Safety Door Interlocks?

What is the proper standard for safety door interlocks? I am a firm believer in safety relay modules over a standard relay system. Yes they do cost a lot but liability plays a big concern.
Should the safety modules be interlocked through the master relay, cycle stop relay, or input PLC VIA output to Master or cycle stop relay? Thanks

For what application - airplanes, trains, ships, jails, nuclear plants,
uh, uh, etc etc.

They all have their own rule book on top of OSHA.

What are you trying to keep people from?
Xray, electrical, machinery ???

Dan Bentler

When I recently worked under contract for a large automotive company, the standard (which I fully agree with) was to use 2 pole door switches with a contact of each switch in series with the others. Each series string was then wired as an input to a safety relay. The output was used as the MCR (master control relay).

The safety relay works by comparing the two inputs for "agreement". This eliminates any "welded" or stuck contacts from creating a problem.

Leave the old style, simple relay MCR's to the Smithsonian. I've seen far to many shorts and welded contacts that would never drop out a traditional safety circuit...

Can the machine stop safely in time if the operator opens the safety shield? If not, then locking safety switches are needed to keep the door from opening until all hazardous motion is stopped.

Safety shields that are not noramlly opened by the operator in the normal course of operating the machine are tied to the emergency stop circuit. Opening these shileds is the same as mashing an E-STOP button and requires a reset of the estop string. Shields that are opened routinely to access the work area are interlocked through a seperate safety circuit which resets as soon as the doors are closed. This should deprive all electrical and stored energy sources (such as comrpessed air) in the work area of the ability to actuate, but once the door is closed the energy is restored without further action of the operator.

This is entierly dependent on the machine. You need to do a thorough safety analysis of the process.

Im also a big fan of 2 pole safety switches on the doors, preferably a switch that cannot be easily defeated. I've seen too many cases of the operator wrapping a bit of tape around a cam type limit switch to defeat the safety shield because he doesn't want to be bothered with opening and closing it.

Several years ago an operator lost a finger in a machine. I was sent to investigate and found an allen wrench jammed into a switch that was intended to keep his fingers attached. The machine was an older model and did not have the more modern tamper resistant interlocks. The operator was terminated for violating company policy by willfully and intentionally defeating a safety device. Nevertheless, he sued, and got paid for being stupid. Every since then I have been a huge proponent of tamper resistant switches, and I also monitor the switch circuit with the PLC (hardwire the safety circuit but monitor it as will with the PLC). If I expect to see the switch cycle, and it does not, then the PLC activates an alarm. That way the supervisor can see it in the alarm log if switch tampering is attempted.

The machine is a Unit Boxer for Automotive Filters.

Mechanical moving parts is the main safety concern. Is there a OSHA standard that would list certain criteria for safety interlocks? I think a double contact switch with its own series circuit is a great idea.

Off the top of my head OSHA specifies switches for
upper travel on cranes and hoists, emergency stops on shears and maybe on rolls. Little vague on this havent read this stuff for a couple years. Oh yeah how did I ever forget this two hand control or lite screen on punches and brakes or shears.

Generally they (OSHA) stipulate for guarding of mechanical drives. The trouble with guards is they don't get put back which is why you are putting in doors to allow needed easy and quick access. This all but forces you to install switches on the door to ensure "the guard is put back" before the machine can restart.

The trouble with this is making the switch setup foolproof - they cant bump the switch and start the machine and grind themselves up. The switch NEVER breaks or welds shut. Operators NEVER cheat the switch either.

I like the double contact switch also -- with contacts in series that is good redundancy.


Dan Bentler