Safety relay Implementation

[Rocboni]

Right, looking for some info from some of you control gurus.

So I'm very interested in control sytems, panels etc have even built a few and have come across safety relays, trying to get the hang of how they are actually implemented into a control system, I've studied plenty of the safety relay datasheets and wiring diagrams so have an idea of how they are fitted in but I was wondering if anyone has a panel design where say a Pilz x3 was fitted and I could have a look at. I'll draw something simple up later and post it on here how I think they are wired, I understand there are different safety categories there are many ways they can be wired.

 

[Bobster]

Firstly how are you designing the system? To what standard? Who has risk assessed it.

The actual wiring of a safety relay is easy. The documentation and risk assessments are the hard part.

You mentioned you are building panels, how are you currently CE marking those? What standards are you building them to?

Please understand that machinery control falls directly under statutory law. The HSE have real teeth in prosecuting an have been doing at an increasing rate over the last 12 months since they were given new powers.

Not meaning to scare or put you off, just something you should know. An the fact you are a trainee will not in anyway protect you, it'll just mean you will pull whoever is responsible for your training into court with you.

I know people will read this and think I'm going over the top, but control system design and integration is my job. I see enough complaining from electricians around 5wws and Bob from the pub doing CU changes.

 

[Rocboni]

I knew this is the reply I would get, panel building is like a swear word on here, let’s forget I mentioned I built a have built few panels. Personally I know they were not to the correct standard and had a few cosmetic breaches but I was under instruction for my manager who designs be them but is old school and is still doing things the old fashioned way, he’s never heard of safety relays.

I’m not talking about a specific application here nor do I have a panel to build incorporating a safety relay, I am looking to expand my knowledge on the subject, when I’m through my time i intend to join a company experienced in panel building, but I want to gain some prior knowledge.

I have bought a machinery safety book by Dave Macdonald which is very good and details the process involved in carrying out the required risk assessments to decide which level of safety is required.

IF the actual wiring is easy can you expand, you clearly have experience in the field, surely you had to learn aswell?

This is how I imagine they are wired in a general sense, I know you can have two contactors for some categories of systems, how would you implement this if you had a panel controlling say 20 motors, would you have to have dual contractors on each or a set of large contactors feeding the panel with feedback loops?

 

[Bobster]

Pretty much correct but you need separate safety relays for each guard switch. Unless the guard switches you use can self diagnose problems with them.

For instance, an operator always opens GS1 first then GS2. Closes GS2 and then GS1, a problem with only 1 contact opening on GS2 will not be diagnosed, or even both contacts being welded closed.

So you would need a safety relay for each GS, that would auto reset. These monitor each channel and will lockout if it detects an issue. Put the 2 safety contacts on each relay into the safety loop on the master safety relay.

Emergency stop buttons can be series up, as they fall under different standards.

The guard switch circuits and e-stop circuits would be each assessed and probably have to meet different performance levels.

 

[Rocboni]

When you say safety loop of the master what terminal do you mean s21 and s22, does this need to be dual channel?

I presume this is how a global stop system work swith single buttons across multiple machines? Separate safety relays feeding one master?

Could you run a 4 core swa through the stop buttons or does it need to be separate cables for each channel?

 

[Bobster]

Duel channel or single channel architecture would be dictated by the risk assessment and the required performance level derived from it.

An yes I mean the auxiliary safety relays would be wired in the safety loop s11-s21 s12-s22 or whatever terminals are designated for a particular safety relay.

A 4 core can be run to each button, as the safety relay will self diagnose short circuits or crossed wiring.

 

[Rocboni]

Trying to relate this to an application I see quite a lot, grain elevators with hatches at the bottom for cleaning out, these often have a magnetic switch arrangement utilising the NC contact in the control circuit.

Should something like this really have a safety relay monitoring the guard switch and the elevator motor have dual contactors at the panel each contactor monitored by the feedback loop? I know it depends on a risk assessment.

 

[Bobster]

Yes, more than likely. The switches would also have to be safety rated.

Should also not just be relying on the safety circuit if the job is taking longer than a few min at a time.

 

[Bobster]

Dependent on screw speed, size and a few other factors. Stand still monitoring with hatch locking safety switches may be needed.

Regulations have also changed on 'fixed guarding' if a guard needs to be removed for regular operation/maintenance then the old rule of having it only accessible with the use of a tool is no longer allowed. It'll need a guard switch.

 

[Rocboni]

Well they are mostly to deal with blockages, obviously isolation and lock of takes place at the motor isolator.

The Elevators don’t have a high inertia and stop within a few seconds.

Good to know though, feel like I’m learning, the tricky bit is the risk assessment and actually determining the level of safety. And there’s knowone to make sure you’ve made the correct decision, presume that comes with experience.

 

[Bobster]

Dependent on the performance level needed, and more so with process safety SIS circuits. Your safety calculations will need to be verified by either someone else qualified in the company. Or if it's a higher level, somebody independent of the company.

 

[Rocboni]

Do you verify your own calculations? What qualifications do you actually need?

I won’t be doing them, there’s certainly know one in the company qualified enough to verify, they dont even use safety relays.

 

[Bobster]

Yes I verify my calculations and other companies. As well as being an independent expert witness when the HSE ask.

I'm accredited with the TÜV, pilz offer training too, I've been through that that's C&G accredited.

 

[Rocboni]

Reading through the book I have it shows a category 3 stop diagram requiring dual redundancy at each stage of the safety function.

For example where a guard switch on a particular elevator was activating the safety relay would dual redundancy on the final actuator for this elevator only be sufficient or does a cat 3 require the whole panel to be shut down using two large contactors, I’m just thinking where a panel controls multiple motors requiring a say 200A supply do you need to implement dual contactors capable of 200A to cut power to all the motors or just on the elevator thus only one extra contactor would be required?


Does this make sense?

 

[Bobster]

Ok, so category ratings is to do with the architecture of the safety circuit. It has nothing to do with safety rating. Even though it does directly effect it.

If you're controlling multiple motors in the situation you describe, it may be cheaper having each motor circuit to have say 2 * 5kw safety contactors. Use one just for safety and the other for safety and control, in the case of a DOL.

Or use 1 200amp contactor, and the second contactor can be the individual ones starting and stopping the motor in normal operation.

Then you have things like STO for larger VSD driven motors.

Doing safety properly isn't a cheap affair. I'm sure the calculations in the book your reading will later start factoring in the cost of a human life. Doing large plant wide safety systems, (different standard to machinery safety but it follows the same basic principles) the cost of human life is very much factored into the designs in using safety systems to eliminate all hazards.

Remember if a circuit is asking for Cat 3 architecture, redundancy isn't just electrical. The limit switch may have 2 contacts in it for a duel channel circuit, however there is still only 1 actuator.