Equipment Safety Circuits

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ernric43

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Location
United States
I have an older piece of equipment with the light curtains tied into the PLC to act as the safety device.
I found the original prints and it was wired that way from the OEM.
The light curtain stops the equipment when broken and the equipment will not restart until the light curtain is free from obstruction.
It was be fairly extensive and time consuming to add a safety relay.

Two questions:
Was this ever considered an allowable practice?

Am I obligated to change it?
 

roger

Moderator
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Location
Fl
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Retired Electrician
First and maybe only question, is the machine listed?

Roger
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
a fair number of plcs are rated as safety plcs. it is also possible that at the time it was built it was not considered necessary to have a safety rated device. in fact it might not be required even today. it depends entirely on what the risk assessment comes up with. have you done a risk assessment? If not how do you know that changing something will not make it less safe?
 

roger

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Location
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Retired Electrician
If it leaves the factory as a listed piece of equipment why would you think it needs to be changed and why would you?

Roger
 

roger

Moderator
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Location
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The PLC will do what it's told to do and if the particular conveyer, slicer, train, etc... is manufactured and listed with a light curtain as a safety device I wouldn't change it but you can

Roger
 

roger

Moderator
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Why would you think it needs to be changed if it's unlisted?
I don't and didn't say it did. Even if it isn't listed removing a safety from a working piece of machinery is scary and I would advise the OP not to change it.

Roger
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It's also entirely possible that what the original poster is describing is not actually a safety circuit. There is just not enough information in his post to determine what an appropriate step to take might be, if anything needs to be done at all.
 

ernric43

Member
Location
United States
I have an older piece of equipment with the light curtains tied into the PLC to act as the safety device.
I found the original prints and it was wired that way from the OEM.
The light curtain stops the equipment when broken and the equipment will not restart until the light curtain is free from obstruction.
It was be fairly extensive and time consuming to add a safety relay.

Two questions:
Was this ever considered an allowable practice?

Am I obligated to change it?
Again this is an older piece of equipment so I does not have a safety PLC. We recently had a safety incident and found out that the equipment operated via the jog push buttons while the light curtain was interrupted. We modified the programming to address this issue.
The question came up from upper management about whether NEC ever allowed an OEM to use a PLC, non-safety PLC, to act as a safety device.
 

ernric43

Member
Location
United States
a fair number of plcs are rated as safety plcs. it is also possible that at the time it was built it was not considered necessary to have a safety rated device. in fact it might not be required even today. it depends entirely on what the risk assessment comes up with. have you done a risk assessment? If not how do you know that changing something will not make it less safe?
When was the requirement established that required a safety rated device? This is not a safety PLC.
 

ernric43

Member
Location
United States
It's also entirely possible that what the original poster is describing is not actually a safety circuit. There is just not enough information in his post to determine what an appropriate step to take might be, if anything needs to be done at all.
All of the safety devices on this piece of equipment, door switches and light curtains are inputs to the PLC and it is not a safety PLC.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
The question came up from upper management about whether NEC ever allowed an OEM to use a PLC, non-safety PLC, to act as a safety device.
This would not be an NEC issue.

Roger
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
This would be more of an OSHA and maybe an ANSI type issue.

Roger
 

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
Again this is an older piece of equipment so I does not have a safety PLC. We recently had a safety incident and found out that the equipment operated via the jog push buttons while the light curtain was interrupted. We modified the programming to address this issue.
The question came up from upper management about whether NEC ever allowed an OEM to use a PLC, non-safety PLC, to act as a safety device.
OEM never had to be NEC compliant beyond or once UL listing was obtained as a complete peice of equipment. If the safety device was an add on to an older peice of equipment it was likely to meet OSHA requirements not necessarily NEC. If in place as an OSHA requirement, the ability to circumvent the safety switch via a manual action as described by pushing the jog button would make the safety switch ineffective and noncompliant from OSHA perspective. Sometimes OSHA allows for maintenance reasons a safety device can be overridden but cannot be perminantly done so that it will "run" under normal operations with safety device disabled.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
All of the safety devices on this piece of equipment, door switches and light curtains are inputs to the PLC and it is not a safety PLC.
If They are truly safety functions then they should be handled by safety rated controls. However depending on the age of the machine safety rated stuff may not have been available at that time.

Really you need to do a risk assessment and determine just what the potential hazards are, if any.
 

tom baker

First Chief Moderator
Staff member
The NEC is a installation standard and it’s scope covers the wiring to the machine. Other standards such as UL508A, NFPA 79, etc, cover the machine wiring.
 
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