Question for the experts:
Let's assume I have (2) pumps on a project. They are guarded - that is there is no safety issue that would cause someone to get hurt, and each pump has a safety disconnect next to it that allows service to change the pump if necessary.
The pumps are on control power - that is when the MCR is off the pumps will also shut off.
If I had Hand-off-Auto switches for these pumps, when control power reset is pressed, the pumps would immediately start if the selector switch is in the hand position.
This doesn't seem allowable per NFPA 79:
"Once active operation of an emergency stop (see
Section 10.7) or emergency switching off (see Section 10.8)
actuator has ceased following a command, the effect of
this command shall be sustained until it is reset. This
reset shall be possible only at that location where the
command has been initiated. The reset of the command
shall not restart the machinery but only permit restarting.
(3) It shall not be possible to restart the machinery until all
emergency stop commands have been reset. It shall not
be possible to reenergize the machinery until all emer‐
gency switching off commands have been reset."
For this reason, I was thinking of two things:
1. Having a Start/Stop button in addition to the H-O-A switch (seal circuit w/contactor that only works in Hand mode)
2. Using a spring return for the Hand portion of the switch so someone would have to be standing there when the machine comes on.
Any experience with this? HOA switches seem incredibly common - so maybe I'm overthinking this.
On a PLC, I suppose I would have any E-stop just automatically put the HOA on the HMI into Off mode.
Let's assume I have (2) pumps on a project. They are guarded - that is there is no safety issue that would cause someone to get hurt, and each pump has a safety disconnect next to it that allows service to change the pump if necessary.
The pumps are on control power - that is when the MCR is off the pumps will also shut off.
If I had Hand-off-Auto switches for these pumps, when control power reset is pressed, the pumps would immediately start if the selector switch is in the hand position.
This doesn't seem allowable per NFPA 79:
"Once active operation of an emergency stop (see
Section 10.7) or emergency switching off (see Section 10.8)
actuator has ceased following a command, the effect of
this command shall be sustained until it is reset. This
reset shall be possible only at that location where the
command has been initiated. The reset of the command
shall not restart the machinery but only permit restarting.
(3) It shall not be possible to restart the machinery until all
emergency stop commands have been reset. It shall not
be possible to reenergize the machinery until all emer‐
gency switching off commands have been reset."
For this reason, I was thinking of two things:
1. Having a Start/Stop button in addition to the H-O-A switch (seal circuit w/contactor that only works in Hand mode)
2. Using a spring return for the Hand portion of the switch so someone would have to be standing there when the machine comes on.
Any experience with this? HOA switches seem incredibly common - so maybe I'm overthinking this.
On a PLC, I suppose I would have any E-stop just automatically put the HOA on the HMI into Off mode.