Roping off wires in gear

[captainwireman]

I was asked this question today and I thought it would make an informative post.

What is the purpose of securing wires in electrical gear with rope of size and number of twists around the conductors specified by the manufacturer.

 

[GoldDigger]

When high currents (fault currents) flow through parallel wires, the magnetic repulsion forces can tear unsecured wires loose.

 

[captainwireman]

This particular application is transfer switches in a large data center, part of a UPS system. The UL testing laboratories certify short circuit current withstand ratings for equipment subject to high available fault currents. The conclusion by UL is " The extreme electromagnetic forces created by a high level fault are capable of bending/deforming metal and destroying insulators." Under the qualification requirements (from UL 891) e) No conductor pulls out of a terminal connector, and there is no damage to the conductor insulation or to the conductor.
It is important to be aware just how powerful and destructive this medium we work with can be and how proper, tested, and approved application can mitigate these hazards.

 

[captainwireman]

Just wanted to clarify, the size of the rope is not important. The tensile strength is the determining factor. We are using jute but nylon is also a UL approved style by this manufacturer to ensure a pass at the UL labs.

 

[SceneryDriver]

Just wanted to clarify, the size of the rope is not important. The tensile strength is the determining factor. We are using jute but nylon is also a UL approved style by this manufacturer to ensure a pass at the UL labs.

I wouldn't use jute - a natural fiber - in an application like this. Natural fibers are prone to drying out and loosing strength over time. Synthetic fiber rope would my choice here. Properly tying knots is also important too.



SceneryDriver

 

[captainwireman]

The properties for jute ( manilla) rope were chosen in the specs for their low stretch and knot holding properties. Don't know but I would think if not exposed to moisture, it may last a very long time. Hmmm, good point though. Maybe long life was not a consideration in this high-tech application.

 

[GoldDigger]

By contrast, nylon stretches too much and poly is almost impossible to knot well.

 

[SceneryDriver]

By contrast, nylon stretches too much and poly is almost impossible to knot well.

There are plenty of low-stretch synthetics that hold knots well: spectra core with nylon jacket, and Tek-12 (aramid fiber) are two of many. Perhaps if the expected lifetime isn't longer than 10 years or so, jute might be ok. I work in theater and I've seen plenty of nasty, weak, next-to-useless manila rope that never saw any moisture; just age.

'Twer it me, and the conductors were to be roped off in the enclosure for a permanent install, I would strongly consider a synthetic such as I listed above. GoldDigger is right; nylon is a poor choice for many reasons. Too much stretch, and doesn't deal with elevated temperatures over time very well. If using other than jute rope were a listing issue, I'd definitely think about contacting the manufacturer of the equipment to discuss alternate rope types.


SceneryDriver

 

[iwire]

Using any other rope other than what is specified by the manufacturer would be a violation and should cause a failed inspection.

I would say the chances of a manufacturer approving a different rope are absolutely zero unless you pay for the testing required to prove they would do the job.

 

[jumper]

Here is a similar thread on the subject if anyone is interested.

http://forums.mikeholt.com/showthread.php?t=137641

 

[GeorgeB]

By contrast, nylon stretches too much and poly is almost impossible to knot well.

I 100% agree on the nylon, but "poly" is a different issue. Polypropylene is the cheap stuff at home stores (and other reliable sources), but typically has those disadvantages. It is also what our POCO (Duke Power then) used to pull lines, and discarded after some number of uses or time; we used it for many noncritical uses as youngsters.

Polyester, on the other hand, is an excellent product, can have its "hand" (surface feel and to some extent knot holding) controlled by the manufacturer. Prior to the newer fibers and blends, it was used, almost exclusively, for low stretch applications on sailboats where wire was objectionable. Today, Spectra and Kevlar seem to have the high end of that market.

DuPont's tradename for polyester fiber is Dacron; there are many proprietary and generic alternatives ... polyester is not 100% definitive; nor is nylon. Each can be optimized for a purpose.

 

[Barbqranch]

Here is a picture of what high magnetic fields can do to a quarter (change of field).

 

[big john]

Here's a good demonstration on why lashing is important in switchgear with a high short-circuit current:

 

[GoldDigger]

Here's a good demonstration on why lashing is important in switchgear with a high short-circuit current:

Nice! The metal straps did not help much, did they?

 

[captainwireman]

I can't seem to find any info on the type of knot. We are using a lashing style that utilizes several wraps securing the working end.

 

[templdl]

When high currents (fault currents) flow through parallel wires, the magnetic repulsion forces can tear unsecured wires loose.

Also, one can't overlook the weight of the conductors themselves and the stress that it often places on the terminals which may not be able to support the weight.

 

[GoldDigger]

Also, one can't overlook the weight of the conductors themselves and the stress that it often places on the terminals which may not be able to support the weight.

But in general the lashing prescribed by the manufacturer does not include any strain relief function and would not help in the situation you suggest.

 

[gadfly56]

I can't seem to find any info on the type of knot. We are using a lashing style that utilizes several wraps securing the working end.

Call your local Boy Scout troop.

 

[SceneryDriver]

If you have an iDevice, look for "Grog Knots" in the App Store. It's a $4.99 app, and it has animations to show how to tie many useful knots.


SceneryDriver

 

[gadfly56]

If you have an iDevice, look for "Grog Knots" in the App Store. It's a $4.99 app, and it has animations to show how to tie many useful knots.


SceneryDriver

That app was one of three highlighted in this month's issue of Scouting Magazine. The other two are "How to Tie Knots 3D" for $1.99 for iOS, free for Android, and "Knots 3D" for $0.99 for iOS, free for Android.