PLC Input Wiring

The 50 or so PLCs installed where I work are all wired using 120 VAC control circuits. Limit switches, manual switches, proximity switches and such all source 120 volts to their particular PLC inputs. When not closed, the wire to each input floats (actually, it is capacitively coupled to the PLCs' modules' common terminal - 120 V neutral, in our case - through the modules' input circuit). The PLCs are installed in remote cabinets, off the production floor.

The operator controls' environment is wet, so there have been occasions that a water short across a switch's terminals conducted enough current to turn on an input. This condition is made apparent by a dimly lit annunciator LED on the input module corresponding to the partially shorted switch. Lately, some inputs have been turning on without the tell-tale dim LED. With a 'scope, we traced that to intermittent electrical noise spikes on the input wire induced by a new style of frequency drive we began using.

It's clear there is a design flaw in the control circuits for the environment in which they operate. (My bad). I believe the solution is to add a contact block to each manual switch such that the input is grounded in the switch's open state. Mechanical limit switches of the type we use typically have DPDT contacts, and they can be rewired to ground the signal wire when not actuated. Electronic proximity switches are more problematic, as we usually use 2-wire AC switches. I plan to load the inputs connected to the proxes to reduce the chance of water short actuation.

I don't know of cheaper solution to the problem. Water in a food plant is a fact of life, whether it comes from high pressure hoses or from condensation during rapid and drastic temperature changes. We have used petroleum jelly (food grade acceptable) on the switch terminals with limited success. Turning a switch's contact block such that there is no horizontal plane between input and output doesn't work due to the bridging film of condensation that forms after each cleaning shift. Maintaining sealed, pressurized operator control boxes is prohibitively expensive and time consuming. I did toy with the idea of using magnetically actuated, hermetically sealed reed switches for manually inputs, but there are limitations in available contact arrangements. And that would be a lot of magnets. :)

I just thought I'd pass this on and invite comments.

Reply to
bargepole
Loading thread data ...

120 VAC control

switches and such all

closed, the wire

to the PLCs'

through the

cabinets, off the

been occasions that

current to turn

annunciator LED

switch.

tell-tale dim LED. With

spikes on the

began using.

for the

solution is to

input is grounded in

type we use

ground the signal

more problematic,

inputs connected to

food plant is a

from condensation

petroleum jelly

success.

horizontal plane

film of

Maintaining sealed,

expensive and time

actuated,

there are

would be a lot of

That stuff should be in vapor tight enclosures... you sound quite bright, but when you are gone the new guy will likely not be able to address these issues you take for granted so easily, each glitch will be a major problem.

Its false economy imho to leave that sort of equipment exposed to wash down hoses and fruit acids etc.

Phil Scott

Reply to
Phil Scott

problematic,

condensation

What kind of control wire did you use? I always use twisted shielded wire. Ground the shields only at the PLC. Make damn sure that you have an low ohm to ground relationship for the main control cabinet. Use an dedicated transformer or source for all of the controls. I use double regulated isolation transformers for my PLC installations. I also add in surge protection, we have periods of lightning here so it is cheap insurance.

I had an waste water plant where the controls were run in conduit. Thhn or thwn wire and some shielded twisted pair. Voltages from 24 vdc to 277 vac. It was a mess. The operator could turn on one motor and a couple of others would turn on as well. I got them to ground the shielded twisted pair at the main control cabinet. Over the objections of all the electricians and the electrical engineer. Seems they did not see the logic in my method. Two weeks later an consulting engineer for the electrical contractor called me and said the problems went away. I never was asked to go back there for any more OEM problems. I was an OEM field engineer at the time.

As for the moisture issue, water proofing is sometimes pretty hard maybe an cover over the device so that water can only come in from the bottom. The grounding of the wires does not sound like an good idea to me. What happens if one of the grounds gets back to the IO board and shorts out the whole card.

Relying on the indicator lights is an poor practice in my opinion. Use control lights or LED's. The isolation will stop the flickering that your experiencing. Unless there is an enough current and voltage to pick the relay. If that happens you have other problems to work on first.

Reply to
SQLit

using 120 VAC

switches and such all

closed, the wire

to the PLCs'

through the

cabinets, off

been occasions

current to turn

lit annunciator

switch.

tell-tale dim LED.

spikes on the

began using.

for the

the solution is to

input is grounded

type we use

ground the signal

more

inputs connected

a food plant is

from

petroleum jelly

limited success.

horizontal plane

film of

Maintaining sealed,

expensive and time

actuated,

there are

would be a lot of

shielded wire.

have an low ohm

dedicated

regulated

in surge

insurance.

conduit. Thhn or

vdc to 277 vac.

couple of others

twisted pair at the

electricians and the

method. Two

contractor called me

back there for any

pretty hard maybe an

the bottom.

to me. What

shorts out the

opinion. Use

flickering that your

to pick the

first.

A useful and informative bit of information... thanks

Phil Scott

9/6/2004
Reply to
Phil Scott

We've had to load some 120 vac inputs (at the modules) with suitably chosen resistors to eliminate sporatic actuation by coupled noise.

I know you can't replace all the problem switches but when you must, use good quality factory sealed limit switches or better yet - replace them with proxes if possible. For proxes in washdown environments I'm sold on Allen Bradley's

871TM series. We had a large, multistage washer (17 station - for washing 16 liter diesel engine blocks) that used 400 psi water/detergent blasts from hundreds of nozzles. That environment quickly destroyed the original proxes and anything we'd put in there. Plus, it give us fits with sporatic switch turn-ons at the PLC throwing everything out of cycle. We bit the cost bullet and tore all 50 or 60 of the old proxes and their cables out and replaced them with the 871TM types with Toughlink cables. That washer ran for three years (I'm serious!), 24 hours a day, 7 days a week, and we NEVER had to replace a single 871TM. Believe me - they're worth the cost. Here's some info from
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*Stainless steel face and barrel *Available with abrasion resistant, fire retardant ToughLinkT cable that exceeds SOOW-A requirements *1200psi (8270kPa) washdown capability for contaminant and fluid ingress resistance *Electrical protections against short circuits, overload, transient noise, false pulses and reverse polarity (DC models) to help reduce downtime and maintenance costs *Extended range models available

Back in the early 90s I had worked in an ice cream plant for a couple years and I sure wish they would have used those proxes instead of the crap that the OEMs used. I know about food plants and electrical problems - believe me.

It's a lost cause with water. I had some limited success with just potting the existing switch bodies (where that's feasible) with an epoxy filler - like what's used in certain direct burial cable connections - and then using factory sealed (they pot them with an epoxy also) switches for replacements. Still, the water gets into the head and cruds and corrodes the mechanical parts which is why I prefer proxes. Something else- don't ever be tempted to use modular proxes in a washdown environment. I don't care what the manufacturer says their product can handle - eventually it won't.

Perion

Reply to
Perion

Another tip...drain holes...drill drain holes in conduit bodies and device junction boxes. In your environment conduit and box seals only keep the water IN.

Reply to
Fred

Drain holes also allow for physical damage of the wire when pulled and allow for air exhange in the conduit.

IMO sealed is the best way.

I have not seen the "drilling drain holes" in 25 years.

Of course it is your project.

Reply to
SQLit

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