Fault current paths

[stroker]

Unless I was dreaming I once heard that an EMT
SS connector was superior to the compression
connector for carrying a fault current.The
compression connectors gland would vaporize
before faulting high loads.When an insulated
grounding conductor was thrown into the mix, the
ss connector path was better because of the surface area on the EMT conduit to the fitting.

Can anyone back me up on this one?

 

[bdarnell]

Re: Fault current paths

I've always specified compression, steel connectors for all of the jobs I design. My reasoning is that a compression ring makes contact all the way around the conduit whereas a set screw fitting leaves gaps where it indents the conduit. I've seen conduits in forensic investigations that were subject to a high magnitude fault and there is usually always a burned mark or pitting around the set screw (and usually the teeth of the locknut).

Just a preference but it makes sense in my mind.

 

[bphgravity]

Re: Fault current paths

UL lists all EMT fittings as suitable for "grounding". I would imagine certain types perform better than others, but all must (should) perform effectively.

 

[infinity]

Re: Fault current paths

As an installer I hate compression fittings. The smaller sized ones aren't a problem but when you get to the 1 1/2" and above range they're useless. For one you can never get them tight. And if you do they still aren't as tight as a steel set screw fitting with the screws indented into the conduit. Try to get 2 pairs of 460 channel locks, or worse, two chain wrenches, on a 3" coupling on a rack of conduits and you'll die trying to tighten it. Besides while you're struggling and cursing at them, you're taking far longer to perform a simple task than you would if you were using a fitting with set screws. And let's not forget all of the times that you have to disassemble the stupid fitting to get the conduit into it.

There was a study done years ago after an earthquake in California and the only fittings that were still together were the steel set screw type. The compression types pulled apart and the die cast types broke apart. Thanks for letting me rant for a bit.

 

[thinfool]

Re: Fault current paths

Originally posted by infinity:
[Q]
There was a study done years ago after an earthquake in California and the only fittings that were still together were the steel set screw type . The compression types pulled apart and the die cast types broke apart. Thanks for letting me rant for a bit. [/Q]

I was ready to type the same thing. It was in EC&M that I read this. I know engineers like compression, but to me, this is a symptom of their inexperience in the real world. All too often compression fittings are made "hand tight" and never wrench tightened later. I admit that there is no guarantee that set screws will be tightened either.

However, I will support the idea that a tightened compression fitting is more likely to separate under stress than a steel set screw fitting that has been properly tightened.

I recently had an engineer tell me that compression is preferred is because, on smaller sizes of EMT, the set screws indent the pipe, raising the possibility that insulation will be damaged when the wire is installed. Possibly from a sharp edge if the EMT is not seated fully into the fitting.

In 40+ years in this business, I have yet to see this happen.

 

[infinity]

Re: Fault current paths

I recently had an engineer tell me that compression is preferred is because, on smaller sizes of EMT, the set screws indent the pipe, raising the possibility that insulation will be damaged when the wire is installed. Possibly from a sharp edge if the EMT is not seated fully into the fitting.

I have heard statements like this too in the past but if the fitting is tested and listed than I can't see how this argument has any basis in fact. As an installer I can base my opinion on the field application of these fitting, not on opinion of someone sitting behind a desk. And by saying that I'm not slamming engineers, I'm just stating that the practical application of things are sometimes different than their intended design applications.

[ February 26, 2006, 11:26 AM: Message edited by: infinity ]

 

[macmikeman]

Re: Fault current paths

Too bad there is no combination type. I would like to see that in use. Wrench tight, and set screw tight. Finding a seperated conduit in a drop ceiling is such an eyesore, ss or compression. One of my major personal pet peeves.