Equipment Safety Circuits

[ernric43]

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]

First and maybe only question, is the machine listed?

Roger

 

[petersonra]

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?

 

[petersonra]

First and maybe only question, is the machine listed?

Roger

why would this matter at all?

 

[roger]

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]

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

 

[petersonra]

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

Why would you think it needs to be changed if it's unlisted?

 

[roger]

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]

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]

First and maybe only question, is the machine listed?

Roger

UL listed?

 

[ernric43]

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]

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]

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.

 

[ernric43]

UL listed?

So if by listed you mean catalogish, then no it's not listed. This is a custom piece of equipment made for our specific product.

 

[roger]

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]

So if by listed you mean catalogish, then no it's not listed. This is a custom piece of equipment made for our specific product.

No, I meant an NRTL such as UL.

Roger

 

[roger]

This would be more of an OSHA and maybe an ANSI type issue.

Roger

 

[Fred B]

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]

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]

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.