Equipment Safety Circuits

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
There are a lot of older machines out there that would not pass muster according to today's safety standards. Your risk assessment will tell you what if anything needs to be done to make the machine safe by today's standards.

It is probably not as simple as adding some safety relays. It almost never is. We have turned down work because we did not think we could make an existing machine "safe".

On the other hand, often it is pretty straightforward.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Although there are specific agencies that do certify machines from a functional safety perspective, machine safety controls / relays etc. are not yet mandated here in the US like they are in other parts of the world. OSHA is more vague about this issue in that they mandate general safety / guarding and accept machine safety control systems as a way to get there, but they stop short of actually requiring them so long as operator safety is achieved.

That said, many companies, especially those with operations in other countries or who are headquartered overseas, will require safety certification, which will mean compliance using safety controllers / relays etc. But given that your machine has already been in operation without that we can assume this is not the case, so anything more would be basically voluntary. There are good arguments to be made for upgrading, but again, no mandate.

One of those arguments that I have indirectly experienced is one of liability. If a worker is injured using a machine and their lawyer finds out there WAS a safer way to do the controls, they may go after the owner for not taking accepted measures to protect workers using available machine safety systems. I friend did work retrofitting a machine that makes rebar reinforcing cages for concrete pillars and although he used safety light curtains and through-beam systems to keep workers away from the moving parts, the workers found a simple way of tricking the system and someone got skewered. Their lawyer wanted to know why they didn't do a safety assessment and get certification, the owner claimed ignorance, but lost. My friend, the EC that did the work, was not found responsible because he was contracted by the owner without stipulation of providing a certified system, but his lawyer still cost him over a year's worth of income.
 

BillyMac59

Member
Location
Wasaga Beach, Ontario
Occupation
Industrial Electrician
I've read through the responses and can see some logic in most arguements. From a Canadian perspective....if the machine came from an OEM with the light curtain tied to a non-safety PLC AND was approved by some governing body as safe, then it's safe. The big BUT is in entry #11. The safety circuit was modified due to jogging being enabled withh the light curtain blocked. Up here, this would be interpreted as a significant change to the machine's safety characteristics resulting in a PSR - Pre Start Review, as mandated by provincial health and safety.
As far as using a "regular" PLC for part of a safety circuit, I don't know if it was ever illegal. Every PLC manual I ever read - dating back to the mid-80s - strongly advised against this practise.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I've read through the responses and can see some logic in most arguements. From a Canadian perspective....if the machine came from an OEM with the light curtain tied to a non-safety PLC AND was approved by some governing body as safe, then it's safe. The big BUT is in entry #11. The safety circuit was modified due to jogging being enabled withh the light curtain blocked. Up here, this would be interpreted as a significant change to the machine's safety characteristics resulting in a PSR - Pre Start Review, as mandated by provincial health and safety.
As far as using a "regular" PLC for part of a safety circuit, I don't know if it was ever illegal. Every PLC manual I ever read - dating back to the mid-80s - strongly advised against this practise.
You can tie the output of a safety circuit to a non-safety related device or circuit.

You can also put a non-safety element into a safety input circuit as long as it does not impair the safety function.

Before I would condemn any of this as unsafe, I would want to know a lot more about what is actually being done.

It is not as common as it once was but at one time it was fairly common to use light curtains as a replacement for cycle start push buttons. They were not intended to be a safety element just an operator convenience and a way to speed up the cycle that did not require the operator to use a cycle start button.
 
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