HOA Switch

[rivecar44]

Please tell me the reasons why "OFF" is in the middle. Could it be HAO (i.e. transition from one control location to another without turning the system off)?

 

[jim dungar]

Please tell me the reasons why "OFF" is in the middle. Could it be HAO (i.e. transition from one control location to another without turning the system off)?

Because grandpa did it like that.

I have never seen any published standard for this, although it may have come from the automobile industry. I do know that 'selector switch' manufacturers offered standard legend plates like HAO some 50 years ago.

 

[retirede]

I suspect it’s because when you begin in the off position, you want to go directly the desired mode. Since most contacts are break before make, you could “bounce” a motor starter on the way to the far position.

 

[TwoBlocked]

Because the 3 position selector switches I usually see have a set of contacts that close when turned to the left, another set that close when turned to the right, and none when in the center.

 

[don_resqcapt19]

Because the 3 position selector switches I usually see have a set of contacts that close when turned to the left, another set that close when turned to the right, and none when in the center.

You can get any combination of closed contacts that you want for a 3 position selector, but the set up you mentioned is the most common.

 

[TwoBlocked]

Please tell me the reasons why "OFF" is in the middle. Could it be HAO (i.e. transition from one control location to another without turning the system off)?

The Hand mode often allows a start/stop/reverse station to have control, or it may simply start a system regardless of the system's condition. The Auto mode will run the system according to other inputs. Either way if you want to turn the system Off, and the Hand mode is next to the Auto mode, the system may start when you don't want it to while switching through the modes to get to the Off mode. That won't happen with the Off mode in the middle.

Hadn't really thought about why before. Good question!

 

[jim dungar]

The Hand mode often allows a start/stop/reverse station to have control, or it may simply start a system regardless of the system's condition. The Auto mode will run the system according to other inputs. Either way if you want to turn the system Off, and the Hand mode is next to the Auto mode, the system may start when you don't want it to while switching through the modes to get to the Off mode. That won't happen with the Off mode in the middle.

Hadn't really thought about why before. Good question!

Yeah, but if you want to smoothly transition between manual and PLC control a HAO sequence makes more sense.

 

[TwoBlocked]

Yeah, but if you want to smoothly transition between manual and PLC control a HAO sequence makes more sense.

And if you do not want such a transition? Suppose you have a tank with a level switch, a pump, and piping with a strainer. Something is going on and the operator switches to Hand mode and operates the pump at the local control station. He discovers that there is a leaky gasket on the strainer and turns the pump off at the local station. The HAO switch at the main control panel is still in Hand. The tank is at a level where if the system is in Auto mode, the pump will start and spray fluid all over the place. If he switches from Hand to Off, he must go through the Auto position causing this to happen. Sure, the operator can do this, that, and the other to prevent it, but this is just an example.

If you want a smooth transition between Hand and Auto and not have possible bad scenarios, might be better to have two selector switches, one On/Off and the other Hand/Auto instead of HOA. I've seen both installations. Can't remember ever seeing a HAO or any other combo.

 

[petersonra]

Hard to say. Why do some places use red lights for running and others use green?

 

[petersonra]

I can only remember one time I dealt with people who used HAO. I asked why and they said so that they wanted it to be more obvious if a switch was not in auto.

 

[ptonsparky]

I can only remember one time I dealt with people who used HAO. I asked why and they said so that they wanted it to be more obvious if a switch was not in auto.

IMO, it should be made very obvious Hand vs Auto has been selected and not only by looking at selector switch positions. PLCs certainly make that easier.

 

[jim dungar]

So much switch shaming.

The point is: it is up to each company to make their own choice as there is no over riding standard that must be followed.

 

[kwired]

You can get any combination of closed contacts that you want for a 3 position selector, but the set up you mentioned is the most common.

As well as different operating characteristics for the cam that actuates those contact blocks, or even an operator with maintained position one way and spring return the other way from center, though that would typically be more for something like jog off auto instead of hand off auto when it comes to describing the function - electrically they would basically be wired the same.

 

[don_resqcapt19]

As well as different operating characteristics for the cam that actuates those contact blocks, or even an operator with maintained position one way and spring return the other way from center, though that would typically be more for something like jog off auto instead of hand off auto when it comes to describing the function - electrically they would basically be wired the same.

In the plant I did a lot of work at, the hand was only for lockout verification in most cases, and was spring return to center.

You would go to the equipment, use the hand position to see the equipment run, go to the MCC and lockout the starter, and then return to the HOA to verify that the equipment would not run. If the motor was on a VFD, the hand would go to a separate input in the VFD that was set to run at 15hz. The hand position would bypass all interlocks with the exception of the overload relay in the starter bucket.

There were a few cases where they wanted to use the hand position to keep the equipment running and those did not have spring return. One example was the cooling water pumps on the reactors. The hand position was maintained on those so that in the event of a process control failure they could run the pumps in hand. There was an additional contact on those to put the temperature mixing valve to cooling water only.

 

[kwired]

In the plant I did a lot of work at, the hand was only for lockout verification in most cases, and was spring return to center.

You would go to the equipment, use the hand position to see the equipment run, go to the MCC and lockout the starter, and then return to the HOA to verify that the equipment would not run. If the motor was on a VFD, the hand would go to a separate input in the VFD that was set to run at 15hz. The hand position would bypass all interlocks with the exception of the overload relay in the starter bucket.

There were a few cases where they wanted to use the hand position to keep the equipment running and those did not have spring return. One example was the cooling water pumps on the reactors. The hand position was maintained on those so that in the event of a process control failure they could run the pumps in hand. There was an additional contact on those to put the temperature mixing valve to cooling water only.

Plant I've done a lot of work in often was Run (or sometimes maybe auto)/Off/CIP Left position being the "normal operating method whether automated or not beyond just turning the switch there and CIP (clean in place) was basically an auto position but let the CIP system control the item when needed during the cleaning process.

True HOA I guess normally bypasses most other control components when in hand. Though I've seen machines that maybe still have limits or interlocks that still operate in hand, like say the machine needs an oil pump to run anytime the main machine is in use. The "auto" control position in such case is more for coordinating that machines use with other equipment in a production process and not so much for making sure the oil pump is working, if it isn't the machine doesn't run either way. But then I've also seen Hand positions that even bypass motor overloads on some equipment and actually were factory installed that way in a listed control panel.

 

[ATSman]

Hard to say. Why do some places use red lights for running and others use green?

If I remember right when working on Telco central office switchgear, their breaker ind. light logic was the opposite to industry standards. Red meant the breaker was closed while green meant breaker open. I guess it is what your reference is. Red (closed) means caution, load circuit is hot, while green (closed) means load circuit is hot and condition is a go or normal.

 

[TwoBlocked]

For switch gear, I have always seen Red for closed, Green for open. Recently had a customer that wanted it changed the other way around. I made an excuse not to. For motors, usually Green for running, Red for stopped, but not always. For switchgear, I bet there is a standard; for motors, maybe not.

 

[Jraef]

IF you want an Off position (and most people do), you want it in the middle. The middle OFF position ensures that you have to pass through Off to change the operating mode and that helps to prevent it from being inadvertently left in the wrong position. So for example if the switch was "Off - Hand - Auto", and you had it Off while testing something, you might flip it to Hand and not ALL THE WAY to Auto, then leave, thinking it was in Auto because it was running. That could be bad. Otherwise, just use a 2 position switch that is just Hand or Auto, but for safety reasons, most people don't want that.

In a lot of irrigation pump panels, the switch is "Auto - Off - Hand - Start" with a spring return from Start to Hand.

 

[jim dungar]

If I remember right when working on Telco central office switchgear, their breaker ind. light logic was the opposite to industry standards. Red meant the breaker was closed while green meant breaker open. I guess it is what your reference is. Red (closed) means caution, load circuit is hot, while green (closed) means load circuit is hot and condition is a go or normal.

Square D and Allen Bradley had different standard colors for their ON pilots. We had a price adder if you wanted the other person's color from the factory. We gave away a lot of colored caps for the EC to field install.

 

[ptonsparky]

Hand Off Auto with a Start PB above for most of the PPs we used.

We worked a couple old MCCs that had none. The HMI we added had radio buttons for the three plus a Jog worked in somewhere.