Switching 0VDC on valve bank?

[JOLTRON]

GenerL question. I dont have the part number infront of me. But i am looking at a valve with an M12 connection on it. Pin1 - not used, pin 2 - 24VDC A, pin 3 - 0vdc, pin 4 24vdc B.

Generally if i am switching safety voltage i would do it on the 24vdc side. But due to limited space i am wondering if it is still safe if the 0vdc side is switched. There is no ground wire that the 24VDC would use as an alternate path.

Just curious since i have never wired a machine this way.

Thanks for any advice!

 

[rupej]

I wouldn't do it unless there was no way around it. It could be a real pain for the next guy to troubleshoot.

Can you explain why you can't switch the 24VDC? Just curious.

 

[JOLTRON]

I wouldn't do it unless there was no way around it. It could be a real pain for the next guy to troubleshoot.

Can you explain why you can't switch the 24VDC? Just curious.

Due to limited room in the panel could only fit 1 really and it doesn't have enough contacts. Switching 0 cuts the number of contacts required in half. Gets us down to using only one expansion relay.

 

[Bullzi]

I am with rupej, I would not do it that way. Even if you you have to add a little box on the outside of the panel it would be worth it in the end. If you are truly left with no choice then make sure you document the hell out of your drawings and label the system well.

 

[BobB]

Why not? It is just a switch. Many PLCs switch 0VDC - I use them quite regularly. It is common in Japan. I do not ground my 24VAC neutral from an isolation transformer or my 0VDC from a switchmode power supply either - if you do you lose isolation. Also then there is no problems with anything shorting to ground in your control circuit.

 

[Bullzi]

Bob,
Not saying that it won't work or even that it is "weird" in any way. I get the impression from the OP that this is not how the rest of the panel is done. It is one thing if the if the whole system is designed to switch the DC Negative but if this addition will the only part that is set up that way it will cause lots of "head scratching" during troubleshooting.

I am just encouraging the OP to think about the complete system, future work and troubleshooting before implementing a solution.

 

[BobB]

Understand - although I often see both in a control panel. One has to be up with it that is all. Having a look at a schematic is imperative.

 

[shooter]

You have to switch the control signal, as the valve needs power to close, when estop is pressed.
However you can put the supply of all the analog signals to one contact, so switch before the PLC not after.

 

[dwoodlock]

Shooter, I don't think the valves are analog.

If its the only way to do it ok, but if this is something you are building for someone, I would try and come up with a better way.

Having a set scheme for a system, except for one device because of a planning fault is not a good look.

If its a production machine, I understand. sometimes you have to do whatever it takes to get it going, but if you're in the build phase, I would re-approach.

 

[boneless]

Just throwing this out there.

What if you switch the 0, something happens to the cable or your valve and the 0 gets exposed to ground.

In this case, that would engage multiple valves at the same time?

 

[kamenges]

Originally posted by boneless:

In this case, that would engage multiple valves at the same time?

This would only engage valves that were being commanded to be on anyway. any valve that was not commanded to be on would stay off. In addition, in his original post, JOLTRON indicated that the power supply negative was not tied to ground. So a valve common short to ground should have no effect.

Originally posted by Bullzi:

If you are truly left with no choice then make sure you document the hell out of your drawings and label the system well.

Unless the drawings are already so convoluted that they are near useless anyway, I don't think you need to do any special with indicating this design other than showing it accurately on the drawing. There are no electrical tricks being exploited here. It is really no different that running the return of both coils of a reversing motor starter through a single overload contact. If that doesn't confuse someone then switching a dual solenoid valve common shouldn't either.

On a related note, I don't know why electrical designers are expected to constantly design for the least common denominator. When I signed on for this gig I was never told it was my job to protect OTHER COMPANIES from poor hiring practices. If a guy can't read a drawing he shouldn't be troubleshooting a machine. How many architects do you you think worried about the competence of building maintenance before they started tossing tuned mass dampers 1200 feet above grade?

Keith

 

[boneless]

Well, he did mention that he would need less relays, so I am assuming that he is controlling multiple valves with a single contact.

He did not say the supply is not grounded, he only said there is no ground wire in the cable.

 

[Timbert]

On a related note, I don't know why electrical designers are expected to constantly design for the least common denominator. When I signed on for this gig I was never told it was my job to protect OTHER COMPANIES from poor hiring practices. If a guy can't read a drawing he shouldn't be troubleshooting a machine. How many architects do you you think worried about the competence of building maintenance before they started tossing tuned mass dampers 1200 feet above grade?

I will disagree with this. We already think of the end user and maintenance workers in our designs. How many designs have safety systems built into them? Bubba shouldn't put his hand in the guillotine machine but you'd be negligent not to consider the possibility. We could say that the other company shouldn't hire a Bubba, but they do.

We have the same duty to make the machine as easy to maintain as possible. Besides it's not always some other company's Bubba we have to deal with. You might have to modify the machine a few years down the road. It is a lot easier to understand and deal with if controls were connected in an obvious, logical and consistent pattern.

I will grant that sometimes it is unavoidable, but the first thought should not be "Tell 'em to read the schematic."

To the OP, in this case, since it is a safety circuit, I would definitely try not to be 'cute' and do something different. Someone, somewhere, may connect that isolated power supply to ground and now you've compromised the safety of the system.

 

[JOLTRON]

Hey guys,

We were able to find more compact safety relays to use so we are planning on switching the 24VDC signal side.

The plan is to add 3 valves with M12 connectors on each. Each m12 connector has a single 0VDC and a 24VDC for each direction. So planning on breaking each channel would require 6 channels through the safety relay, were as if we broke the 0VDC it would only require 3 channels.

Hopefully that cleared some of the things up. As far as trouble shooting I always document items to the best of my abilities, but there are a few machines (like this one) that came in and we do not have a software copy of the electrical schematics, so whatever changes we make have to be red-lined in. Even if we did have the soft copies I believe it is done in E-Plan, or something other than AutoCAD which is what we use.

I agree it would have been confusing and crappy to say the least, but it is one of the scenarios where you get a call on a Tuesday about a project that needs to be completed by Friday.

We typically use valve banks that are on a bus system and we just run a single AUX / switched power to the bank and call it done. But this is an older system with a Brand of PLC and Fieldbus we are not used to

Anyways thanks for the help and advice!