# PLCs abs troubleshooting



## micromind (Aug 11, 2007)

One of the best troubleshooting skills is to figure out how something works. Also, always be ready to look at things that 'never fail'......like an O/L. Just because the computer says this, this or this doesn't mean that's what's actually happening. 

I was called to a rock crushing plant a few years ago for a 10HP conveyer motor controlled by an MCC that blew the fuses on the bucket. 

I got out my trusty megger and yes, it failed the test. The circuit went underground to a can with lug blocks but of course, nothing was labeled. All the motors in the can were started across-the-lines and nothing was energized so I found which block it was with the merger. I took the tires out from the lug block and was surprised that the fault was toward the MCC. 

I figured the underground wire had a fault but decided to pull the wire off the bottom of the starter. It megged onto the can. I reconnected the lug block and it still megged ok. 

So I megged the starter lugs. It failed. 

Basic Square D size 1 starter with heater type O/Ls. 

Pulled the heaters and the bottom lugs still failed. Pulled the starter (On Square Ds, you have to pull the starter in order to remove the O/L block; the screws are in the back, not the front.......grrrrr.........). 

I couldn't see anything wrong with the O/L block. Replaced starter, passed the megger test and it ran fine. 

I've never seen an O/L block short to ground before. After it was out, I set it on a steel plate and it failed the megger test.


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## SWDweller (Dec 9, 2020)

I have had a plethora of problems with Triac contacts. I typically use relay contacts and some of the issues go away. Most of the time as you so elegantly stated the problem is not in the PLC.

Good read thanks for sharing


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## just the cowboy (Sep 4, 2013)

Some of my rules.
A PLC program is only as good as its programmer. 
PLC programs don't change themselves.
Go for the basics first. Stop look and listen, oh yea and smell.


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## paulengr (Oct 8, 2017)

SWDweller said:


> I have had a plethora of problems with Triac contacts. I typically use relay contacts and some of the issues go away. Most of the time as you so elegantly stated the problem is not in the PLC.
> 
> Good read thanks for sharing


Triacs don’t survive inductive kick very well. Those cheap little surge protectors on the coils will stop this from happening. No inductive loads are fine.

The advantage of solid state is no moving parts so it doesn’t wear out. Instead of say a million cycles or less, you can do as many as you want. Great for twitchy operators. The downside is it’s electronic so you have to protect it from surges.


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## paulengr (Oct 8, 2017)

micromind said:


> One of the best troubleshooting skills is to figure out how something works. Also, always be ready to look at things that 'never fail'......like an O/L. Just because the computer says this, this or this doesn't mean that's what's actually happening.
> 
> I was called to a rock crushing plant a few years ago for a 10HP conveyer motor controlled by an MCC that blew the fuses on the bucket.
> 
> ...


I’ve seen breakers do this. Once it arcs and sprays metal on the insulator or out the back, it’s done. Time to replace.


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## joab (Dec 28, 2019)

Had one of these today. A recycling facility called. “The main cardboard baler is down, HMI says there is an E-stop” I’m like did you check the the E-Stops? Yup, everything is fine. Did you check ALL of the E-stops? Yes, we opened everyone and checked the contacts with a ohm meter. We need you to come out, there’s something else wrong.

I show up, E-stop circuit is open. I start tracing it out, there’s something on the other end of this conduit that’s not closed, do you have another button over there? Walk under the conveyor, twist the knob... Snap! And the error clears.

Then of course... We checked that one! It was working before! Sure, sure, could be, I left them a spare button.


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## gpop (May 14, 2018)

It always amazes me when people are un-willing to follow the meter and have already ruled out a problem with out testing.


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## oldsparky52 (Feb 25, 2020)

joab said:


> Then of course... We checked that one! It was working before! Sure, sure, could be, I left them a spare button.


I would have been tempted to say I better replace it as a precaution (and charge large).


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## joab (Dec 28, 2019)

oldsparky52 said:


> I would have been tempted to say I better replace it as a precaution (and charge large).


Oh, it’s costing them. I was in a hurry. I was supposed to be an hour and a half away from that site, setting up for an after hours wire pull.


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## SteveBayshore (Apr 7, 2013)

I went to a marine fuel tank fabricator last week. Foot pedal on 1950s Cincinnati 50 ton break wasn't working. All were* positive* it was a problem with the foot pedal. OK, opened up the foot pedal; all OK. Did anybody body touch anything?? *NO*, just stopped working while we were using it. Waited for 45 minutes while they cleared away a pile of new and scrap aluminum plates so I could lay in the oil on the floor and see in the starter panel. All wires oil soaked and number tags laying on floor of panel. All fuses OK. Jumped out motors one at a time, all OK. No power coming back from operator panel. Traced power wire back from key switch. Hey, what is this key switch for? I don't know, we never touch it. Turned key switch and AHHH works like new. Oh, I bumped the key switch while setting table measurements this morning but I put it right back where it was.


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## emtnut (Mar 1, 2015)

Back when I was automating water/sewage plants, I'd get called out because "our RTU" wasn't working. (Remote terminal unit, very much like a PLC)

Get there, they'd show me motor doesn't start .... SEE !!!

I'd short out the Digital Output, still wouldn't start. That's all the RTU would do, close that contact, so no way was it the problem. Then explain to the plant electrician how to troubleshoot HIS controls.

Oh well, they paid me well


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## paulengr (Oct 8, 2017)

emtnut said:


> Back when I was automating water/sewage plants, I'd get called out because "our RTU" wasn't working. (Remote terminal unit, very much like a PLC)
> 
> Get there, they'd show me motor doesn't start .... SEE !!!
> 
> ...


Most water plant maintenance guys are good at plumbing, but not when those teeny electrons go through those solid copper pipes.


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## gpop (May 14, 2018)

paulengr said:


> Most water plant maintenance guys are good at plumbing, but not when those teeny electrons go through those solid copper pipes.


Thank you for your vote of confidence


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## 460 Delta (May 9, 2018)

gpop said:


> Thank you for your vote of confidence


Channel your inner MTW with a, Is that so?


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## emtnut (Mar 1, 2015)

gpop said:


> Thank you for your vote of confidence


Lol

By far, some of the best electricians I've ever met were plant electricians.


edit* I became one for a long time too. I was headhunted after automating the local water plant !


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## mburtis (Sep 1, 2018)

It would seem that there is an unproportionate number of poeple on this site that have experience with the water treatment industry.


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## 460 Delta (May 9, 2018)

mburtis said:


> It would seem that there is an unproportionate number of poeple on this site that have experience with the water treatment industry.


Where I work we treat water with Portland cement, sand and stone. We always get the same results though, really hard water.


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## paulengr (Oct 8, 2017)

mburtis said:


> It would seem that there is an unproportionate number of poeple on this site that have experience with the water treatment industry.


Think about it. What’s the difference between a water plant and an industrial plant? The biggest difference is the water plants are very good at paying their bills.


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## SteveBayshore (Apr 7, 2013)

Emergency service calls anywhere from $135 to $75,000 and always paid within 25 days.


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## splatz (May 23, 2015)

mburtis said:


> It would seem that there is an unproportionate number of poeple on this site that have experience with the water treatment industry.


I think it may be proportionate, it's a huge part of the industry - utility water, storm water, and waste water are everywhere - mines, mills, marine fuel tank factories, a lot further between those.


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## paulengr (Oct 8, 2017)

mburtis said:


> It would seem that there is an unproportionate number of poeple on this site that have experience with the water treatment industry.


Seriously you would be amazed if you haven’t dealt with them.

The biggest problem with sewage plants is the smell. You get used to it. I’ve dealt with a lot worse in other places. They are inspected with bacteria counts (white glove test stuff) so they are regularly disinfected and more or less spotless.

Aside from that they don’t hesitate to spend money to get things fixed and you get the municipal worker attitude...generally friendly and just doing their jobs. Rarely is anyone in your face. They are really the ideal customers to have.

The nastiest part of the business is when you have to pull a submersible out in a lift station. They will have water to wash it down. Bring the thicker nitrile gloves and a bottle of disinfectant for your tools though and you’ll be fine. If you make regular stops might want to think about a hepatitis C shot.


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## mburtis (Sep 1, 2018)

Think you mis understood me... I am one of those municipal workers. Plant electrician/mechanic/operator for drinking water department in Wyoming. I like seeing poeple who have experience in this industry. I love the water plant. Let's me do the work I want without some line boss breathing down my neck wondering why I'm not done, or having 14 other jobs that I have to get down the road to. I can take my time and make it pretty and make it last. I help the waste water guys out a bit as well, our waste water crew runs a heck of an operation and you wont find a better crew anywhere. I dont hesitate a second to go down there.


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## paulengr (Oct 8, 2017)

mburtis said:


> Think you mis understood me... I am one of those municipal workers. Plant electrician/mechanic/operator for drinking water department in Wyoming. I like seeing poeple who have experience in this industry. I love the water plant. Let's me do the work I want without some line boss breathing down my neck wondering why I'm not done, or having 14 other jobs that I have to get down the road to. I can take my time and make it pretty and make it last. I help the waste water guys out a bit as well, our waste water crew runs a heck of an operation and you wont find a better crew anywhere. I dont hesitate a second to go down there.


If you have 14 jobs that day, you are losing money.

First that gives you roughly 30 minutes per call including drive time and tracking down the customer who already took time out if their day. You’re not going to be doing repairs, not even changing a receptacle. So you schedule another crew and basically charge them for a quote call and make them take another day off work. This is if everything goes well,

On electrical work exactly who do you want to disappoint? The person who’s job you rushed through and did a crap job or the one where you showed up late or rescheduled? Neither one is going to give you repeat business. And word gets around. And how do you even keep a staff?

My company generally schedules one job per day. If we do two or more it’s because it was a predictable job and we have plenty of time or we take an emergency call after the first one is done or we have enough time to do another call. I can’t remember how many “simple repair” jobs turn out to be far more than something simple. If that happens you are going to irritate 13 of those 14 customers.

Plus when I’m on your site you are my number one customer. You are all I care about that day. I don’t watch the clock or spend half my time on the phone. Customers are less stressed. Techs are less stressed. Everybody wins.

And you said Wyoming. Pray tell how are you going to do 14 calls a day when it takes over a half hour just to get from one customer to the next? Even in the densely populated Eastern US where I live it’s 15-25 minutes just to get across town and we don’t have “wildlife jams”.

On a rare day I do 3 calls.


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## mburtis (Sep 1, 2018)

I'm not ragging on contractors, I'm sure most of you guys do exemplary work, just saying being a water plant employee is a pretty sweet gig. I get paid to be here anyway so as long as the plant is operational I can take as much time as needed to study an issue and understand it then make a fix. I could probably make more money going to work for a contractor but they are all so busy around here with construction there is very much a sense of constant urgency with all of them. Now that we have completely derailed this thread....


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## paulengr (Oct 8, 2017)

mburtis said:


> I'm not ragging on contractors, I'm sure most of you guys do exemplary work, just saying being a water plant employee is a pretty sweet gig. I get paid to be here anyway so as long as the plant is operational I can take as much time as needed to study an issue and understand it then make a fix. I could probably make more money going to work for a contractor but they are all so busy around here with construction there is very much a sense of constant urgency with all of them. Now that we have completely derailed this thread....


Customers hate it when you milk a job so you need to look busy no matter how busy things are. If I slack off at all it won’t be in front of you. If I get done early I get to go home early. In the original post that customer was 4 hours away. On site time was 2 hours. That’s a 10 hour day but it was an emergency call...they called at lunch time so I didn’t get home until 9 PM. Overtime is so plentiful nobody wants it. And I had another job the next day. So you bet I’m going to try to get done reasonably quick. Don’t forget though that many of us make more money the faster we get things done. Service calls are usually hourly so more relaxed.


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## macmikeman (Jan 23, 2007)

Hell, even Macmikeman has had to go out to fix the toothbrush extractor machine a couple of times...........


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## pokeytwo (Dec 6, 2015)

Was called to the local newspaper one afternoon at press time and the press won’t run. “Got to have it running” - you all know the story. Computer showed me it was in the e-stop circuit so I asked the operator and his second if the estop buttons were all checked. Was assured each of them had separately checked them all. There’s about 9 down each side of the printing press and a few more on the cross over bridges. I start at one end taking apart the e-stops to check for loose wires, bad contacts/ blocks - the usual stuff. Fella in white shirt and tie comes to ask if I know whats wrong, how long (it will be a while) and finally can I make the press run. Sure - you get me company letterhead signed by the manager directing me to bypass the safety circuit and you’ll be running. In 5 minutes the manager asks me “can I get more help?”
About half way down one side the help arrives, I scope him out and he starts on one of the bridges when I recall there’s an e-stop hiding underneath the far end of the press and it was backwards to the others - a pull-to-stop that nobody had wanted to spend the few dollars to change. Checked it to find it pulled Instead of pushed. So 2 1/2 hrs straight time, 2 men x 1 hr double time plus lost time on the press run.
The moral? Check everything yourself.


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## JRaef (Mar 23, 2009)

paulengr said:


> ...
> Oh yeah we forgot to say we cranked up the controls to push production.


Classic... 

"We don't need a bigger motor... we just crank her up, turn up the Overload setting, reset the faults and keep replacing the fuses. What's the problem?"


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## JRaef (Mar 23, 2009)

My favorite "useless" service call was one where I had built a bunch of custom soft starters in NEMA 4 boxes that ended up at a dam construction site in SoCal (my shop was in Seattle at the time). Got a call that the bypass contactors were chattering loudly an they looked "rusty", so I was being accused of using old used parts to build these (I had not). Flew to LA, rented a car and dove 4 hours in LA traffic to get to the site out east of Riverside (Hemet), opened up all of the panels and sure enough, all of the ABB contactors were rusty and chattering. I'm wracking my brains trying to figure this out, thinking "Well, the panels went over the mountains, maybe it was humid the day when they were packed up and in the trip over the mountains, the cold caused that humidity trapped in the box to condense on the pole faces of the contactors and that rusted them?" I was really stretching though... After about an hour of head scratching and replacing the contactors with new ones that I had shipped down ahead of time, a maintenance guy drove by and stopped to talk.

"Oh, looks like you are finally changing out those contactors that got sprayed down by the water truck!"

What?!!

"Oh yeah, when we were hooking them up, the water truck drove by and hosed us down while the doors were open. We dried off all the electronics, but the contactors started rusting a few days later..."

Ca-ching! $$$
Got paid for a 2 day portal to portal service call, parts and travel expenses. The site manager was PISSED when he found out one of his guys spilled the beans.


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