HOA with Start / Stop

Stationmaster said:
That is: "HAND" is a completely independent circuit, separate from the PLC. You may consider including some safeties in this "independent" circuit by running "control power" through a set of contacts on the phase monitor and through a set of contacts on the low level sensor before it goes to the N.O. "HAND" switch contacts on its way to the contactor (or VFD).

Stationmaster

I have been waiting for someone else to chime in on this, but it has been bothering me all morning. The HAND should be independent, as so elequently indicated by Stationmaster.

JEldreth said:
After reading the GDS it is critical to know all positions of all switches because there are a few valves that have to cycle before the pump can be started, even in the Hand position. I also need feedback to the HMI to know if it is Auto, Off or Hand so it looks like I will eat up 4 I/O for each station.

If you are requiring permissives for this pump to run in HAND, then you should have a hardwired circuit. HAND should be available for use when the PLC is out of service - therefore cannot make the determination if the pump could/should run in the first place.

If the process dictates that the pump should not be run manually, then the HAND should either be removed or substituted with JOG functionality.
 
I have a system that uses "Hand" via the PLC; that is, it runs the pump out of sequence if necessary. It only uses oil pressure as a safety.

I guess it really isn't "Hand' as in a hard wired start, but that's not a typical example.
 
After reading the GDS it is critical to know all positions of all switches because there are a few valves that have to cycle before the pump can be started, even in the Hand position.
J Eldreth, You kind of missed the point. Either it is a Hand (Manual) operation, or it is NOT! In a true "Hand" operation, the PLC should not need to be involved - at all. The whole purpose of a Hand switch is to be able to operate the pump even if all external controls (PLC, valves, interlocks, whatever) have failed. In other words, if the "Auto" PLC Input is NOT ON, then the PLC should do nothing. In that case, either the manual-operated "Hand" or "Off" switch is in control.

If you cannot do that due to necessary interlocks, then forget the Hand switch position and have only Auto and Off. If that is the situation, then you do not actually have a true "Hand" function, so why lie to the poor operators? That could lead to some legal liability down the road, if someone gets hurt as a result of your non-Hand operation being labeled incorrectly. There are thousands of lawyers out there that look for just this kind of thing, after a fatality.

If you are going to design this type of circuit, then you need to understand your obligations, both moral and legal, to those that will use it. You must make it true, real, and transparent. No hidden surprises, like "I turned the HOA to Off and tried to shut it down, but it kept running because the PLC was actually in control". A trial lawyer's dream-come-true!

I also need feedback to the HMI to know if it is Auto, Off or Hand so it looks like I will eat up 4 I/O for each station.
You seem to be looking at the trees and not seeing the forest.

For a H-O-A switch, "Off" is OFF, and nothing further need be said or done about it. AS OTHERS HAVE TOLD YOU: As for "Auto" and "Hand", those are two pieces of information. Theroretically, in Boolean logic, how many conditions can be indicated by 1 binary bit? Two - that is one reason it is called "binary logic". So use an ON bit to indicate Auto, and the same bit OFF to mean Hand. Having more than ONE PLC input for Hand-Off-Auto will not add any more relevant information to your PLC program.

PS: You still probably need 4 Inputs: Hand/Auto, Start, Stop, and Motor Running.
 
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I think he's got something that's more like an "auto hand" switch, rather than a real wired hand switch. It's not really "hand", but it's not "auto" either.

There are some applications where having a "hand" switch is really impossible. The rest of the process needs to know when the pump/process is on.
 
I think he's got something that's more like an "auto hand" switch, rather than a real wired hand switch. It's not really "hand", but it's not "auto" either.
I agree, and these days it is very risky to label any function something that it isn't. My concern is caused by a personal experience, where an electrican was roasted while operating a breaker for a large 2300 volt pump, probably similar to these that JEldreth is working with. I didn't have to go to court, but my boss spent several years in-and-out of the trial.

Courts have held the owner, the companies that made the equipment, and the employee that did the mis-labeling as being financially libel for the mistake. If it does not fit into a well-defined, well-known traditional niche, it is best to leave it out altogether.

Certainly you do not want to use an H-O-A switch simply because you can buy a nameplate with that label. There are people who will make you custom nameplates at a very inexpensive price, much cheaper than the potentially $10-million court case. Instead of Hand-Off-Auto, how about something like "Control A-Off-Auto"? At least then you do get to define what Control A means. With "Hand", nearly everyone already knows what it is supposed to mean. Use a "Hand" that is not really a manual operation only if you know the risks you are taking.
 
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Well Hello there Lancie.

It's good to see someone with the a similar perspective weighing in on this......

I would also like to say (and hear you comments on) that HAND can still mean that requirements (permissives) other that the HAND switch may have to be met before the motor is started. The OP asked about "industry standards", and I must say that I have seen MANY factory engineered, UL underwritten, pump system set-ups where some safety or limit switch, or combination of them must be satisfied before the motor is STARTED even in the HAND position.

The key here, both semantically AND legally, may be that the HAND circuit has no way to KEEP the motor running (erroneously) when the HAND switch is turned OFF, (such as a stuck-on PLC output).

The industry standard as I have been exposed to it, is that HAND is independent of the PLC, NOT necessarily independent of all other circuitry.

Did you see the investigation into the cause of the accident where the cargo ship crashed into the shopping mall on the waterfront of the Mississippi? The engine shut down because the electric motor that circulated the luricating oil shut down. The crew put the oil pump in the "HAND" mode but it still wouldn't run because of a safety circuit designed to protect it. (The CREW, by the way, not the equipment manufacturer, was found to be at fault for not maintaining the oil FILTER properly.)

How's life in the woods?

Stationmaster
 
I still prefer the terms I learned in the Navy.
The thing that must be remenbered about this is we had lots of people standing watch on equipment so that it was closely monitored while operating.
MANUAL meant you had to do the controlling.
AUTO meant that some automatic gizmo did the controlling.
We also had EMERGENCY RUN generally all this was was a hand override on the overloads, but I recall a piece of equipment where it also bypassed the safeties switches.

Generally most equipment was started in hand then switched to auto.

It seems to me in this case that he should have a manual function (with necessary equipment safeties) that would override the PLC. Auto would let the PLC control and may should be labeled PLC CONTROL.

In either case I would have a start and stop button with dual contacts on each --
one for manual control
and one for PLC control.
The stop switch would stop the unit regardless if in manual or PLC control.
IF needed for personnel safety then there would be an E stop that would deenergize all control power and maybe even line power to the starters.

IF he really needs it he should have an EMERGENCY RUN function.
Dan Bentler
 
leitmotif said:
I still prefer the terms I learned in the Navy.
The thing that must be remenbered about this is we had lots of people standing watch on equipment so that it was closely monitored while operating.
MANUAL meant you had to do the controlling.
AUTO meant that some automatic gizmo did the controlling.
We also had EMERGENCY RUN generally all this was was a hand override on the overloads, but I recall a piece of equipment where it also bypassed the safeties switches.


Dan Bentler


Well Dan, I was never in the Navy, but I've seen a LOT of equipment. I can truthfully say I have never seen a "hand override for overloads". That's a new one on me. I agree that some industries use the term "manual" instead of "hand", I've seen it a lot in municipal water and wastewater applications.

Stationmaster
 
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This was on submarine and I believe also on other combat vessels.
The hand override was only for very vital equipment that had to run to ensure ship safety. As an example hydraulics was extremely vital for steering and depth control.

I cannot recall ever seeing it in industry.

Dan
 
Hey, Steve,

Life here in the woods is good. Today I managed to get up enough steam to change the oil in my truck, and rotate all the tires. I noticed my front ones were wearing on the inside edge, so I decided to swap the new spare for one of them. Boy, it has been several years since I pulled a tire off the rim, swapped it with another, and remounted and balanced both. I mispent my youth working at a garage, but I think now that I am at the age that I better take it to some young ape at the local garage next time.

About the Hand switch, I agree that it is common practice to have interlocks with a Hand switch. But like Dan pointed out, there are some situations where it should be a true Hand circuit, with no or minimal interlocks, like for a fire-water pump, or on a military vessel, or anywhere that it is more dangerous for something NOT to run than it is to risk burning up the equipment. I am sure that a submarine would fall squarely in that category!

I don't know if these four pumps under discussion are fire pumps or not. A little more information is always useful, but for some reason rarely provided here.

As always, it comes down to a matter of judgement based on experience. I do think that it will be increasingly risky, legally speaking, in the future to label a circuit as a "Hand" function if it really is more of an Automatic or computer-controlled function as opposed to a true human-operated device.

I just wanted to point out that for equipment that may at times need to run to prevent a worse catastrophe, it may not be wise to interlock the Hand circuit with a bunch of other stuff. I always try to visualize being in the witness box and imagining what questions the prosecuting attorney will ask. If I can't answer them without incriminating myself, it is time to re-design.
 
Lancie1 said:
I just wanted to point out that for equipment that may at times need to run to prevent a worse catastrophe, it may not be wise to interlock the Hand circuit with a bunch of other stuff. I always try to visualize being in the witness box and imagining what questions the prosecuting attorney will ask. If I can't answer them without incriminating myself, it is time to re-design.

I agree completely about the circumstances requiring certain equipment to run no mater what to prevent a worse catastrophe. I think that would qualify the "interlocking stuff" to be a judgment call.

Another point to consider would be the degree of skill and training possessed by the potential "operator". Many of the operators of the irrigation pump stations I build probably shouldn't be trusted with a HAND switch that would force a pump to run with a "phase loss" or "low water". These conditions would cause imminent destruction of the equipment. I think "I" would end up in the witness box trying to explain why MY control scheme allowed the destruction of thousands of dollars worth of equipment because this week's irrigation tech put a switch in the wrong position.

By contrast, we would like to think that the operators of "military combat vessels" would be well trained on the appropriate protocol.

I have used exactly the same litmus test time and again (how would I explain X on the witness stand) to determine how a particular piece of gear should work or how control switches should be labeled.

And I have a young tire shop ape do my mounting and balancing.

: )

Stationmaster
 

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