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Joined: Feb 2004
Posts: 5
EvanH Offline OP
Junior Member
When assembling rigid conduit in a Class I Division 1 location, is there a specific type of thread lubricant that's allowed or recommended? Or is there no rule either way?

Joined: Jan 2005
Posts: 5,369
Likes: 1
Cat Servant
I am not aware of a specific rule that addresses the issue...BUT...

Many folks don't realise it, but Haz Loc stuff is generally designed to "leak," and NOT be gas tight, as a means of venting the pressures from an internal explosion. Quite often this venting is through the threads- so there is a rule that five or more threads must be engaged.
This venting is at a much slower rate than the speed of the "flame front" of an explosion, so the explosion is extinguished before it exits the vent.

So anything that might seal a threaded connection is to be avoided. Even the slight delay required to push out the oil might be enough to cause the internal pressures to build up, and the device fail.

The best practice is to have threading equipment kept up, so that the threads can dry-fit without the help of any oil, lube, anti-seize, or whatever. Even your cutting oil ought to be cleaned off before assembly.

Joined: Apr 2002
Posts: 7,333
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Amen to the above statement!
100% true, and I also don't have an NEC reference.
THe thread path is the 'vent' means.


Joined: Dec 2003
Posts: 886
Quite often this venting is through the threads- so there is a rule that five or more threads must be engaged.

I admit I know little about haz location stuff but this is a bit confusing and you are right, I never did realize it.

I would think that the rule would be to limit the number of threads engaged but still, when tightly made up even dry a threaded connection will not leak even under considerable pressure. How can this be reliable?


Joined: Mar 2001
Posts: 2,056
I understand the principal of E-P enclosures, but I used to use this stuff (the HTL) on aluminum fittings in a Class1 Div.1 location.

Sometimes the aluminum would begin to gall before you could meet your "5 threads and wrenchtight" requirement.
I thought the idea was to force the flame from an explosion through the tight tolerances of the threads (on pipe & conduit bodies) or the polished faces (on enclosures)thereby cooling the flame to the point of extinguishment before it can reach the flammable environment?

[This message has been edited by Redsy (edited 07-14-2005).]

Joined: Feb 2004
Posts: 5
EvanH Offline OP
Junior Member
And it's interesting that in the picture with the lubricant, there's a cap to an explosion-proof junction box shown. I've also noticed a silvery-gray lubricant often present on these caps on switches we purchase. If it's ok there, wouldn't it be ok on pipe threads also?

Joined: Apr 2004
Posts: 156
I have always used the STL lube mentioned in the reply above. It prevents the threads from rusting together (thereby blocking the vent path). A lot of people I have worked for think that the oil put on during threading is good enough, but a couple of years down the line it isn't. I have also seen the thread lubricant specified on many jobs to "prevent infiltration and rust" in corrosive environments.

Joined: Oct 2004
Posts: 55
Great job renosteinke, Crouse Hinds used to provide a lot if useful info on explosion proof installations...I once got a free video to present to the other electricians in the plant that showed what can happen if the installation is done right and what it looks like when it's not done right.
The basic idea is, what we loosely refer to as an “Explosion Proof” installation does not prevent or contain an explosion at all, rather it is an installation that is designed to contain the flame front and cool the escaping gasses to a point where they will be rendered harmless to the outside environment.
One thing that should be made very clear to anyone installing or maintaining a hazardous location is that ANY mated surface should be clean and free from excessive lubricant. Machines surfaces, including threads should be smooth and free from gouges, or pits caused by age or mistreatment. If your die isn’t sharp the threads may be incomplete (ripped) and provide a path for the flame front. If you dissemble fittings and connections and those threads or mated surfaces are old and rusty they may not provide an adequate seal if you reassemble them using the old parts without dressing the threads and or installing new machined surfaces.
On starters and other enclosures, the mating surfaces of the door and other sealing surfaces are machined to a tolerance of several ten thousands and if the enclosure is not treated properly those surfaces will not perform as designed in the event of an explosion.
Also the improper installation of seals and drains (EYD, EYS) can lead to a failure of the equipment to contain flame front of the explosion. If a seal is not properly poured an enclosure on the other end of the conduit in a non hazardous location can be blown off the wall if an explosion occurs in the Haz Loc.
As an example imaging a conduit leaving a Haz Loc and going to the raceway in an MCC, if hot gasses explode into the MCC other, possible critical, equipment could be shut down because the insulation on wires in the raceway are heated above the rated temp of the insulating material which resulted in those wires shorting and the circuit tripping of line or failing to start when needed.
As with any electrical installation, lives can hang in the balance, all the necessary precautions should be taken to insure that the equipment is installed and will operate properly.

Joined: Jul 2002
Posts: 680
If you can't coat threads, how do you deal with 300.6A "Where corrison protecton is necessary and the conduit is threaded in the field, the threads shall be coated with an approved electrically conductive corrosion-resistant compound."

Joined: Mar 2005
Posts: 1,803
The gasses must escape through a helical clearance 'slot' created by the tolerancing of the male/female thread form. An oil-film need be only a few microns thick to be effective for lube/anti-corrosion - but how to judge the quantity to apply? How will the lube age? This all sounds a bit 'iffy' to me, if you're relying on a known gap to create a leak path and then, effectively, applying something which could change the path dimensions. Oh, and as an apprentice I was taught to NEVER apply oil to a threaded bolt or nut, as it affects the holding capacity; a thread is in reality a helical wedge.

Wood work but can't!
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