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Safety relays and intelligent MCCs

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Fnewman

Senior Member
Location
Dublin, GA
Occupation
Sr. Electrical Engineering Manager at Larson Engineering
While I was in semi-retirement, intelligent MCCs appeared on the scene. So, now I am wondering about general requirement for safety relays with these starters since the starters are now commonly solid state themselves. For example, does a conveyor rope pull switch require a safety relay circuit? If so how is it interfaced to a solid state starter?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I don't recall seeing any solid state starter. There are soft starters and there are vfds and there are well-defined ways of dealing with the safety requirements for these devices.

The reality is that a safety relay doesn't make a semiconductor based device any safer.

The so-called intelligent MCC has nothing to do with the power switching side of things. It is solely about the control side.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Aside from what Bob said, a rope pull CAN be used with a "Safety Relay" for sure, and one can make a logical argument for it based on the redundancy and reset procedure if nothing else. But in the US, this is not (yet) mandated. Most people I know using rope/cable pull switches for safety on conveyors still do so with a Master Control Relay (MCR) as it has been done for decades. Same basic concept as a safety relay really, just not as involved in certifications and no built-in cross checking or redundancy.

But back to the Snart MCC aspect; SOME devices inside of smart MCCs can be fitted with "Network Safety" cards that can be used in conjunction with "Safety PLCs" and things like rope pull switches so that the E-Stop or MCR circuit wiring to each device in the MCC can be eliminated. But it's not a mix and match situation, the entire safety system but be designed and validated as a complete entity. Some people do it because there are multiple safety systems involved and they can all be coordinated better that way, but for most people this is an expense they tend to avoid. If the EU type safety regulations ever make it into our law books, this may change, but so far no.
 
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Fnewman

Senior Member
Location
Dublin, GA
Occupation
Sr. Electrical Engineering Manager at Larson Engineering
Misuse of terms aside, thanks for a useful response. With respect to rope pulls, I am used to seeing models with independent two circuits and contacts - one to interrupt control power to the starter and a second to provide an input to the PLC. Based on what you just said, I assume this approach is still legal in the US?
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Misuse of terms aside, thanks for a useful response. With respect to rope pulls, I am used to seeing models with independent two circuits and contacts - one to interrupt control power to the starter and a second to provide an input to the PLC. Based on what you just said, I assume this approach is still legal in the US?
Yes, still legal.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
keep in mind that just what you do is governed by your risk assessment.

it may be that your assessment concludes that no safety relay is required at all.

it might be that a minimal safety relay system is determined to be appropriate.

it might be that a more elaborate system is determined to be appropriate including redundancy and feedback checking.
 
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