Safety PLC functionality

Has anyone experience with controlling a maintenance via a safety PLC. Which safety category is applicable for a normal maintenance switch of a motor.

Don't make any changes in Safety-Rated PLC.s

You are personally liable for any consequences that occur after your change, in the event of an accident.
Usually, there are passwords put in place to limit any changes that may be required, but normally a well designed system does not require change.

There are versions of software, depending on OEM, that allow your maintenance staff to "MONITOR" the process, but not modify the logic.

Depends on your "maintenace" staff, and their abilities. Personally, I would maintain safe distance from the engineered solution unless asked to modify it from a higher level.

Keep yourself protected, and the others around you.

I do believe there is no safety category for a motor.

There maybe for whatever the motor is doing though.

You can have a safety plc and a frequency converter that is CAT 4 but still have power in the circuit. The safety plc makes sure that it cannot move but its not for maintenace.

To do maintenance turn the power off and lock the circuit breaker.

Bratt said:

You can have a safety plc and a frequency converter that is CAT 4 but still have power in the circuit. The safety plc makes sure that it cannot move but its not for maintenace.

To do maintenance turn the power off and lock the circuit breaker.

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Fully agree... Maintenance is done with all the power and air/ hydraulic supples isolated.

additional info

This is the case:
Safety PLC installed. Emergency stop pushbutton's double channel to inputs safety PLC.
Safety PLC double outputs to contactors switching off power.
Maintenance switch single input to safety plc.
Same double output switching of contactors and power supply.
Is this common practice, or should the maintenance switch have a double input to the safety plc as well?

Maintenance is normally an Isolator, which is locked off when an engineer is doing work.

I suppose it is possible to wire a selector/key switch into a safety PLC to act in a similar way to an E-Stop.

BUT... I would never do it.

If someone is working on a peice of equipment, he MUST HAVE PIECE OF MIND that NO_ONE can switch it back on.

This means ISOLATE the power and LOCK-OFF and have key to lock in pocket.

Off course the mainenance switch is padlockable.

toone said:

This is the case:
Safety PLC installed. Emergency stop pushbutton's double channel to inputs safety PLC.
Safety PLC double outputs to contactors switching off power.
Maintenance switch single input to safety plc.
Same double output switching of contactors and power supply.
Is this common practice, or should the maintenance switch have a double input to the safety plc as well?

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Question is hard to answer without knowledge of what the machine is and what it does.

What it sounds to me is that some maintenance procedure requires that power is maintained to machine or a portion of it.

Therefore lockout of main disconnect is not possible.

What is also needed is a description of what this maintenance task is.

Dan Bentler

The total machine consists out off pallet conveyor sections, linedrive section (aluminum web), cut and piling section.
The maintenance switch is only used during preventive maintenance activities (mechanical activities like roller, bearing exchange.
No electrical maintenance activities like motor disconnect.

toone said:

The total machine consists out off pallet conveyor sections, linedrive section (aluminum web), cut and piling section.
The maintenance switch is only used during preventive maintenance activities (mechanical activities like roller, bearing exchange.
No electrical maintenance activities like motor disconnect.

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Hmmmm I'm gonna have to come over there to learn new maintenance and repair techniques. Every time I change bearings, rollers etc I am required to take things apart so I pull the main disconnect and lockout and tag it.

What I had in mind when I responded WAS PM kind of stuff ie greasing, checking alignment, proper operation of linkages etc. In the maintenance end of things I had in mind setting pressure switches, limit switches, etc etc.

What I think you need or seem to need is a
"maintenance OK to RUN or JOG (?)" kind of bypass switch which may bypass some safety functions to allow special maintenance activities. An example is bypassing the door safety switches on machines.

Dan Bentler

Dan Bentler

I have quite often used a category 4 safety PLC with a lockable isolator feeding into the safety PLC for lock out / tag out.
When the safety PLC sees the isolator off and both safety contactors de-energised then it energises a lamp that indicates the system is isolated.
If the operator has locked the isolator out and also seen the "isolated" lamp come on then they are safe to work on the machine.
This method is totally dependant on the safety PLC program being correct.
The reason we do this and do not trust full current isolators for this is below. A few years ago I was in a sawmill and had to do some work on a trim saw box. (5 metre long metal box full of saw blades) I turned off the rotary isolator but it did not feel right. Upon checking I found the only thing was happening was the shaft turning in the switch. The fitters did not notice this and I have no idea how long they had been working in this machine changing blades without the saw box being isolated.
If you read the rules/regs etc a full current isolator does not satisfy the requirements for isolation of a cat 4 hazard because it is not provable. OK you can say an electrician can check his meter works, turn the isolator off, then prove the isolator is off but does this happen if a fitter is required to work on a machine.
Some isolators are now being made with a visible section where you can see an air gap when the system is isolated but these are a bit rare.
I trust my life to a well designed and implemented safety PLC controlled machine then a single $20 isolator. Then the question is how do you know the safety PLC system is well designed????
Regards Alan Case

Safety PLC system will be tested and verified thoroughly and only well proven equipment will be used. The code of the plc will be secured. In case changes are required and executed the complete safety system needs to be validated again.
Feedback from the status of the maintenance switches will be indicated on the HMI system. But I agree it would be more convenient to have direct feedback at the maintenance switch.

With apologies to those who are in no doubt,

I would suggest where an isolator is used as isolation
The absolute minimum requirement for making safe is :

1, check your test lamp on a known source
2, check the incoming mains to isolator is live using approved test lamp
3, Turn isolator off
4, check isolator is dead on all outgoing phases
(between phases and all phases to earth, then all phases to neutral where present)

5, re check test lamps on known source
( usually seperate portable source )

6, Tag , Lock off using multi hole hasp and lock and retain key

7, If other trades are working on same kit , there seperate lock should be put through another hole on multi hole hasp

I have also known and experienced the shaft of an isolator turn and not actually actuate the isolator , however if tested properly , would be apparent at testing

Obviously there are some maintenance procedures that may have to be done whilst switched on , but these should be covered by a sop or standard operation procedure

The original question was about safety plc,s , sorry to pull in a slightly different direction but i feel the basic disconnection of a machine or part of one should be very carefully considered , especially as you need to prevent the accidental switch on by others or the potential for automatic startup

toone said:

Has anyone experience with controlling a maintenance via a safety PLC. Which safety category is applicable for a normal maintenance switch of a motor.

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For maintenance and repair - turn the thing off! and lock it out!
For a machine that requires access to dangerous moving parts during operation so that personnel can clear jams, tangles etc. then the interlocks need to be at the same safety category as the during normal operation.

Also a safety category to EN 954 or EN 13849 doesn't apply to safety PLCs these sytems should be designed and validated to EN 60261 and be assigned a SIL value.

The only use for a single channel input from a keyswitch would be as a mode control switch say for machine setting or robot teaching where access is required but extra safety precautions are taken such as limiting movement or very slow speeds.