Broken wires_ASCE 74-03 Method 1 and App.J-4

[JJOO]

I have several questions and would like to hear from experts here.

Given:
ASCE 74 states, in Appendix J Section J.4, the Broken Wire Load (BWL) is equal to the everyday bare wire tension (EDT) of the ground wire and about 70% EDT of a conductor. It also says in the next paragraph, the single BWL, applied as a static bare wire load in no wind/ice conditions has proven adequate for protection after a wire breaks in all conditions except for breakage under heavy ice conditions or where the supports were relatively brittle.

Questions:
#1) If I have a continuous ground wire at the top of a tower and (2) power cables at a cross arm, should I assume that the wires to be broken would be the ground wire and just one power cable? The remaining intact wire would exert vertical/radial loads; should I estimate those vertical/radial loads of the intact wire in a 60 deg F normal condition with no wind and no ice? Section 3.1.2.1 says "the other support points all carry full bare wire vertical loads with no wind"; so my interpretation is, ASCE 74 suggests wire break in a 60 deg normal condition only. Why? Is it an overkill to consider wire break happenning in NESC Heavy ice loading condition (or in Extreme wind)? Sectoin 3.1.2.1 says in the end, "utilities in areas of known severe icing may consider an RSL calculated at some iced condition," and from this sentence, I think I may have to consider wire break in NESC Heavy ice.. [I'm in NYC.]

#2) My guess is, those ground wire and a power cable that are assumed to be broken would still have vertical/radial loads from the other side; is this a correct assumption? If so, for those vertical/radial loads estimation, should I assume the horizontal tension in a 60 deg normal condition only? [back to Question 1]

#3) If the two power cables are supported by a cross arm, one on each side of the pole, assuming one cable broken would create torsion. Then, cross-arm and the pole shall be designed for this torsion? [I'm just a structural engineer..]

#4) If the wire support is of a suspension-type, which gives flexibility, BWL would be as explained in Section J.4, right? Section 3.1.2.1 mentions Risidual Static Load (RSL), and it sounds like RSL means the same thing in BWL described in Sect.J.4; am I right or wrong?

#5) How about the wire attachment that is of the dead-end type, i.e. terminations at thru vang? Wire tensions may be different depending on the span/installation tension, etc.. In this case, is the concept of wire break even applicable? If so, BWL should be estimated differently? What if those power cables are carried by EHS messenger?

#6) What if I have a cont. GW at the top and (4) power cables instead of two? Still assume one GW and one conductor broken? Sect.3.1.2.1 Method 1 states, apply RSLs to a nominal one-third of the conductor support points or to one ground wire support point, or both. Does that mean I need to assume one GW and at least two conductors broken? What is the difference between a single-circuit line and double-circuit lines? [probably a dumb question from someone who has little electrical background..]


Any input would be greatly appreciated.

 

[transmissiontowers]

Sorry for not replying sooner. Not much traffic on this forum and I don't check here too often. I'll have to reply tomorrow at work when I get a chance or bring my book home and try to remember what we were thinking when we wrote the broken wire sections. I was on the re-write committee and it has been a while since we hammered the guide out.

Please remember that the document is a guide and not a standard so you are free to pick what you like for a broken wire case. If your company has cases that you have used in the past, I would say that you can use that. The broken wire sections are meant to give the tower or pole some longitudinal capacity to prevent a cascade.

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I have been called "A storehouse of worthless information" many times.

 

[transmissiontowers]

I found my old version from 1984 but can't find the my 1991 version of ASCE 74. The latest version is the Third Edition published in 2009 and Section J.4 is Failure Investigations. The BWL comments are now in Appendix I. You might want to get a copy of the 3rd edition of ASCE 74 and read over the sections.

I have to get to a meeting now but will try to read the section later and comment.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[bacon4life]

#1) Is this for normal 3 phase AC? Two power cables are a bit odd unless this is for HVDC or electric trains. Perhaps you are referring to bundled conductor where each of the three phases have multiple cables separated by about 12 inches? If so, the broken wire load should be the loss of an entire bundle. Each bundle often shares common hardware such as yolk plates or insulators that could result in the entire bundle failing.

#3) Yes, the tower must be designed for some torsion. The longitudinal conductor case often drives the sizing of some crossarm members.

NESC 252C does not require a specific broken conductor case for tangent towers, leaving it up to the designer to provide “longitudinal strength … at reasonable intervals.” In many areas, using 60 F nominal conditions has historically provided some resistance to broken conductors while also providing strength to deal with other types of unequal loading such as stringing, unequal spans or unequal ice.

Other alternatives to using a severe ice loading broken conductor case could include sacrificial members that allow for a reduction in tension without actually dropping the conductor or using full tension deadend placed every few miles. When using residual static loads, be aware that an actual broken conductor event has significant dynamic forces involved, so using the RSL does not necessarily mean the structure can withstand a broken conductor in the adjacent span. For a highly flexible structure that has a low calculated RSL, the RSL may not provide enough longitudinal security.

You can’t know if considering iced broken conductor is overkill until you find out the transmission owner’s plan to deal with damaged structures and their philosophy on containing damage versus preventing damage.

#4&5) As an electrical, I am not up to date on the details of ASCE 74, however, historically the utility I work for treats deadend attachments & ground wires as full tension and reduces the tension on suspension type insulators to 75% of full tension.

If the structure is intended to be a deadend, it would usually be designed for full loads for all conductor on one side and no loads for the other. Using through-vangs (Deadend type attachments) on a tangent tower may set up potential future problems. If the tower attachments are not clearly identified as tangents, a future field crew or line designer
could incorrectly assume they are suitable for deadend loading.

#6) Usually a single circuit structure would have 3 power wires (or 3 bundles) and 1 or 2 ground wires. A double circuit tower would have 6 power wires (or 6 bundles) and 1 or 2 ground wires.

 

[transmissiontowers]

I'm guessing we need to know the voltage on this line and if it is AC or DC. The configuration would also help. If it is a single circuit horizontal like a wood H-Frame, or a single circuit vertical with the conductors stacked above each other. The middle phase in this case is usually moved over to prevent wire clash as the span gallops. A single circuit delta may have one phase in the center of the tower and the other 2 on the outside. A 2 circuit vertical is more common in my part of Texas with the circuits on either side of the pole or tower on arms.

The Owner or Utility that is building the line should be able to provide guidelines for your longitudinal strength.

In many cases, the in-line deadend structure is not meant to be a full deadend but only a partial deadend where you might have 2 wires per bundle on one side and 1 wire per bundle on the other.

I will bring my ASCE 74 3rd edition home and try to reply after I re-read what we wrote.

_____________________________________
I have been called "A storehouse of worthless information" many times.