Guarding and safety relays

Greetings all.

I've been trying to narrow this down and come up with a more specific question, but I'm having no luck, so here goes -

When do you have to use a safety relay on guarding? I can't think of 5 machines I've ever seen that had them at all (e-stops usually just cut the control power circuit).

I went to the OSHA website to knuckle down and read the regs, but is it me, or do they not show only the title, no text on the article?


Thanks!

TM

Ah, I thought of something specific

Specifically, what ought the safety relay DO? Throw an E-Stop? Cut control power? Kill main power to anything that moves? Initiate self-destruct?

What brings this up is, I've got a machine that is going to need some guards. Obviously, the guards must be opened for routine setup tasks, which raises the issue of bypassing the guards.

Now, if I use a PLC contact to bypass the guards, and the contact fuses shut, safety problem. If I use a manual switch, and the switch sticks, safety problem.

So how do I approach this by any means short of just cutting control power to the entire machine whenever a door opens, regardless of when?

Or should I be doing precisely that?

Thanks!

TM

The main purpose of a safety relay is to provide redundancy and cross checking. It can monitor the inputs (naturally) but also the auxilary contact(s) of outputs to ensure they are in a safe state (not welded) before starting.

The main reasoning to judge whether increased safety is needed is in 3 parts. 1) What could happen to the operator if the safety system fails? 2) How often is an operator exposed to this situation? 3) If the situation occurs, how likely is it that the operator can escape from harm?

The more scary the scenarios get (Death, multi times a day, can't escape at all) the more you will want redundancy on the guard switches, a guard relay, force guided contacts on the contactors etc. I guess it's your call.

Many times the safety relays are used when locking guard switches are used and especially when you need to delay the opening of the locking switch.

This page may be helpful:

http://sti.com/safety/index.htm

Start with document 819 under the Safety Interlock Switches and monitoring relays section. Much of the other stuff is product specific.

Also found a document at Rockwel/AB their number
SHB-900 at the bottom of this page which may be helpful:

http://www.ab.com/manuals/pd/safetys.htm

Regarding your second post, it may depend on the type of machine you are building and the specific hazzards present (if there is a robot or press to deal with there are specific standards you can turn to)

The following is for education use only
On a previous machine I worked on, we put a key switch inside the electrical encosure for overriding the guard door switch interlock to allow setup of a gage. A second set of contacts from this switch went to the PLC to prevent automatic operation when in the bypass position. Note that even if this machine had no guarding, the worst case injury would have been a cut or a broken finger. (Of course, if you are a hemophiliac, a cut can be deadly).

It can definately be a challange to design machines to be safe and to be user friendly. Many times these goals are in conflict and given the current direction of regulatory agencies I see this only getting more challenging in the future.

nOrM

Bernie wrote:

I guess it's your call.

Click to expand...

You will find this is the about the MOST help you will generally get from any of the safety component suppliers.

Due to the ease with which one party can sue another for seemingly any reason, you won't find many people who will take any kind of responsibility for your system. My experience has been the suppliers say, "here are our products, here is our documentation, determine what you need and we'll sell it to you."

Maybe other's have had better luck?...

Yeah, i agree with everything your man bernie says. It's down to the engineer responsible for the design of the system to determine risk levels (or 'categories'). I don't know if it is similar over there, but here, if there were to be an accident involving an operator and some form of machinery, the first thing the HSE would demand is all supported documentation relating to the machines design (especially the RISK ASSESSMENT carried out on said machine).

Once the risk assessment has been carried out, a system can be implemented and the necessary category level achieved, whether it be category 1/2/3/4 (4 being most stringent).


If YOU decide the machine is perfectly safe (from an operators point of view), with a single e-stop contact, then ok, but you have to ask, what is the worse that can happen if that contact welds shut, or wiring becomes faulty?


Over to you Tim!

TimothyMoulder said:

I went to the OSHA website... is it me, or do they not show only the title, no text on the article?

Click to expand...

They'll show more for dollarzzzz. Safety manuals are expencive
and there are so many of them. I think this is one of the major
sources of income for CSA for example - although they just reprint
the European standards and add their own logo on the cover.