Safety Relays with PLCs

curlyandshemp

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curlyandshemp
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I've searched and searched, but cannot seem to find wiring diagrams of Safety relays being used with PLC outputs. We have inherited an entire package line moved up from the US to Canada. A Prestart Health and Safety Report was done, and Category 3 & 4 safety circuits are required on almost each motor and drives.

Upon a search of ABs safety website, Pilz and Siemens, it seems I can only find wiring examples with the assumption a Pushbutton Start/Stop will be used to control motors. In our case, plc outputs are being used. Not all motors run at the same time , but under program control. I was thinking of using the safety relays ( AB Guardmasters ) K1 & K2 outputs to condition the PLC output to reach the required motors's contactor K1 & K2 contactors.
Contactors must be in series.

Anyone have a wiring example for Cat 3 & 4 circuits using a PLC ?

Thanks
Ian
 
This is just an assumption but could you use an interposing relay instead of a Pushbutton to start the machine? Just pulse it on for 2 seconds and turn it off using the PLC output. If you need something like dual contacts o na PB...then put 2 relays in series and use 2 PLC outputs.
 
Christoff84 said:
This is just an assumption but could you use an interposing relay instead of a Pushbutton to start the machine? Just pulse it on for 2 seconds and turn it off using the PLC output. If you need something like dual contacts o na PB...then put 2 relays in series and use 2 PLC outputs.
Click to expand...

Don't know

That is what I'm asking . How do you have a PLC's enable ouput fit into the safety scheme of things. I have seen previous posts where a safety relay enabled MCR drops out the control voltage to PLC outputs, but what if the relay output is a dry contact , such as in most VFD 2 wire start/stop applications ?
 
Is the PLC itself rated cat 3 or 4 ?

If not no matter how many I/O you will use in parallel/series still the machine will not pass.

If you still want to go with PLC have to use safety PLC's which usually are cat4
 
The safety relays should be hard wired and not via the PLC.

The PLC outputs should control as normal but the safety relay contacts should be used to remove the power.

Depending in your system, you may need timed delay on the safety relays (where a powered stop is quicker and safer than an imediate removal of power).
 
Ian,

It sounds like you are referring to a SIL level application. As the application of this sort of system has been raised to a rather high art including risk assessments and the analysis of health and safety exposures of personnel and equipment, the system is required to be designed by a certified person.

Also, there are SIL rated PLCs that are red in color that required dedicated software. If an official SIL rated installation is determined to be necessary, I think that you will find that a regular PLC no longer fits the application.

I just built a fairly straight forward plant-wide e-Stop system to kill the machinery in case of a fire or broken hydraulic line, etc. Air compressors, some mixing stations and a bank of hydraulic pumps, etc were hardwired to about 6 e-stop stations through special monitoring relays that detect equipment failures. Also the e-stop stations have internal monitoring provisions to detect if they fail. The monitoring provision requires seperate wiring back to the relays in addition to the actual e-stop wiring. All contacts in the system are doubled up to improve reliability.

The analysis of the requirement, the design of the system and the specification of the equipment was done by a certified designer.

I was only permitted to build it, but that was about all. Due to the leaglities and liabilities, this was not a fun job. I don't recommend it.

Bob A.
 
Safety control circuits must be independent of the BCS (Basic control system).
Typically the Safety relay would control power to your output cards, and the output cards would control the system. The enable contact on drives would also be run through the safety relay, as well as the control power for across the line starters.

The two system must be independent to prevent injury, you cannot rely on the PLC, because an unknowing person my alter your code at some point in the future and disable the safety logic.
 
Bob A. said:
Ian,

It sounds like you are referring to a SIL level application. As the application of this sort of system has been raised to a rather high art including risk assessments and the analysis of health and safety exposures of personnel and equipment, the system is required to be designed by a certified person.

Also, there are SIL rated PLCs that are red in color that required dedicated software. If an official SIL rated installation is determined to be necessary, I think that you will find that a regular PLC no longer fits the application.

I just built a fairly straight forward plant-wide e-Stop system to kill the machinery in case of a fire or broken hydraulic line, etc. Air compressors, some mixing stations and a bank of hydraulic pumps, etc were hardwired to about 6 e-stop stations through special monitoring relays that detect equipment failures. Also the e-stop stations have internal monitoring provisions to detect if they fail. The monitoring provision requires seperate wiring back to the relays in addition to the actual e-stop wiring. All contacts in the system are doubled up to improve reliability.

The analysis of the requirement, the design of the system and the specification of the equipment was done by a certified designer.

I was only permitted to build it, but that was about all. Due to the leaglities and liabilities, this was not a fun job. I don't recommend it.

Bob A.
Click to expand...

This is one of these situations where a plant has purchased used equipment off a broker, then brought the equipment to their facility to make a new production line out of old equipment. The main machines at issue are a palletizer at the end of the line and the de-palletizer at the beginning. The company has already paid to someone to replace all the old obsolete TI PLCs with 'brand spanking new' SLCs.

We have been subcontracted my the main contractor to get the equipment running and CSA approved. The PHSR was an 'oh by the way' later.
 
curlyandshemp said:
... but what if the relay output is a dry contact , such as in most VFD 2 wire start/stop applications ?
Click to expand...
* Feed the power to the VFD via two contactors in series.
* Contactors driven by two separate PLC outputs.
* Power to PLC outputs via Safety relay circuit.
* Auxilliary contacts from contactors fed back to PLC inputs to monitor status and shut down outputs if not coming on and off together.
* Never rely on interposing relays to shut down a VFD on emergency stop, even if they're fed from a PLC output fed from an e-stop relay. (Single point of failure - a safety no-no.)

The Guardmaster and Pilz relays have multiple points of failure and internal cross-checking every time they power up. I have just been changing a circuit left for me on an "in-house" design by a predecessor. He's used a Pilz relay alright - but then slaved two Omron MY4 (standard plug-in) relays from it, one for each power circuit. Words fail me. o_O
 
ToolGuyFred The Guardmaster and Pilz relays have multiple points of failure and internal cross-checking every time they power up. I have just been changing a circuit left for me on an "in-house" design by a predecessor. He's used a Pilz relay alright - but then slaved two Omron MY4 (standard plug-in) relays from it said:
Must be a common mistake. Our in house panel designer just had a panel built in our panel shop sent to the job site I am on. Done the same thing, all thing safety stuff to activate a Phoenix din rail mounted relay that conditions the PLC signal to a VFD
Click to expand...
 

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