Underground Gas Station Conduit

[Michael53]

This is a duplicate of a post I made in November. I'm reposting because I received no comments on the original post which is very unusual. Is the question too long or confusing? Please let me know and I'll restate it if it's confusing. Please see below. Thank you for your input. Michael

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I've wired several gasoline dispensing stations in my time and always used PVC conduit utilizing 514.8 Exception 2.

The organization I work for now is updating its fuel dispensing system. One of the reasons is becasue the GRS conduit used in the last few feet has rusted out. (The rest of the system is in PVC) The winning contractor said he was required to use rigid steel conduit throughout the system for warranty purposes. The companys making the demand are Fuel Master and Bennett Pumps. I pointed out my past experience with PVC and told the contractor I would research the information.

Both FM and Bennett Pumps stated originally PVC was okay as long as I understood an "Act of God" (Lightning is considered an Act of God) would void the warranty. The contractor then contacted the companies and the next thing I knew... I received retractions from FM and Bennett Pumps stating the system most use GRS throughout.

Their installations instructions advise that if there is a reason GRS cannot be used... contact them for a workaround. So I called them back. I was told it was okay to use PVC coated GRS for the system. I have a hard time understanding this.

The reason they first cited for using GRS was to dissapte any lightning induced surges to ground through the walls of the GRS and that PVC wouldn't allow that. So I asked how PVC coated GRS would be any better than PVC. I received no answer.

We now have a very expensive PVC coated GRS conduit system in place. I keep trying to get an answer from FM and Bennett Pumps... becasue I would like to know the technical reasoning behind their decision.

Have any of you ran into this issue? Does anyone know the reasoning behind PVC coated GRS as opposed to PVC conduit? Any technical explanation would be appreciated... or any agreement with me would help me not feel so foolish. Any opinions?

 

[readydave8]

I'm wondering also, and know how frustrating 0 replies can be, so here you go back to the top.

 

[north star]

= $ = $ =

Michael53,

My recommendation is to call FM and Bennett Pumps again,
and pursue it until you a defined, code compliant answer.
Unfortunately, this is going to take some time and effort
on your part......Probably a significant amount of time
at playing telephone tag and unreturned calls......Try to
talk with the engineers or the Technical Dept.; wherever
that may be and get an e-mail address, so that you can
limit your actual speaking time on a telephone call.
They will not care about your in-ground system &
problably won't want to hear about your woes.
Approach this as performing the required "due diligence
research" on something that will occur, not that has
already occured!

As always, please get back to us on this Forum and
let others the outcome of your research.

...And shame on us all for not at least providing some
type of direction for you back in November. hmy:

$ = $ = $

 

[iwire]

I have moved this thread into the haz locations forum in the hope someone will be able to help you.

 

[kbsparky]

PVC conduits can allow lightning strikes to penetrate the wiring inside, and cause much havoc with sensitive electronic equipment connected to such wiring.

The use of a metal conduit system can prevent this migration of stray static voltage, and every manufacturer has the right to specify this type of protection as a condition of them providing warranty coverage of their equipment.

It has NOTHING to do with the minimum standards set forth in the NEC.

 

[hardworkingstiff]

I've had the same argument (uh, I mean discussion) with both Fuelmaster and Bennett. They don't have good engineering reasons, they just say the warranty is voided if not installed in rigid METAL conduit.

I was in a meeting a few years ago where a local distributor was discussing installing a lot of Fuelmaster card readers and the conduit issue was discussed. Their decision was since acts of God are not covered by warranty, then they would not worry about using the PVC as allowed by the NEC.

I wired some Bennetts on a wooded floating dock once and used PVC. They seemed to have a lot of "ghost" type problems. Customer called saying it didn't work, then when the tech got there it worked. Seemed they were replacing a lot of boards. I suggested they install a power integrity surge suppression module in the head of the dispenser before the power supply and they did and the problems seemed to clear up a bit.

As far as I know, the Fuelmaster's installed with PVC are fine. I always used a shielded twisted pair for the telephone wire. On the Bennett's I pulled a shielded twisted pair (like the Belden 88760) for the communication.

I feel your pain on this. I argued with Gasboy (back in the '80's) about this too.

I agree with you that if PVC coated GRS is OK, then PVC should be fine too (I would use the shielded twisted pair for the communication).

Grounding is the biggest thing, and I can see where the metal conduit would be a better path for lightning.

You know your choice, follow their rules and get the perceived warranty (maybe) or just do it your way.

 

[Michael53]

Thank you

Thank you

Thank you readydave8, north star, iwire, kbsparky and hardworkingstiff... for the help and comments. Sorry for the long response time... it was unavoidable, and I didn't want you to think I was unappreciative. Thank you all again.

Hardworkingstiff - yes, a lack of engineering reasoning from Fuelmaster and Bennet is why I came here. I had two conversations with them... and each yielded a different answer. One saying PVC was okay, the other saying it wasn't.

kbsparky - I can understand how a lightning strike could damage sensitive equipment and agree with you about the conduit to use being their call. What piqued my interest was when they said PVC coated rigid was okay, and PVC could not. Seems to me the PVC coated rigid would isolate the conductors and prevent their exposure to current. Also... wouldn't an equipment ground be just as practical as bare rigid conduit in bleeding off the induced field if it were properly grounded?

north star - I spoke with both the companies and various reps for a total I'm sure of over a couple of hours. Eventually... I got tired of "discussing" and asking for engineering reasoning, and they got irritated by my persistence. My boss was irritated because I was not getting results.
So I decided this wasn't a sword I'd fall on.

Again... thank you all. Michael

 

[GoldDigger]

Seems to me the PVC coated rigid would isolate the conductors and prevent their exposure to current. Also... wouldn't an equipment ground be just as practical as bare rigid conduit in bleeding off the induced field if it were properly grounded?

PVC will, for all practical purposes be the same as no conduit, with empty space around the conductors. If an EGC runs next to the wires, a mega-current there can induce a voltage in the other wires, which could cause damage. (Think of it as a 1-1 single turn transformer). On the other hand if the wires are INSIDE the conductor which carries the fault current there will be no magnetic field where the data and power wires are.
So you/they need to consider two sources of damaging voltages, namely magnetic induction and current related changes in the potential at the points where the EGC is connected (electric field). EGC in contact with earth may help the second problem, while an EGC which surrounds the wires will remove the first problem.
Straight PVC does not mitigate either of them.

 

[hardworkingstiff]

Straight PVC does not mitigate either of them.

What about using a shielded cable for the data lines?

 

[GoldDigger]

What about using a shielded cable for the data lines?

A shielded cable will protect against potentials induced by electric fields. It will be equivalent to tissue paper for magnetic fields.
Magnetic shielding is very expensive and has a maximum field against which it can protect.

While we are on the subject, a twisted pair for data will insure that the same voltage is induced on each of the two wires, so that a receiver which detects just the difference in voltage will not see noise. (A twisted pair for an AC power run will reduce its ability to induce stray voltages in other wires.)
But a lightning current surge could symmetrically increase the voltage on both wires beyond what the equipment could withstand.

The two ways to avoid the magnetic induction problem are either to keep all lighting related currents far from the wires or to put the wires inside the current carrying conductor.
The first option would not, by itself, help for lighting induced changes in the "earth" potential at different points in the system.

 

[hardworkingstiff]

A shielded cable will protect against potentials induced by electric fields. It will be equivalent to tissue paper for magnetic fields.
Magnetic shielding is very expensive and has a maximum field against which it can protect.

While we are on the subject, a twisted pair for data will insure that the same voltage is induced on each of the two wires, so that a receiver which detects just the difference in voltage will not see noise. (A twisted pair for an AC power run will reduce its ability to induce stray voltages in other wires.)
But a lightning current surge could symmetrically increase the voltage on both wires beyond what the equipment could withstand.

The two ways to avoid the magnetic induction problem are either to keep all lighting related currents far from the wires or to put the wires inside the current carrying conductor.
The first option would not, by itself, help for lighting induced changes in the "earth" potential at different points in the system.

Thank you, your explanations are the best I've heard from anyone I ever talked to about the why's for when all metal conduit has been spec'd. Thanks again.

 

[GoldDigger]

Thank you, your explanations are the best I've heard from anyone I ever talked to about the why's for when all metal conduit has been spec'd. Thanks again.

Glad I could help. I am pretty good on theory but appreciate the practical experience others can provide.

 

[north star]

( % )

Sounds as though the install should be run in RMC and be
done with it.......This is/ was a good topic & discussion.
Thanks **Michael53** for asking on here!

( & )

 

[hardworkingstiff]

The two ways to avoid the magnetic induction problem are either to keep all lighting related currents far from the wires or to put the wires inside the current carrying conductor.

Thinking about this a little more, would pulling a ground conductor bonded at each end of the metal conduit run have any negating effects on "putting the wires inside the current carrying conductor" phenomena?

 

[GoldDigger]

Thinking about this a little more, would pulling a ground conductor bonded at each end of the metal conduit run have any negating effects on "putting the wires inside the current carrying conductor" phenomena?

If the internal ground conductor was of a low enough resistance relative to the conduit (certainly possible, depending on the details of the conduit) that it carries a large part of the current, then yes, it would be almost as bad as the same grounding conductor inside PVC.

 

[rbalex]

Having used 501.10(A)(1)(a) Exception (even long before it was “official”) for miles of PVC underground raceway in many refineries both foreign and domestic, this thread has caused me to ponder why I’ve had no noted problems, including the instrumentation systems.
At this point, it seems that a properly installed lightning protection system with a single-point connection to the grounding electrode system also comes into play.

 

[GoldDigger]

At this point, it seems that a properly installed lightning protection system with a single-point connection to the grounding electrode system also comes into play.

I think it really does depend very strongly on what you have done to reduce the lightning-induced voltages between opposite ends of EGCs. If there the voltage difference is small, the currents will be low enough that they will not cause problems.

In some of the Solar PV forums there is a lot of discussion of whether (regardless of code interpretation) it makes sense to run grounded wires which serve to connect widely separated ground electrodes, and may carry high lighting-induced currents, along the exact same path as the current carrying wires. This discussion includes particularly running a ground wire along with the current carrying conductors all the way to an interior panel instead of running the ground wire outside the house directly to the ground electrode in question. Some of the concerns definitely result from one conductor having to serve two or more different purposes.
Is it an EGC, a GEC, Electrode Bonding Jumper or other? Or some combination? And is it there for lighting protection, fault clearance or both?