HOA or OAH switch 1

[markstg]

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?

 

[waross]

The majority of applications generally alternate between auto and off. Hand is used by the maintenance workers.
They are common and the often same contact blocks as are used in push button stations are used.
If there is a problem with a HOA switch in a particular application then don't hesitate to use an OAH switch.

Bill
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"Why not the best?"
Jimmy Carter