I'm doing a job for a small operater in a rural airport. Its an aboveground tank with a retail type gas pump piped to it. The owner has been talking to an engineer who has helped set this system up. I have no problem with the regular stuff, pump, tank, tank monitering but the guy wants the pump, hose reel and static strap bonded together and then to a ground rod driven beside the concrete pad. He has no concern about the size of the EGC I have pulled to the pump(#12). What code am I violating if I drive the rod without the proper size conductor going back to the service??
You're violating several code sections, and defeating the purposes of grounding as well. I reccomend the engineer read Soares book on grounding, as a start.
No matter what you do, you need that green wire going back to the service. The purpose of this wire is to allow fault currents to be large enough to trip the breakers.
A ground rod is another matter. That is there for lightning protection. While the rod can be argued to "not accomplish anything," it won't hurt either.
Static will be primarily drained through the green wire, back to the service.
Likewise, the ground rod needs to be electrically connected to the green wire. All grounds at the same potential... AND the whit wires kept separate fron the grounds, until you get back to the service.
In fairness to the engineer, Section 250 has more contradictions, confusing terminology, and twists to it than the rest of the code combined! Much was done in 2005 to address this, and more will be seen in the 2008 code.
Re: Static and airplane refueling#68840 08/17/0609:47 PM08/17/0609:47 PM
I believe 250.54 is as close as a clear answer as you can get. Using #6 not prohibited or required. Earth only as a return is prohibited. Otherwise size the bonding conductor from the pump to the panel based on the size of the circuit that may energize the equipment. Check sections 810.21 (J) and 820.100 (D) which are seperate structures that have ground rods. They require # 6 I would recommend # 6. If the engineer complains about the cost of the # 6 explain that it is a pre-emptive legal fee. Alan--
Alan-- If it was easy, anyone could do it.
Re: Static and airplane refueling#68842 08/18/0604:24 PM08/18/0604:24 PM
A Piper Warrior can hold 50 gallons of 100LL, of which 48 gallons are usable. If you use the fuel to set the airplane on fire via a spark from the nozzle, all 50 gallons are usable. I would imagine that the grounding scheme he had in mind also includes a clamp to attach to aircraft.
Re: Static and airplane refueling#68844 08/18/0608:19 PM08/18/0608:19 PM
Rabbit, I understan where you are coming from. This is one area where the code can be read multiple ways. Last year, the IAEA (the Inspectors' group) asked three real experts- and got four distinct answers!
Look through the proposals to the 2008 NEC. Several attempts are being made to clear this mess up. They also address Larry's point.
Larry, as I see it, if you use #6, then you may consider the second rod (if you must) in attaining the 25 ohm requirement. If you don't you can't. A pretty slim distinction, but that's the way the code seems to work it out.
Re: Static and airplane refueling#68845 08/19/0605:45 AM08/19/0605:45 AM
Part of the equipment I'm bonding is a wire reel with a large clamp on it, the clamp is connected to the plane to drain static. It was my assumption that the static came from the plane but thats wrong. The static comes form the fuel being pumped thru a filter.The filter housing is metal but its lined with plastic. In this case the fuel won't be pumped so fast(10 or 12 gallons a minute) but generally jet fuel is pumped at 200 gallons (or higher) a minute. Once I talked to the engineer he thought about it for awhile and he decided the most important thing was to bond everything and forget about the ground rod until he(we) come up with the answer. Apparently he has done this many times with the rod. I'm going to try the states head electrical inspector and see what he says. The place isn't open yet so I have time. Bringing a piece of #6 back to the service wouldn't be very practical at this point and it was one of the first things I suggested.
Re: Static and airplane refueling#68846 08/19/0607:00 AM08/19/0607:00 AM
Check out the difference between "supplemental" and "supplementary" grounding electrodes It's in 250.53(D)(2) and 250.54. A supplemental electrode is a required electrode. A supplementary electrode is one that "shall be permitted", and isn't subject to the same requirements See if that doesn't help.
Also, one of the moderators told me the other night on the chat that the idea behind static discharge was to get the static to the closest grounding plane (not airplane).
Re: Static and airplane refueling#68847 08/19/0612:12 PM08/19/0612:12 PM
I could care less if the airplane wing and nozzle are at ground, just that they are at the same potential before fuel flows or caps are removed. So if it's a fuel truck on rubber wheels or a stationary fueling island, the clamp needs attached first. Then, I still want to see the closed fuel nozzle touching the wing skin before the cap is removed.
I don't think that your assumption was wrong. There might be a charge on the airplane or fuel truck. The fuel contacts the metal nozzle after passing through the filter. But who has ever seen a lineman blasted or running across the tarmac on fire screaming, "Gee, I wonder what caused that static discharge?"
Many small airports don't even carry Jet A. Even if they do, the fuel trucks and delivery rates are similar. Here again, you won't see them gounding anything, but just attaching a clamp from the truck to the aircraft. Joe