Safety Interlock Question

[machfive55]

I was interested in doing a safety interlock like Kal did so the elements don't fire when the system is powered up.

My question, is there a way to do this without the relay. For instance, feed the power from the keyed switch through the element switch NC contacts, then into the main power contactor. So, if the switches are closed and the system is powered up, the main power contactor gets hot on the coil. Now, in this configuration, as soon as one of the element switches opens, the main contactor coil shuts off. However, what if you you feed the coil with a short from a power bus that is after the main contactor. This would essentially feed the main contactor coil from power running through the contactor. Would that work?

So, when the main key is thrown, the startup "check" happens and feeds the main contactor which allows power to the main bus. Once power gets to the main bus then the contactor coil is fed by the main bus, so if you switch the element switches the main contactor stays on. Does this make sense?

I'd also put an e-stop on the main contactor coil, so if you mash the e-stop everything shuts down.

 

[reynolds5520]

What you are suggesting makes some sense in terms of eliminating the relay component but there is one thing missing.

Your circuit concept basically uses the key switch and the interlock switches to turn on the main power contactor which then latches itself on. Once the main power contactor has been latched on, the key switch has been bypassed and won't turn things back off. You would need another set of contacts in the key switch to break the latching circuit when you switch it off.

You could also use the e-stop as the power off, but that doesn't give the behavior most users would expect from turning the key to off.

 

[machfive55]

Got it, that makes sense. I knew I was missing something, couldn't put a finger on it.