If somebody can tell me what is the function of HOA and what is the purpose HOA station combined with start/stop switch while the signal is coming from MCC. Can we use HOA alone with MCC to stop motors localy.
HOA = Hand - Off - Auto
Hand allows for local control via pushbutons
Off is self explanatory
Auto allows for control via a remote source (eg. PLC)
The HOA is a device that has switches that maintain their position.
Start and Stop buttons are devices that have momentary actions.
The HOA switch should be used in a situation that has a single point of manual control to allow the motor (or other device) to A - operate from an automated system, O - not operate, or H - operate with no safe guards or automated control.
The Hand position should be used to bump the motor or to operate for shorts times while being observed by operating personnel. Of course the circuit protection and overloads in the MCC will protect the motor.
The Off position will prevent any operation. This should not be the only method of preventing operation when maintenance is being performed. A Lock-Out-Tag-Out scheme should be used.
The order of the positions should be OAH. This allows for moving to any position with out going to an unwanted position.
Start and Stop buttons are intended to allow control of a motor from momentary actions. More than one button can be used for these functions and other controls elements can be placed in the circuit for permissives and/or control.
It's not a good idea to use both a HOA and Start/Stop Buttons at the same location. It confuses the operators.
When controlling a motor or other device from a PLC or other type of controller, you would want to use the HOA switch. Start and Stop buttons could be wired to the inputs and one output from the PLC would be routed through the Auto position of the HOA to control the motor.
So, Yes you can use an HOA switch alone with an MCC to stop motors locally.
You have listed a good description, but I wonder if you really mean what you say in the paragraph below:
> The order of the positions should be OAH. This allows for moving to any
> position with out going to an unwanted position.
My experience shows that most often I need just what it's called: HOA. That way you don't have to go through Auto to get from a manual operation (in Hand) to Off.
How about OFF-HAND-AUTO - that way, you don't have to stop the motor if it has been manually started. (In one plant I have worked on, pump motors were started manually where possible, as operators were expected to check for leaks and strange noises.)
Also, check the definitions of Local/Remote - many operators will regard "Local" as being "Local to the motor" rather than "Local to the MCC".
HOA (Hand-Off-Auto)...Hand means controlled locally at the MCC with the Start/Stop buttons at the MCC bucket (depending on MCC bucket content & wiring). Off is self-explanatory. Auto means it is controlled remotely... either by a PLC, level switch, pressure switch, and/or other devices. Sometimes HOA is referred to as Local-Off-Remote. Local means controlled locally (manually) at the MCC and Remote by the PLC or other
external devices. Sometimes in Remote, a software HOA on a SCADA or HMI screen is used in conjunction with the physical HOA switch at the MCC. This allows an operator to manually control the system from the HMI screen
(override the control logic), without going back to the physical MCC and putting a switch in Remote. In this case, the MCC HOA might be referred to as LOR (Local-Off-Remote).
H = Hand = Manual/Local Start, Inhibit Auto (or allow Start/Stop Permissive if combined with Start/Stop PB station)
O = Off = Manual/Local Stop, Inhibit Auto (and Inhibit Local Start/Stop if PB station is installed)
A = Auto = Allow Start/Stop from Remote (inhibit Local Start/Stop PB
Yes, HOA can* be used alone (without PB's) for Start/Stop locally. You didn't mention auto control... If there is no "Auto" contact, you have no need for HOA, just start/stop PB station(s).
* However, this may depend on the AHJ (Authority Having Jurisdiction) for the facility, since there is some inherent lack of safety.
There is no latch contact with HOA only. Therefore, if the device control circuit power is lost (O/L or CB trips, etc.) with switch in "H", the device will stop. If it is then reset while in "H" - - - the device immediately starts again, possibly without human knowledge or intent.
For any application where human oversight is needed, a Standard Stop/Start PB station should be combined with the HOA.
Patrick W. McGinnis
The function of a HOA (Hand-Off-Auto) selector switch is to control whether a motor is in hand (manual) mode, auto mode, or off. To operate the motor while hand mode is selected, start and stop push buttons are usually used. Auto mode allows a separate control signal to operate the motor, while the off position prevents the motor from being started. "HOA stations" can be located at the MCC or remotely. Hope this answers your question.