators to approach a hazardous conveyor (more would require full LO/TO):

When is it ever accecptable to enter a hazardous zone without locking out?
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CSM Engineer
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#5
In reply to #4

Re: Relay logic and SIL level

12/06/2008 10:04 PM

Under conditions that satisfy OSHA's "alternate means of lockout" as outlined in their regulations. So, technically, you are correct - it is still a lockout condition, but not as most people in industry think of it. This circuit is an example that satisfies OSHA's requirements so long as the activities are "routine, repetitive and integral to the use of the equipment for production". Maintenance always requires full LO/TO.

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Garthh
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#6
In reply to #5

Re: Relay logic and SIL level

12/09/2008 10:31 AM

Thanks CSM,

The ambiguity is bad, mgt. gets the idea the lessor standards apply to any & all situations.
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