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Old 04-11-2017, 08:54 PM   #1
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Default Commercial fuel dispenser communication

We had a trouble call at one of our locally owned public fuel stations. One of the other local contractors had repaired a broken underground rigid conduit feeding one pump with some smurf tube and a C conduit body, spliced the wires in the C and then buried it under concrete. Splice failed and we the the call to repair. The conduit has 1 208v 20a circuit for the pump motors, 1 120v 20a control circuit and a 16/2 twisted cable for comm. We repaired the conduit with a piece of rigid and pulled new wire from the building to the pump. I found a 18/2 twisted shielded cable with a 300v rating to pulling in along with the thhn. I get a call today from the owner saying the pump company wants us to pull out the 18/2 twisted and shielded and install a field twisted 14/2. They are having an intermittent comm problem with the pump, but I do not think it is a wire issue. The shielded is bonded on one end to the egc. With only 2 wires, it should be rs485 comm correct? Which I did not see any terminating resistors, but this is specialty equipment and may be built in. My 18/2 is not twisted as much as the existing 2 wire a I pulled out, but the wire I pulled out did not have any shield or outer jacket and they pretty much are blaming my shielded cable for all of the problems, any thoughts?

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Old 04-11-2017, 09:00 PM   #2
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Originally Posted by mbednarik View Post
We had a trouble call at one of our locally owned public fuel stations. One of the other local contractors had repaired a broken underground rigid conduit feeding one pump with some smurf tube and a C conduit body, spliced the wires in the C and then buried it under concrete. Splice failed and we the the call to repair. The conduit has 1 208v 20a circuit for the pump motors, 1 120v 20a control circuit and a 16/2 twisted cable for comm. We repaired the conduit with a piece of rigid and pulled new wire from the building to the pump. I found of 18/2 twisted shielded cable with a 300v rating to pulling in along with the thhn. I get a call today from the owner saying the pump company wants us to pull out the 18/2 twisted and shielded and install a field twisted 14/2. They are having an intermittent comm problem with the pump, but I do not think it is a wire issue. The shielded is bonded on one end to the egc. With only 2 wires, it should be rs485 comm correct? Which I did not see any terminating resistors, but this is specialty equipment and may be built in. My 16/2 is not twisted as much as the existing 2 wire a I pulled out, but the wire I pulled out did not have any shield or outer jacket and they pretty much are blaming my shielded cable for all of the problems, any thoughts?
Please correct your OP.

Read slowly.

What did you pull out ?

What did you pull in ?

Thanks.
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Old 04-11-2017, 09:13 PM   #3
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None of the supply houses had a unjacketed unshielded comm wire like I removed. The dispenser company suggested simply using thhn and a drill to twist, this sounds like a terrible idea. I don't mind repulling in same thing, but I don't understand how an unshielded twisted cable will outperform a twisted shielded with a sheathing bond. I have used this same cable for rs485 with plc comm without issue. Rs232 would be 3 conductor correct?

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Old 04-11-2017, 10:11 PM   #4
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RS 232 would be at least three wires, but there are other 2 wire protocols besides RS 485. You really can't do anything but follow their instructions if you're just guessing what it is. Both RS 232 and RS 485 have been fairly resistant to noise for me.
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Old 04-11-2017, 10:27 PM   #5
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They were pretty vague instructions to start. I inquired about cable type before we installed it. Originally the tech instructed that twisting was not required unless they are running their diag software from the store, still no cable spec as to how many twist per ft, size, etc. There was a transformer and one board failure in the pump went we reenergized the pump, I suppose from the previous control circuit issue. I think there is more damage to their control system and they are blaming the cable because it's not identical.
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Old 04-11-2017, 10:34 PM   #6
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If they are Gilbarco pumps they don't like you to use shielded cable for the pump communication. Most likely it's a current loop protocol. But if they blew some boards it could be that parts of other ones were damaged.

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Old 04-12-2017, 07:58 AM   #7
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I agree with BH, you probably have a current loop communication circuit. I spoke with a Gilbarco engineer 15 years ago about the shielded vs twisted pair thing. I was told that Gilbarco did extensive testing and they found the twisted pair (10-12 twists per foot) had much better results than a shielded cable (that has about 6-7 twists per foot).

I believe you can purchase this from Southwire, I think they will twist it for you and put it on a spool (a lot better than doing it in the field, I know from experience). Don't be shy about asking for special put ups. I recently purchased a combination of Beldon equivalent wire from them and got it in the put ups I needed ([email protected],000', [email protected]', [email protected]'). In the past I always purchased the 1,000' rolls not knowing they would custom put up (and they did not charge extra).

Also, I don't think you need 14, usually 18 will do. But, I think it would be wise to do what the service company request in order to remove you from the blame game.

Call your Southwire distributor. Let us know what they do for you.
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Old 04-12-2017, 08:05 AM   #8
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Also, I don't think you need 14, usually 18 will do. But, I think it would be wise to do what the service company request in order to remove you from the blame game.
It sounds to me like it's their party. (The service company's.) I'd put in what they paid me to put in. If they asked me to learn more about the system and recommend what will fix the problem, I'd start with the manufacturer and their specs. I'd definitely not stick my neck out second guessing them without being invited (and of course paid) to do so.
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Old 04-12-2017, 08:17 AM   #9
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It sounds to me like it's their party. (The service company's.) I'd put in what they paid me to put in. If they asked me to learn more about the system and recommend what will fix the problem, I'd start with the manufacturer and their specs. I'd definitely not stick my neck out second guessing them without being invited (and of course paid) to do so.

No one would be sticking their neck out.
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Old 04-13-2017, 04:18 AM   #10
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Troubleshooting comm issues can be quite difficult, and is pretty much "guess and check" without putting an oscope on the signal.

Now, if this was RS232, length would be an issue, so I suspect RS485 or some derivative like DeviceNet, for instance, and agree that this is likely a current-loop type communication protocol. I have no seen it mentioned here, but you really should not have this in the same conduit paralleled with your 120. Usually with an unloaded circuit (for instance switching to PLC or Controller IO and *NOT* pulling any coils and such), this wouldn't be much of an issue, but with it being in the conduit WITH the motor power circuit, there is going to be induction, shielded or otherwise.

Was the wire you pulled out in a blue sheathing by chance?

Anyway, back to the intermittent com issue:

How often does this happen? If it is every now and then, it is tougher to justify another race. If it is a daily occurrence, just temp in a wire above ground and see if that resolves the issue. If so, new raceway it is.

The next step is to check terminations. Check for frays. A single small fray touching the other terminal occasionally due to vibration can cause this, and is quite common. Especially after a long job where everyone is tired and ready to go home. You said you bonded the shield on only one end, which is great. Does this wire have a shield AND a drain? A common mistake I have seen made is that the drain wire and shielding are both landed on ground. You do not want the shielding bonded, only the drain wire.

As far as a field-twisted wire, this is quite common in com cables, as the twisted pairs eliminate crosstalk.

Anyway, good luck troubleshooting the issue.

Last edited by sparkiez; 04-13-2017 at 04:20 AM.
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Old 04-13-2017, 04:43 AM   #11
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Now, if this was RS232, length would be an issue, so I suspect RS485 or some derivative like DeviceNet, for instance,
Why do you say that? I have lots of RS 232 running over 1000' on 24 gauge UTP.

Quote:
and agree that this is likely a current-loop type communication protocol. I have no seen it mentioned here, but you really should not have this in the same conduit paralleled with your 120.... but with it being in the conduit WITH the motor power circuit, there is going to be induction, shielded or otherwise.
I hear this all the time, but I don't think it's true. If the insulation rating is sufficient, it's compliant to run it in the same conduit. The great thing about 4-20A devices is that no matter what's happening with voltage drop, induction, or etc., which would obviously mess up 0-10V and others, a CURRENT loop sensor will make the CURRENT right no matter what's happening to the VOLTAGE. Unless there's some short to ground or parallel path draining the current that is supposed to be returning to the inputs, what goes out must come back.

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If it is a daily occurrence, just temp in a wire above ground and see if that resolves the issue.
That's always a good idea although it might not be practical at a filling station.

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The next step is to check terminations. Check for frays. A single small fray touching the other terminal occasionally due to vibration can cause this, and is quite common.
This is a good suggestion too. I can't even count how many times one stray strand in a termination has made a monkey out of me. I really should be using those ferrules at all times, so far it's more something I mean to do than something I've actually done, but I am a slow learner.

I am sure a lot of times when a device is replaced and it fixes a problem it was really the retermination that fixed the problem, there was no problem with the device.

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You said you bonded the shield on only one end, which is great. Does this wire have a shield AND a drain? A common mistake I have seen made is that the drain wire and shielding are both landed on ground. You do not want the shielding bonded, only the drain wire.
I have not heard this before. I do it just for neatness, once you consider the drain wire is in contact with the shield the whole length of the cable, there's really no reason to worry about twisting up the foil and grounding it. But could it actually cause trouble doing so? I don't bother to remove the foil where I found someone else has grounded it.
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Old 04-13-2017, 05:58 AM   #12
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It is a common practice to install the twisted pair comm wires in with the power wires, done all the time with minimal to no problems.

Y'all are making this too complicated for the OP. He is not hired to troubleshoot the comm issues, just to install it per manufacturer specs. Pull the twisted pair (14 since the guys that are responsible for the troubleshooting are calling it out) and move on. Maybe megger the wiring as an added insulator to the problem (hey, I made a funny, ).
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Old 04-13-2017, 07:05 AM   #13
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Quote:
Originally Posted by sparkiez View Post
Troubleshooting comm issues can be quite difficult, and is pretty much "guess and check" without putting an oscope on the signal.

Now, if this was RS232, length would be an issue, so I suspect RS485 or some derivative like DeviceNet, for instance, and agree that this is likely a current-loop type communication protocol. I have no seen it mentioned here, but you really should not have this in the same conduit paralleled with your 120. Usually with an unloaded circuit (for instance switching to PLC or Controller IO and *NOT* pulling any coils and such), this wouldn't be much of an issue, but with it being in the conduit WITH the motor power circuit, there is going to be induction, shielded or otherwise.

Was the wire you pulled out in a blue sheathing by chance?

Anyway, back to the intermittent com issue:

How often does this happen? If it is every now and then, it is tougher to justify another race. If it is a daily occurrence, just temp in a wire above ground and see if that resolves the issue. If so, new raceway it is.

The next step is to check terminations. Check for frays. A single small fray touching the other terminal occasionally due to vibration can cause this, and is quite common. Especially after a long job where everyone is tired and ready to go home. You said you bonded the shield on only one end, which is great. Does this wire have a shield AND a drain? A common mistake I have seen made is that the drain wire and shielding are both landed on ground. You do not want the shielding bonded, only the drain wire.

As far as a field-twisted wire, this is quite common in com cables, as the twisted pairs eliminate crosstalk.

Anyway, good luck troubleshooting the issue.

It is a daily occurrence, but there are 3 other pumps with the same control system that work fine with the comm wire in the power raceway. I did some questioning yesterday as I do not like the blindly change out material, through the years I found this is typically someone else not wanting to do their job as I suspect in this case as well. The existing wire in the other 3 pumps in unshielded, unjacketed twisted cable and they all 3 work fine, so this is not an issue of comm wire in the power conduit. The customers "comm" problem consists of the pump control system randomly showing fuel sales when the camera shows no vehicles have been at the pump. After we originally repaired the branch circuit, the control transformer and output board in the pump was bad. I suspect there is an issue with the comm card as well, to have interference just right to induce a signal which can be read as a completed sale repeatedly throughout the day is pretty much impossible. I did pull out 1 of the new comm cables and pulled in a 16/2 field twisted cable. I left my other shielded cable in the conduit as a spare. Ill keep you guys updated as to what turns up, thank you.
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Old 04-13-2017, 11:02 AM   #14
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"You said you bonded the shield on only one end"

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I have not heard this before. I do it just for neatness, once you consider the drain wire is in contact with the shield the whole length of the cable, there's really no reason to worry about twisting up the foil and grounding it. But could it actually cause trouble doing so? I don't bother to remove the foil where I found someone else has grounded it.
That is how I was taught to do it originally and have always done it that way, never questioned the reasoning.
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Old 04-13-2017, 11:17 AM   #15
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"You said you bonded the shield on only one end"



That is how I was taught to do it originally and have always done it that way, never questioned the reasoning.
I am of the understanding that you don't bond both ends because you don't want to have a parallel path for current and create a ground loop.
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Old 04-13-2017, 11:41 AM   #16
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My question was whether bonding both the shield itself (foil) and the drain wire was actually a problem. I figured it is unnecessary to bond the shield when there's a drain wire, but was I think @sparkiez was saying it's a bad thing.

As far as bonding one end or both - my unenlightened thinking is to ground one end, and if there is any question about the grounds being at different potentials, deal with that with big boy equipotential bonding rather than by bonding a puny drain wire at both ends. There are exceptions depending on the gear and the comm protocol.
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Old 06-07-2017, 01:54 PM   #17
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For this situation, the only solution is to pay some guy with working brain.
All I see is no information, just empty talk, like:
- What kind pumps are those?
- Well, they are blue...
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Old 06-07-2017, 03:08 PM   #18
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For this situation, the only solution is to pay some guy with working brain.
All I see is no information, just empty talk, like:
- What kind pumps are those?
- Well, they are blue...
Maybe THIS forum may be of more use to you then.
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Old 06-07-2017, 06:24 PM   #19
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The field twist did the trick. Hasn't missed a beat since.
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