MCP door interlock

We have some MCPs that have a electrical door interlock that shut off the control voltage when the door is open while the disconnect is in the on position is there a regulation that require labeling to warn of opening door while energized will shut equipment off?
 

kwired

Electron manager
Location
NE Nebraska
A little odd that it is arranged to only open control voltage, often you can't open door without opening all voltage inside, other than maybe class 2 control voltages.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
We have some MCPs that have a electrical door interlock that shut off the control voltage when the door is open while the disconnect is in the on position is there a regulation that require labeling to warn of opening door while energized will shut equipment off?
This used to be a common practice. There were actually special door interlock switches that opened up the Estop circuit if the door was opened. The special switch was three position. When the door opened, a spring would send the piston forward and open the estop circuit. With the door closed, the position was in the center position and the contact closed. It also had a third position where one could pull the piston out with the door open and the contact would go back on, but this was a maintained position.

I have no idea what they were trying to accomplish. Have not seen it done that way in quite some time.
 

kwired

Electron manager
Location
NE Nebraska
This used to be a common practice. There were actually special door interlock switches that opened up the Estop circuit if the door was opened. The special switch was three position. When the door opened, a spring would send the piston forward and open the estop circuit. With the door closed, the position was in the center position and the contact closed. It also had a third position where one could pull the piston out with the door open and the contact would go back on, but this was a maintained position.

I have no idea what they were trying to accomplish. Have not seen it done that way in quite some time.
Makes some sense if the E stop operates a contactor or shunt trip breaker that supplies the enclosure being opened, but if it cuts control power only and leaves the power circuits inside that enclosure energized it doesn't make much sense.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Makes some sense if the E stop operates a contactor or shunt trip breaker that supplies the enclosure being opened, but if it cuts control power only and leaves the power circuits inside that enclosure energized it doesn't make much sense.
It is roughly equivalent to tripping the estop pb, but why it was considered a good idea escapes me.

It also used to be common to power the estop circuit through a PLC output, This was explained to me as sort of a watchdog. If the PLC crapped out, supposedly the output would shut off and trip the estop. Made more sense, at least back when PLCs were not as reliable as there are these days.
 

kwired

Electron manager
Location
NE Nebraska
It is roughly equivalent to tripping the estop pb, but why it was considered a good idea escapes me.

It also used to be common to power the estop circuit through a PLC output, This was explained to me as sort of a watchdog. If the PLC crapped out, supposedly the output would shut off and trip the estop. Made more sense, at least back when PLCs were not as reliable as there are these days.
E-stops are typically more focused on shutting down mechanical energy hazards when actuated and not so much electrical energy hazards. Often shut down pneumatic as well as it somewhat directly associated with mechanical energy in many instances.

Opening a control panel door usually doesn't have much for mechanical energy hazards and mostly has electric shock and arc flash hazards., which shutting down control power doesn't necessarily remove those hazards within the panel.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It was not so much about eliminating energy as the thought was that the machine would shut down because the estop would be tripped. The fear was that the PLC would go crazy and who knows what it would do so hopefully it would just shut down the machine by tripping the estop. I don't know that ever happened but that was the idea anyway.

There were even elaborate watchdog timers created where you had to turn on and off a sequence of PLC outputs to keep the timer from tripping the eStop on the theory that if the PLC went nuts The sequence of outputs would be interrupted and the timer would trip the eStop.
 

kwired

Electron manager
Location
NE Nebraska
It was not so much about eliminating energy as the thought was that the machine would shut down because the estop would be tripped. The fear was that the PLC would go crazy and who knows what it would do so hopefully it would just shut down the machine by tripping the estop. I don't know that ever happened but that was the idea anyway.

There were even elaborate watchdog timers created where you had to turn on and off a sequence of PLC outputs to keep the timer from tripping the eStop on the theory that if the PLC went nuts The sequence of outputs would be interrupted and the timer would trip the eStop.
Have they been putting the PLC's on Zanax and no longer is :) a problem?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Have they been putting the PLC's on Zanax and no longer is :) a problem?
I don't know that it was ever more than a theoretical problem.

Another thing they used to do was make you use a relay on solid state outputs on the theory that the solid state output might fail on otherwise. I don't know what they thought the relay was going to do for you. if the solid state output failed on, the relay would be on, and the ice cube relays often used for this purpose were notorious for welding closed.
 
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