Electrical vs Logic Interlock

583 Views 18 Replies 13 Participants Last post by Mike in Canada, 6 d ago

mburtis

Discussion Starter · 24 d ago

Want to gather some opinions on a design decision for a project that I'm working on. Basic idea is that there are 2 pumps (pump 1 and pump 2) that pump wastewater grit to a screw conveyor (Grit Classifier). The current wiring (which is a mess) has contacts from the motor starter of the grit classifier wired into the start circuit of pump 1 and pump 2 as an interlock. Neither pump can run if the grit classifier isn't running.

This system is pretty much stand alone right now, and they want to integrate it into SCADA. Control and feedback from all the equipment via PLC, etc. I'm left overthinking whether or not it would be better to leave this interlock electrically hardwired or just integrate it into the control logic.

Moving it into logic cleans up the wiring a little and eliminates cross connections between different bucket in the MCC (multiple sources). There is really no safety concern if the logic was to fail (not that it should) to either people or equipment. I don't really see much of a downside to integrating it into the pump control programming.

Leaving it hardwired pleases my old school loving personality, but leaves more complicated wiring with multiple sources in cramped MCC buckets. The fact that it is already wired doesn't really factor in much because like I said the existing wiring is a disaster.

What's your opinions on control interlocks (ie not personnel or equipment protection related).

1 - 19 of 19 Posts

SWDweller

· #2 · 24 d ago

Are you going to need an E stop?

I worked in a chilled water plant and we had to install disconnects at every air valve so when they operated a valve for maintenance there was a way to prevent all signals from the SCADA to the valve and a way where you could physically lock out the valves.
Disconnects in this situation were switches when the valve was locked out it opened the signals from the SCADA system.

Might be time to call the safety officer if you get along with them.

ScooterMcGavin

· #3 · 24 d ago

I’d move it to logic 100% without giving it a second thought. This is a very simple interlock application.

Reactions: 3

splatz

· #4 · 24 d ago

I could see doing it either way. There is something to be said for leaving a simple straightforward thing like an interlock in the wiring rather than in the PLC.

In the wiring, it can be jumpered to override during troubleshooting. (This can be a plus or a minus, depending if it's used or abused.)

You can set up HOA operation for when the PLC is offline and still have your interlock.

If you keep simple stuff out, the PLC program is shorter and maybe easier to maintain.

Reactions: 5

billymac93

· #5 · 24 d ago

Running everything back to a plc would probably neaten the wiring somewhat. An off-line plc would shut the system down. Personally, I would leave as is but add wiring labeling and drawings in both buckets. The plc would add the flexibility of making changes but would you ever need to do that.

Reactions: 1

gpop

· #6 · 23 d ago

Should happen = wire everything to the plc (before wiring work out what how it should fail safe, E.g motors stop, valve open or shut, etc)

Has to happen = hard-wire then add a extra contact to signal to the plc what has happened. (e.g E-stop hard-wired with a extra set of contacts that tells the plc the location of the stop button)

In your case i would let the plc take care of it.

Reactions: 2

dvcochran

· #7 · 15 d ago

mburtis said:
Want to gather some opinions on a design decision for a project that I'm working on. Basic idea is that there are 2 pumps (pump 1 and pump 2) that pump wastewater grit to a screw conveyor (Grit Classifier). The current wiring (which is a mess) has contacts from the motor starter of the grit classifier wired into the start circuit of pump 1 and pump 2 as an interlock. Neither pump can run if the grit classifier isn't running.

This system is pretty much stand alone right now, and they want to integrate it into SCADA. Control and feedback from all the equipment via PLC, etc. I'm left overthinking whether or not it would be better to leave this interlock electrically hardwired or just integrate it into the control logic.

Moving it into logic cleans up the wiring a little and eliminates cross connections between different bucket in the MCC (multiple sources). There is really no safety concern if the logic was to fail (not that it should) to either people or equipment. I don't really see much of a downside to integrating it into the pump control programming.

Leaving it hardwired pleases my old school loving personality, but leaves more complicated wiring with multiple sources in cramped MCC buckets. The fact that it is already wired doesn't really factor in much because like I said the existing wiring is a disaster.

What's your opinions on control interlocks (ie not personnel or equipment protection related).

I agree with @splatz , add a HOA and leave the current wiring for the Hand selection. This, along with the normal PLC motor control logic, will be safer and reduce the 'two points of control' issue.

Reactions: 1

mburtis

Discussion Starter · #8 · 15 d ago

These pumps already have an H-O-A switch in which the hand position completely bypasses this interlock (basically a hot wire straight to the starter coil).

I think I will probably end up moving this interlock into the plc for auto mode so I can repurpose some of the auxiliary contacts for feedbacks to SCADA and such. There is no danger to people or equipment so having it in the plc I think will give more flexibility and make the system simpler overall. That was kinda the entire point of this project was to increase flexibility and adjustability of the system so it makes sense to me to move everything into logic.

I still need to discuss things with the guys down there and see what way they want to go, they are the ones that run it and work on it.

Reactions: 2

dvcochran

· #9 · 14 d ago

mburtis said:
These pumps already have an H-O-A switch in which the hand position completely bypasses this interlock (basically a hot wire straight to the starter coil).

I think I will probably end up moving this interlock into the plc for auto mode so I can repurpose some of the auxiliary contacts for feedbacks to SCADA and such. There is no danger to people or equipment so having it in the plc I think will give more flexibility and make the system simpler overall. That was kinda the entire point of this project was to increase flexibility and adjustability of the system so it makes sense to me to move everything into logic.

I still need to discuss things with the guys down there and see what way they want to go, they are the ones that run it and work on it.

If you move thigs to the PLC, will that eliminate the manual override? If so, I would see that as a problem if I was the operator.

mburtis

Discussion Starter · #10 · 14 d ago

No the hand position will remain unchanged. In my mind this is how a hand function should work, nothing between the switch and the starter.

Reactions: 1

splatz

· #11 · 13 d ago

mburtis said:
In my mind this is how a hand function should work, nothing between the switch and the starter.

This is another good point, what should hand operation be?

If you're testing to see if the motor will run, hand operation that's totally direct makes sense. Of course the person operating the switch better know enough about how things run to interlock himself if necessary.

If you're looking for a hand switch that's an alternate way to run the plant when the automation isn't working, it may make sense for hand operation to have the interlocks in the wiring so the system is safe to operate in hand mode with less reliance on the operator to operate it safely.

Reactions: 2

micromind

· #12 · 13 d ago

Here's how I usually design my systems that are controlled by PLCs.

Every MCC bucket has a HOA switch. If running in hand can cause damage to equipment and/or persons, then hand is spring-loaded and must be physically held in hand. An air compressor is a good example of this.

In the auto position, the PLC has complete control. Often there is a local on-off or start-stop switch near the equipment and the PLC becomes a permissive rather than a control.

One exception is that if there's a good chance of wrecking stuff and/or people when 2 or more things run at the same time, then I will hard-wire interlocks. This is quite rare but I have done it. In these cases, the hand position also includes a key switch to start the equipment.

Further, I use a different color for interlock wiring. This way if you see, for example, a purple wire in a bucket, you know it's interlocked with another bucket. This helps with troubleshooting. Also, no wire is installed without a label.

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Reactions: 5

460 Delta

· #13 · 12 d ago

micromind said:
Here's how I usually design my systems that are controlled by PLCs.

Every MCC bucket has a HOA switch. If running in hand can cause damage to equipment and/or persons, then hand is spring-loaded and must be physically held in hand. An air compressor is a good example of this.

In the auto position, the PLC has complete control. Often there is a local on-off or start-stop switch near the equipment and the PLC becomes a permissive rather than a control.

One exception is that if there's a good chance of wrecking stuff and/or people when 2 or more things run at the same time, then I will hard-wire interlocks. This is quite rare but I have done it. In these cases, the hand position also includes a key switch to start the equipment.

Further, I use a different color for interlock wiring. This way if you see, for example, a purple wire in a bucket, you know it's interlocked with another bucket. This helps with troubleshooting. Also, no wire is installed without a label.

I was completely with you until the wire label, then I got lost. Lol.

Reactions: 5

mburtis

Discussion Starter · #14 · 12 d ago

460 Delta said:
I was completely with you until the wire label, then I got lost. Lol.

I've spent the last week tugging on wires and following conduits trying to sort out our intake facility. Any labels that are there are unreadable or useless. Better than a cabinet I was in down at the poo plant though. Each end of the wire was a different tag supposedly identifying where the other end was, what a PITA.

Reactions: 3

glen1971

· #15 · 12 d ago

mburtis said:
I've spent the last week tugging on wires and following conduits trying to sort out our intake facility. Any labels that are there are unreadable or useless. Better than a cabinet I was in down at the poo plant though. Each end of the wire was a different tag supposedly identifying where the other end was, what a PITA.

I've heard of that convention a couple of times in the past, but have never seen it in the industry that I can recall.

mburtis

Discussion Starter · #16 · 12 d ago

I hate it, especially when there are tags like TB6-05 but none of the terminal blocks are marked anyway. I can see how it seems like a good idea, but I prefer just a dumb simple number that identifies the wire.

Reactions: 1

joe-nwt

· #17 · 10 d ago

glen1971 said:
I've heard of that convention a couple of times in the past, but have never seen it in the industry that I can recall.

I use that system for panel wiring. For field wiring I prefer cable/conductor type markings.

Never saw the need to mark a conductor with the terminal number it's attached to.

Reactions: 1

JRaef

· #18 · 9 d ago

mburtis said:
No the hand position will remain unchanged. In my mind this is how a hand function should work, nothing between the switch and the starter.

So, a simple way to implement “interlocking” for a Hand function, if deemed necessary, is to use a keyed selector switch in each bucket, and only one key that is not removable in the Hand position. You can get that from most selector switch providers. That leaves you with zero reason to leave the hard wired electrical interlocking in place.

Reactions: 2

Mike in Canada

· #19 · 6 d ago

splatz said:
I could see doing it either way. There is something to be said for leaving a simple straightforward thing like an interlock in the wiring rather than in the PLC.

In the wiring, it can be jumpered to override during troubleshooting. (This can be a plus or a minus, depending if it's used or abused.)

You can set up HOA operation for when the PLC is offline and still have your interlock.

If you keep simple stuff out, the PLC program is shorter and maybe easier to maintain.

Additionally, you only have SO MANY inputs and outputs to a PLC. Yes, you can add I/O exptenders IF YOU HAVE ROOM, but I try not to gratuitously add inputs and outputs that don't need to be there. The spare you leave today may save your whole week in the future.

Reactions: 3

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