Is this Circuit Safe

Tim Ganz · Feb 17, 2014

Is the attached circuit safe? They have the Hopper door switch and the floor door switch which are both dual contact tonged style limit switches and are both red but they are only using 1 of the legs in this circuit and nowhere else in the print except as an input to the plc so it will show on the HMI if it's the floor switch or the hopper switch.

Is this single leg breaking power to the output module really safe? Does it comply with safety standards?

This is OEM equipment and only a few years old.

cjd1965 · Feb 17, 2014

I would say NO

I would take a 2 channel circuit into a dedicated monitoring relay and a contact of that relay into the PLC

Obviously the relay would also lockout the hazard such as agitator/ feed conveyor /hopper valve /whatever

Tim Ganz · Feb 17, 2014

Does it comply to any category?

JDCROCKETT724 · Feb 17, 2014

What you have here is a single channel safety circuit that is in an auto start configuration. Meaning, as soon as you correct the floor switch or hopper switch then your loads are live without having to reset the circuit via a PB. The safety question really depends on how severe someone could get hurt if one of these items were to fail. I typically always default to a safety relay with 2 channel safety switches for protection. I set the relay to auto start if it is a light curtain type application and a monitored start for all e-stop and guard door applications.

JDCROCKETT724 · Feb 17, 2014

It may be category one, but I don't have any safety manuals with me right now.

brendan.buchan · Feb 17, 2014

Looks like it only meets the requirements of safety Category B or 1 at best.

It all depends on the results of a risk assessment on this machine as to whether this is acceptable.

Generally (and I mean very "generally") speaking if someone can get hurt enough that normal first aid (ie. a bandaid) won't fix, then you should be looking at Cat. 2+ as a minimum depending on the frequency of exposure to the hazards.

silva.foxx · Feb 17, 2014

Cat 1. The safety category should satisfy the risk assessment. We do not have enough info to say whether it is safe enough as we can't ascertain what the danger is. I'm guessing that the desired outputs from the card is set by a reset pushbutton input, perhaps from the HMI.

cjd1965 · Feb 17, 2014

I would never wire a safety switch directly to a plc ....

silva.foxx · Feb 17, 2014

In this instance using what "sound" like proven safety devices (unknown) to switch voltage to the common of the output card to be switched to final control elements. If one of the two safety switches open then the supply is removed even if the outputs discrete switch fails closed or programmed in a unsatisfactory manner. But the final control element may fail closed, regardless of the single channel safety chain, and compromise the safety system integrity!

Doug_Adam · Feb 17, 2014

It depends.
If the door is never opened except for maintenance when the machine is shut down and isolated, then it may be OK.
If it is opened every 5 minutes, the operator putting his hand in to get something out of the whirling blades of death, and that is the only method of shutdown in use, then no.

Tim Ganz · Feb 17, 2014

It's a Grinder that sits in a pit below grade and the hopper top is about 5.5 feet off the floor.

The hopper door can be opened to access the machine to remove jams,etc.

The pit door is primarily for maintenance access and is machine is locked out in most instances when it's used.

It does not show on the print but there are about 6 other safety switches that protect doors on the granulator portion below the main grinder and again all this is below grade.

We are about to buy another one of these and it's wired the same way. Just wanted to make sure it will comply with current safety standards.

Tim Ganz · Feb 17, 2014

All the safety switch are in series and kill power to the output card when opened. and the other contact goes to plc input card to let touch screen display which switch opened.

daba · Feb 17, 2014

Tim Ganz said:
The pit door is primarily for maintenance access and is machine is locked out in most instances when it's used.

need i say more ...

Doug_Adam · Feb 17, 2014

Ok, most places now days require (by law) a written paper trail of risk assessment and risk reduction methods.
I would suggest you request this from the supplier.
This should also show the correct operation methodology (when to lockout, tag out), and correct test schedule.
Generally, the method you use is dependent on risk.
Low risk, means that you don't need much in the way of safety.
High risk requires extensive safety.
You should check the standards for your locality.

Is this Circuit Safe

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