HOA or OAH switch

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markstg

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A young engineer has asked a question and I don't have a good answer other than that is how I've always done it.

A motor that has an Auto Start function, uses a Hand - Off - Auto, 3 position selector switch. If switch is in Hand (motor running) and it needs to be placed in Auto, if Auto is calling for the motor to run, by going through the Off position, the motor starter will drop out and reenergize. Generally not a good thing to do on large motors.

If an Off - Auto - Hand switch is used, where the contacts are make before break (Allen Bradley makes these), the juking of the starter will not occur.

Why is HOA the standard when it appears OAH is a better solution?
 
jim dungar said:
Mostly, because that is how grandpa did it.
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I agree. Also is application dependent IMO. There may be some applications where you would/should never go from hand to auto or vice versa, as well as some where maybe OHA may be more desirable.

If the control needs are really complex enough you are probably using a programmable controller with a program that will lock out such functions from happening if they are not allowed by the process.
 
There are a couple of schools of thought about this.

One is that having the "off" position be the center position you can always only be one position away from "off". It is also fairly easy to look at a bank of selector switches and see if they are all in the same center position.

Some places do not allow an "on" position. The "hand" position has a corresponding start and stop PB so that on power failure the motor does not automatically restart.

A lot of places require feedback from the HOA switch so that the PLC program knows that the switch is not in the "auto" positions so it can take appropriate action.

Personally, I do not think it is appropriate to be screwing around with running motors in manual all that much so I do not think it matters.

HOA is the de facto standard for most places.

These days it is not all that uncommon to not even have such a switch since it is all but impossible in many plants for an operator to actually run things manually anyway, and not having the switch discourages them from trying.
 
Many times the hand function is only for manually running something when testing, setting up, or other procedure that is not a part of normal production. Whether or not this hand control feature should even be something the normal operator has access to using could even be something to consider when designing the controls. Running the item in hand during normal production can have bad consequences sometimes if that piece of equipment doesn't shut down when it otherwise automatically should have.
 
petersonra said:
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These days it is not all that uncommon to not even have such a switch since it is all but impossible in many plants for an operator to actually run things manually anyway, and not having the switch discourages them from trying.
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I have installed lots of them for lockout verification purposes. Typcially they are HOA with spring return from left to center. The lock out procedure for a "mechanical" lockout is to stop the process, go to the HOA and make sure the motor runs in hand, go to the MCC and lock out the starter, and then return to the HOA and make sure the motor does not run in hand. Try-lock-try...kind of like live-dead-live.

In some cases, like for cooling pumps and reactor agitators, the hand position is not spring return so that they can manually keep that type of equipment running if there has been a PLC or DCS failure.
 
kwired said:
Many times the hand function is only for manually running something when testing, setting up, or other procedure that is not a part of normal production. Whether or not this hand control feature should even be something the normal operator has access to using could even be something to consider when designing the controls. Running the item in hand during normal production can have bad consequences sometimes if that piece of equipment doesn't shut down when it otherwise automatically should have.
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It is not unusual to have some kind of manual function available via the HMI or SCADA system to handle this kind of situation. Or for bumping the motor to verify rotation. I have even seen a "bump" PB on HMI or SCADA screens now and then. Even programmed a few that way.

Makes it a lot easier to keep track of what is actually going on in your plant since all this stuff gets logged.
 
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