Hand-Off-Auto switches

Rson

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Join Date
Jun 2017
Location
Michigan
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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.
 
Can you put a latching relay in for hand or auto, such that if power drops off, the relay drops out, and once power is restored, they have to switch between off and hand or auto again to restart?
 
1. Having a Start/Stop button in addition to the H-O-A switch (seal circuit w/contactor that only works in Hand mode)
I'm hardly an expert, but the HOA setups I've got all have separate stop/start buttons.
 
Why the need of HOA?

Estop and Reset bring on MCR and hold the reset.
MCR ON + Start + Stop bring Pumps ON and hold Start.
 
Why the need of HOA?

Estop and Reset bring on MCR and hold the reset.
MCR ON + Start + Stop bring Pumps ON and hold Start.

Well, in this particular case, the HOA switch is really more of a 'Local/Remote' switch.

Local/Hand = run pump from panel
Remote/Auto = run pump from control enclosure elsewhere
 
Well, in this particular case, the HOA switch is really more of a 'Local/Remote' switch.

Local/Hand = run pump from panel
Remote/Auto = run pump from control enclosure elsewhere

Hand and Estop and Reset bring on MCR and hold the reset.
MCR ON + Start + Stop bring Pumps ON and hold Start.

Auto connect to Remote
 
As others have mentioned, most of the HOA units I have seen have a start/stop circuit with a latching relay around the start button that would drop out in case of power loss and require, if in HAND position, the start button to be pressed again to resume operation.


If yours doesn't have that maybe order one of these and it should fix your problems.



Of course, since you said that here is no hazard to man/machine if the pumps restart automatically, perhaps this is a non-issue anyways. This is low voltage release vs low voltage protection. Some things are allowed to automatically restart, depends on the equipment and safety assessment.



If you have any doubts just order a HOA switch with a start/stop circuit that will require manual restart if left in the HAND position.
 
you are correct in this regard - a system cannot automatically restart when power is restored. upon power loss or an e-stop, a safety relay must be reset which requires a manual reset push button. Sorry, a safety relay must be used to reset the system, an MCR only is no longer allowed, the contacts can weld /stick and not de energize the pumps.

so you will have incoming power.
main disconnect
control transformer
safety relay with a seal in circuit
e-stop (required - may also require a loto attachment), power on/reset
the hand off auto switch -
then a start and stop switch to start the system using the MCR
a hand and auto indicator push to test light to indicate which state the machine is in.
don't forget your motor protection and field disconnects.
also, do both pumps startat the same time?
if i am incorrect on any point, someone please correct me.
regards,
james
 
Is this PLC controlled?

When the PLC starts have first scan kill all outputs...

But I prefer to only work with momentary switches.
 
Hi, if the pumps operate strictly "mechanically" that is with contactors and an H-O-A with no outside controls except maybe a pressure or level switch--NO PLC--then if you don't want them to start at all in either position on the HOA after a power fail then you need a manual momentary start button at the minimum. You can stop the pumps by turning the switch off.
 

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