# $500 up in smoke, what am I missing



## mburtis (Sep 1, 2018)

So got my butt kicked by a flowmeter today and I don’t understand why. In pipe propeller type meter (McCrometer) with a loop powered 4-20 mA output that runs back to the plc. So wiring is +24vdc from power supply to meter, meter back to terminal block in plc cabinet, terminal block to plc input card, plc input card to 24 return. We had this fail a couple of weeks ago and I went and trouble shot it and figured a surge from lightning or something had blown the 4-20 board in the meter (which it is). Ordered a new meter head and installed it today and the 4-20 card flat blew up.

when I initially troubleshot it I continuity tested the wire and it tested fine as well as checked voltage at the meter end. I chased my tail for a minute on the voltage check. I think what happened was I only hooked up the +24 wire out to the meter and left the return wire from the meter disconnected so it was floating on both ends. When I tested for voltage at the meter I kept getting 0. I’m assuming that since it was floating the meter had no reference? Regardless of what stupidity I was doing that day, today I checked it with everything in the cabinet fully wired just the two wires at the meter disconnected, and had 24v between the two wires at the meter. I went a step further and hooked the fluke 787 up in simulate mode and everything worked just fine, the plc got the signal, it displayed on SCADA, whole nine yards.

I’m at a loss trying figure out what I’m missing that would cause the board in the meter to blow up and I mean literally blow up and cover the inside of the meter in smoke residue, but work perfectly fine with the fluke and not harm the plc. Any ideas?


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## splatz (May 23, 2015)

Did you have the polarity correct at the PLC input? 

This is clipped from one of their manuals, don't know if this matches your model...


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## gpop (May 14, 2018)

If you go to trouble shoot a in loop meter and measure between + and - you would expect a reading between 1-5v if everything is working correctly. 24v would indicate a meter that has a open card (same reading if the meter was disconnected and you measure the wires) or 0v indicating the meter has a short or you have lost the power supply.

The plc has a resistor built into the card if you put 24v to the meter and do not loop in the plc card the meter will smoke as it has nothing to stop it pulling the max amps of the power supply (no resistor in series). Normally we would install a 250ma fuse in the loop to prevent a problem like this.

Problems can occur when you are hooking up 4-20 in loop as a wire that is marked dc- one end will terminate to a wire marked dc+. Make a mistake on a terminal strip and land dc- to dc- and you just un-wittingly looped out the card and let the magic smoke out.


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## 460 Delta (May 9, 2018)

I’m curious as to how a 4-20 ma loop works on a turbine meter. The r- mix industry uses primarily Badger, and to a lesser extent Neptune turbine flow meters in the batch water and the ad-mix system. Ours are pulse counters operating on Hall effect pickups. They can be a little finicky to calibrate, but are pretty steady once dialed in.


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## mburtis (Sep 1, 2018)

Well crum @gpop you just educated me and made me realize I don't know didly. When I was trouble shooting the power supply I had unhooked the plc wires and twisted them together to take it out of the loop, as a test to see if the card was burned out. I was in a hurry today and didn't look in the cabinet until I had already hooked the meter up. For some reason I always thought the transmitter would limit the current itself, most everything I work on is a source type set up, these meters are about the only loop powered things.Well at least I know what I did now, expensive lesson though and I can tell the boss I'm just an idiot. Bet I won't do that again. I'll head to my corner to hang my head now.


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## mburtis (Sep 1, 2018)

@460 Delta these are a similar set up probably. These can do a pulse output as well but can be ordered with an optional 4-20 output, little add on board deal. That lets out surprising amounts of smoke apparently.


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## ScooterMcGavin (Jan 24, 2011)

mburtis said:


> Well crum @gpop you just educated me and made me realize I don't know didly. When I was trouble shooting the power supply I had unhooked the plc wires and twisted them together to take it out of the loop, as a test to see if the card was burned out. I was in a hurry today and didn't look in the cabinet until I had already hooked the meter up. For some reason I always thought the transmitter would limit the current itself, most everything I work on is a source type set up, these meters are about the only loop powered things.Well at least I know what I did now, expensive lesson though and I can tell the boss I'm just an idiot. Bet I won't do that again. I'll head to my corner to hang my head now.


The transmitter doesn’t “need” the plc card to function properly. The transmitter is inherently current limiting that’s how it works. I usually do exactly what you described because I’ve done the opposite and have smoked plc cards by short circuiting the input by getting my wires crossed. (way more than $500 in damage BTW so don’t feel bad). Transmitters don’t care about the load resistance as long as it’s less than the maximum that it can drive for a given voltage.

Sounds to me more like a failed surge arrester or blown transmitter causing it to go to ground. Your test meter isn’t grounded so it will work fine since it’s floating. The power supply is probably grounded so it’s getting a ground fault when you try use it.

I’ve seen this a few times where the loop tester works but real power supply doesn’t.

considering the first card did the same thing your second card is probably the victim of the problem that blew up the first card. Might need a whole new transmitter.

The 1-5V thing is mostly true. That assumes a 250 ohm resistor. Some cards/devices only use a 50 ohm resistor so you will have .2-1 V of voltage drop across it.


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## mburtis (Sep 1, 2018)

I was suspect as the first meter had the same failure, took it apart and the 4-20 board was blown up, just not as spectacularly as the second. Another reason I at first didn't consider the plc being out of the loop. Pretty impressive damage for 24v, I mean it blew chunks of chips off the circuit boards. However it is hooked to a fair sized power supply. The power supply is tied to ground, but is running a bunch of other stuff as well, including another of the same type of meter. I would think I would see issues elsewhere. There is no lightning arrestor on this line, although I need to add one. The manufacture makes a pretty big deal about having an isolated power supply for these but this has been running since 2015 like this. I need to clean up this cabinet and may install a isolated power supply for this meter.


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## gpop (May 14, 2018)

scameron81 said:


> The transmitter doesn’t “need” the plc card to function properly. The transmitter is inherently current limiting that’s how it works. I usually do exactly what you described because I’ve done the opposite and have smoked plc cards by short circuiting the input by getting my wires crossed. (way more than $500 in damage BTW so don’t feel bad). Transmitters don’t care about the load resistance as long as it’s less than the maximum that it can drive for a given voltage.
> 
> Sounds to me more like a failed surge arrester or blown transmitter causing it to go to ground. Your test meter isn’t grounded so it will work fine since it’s floating. The power supply is probably grounded so it’s getting a ground fault when you try use it.
> 
> ...



I have seen a instrument tech burn up a $1500 pressure sensor on a bench simply hooking it to a power supply then for good measure grab another and fry that because he thought it would be current limiting. Life would be so much simpler if it worked that way when doing bench calibrations with a pressure pump but unfortunately its just expensive smoke.


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## gpop (May 14, 2018)

mburtis said:


> I was suspect as the first meter had the same failure, took it apart and the 4-20 board was blown up, just not as spectacularly as the second. Another reason I at first didn't consider the plc being out of the loop. Pretty impressive damage for 24v, I mean it blew chunks of chips off the circuit boards. However it is hooked to a fair sized power supply. The power supply is tied to ground, but is running a bunch of other stuff as well, including another of the same type of meter. I would think I would see issues elsewhere. There is no lightning arrestor on this line, although I need to add one. The manufacture makes a pretty big deal about having an isolated power supply for these but this has been running since 2015 like this. I need to clean up this cabinet and may install a isolated power supply for this meter.



We all make mistakes so spend a extra $8 and get a fuse holder and a fuse (250 ma) next time. If you can swing it throw in a extra $80 and get a surge suppressor.


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## mburtis (Sep 1, 2018)

I need to redo this cabinet anyway, it's bad bad. So may throw fuses and lightning arrestor on all the loops to keep this from happening again.


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## gpop (May 14, 2018)

mburtis said:


> Well crum @gpop you just educated me and made me realize I don't know didly. When I was trouble shooting the power supply I had unhooked the plc wires and twisted them together to take it out of the loop, as a test to see if the card was burned out. I was in a hurry today and didn't look in the cabinet until I had already hooked the meter up. For some reason I always thought the transmitter would limit the current itself, most everything I work on is a source type set up, these meters are about the only loop powered things.Well at least I know what I did now, expensive lesson though and I can tell the boss I'm just an idiot. Bet I won't do that again. I'll head to my corner to hang my head now.



Let you into a little secret. Monday i was hooking up hart on a loop powered ultrasound level indicator and it was being a pain communicating so i added a resistor while not paying attention and now i have a burn on my finger. Little 1/4 watt resistor can get hot as hell hooked across a power supply. Nothing damaged except for my pride and finger.


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## MotoGP1199 (Aug 11, 2014)

mburtis said:


> I need to redo this cabinet anyway, it's bad bad. So may throw fuses and lightning arrestor on all the loops to keep this from happening again.


They make din rail terminal block fuse holders with LED blown fuse indicators for cheap that work great for this.

Here's a 10 pack for $23. Just need to buy the fuses.


https://www.automationdirect.com/adc/shopping/catalog/terminal_blocks/konnect-it_din-rail_terminal_blocks/circuit_protection_blocks/kn-f10l24dc-10


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## mburtis (Sep 1, 2018)

Well proved this morning that @gpop was right on. Took another meter head off a different meter out this morning and hooked it up. Everything worked fine except its off a different size meter so the number is way off and I can't adjust it. However it proves that the problem was the plc being bypassed.


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## paulengr (Oct 8, 2017)

Very strange.

The way these things work is that it’s a variable resistor with a controller that varies the resistance to give the proper current output. So with 24 VDC the total loop resistance must be between 24 / 0.004 = 6000 ohms and 24 / 0.02 = 1200 ohms. For a maximum 1000 ohm load that means the transmitter (flow meter) must vary output resistance from 200 to 5000 ohms. It also needs to accommodate better conditions so even if the total loop resistance is only 100 ohms it still varies between 1100 and 5900 ohms. The lower end is well within the transmitters capabilities…worst case is at maximum loop resistance, not lowest. So electrically The shorted loop thing sounds very doubtful.

Here is what has happened to me quite often. The power supply may be floating (not grounded) or the meter is grounded to another source, or unshielded cable used, or the shield is grounded at both ends forming a ground loop. Often multiple power supplies are involved, and especially the PLC may have non-isolated inputs. Allen Bradley especially is notorious for this. It’s pretty obvious if you start measuring voltages between your various “commons” and grounds and often from return terminal to return terminal, both AC and DC on the PLC input terminals. Very often I’ve seen 120 VAC or worse appear on a 4-20 mA loop.

Now putting 120 VAC input a loop powered transmitter is likely to blow out some components.’

There are a couple solutions. Sometimes you can short all the DC commons to ground. Or you can buy cheap signal isolators that use opto-isolators so each loop is independent.


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## mburtis (Sep 1, 2018)

I wasn't sure I believed it but it's the only thing that explains it. Now this cabinet is a mess but there are other identical meters hooked up the same way and none of them have issues. I would think that if it was something on the ground/shield it would show up elsewhere since the are all tied together. I also haven't managed to find any significant voltage anywhere it shouldn't be.


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## gpop (May 14, 2018)

paulengr said:


> Very strange.
> 
> The way these things work is that it’s a variable resistor with a controller that varies the resistance to give the proper current output. So with 24 VDC the total loop resistance must be between 24 / 0.004 = 6000 ohms and 24 / 0.02 = 1200 ohms. For a maximum 1000 ohm load that means the transmitter (flow meter) must vary output resistance from 200 to 5000 ohms. It also needs to accommodate better conditions so even if the total loop resistance is only 100 ohms it still varies between 1100 and 5900 ohms. The lower end is well within the transmitters capabilities…worst case is at maximum loop resistance, not lowest. So electrically The shorted loop thing sounds very doubtful.
> 
> ...


I'm sure you have some old analog loop powered junk laying around so why not give it a go and see what happen.


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## Mbit (Feb 28, 2020)

I'm confused. For standard 24vdc 4-20mA transmitters you can hook them to 24VDC, no resistors required. I do this quite frequently on the bench.

But I've been wrong before lol


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## gpop (May 14, 2018)

Mbit said:


> But I've been wrong before lol


So have i. 

I agree with the logic that there should be no reason that you could not hook 24vdc directly to a transducer. Personally i have never hooked one up that way as i can remember seeing the instrument tech smoke units on the bench. Then again the guys smoked all sorts of instruments including 2 fluke 744 meters so it wouldn't surprise me if he was doing something else to cause the smoke. 
Im upgrading some ultrasound loop powered devices in the next few days so i will have the old ones i can play with on the desk.


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## Quickservice (Apr 23, 2020)

mburtis said:


> Well crum @gpop you just educated me and *made me realize I don't know didly*.


Don't feel bad... most of us have experienced that many times. Like having your MM set on DC and you are cursing because you can't get a reading on a 120v AC circuit. And it happens more often when you get old as dirt!


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## splatz (May 23, 2015)

I would not have thought the current limiting resistor would be necessary but I could be wrong. In fact I'd bet it depends on the particular make and model. I pretty much read the paperwork with every 4-20ma device I install because there are enough differences you can't assume you know how this one works, I like to confirm what goes where before wiring it into the controls. You never know if you ordered the right one, or they shipped you the one you ordered, etc. 

I have gotten in the habit of bench testing them just to see if I understand the wiring. Actually not usually at a workbench, more likely on a box, bucket, or cart. I usually put the 4-20ma loop in series with an analog meter and power everything from two 9V batteries in series. Most or all that I have used 18VDC supply is within spec. Using batteries is handy, I don't need a receptacle and DC power supply to test. This has enough power to run the thing but is less likely to fry the thing. A fuse of course would be a smarter way to go and I always have fuseholders and fuses around but this has worked so far.


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## mburtis (Sep 1, 2018)

All I know at this point is I stole parts out of anther meter and it works now. The little board that I blew is labeled as a 4-20mA protection board and after talking to the tech it sounds like they might let the smoke out just by looking at them wrong.


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## just the cowboy (Sep 4, 2013)

As said fuse fuse fuse. Now we know what we bust you on. @460 Delta gets it for wire numbers you can be fuses.

We fuse ALL 4-20 in and outputs, also anything that leaves the panel has a transector protection against lighting, they are not cheap. 
We also use a relay with all output and inputs that leave the panel as sacrificial parts, if lightning hits a tank hatch it smokes the relay not the PLC.


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## 460 Delta (May 9, 2018)

just the cowboy said:


> As said fuse fuse fuse. Now we know what we bust you on. @460 Delta gets it for wire numbers you can be fuses.
> 
> We fuse ALL 4-20 in and outputs, also anything that leaves the panel has a transector protection against lighting, they are not cheap.
> We also use a relay with all output and inputs that leave the panel as sacrificial parts, if lightning hits a tank hatch it smokes the relay not the PLC.


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## mburtis (Sep 1, 2018)

just the cowboy said:


> As said fuse fuse fuse. Now we know what we bust you on. @460 Delta gets it for wire numbers you can be fuses.
> 
> We fuse ALL 4-20 in and outputs, also anything that leaves the panel has a transector protection against lighting, they are not cheap.
> We also use a relay with all output and inputs that leave the panel as sacrificial parts, if lightning hits a tank hatch it smokes the relay not the PLC.


I've actually been researching this the last couple of days putting together parts lists to add protection to the cabinets. What models of surge protection are you using? Or could anybody else recommend. Im guess a true surge suppressor is a better bet than an optical isolator/signal conditioner. We have a few on some of the 4-20 loops. Non of our plc wiring is fused much at all and no interface relays or such unless it's an ice cube to step up the power. You said everything that leaves the panel but does that include inside buildings. Obviously if it leaves the building lightning ect is a possibility but if it's just running across the room ?


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## just the cowboy (Sep 4, 2013)

mburtis said:


> I've actually been researching this the last couple of days putting together parts lists to add protection to the cabinets. What models of surge protection are you using? Or could anybody else recommend. Im guess a true surge suppressor is a better bet than an optical isolator/signal conditioner. We have a few on some of the 4-20 loops. Non of our plc wiring is fused much at all and no interface relays or such unless it's an ice cube to step up the power. You said everything that leaves the panel but does that include inside buildings. Obviously if it leaves the building lightning ect is a possibility but if it's just running across the room ?


This is what I use for analog, two channel and they plug in the din rail base for replacement.
I also use Transtector 1100-954 as the first thing in my panel to protect the AC line








https://www.alliedelec.com/product/transtector-systems/1101-680/71261017/


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## gpop (May 14, 2018)

mburtis said:


> I've actually been researching this the last couple of days putting together parts lists to add protection to the cabinets. What models of surge protection are you using? Or could anybody else recommend. Im guess a true surge suppressor is a better bet than an optical isolator/signal conditioner. We have a few on some of the 4-20 loops. Non of our plc wiring is fused much at all and no interface relays or such unless it's an ice cube to step up the power. You said everything that leaves the panel but does that include inside buildings. Obviously if it leaves the building lightning ect is a possibility but if it's just running across the room ?


Inside the building we still use suppressors on instruments attached to water lines that enter the building. Between things like the plc and vfd's on the older plants we don't and i do not see the point of adding them. Newer buildings they simply spec'ed every i/o as requiring protection so the cabinets are huge compared to the amount of i/o. 

A lot of our panels are mounted outside, everything gets a suppressor including network cables. During this time of the year we expect to take 20 to 100 surges a week and probably 30 brown-outs so thousands of dollars worth of suppressors and other protection equipment is a easy sell.


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## just the cowboy (Sep 4, 2013)

gpop said:


> Inside the building we still use suppressors on instruments attached to water lines that enter the building. Between things like the plc and vfd's on the older plants we don't and i do not see the point of adding them. Newer buildings they simply spec'ed every i/o as requiring protection *so the cabinets are huge compared to the amount of i/o.*


Yep in this day and age using a 4'x6' panel with side pans for a 3 well site seems overkill at first, but when you add all the protection and isolation relays it fills up fast. It feels like going back in time but between power blips, lightning and sunshine in our line of work it is needed. 

I'm going to open a new post on UPS's for panels that sort of relates to this but will do better as a new post, see you there.


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## mburtis (Sep 1, 2018)

Thanks for the input, my shopping list just keeps growing.


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## micromind (Aug 11, 2007)

I've never connected any loop-powered instrument directly to 24DC, I've always had a 250Ω resistor plus a 100ma fuse in series with it. 

Years ago, I made up a clip lead that had such a resistor and an inline fuse holder for this very purpose.


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## Jlarson (Jun 28, 2009)

I always use a processmeter on loop power mode when I screwing with odd xmitters, that way it doesn't have a lot of power and I can see what it's doing on the meter lol.


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## gpop (May 14, 2018)

Jlarson said:


> I always use a processmeter on loop power mode when I screwing with odd xmitters, that way it doesn't have a lot of power and I can see what it's doing on the meter lol.



I have a few process meters supplied by the company but my favorite is a altek i brought myself about 15 years ago for $50 used. 









I also have the altek with the switches but i hate that meter as its bulky.


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