Safety Circuits, Force Guided vs. General Purpose Relays.
By Robert Anderson, BSEE
These are all questions that I found myself asking at the beginning of my last project. I've been away from control system design for a few years now, designing software in the interim, and what I found is that safety circuits are more critical and taken more seriously than they were even a few years ago.
There are categories of safety circuits now, (perhaps there were ten years ago too, and I was unaware of them) Category B, 1, 2, 3 and 4. Which indicate, not necessarily the level of safety (as some people think), but how the safety problem is approached. Although a category 4 circuit is generally viewed as more safe than a category 2, this may not be the case. Regardless, that's a topic for another paper, my focus for this paper is the problem that I was confronted with, which was essentially a logic problem using relays for safety. I had a situation where I needed to use quite a few relays to make a safety circuit that was failsafe. I will be speaking of "Force Guided Relays" and a possible alternative to them, General Purpose Relays.
Now, when I started the project I knew little about Force Guided Relays. Unlike traditional relays, the force guided variety don't have a long lever arm for the contact arm. The force acting on a general purpose relay is near the fulcrum of the arm. With a force guided relay, the force acting on the relay is about as close to the contact point as one can reasonably get. This arrangement puts more force on moving a "sticky" contact apart than the traditional variety. Traditional general purpose relays are more likely to "spring" away from a stuck contact, while the force guided relays are actually pushed (or pulled) away. Force guided relays are guaranteed, that when they do stick, they can never have both a normally open and normally closed condition simultaneously. So, the one stuck contact keeps any other contacts whether normally open or normally closed from changing states. Force guided relays are slower. Force guided relays are more likely to stick. Force guided relays wear out quicker. These are facts as I understand them.
I began wondering during my project if it might not be a disadvantage, rather than an advantage of the force guided relays that they are guaranteed not to have a NO and a NC condition simultaneously. Using a normal general purpose relay, using all NO contacts, pole #1 could be open, while at the same time pole #2 could be closed. This is clearly a faulty state. But, could this be to our advantage? Let's design a simple logic circuit.
Of course this is a very simple, made up situation, but reality may not be far removed. It is possible to take any logical safety circuit and using Boolean logic come up with its negative circuit, i.e., one that will always be off when it is on and vice versa. In so doing, a person can always use the unused poles of their general purpose relay for the task and avoid a costly redundant circuit using a full set of redundant force guided relays.
Robert Anderson, BSEE