Gomez_ said:
I'm with you on on everything except this. If you have safety doors on your machine, do you normally run with them open?
"Normally" has
nothing to do with the normal running state of the machine. The sensor has no concept of your machine. It doesn't know or care what your machine is doing. It doesn't know if it's on a $2b NASA spacecraft or a cat flap in my back yard. "Normally" is with reference to the
device itself. What is the normal state of the contacts on a non-contact guard switch, before you actuate it by placing an actuator next to it? Open. Normally open
retirednow said:
I have always believed that “normally open” should describe the status of a contact when its associated ‘actuator’ is not physically ‘forced’
But the text description/comment should always describe the result of actuating it. For example... a n/o push button to start the hydraulics will be drawn ‘open’ but described with what happens in its ‘on state’ eg “Hydraulics On” or “Start Hydraulics”
100% agree. Draw it in the normal (unactuated) state, and describe what it does when actuated. Your mention of hydraulics does raise the other interesting point that hydraulics and pneumatics are (as far as I'm aware) always drawn in the "energised" or "actuated" state. Which is a useful thing to know for anyone playing both sides of the court.
JesperMP said:
If the contacts are mechanically actuated, then N.C.
Argument: For mechanical contact is a safety circuit, they must be forcibly actuated when changing to the safe state (opening of the door).
I see your point, but I still disagree. I would counter that a mechanical guard switch is actuated by the guard closing against it, and that it's exactly the same concept as a magnetic switch being actuated by an magnet closing against it. Think of a simple limit switch on a guard. It's actuated by the guard closing against it, and when the guard is removed, it springs back to the open (safe) position. Of course, you usually wouldn't *actually* use a bog standard limit as a guard switch, but the concept applies just the same. A tongue-and-groove switch is actuated by the tongue. Remove the actuator (tongue) and the contacts open.
Rson said:
Why do safety contactors come with so many normally closed auxiliaries?
:angr:
retuirednow said:
Some n/c contacts can be used in a feedback loop to test that the contactor(s) have physically ‘dropped out’ when de-energised. This would be monitored by the master safety relay.
Quite right. And how many master safety relays do you have, in a typical panel? I know there are certainly instances where you might have several safety relays, but how many
master safety relays, that actually drop out (and thus need to monitor) a particular safety contactor do you have? Right, so in 99% of cases we'll need one NC auxiliary. What do we need the other three for?
Let me put it this way. What is the safe state of a safety contactor? De-energised. What is the safe state of a solenoid, pump, fan, heater, or just about anything else? 99% of the time, it's de-energised. So if you want your solenoids and pumps to enter a safe (de-energised) state when your safety contactors are in a safe (de-energised) state,
what type of contact do you need on your safety contactor?
Tom Jenkins said:
"Normal" in the context of contact status has NOTHING to do with the status of the contact during so-called normal operation.