Thread Sealant on threaded RMC joints?

[dancase]

A few months back we erected a new 800 foot broadcast tower. The obstruction lighting system on that tower used 1 1/4" RMC for the vertical run up the tower, and either cord-and-gland or 3/4 RMC from the cast junction boxes to fixtures.

The manufacturer of the lighting system specified use of pipe thread sealant on all threaded connections, and included several cans of fairly "garden-variety" pipe dope. I can certainly understand why they want the joints sealed. All of the junction boxes are gasketed, and with the exception of the breather fittings it's a sealed system.

However, I don't see anything in the code that specifically allows or disallows use of thread sealants. Nor have I ever seen a "conductive" pipe thread sealant. I'm comfortable that a wrench-tight threaded joint will have suitable electrical conductivity, but it bothers me that there's nothing (that I've seen, at least) that specifically addresses this question.

We're about to replace the lighting system on three 500 foot towers, and run some other above-ground threaded conduit runs at a different site, and I need to know if the use of pipe dope on wrench-tight threaded RMC connections is acceptable, and if there are specific types of sealant that should be used for conduit work. These installs won't be subject to inspection, but our company takes code compliance very seriously -- we want everything to meet or exceed code requirements whether it's to be inspected or not.

Your counsel?

D.

 

[g3guy]

Re: Thread Sealant on threaded RMC joints?

Conductive sealants have been around for quite awhile. Had used them quite often in the past but now that I am retired I'll let the "working class" give you info on what's available now.

 

[petersonra]

Re: Thread Sealant on threaded RMC joints?

I think the local inspector should personally inspect each joint all the way up the 800 feet.

I don't see how the use of thread sealant whether conductive or not is a deteriment. The thread sealant goes in the voids between the threads, not between the threads, so there is no loss of electrical continuity at all.

In fact, it probably promotes electrical continuity by keeping moisture out of the joint thus reducing oxidation.

 

[charlie]

Re: Thread Sealant on threaded RMC joints?

Bob, I agree unless the tape type of sealant is used. Even with that type the threads would eat up enough of the tape to make good electrically conductive joints.

 

[steve66]

Re: Thread Sealant on threaded RMC joints?

Unless the tower is pre-painted, I'm thinking the conduit will also be bonded together by the steel tower and all the conduit clamps.

Steve

 

[allenwayne]

Re: Thread Sealant on threaded RMC joints?

Sounds like to me that it`s a spec. for that job

 

[apauling]

Re: Thread Sealant on threaded RMC joints?

it may have been a few code cycles back, or it is even in the current code, but discussing sealed threaded fittins as impaired grounds does come up in 2 places; jumpers over fittings in water pipe ground rods, and jumpers at water heaters.

this came up before and these were the only two I know about. If anyone knew of others, they weren't mentioning them.

however, i do not see a problem. I would assume that conductive sealants are available and appropriate if the piping is the grounding.

paul

 

[sandsnow]

Re: Thread Sealant on threaded RMC joints?

Anybody remember red lead??

I don't, but an old timer told me about it and showed me a code section. It must be in the '99 or earlier because I can't find it in the 2002.

It was required I think for field cut threads.

 

[dancase]

Re: Thread Sealant on threaded RMC joints?

Originally posted by steve66:
Unless the tower is pre-painted, I'm thinking the conduit will also be bonded together by the steel tower and all the conduit clamps.

Steve

Some of the methodology on a tower like this will seem a bit foriegn to most electricians, just because they haven't dealt with it before.

First of all, the towers we're talking about here ARE painted -- about 55 years worth of it, in fact. Like the majority of towers in this class, these towers (there are three of them in the array)are made of solid rod, with legs 1 1/2" in diameter and crossmembers around 5/8" to 3/4" diameter welded between. The most common means of securing lighting conduit is with stainless steel strapping (Wraplock), strapping it to the crossmembers every few feet. Since these towers are used for an AM radio station, they are insulated -- the tower base sits on a large porcelain insulator, and the guy wires are broken into nonresonant lengths. Rather than the tower supporting the antenna, the entire tower IS the antenna. That means that there is no such thing as "ground" in this lighting system. The conduit is bonded to the tower steel every 25 feet using pipe clamps and #4 copper, making it an integral part of the antenna itself and preventing interaction between the conduit and tower at radio frequencies. Not exactly your typical office wiring job.

D.

 

[LarryFine]

Re: Thread Sealant on threaded RMC joints?

Originally posted by dancase:
The conduit is bonded to the tower steel every 25 feet using pipe clamps and #4 copper, making it an integral part of the antenna itself and preventing interaction between the conduit and tower at radio frequencies.

Wouldn't that mean that the conduit must have an insulating joint where it leaves the tower to avoid grounding the tower?

 

[dancase]

Re: Thread Sealant on threaded RMC joints?

Originally posted by LarryFine:
Wouldn't that mean that the conduit must have an insulating joint where it leaves the tower to avoid grounding the tower?

There are a couple of ways to do that. In most lower-power AM installations, the conductors are brought into the tuning unit (a tuning network that feeds the tower) inside the pipe (usually copper)that connects the tower to the tuning unit. The lighting conductors are then connected to a "lighting choke," a special inductive device that is transparent at 60 Hz, but high impedance at the frequency of the station.

The station where we're doing this work is a 50 kW station, and the height of that towers is such that the RF voltages at the tower base is very high -- over 30 kV on modulation peaks -- high enough that the lighting choke necessary would be large and very pricey. In this case, we use an Austin Ring transformer to couple AC for tower lights across the base insulator. The Austin transformer consists of two interlocking toroidal windings seperated by air. You can see a picture of an Austin-style transformer (the actual one we're going to be using to replace the 55-year old transformers on these towers) at http://www.pwdahl.com/images/tlt.JPG . The tower side then becomes an SDS, nicely isolated.

D.

 

[don_resqcapt19]

Re: Thread Sealant on threaded RMC joints?

If any of the conduit threads are field cut, then the sealant must be an "approved electrically conductive, corrosion-resistant compound". [300.6(A)] I don't think that any standard pipe compound would meet this requirement.
Don

 

[dancase]

Re: Thread Sealant on threaded RMC joints?

Originally posted by don_resqcapt19:
If any of the conduit threads are field cut, then the sealant must be an "approved electrically conductive, corrosion-resistant compound". [300.6(A)] I don't think that any standard pipe compound would meet this requirement.
Don

True enough. But then again, Kopr-Shield isn't a sealant, either.

Is there a product on the market that meets both the 300.6(A) requirements AND will seal the threaded joint?

D.

 

[dancase]

Re: Thread Sealant on threaded RMC joints?

Now that I think about it for a minute (I shouldn't post in my sleep ) Just curious on that one.

D.

 

[don_resqcapt19]

Re: Thread Sealant on threaded RMC joints?

Dan,

IMHO, a properly wrench-tightened threaded joint will have quite solid and acceptable metal-to-metal conductivity no matter what kind of pipe dope is used

I'm not sure that I agree. I would if conduit couplings had a tapered thread, but they don't, so I'm not sure that you would have a solid metal to metal contact if a nonconductive joint compound was used.
If this was my project, I would look at using plumbers couplings in place of the conduit couplings to get a seal without the use of any type of compound. The plumbers coupling has tapered threads. Yes, I know that the plumbers coupling is not listed for this use, but neither is the pipe compound that you are planning to use. It is also my understanding that plumbers pipe compound is not really a sealant, but rather a lubricant. The purpose is to make it easier to tighten the joint to the point where there is solid metal to metal contact between the male and female tapered threads, with the metal to metal contact actually making the seal.
Don

 

[petersonra]

Re: Thread Sealant on threaded RMC joints?

I thought conduit couplings were a tapered thread.

 

[don_resqcapt19]

Re: Thread Sealant on threaded RMC joints?

Bob,
The male threads on the conduit are standard tapered pipe thread, but conduit couplings are straight thread.
Don

 

[dancase]

Re: Thread Sealant on threaded RMC joints?

As a point of interest, I was working today at one of my sites built in the late 40's/early 50's. I found it interesting that there are some straight couplings (look just like conduit couplings)used in the galvanized water pipes. It's been 55 years, and still no leaks at those straight couplings.

Fascinating.

D.

 

[hardworkingstiff]

Re: Thread Sealant on threaded RMC joints?

Originally posted by dancase:
As a point of interest, I was working today at one of my sites built in the late 40's/early 50's. I found it interesting that there are some straight couplings (look just like conduit couplings)used in the galvanized water pipes. It's been 55 years, and still no leaks at those straight couplings.

Fascinating.

D.

How can you tell the difference without taking the piping system apart?