The use of “Spacer Cable” (3 heavily covered conductors supported by a messenger and separated and hung by spacers) versus “Tree Wire” (the same heavily covered conductors strung in an open wire configuration on cross-arms with polyethylene insulators) is mostly a difference in configuration.
Spacer cable has the benefits of being an extremely low profile (about a foot side to side), protects the phase conductors with an overhead messenger (keeps the trees/branches from electrically contacting the conductors, thereby providing mechanical and electrical protection), has improved engineering benefits (lower impedance, 25% improved voltage profile, better voltage regulation, higher power factor, lower losses, etc.), is more suited to multiple circuits, has a better ability to maximize Right-of-Way (ROW) utilization, and has a higher reliability than tree wire, among the primary differences.
Another difference is that Tree Wire usually utilizes ACSR or AAAC since it must support its own weight. Spacer Cable, on the other hand, normally utilizes AAC, since phase conductor tensile strength is not required (all the strength is in the messenger), it has better ampacity, is lighter, and can be compacted (smaller diameter, uses less polyethylene, and has a lower physical profile for wind and ice loading, not to mention pole loading and guying).
The photos below illustrate Tree Wire (left) and Spacer Cable (right).
PLS-CADD wire files have been developed for most conductor and messenger combinations and are available upon request for customers working with us. Contact your Hendrix Regional Sales Manager or authorized Hendrix representative, available at www.Hendrix-wc.com – Aerial Cable Section.
No. The reason is that there is no concern for electrical treeing in aerial covered conductor applications. In UG cable, there is a grounded neutral outside the insulation, so that 100% of the voltage is impressed across that very thin insulation wall. Any moisture available from the stranded conductor is influenced by the extremely high electric field to migrate from the conductor to the neutral. This is electrical treeing, and it can degrade the insulation integrity severely. In aerial covered conductor applications, there is no concentric neutral, there is no severe electric field intensity or voltage drop across the insulation wall, and as such no mechanism for electrical treeing to take place.
Hardware used for Spacer Cable is minimal. Tangent, Angle and Dead end Brackets, angle clamps (same as for bare wire), insulator pins, and covered grips are available from Hendrix. The hardware used for transitions, such as at riser poles, or transitions to bare wire, are not different.
When it comes to terminations, taps, and dead ends, there are a few differences between spacer cable and bare wire that are worth mentioning.
Terminations – Spacer cable is terminated in a coated performed grip, whereas bare wire normally uses a full tension deadened shoe, regardless of technology (automatic, bolted, shoot on, etc., etc.) Hendrix supplies the full range of coated preformed dead end grips required.
Taps – Spacer Cable, since there exists a covering, requires that the covering be stripped before a connection is made. Any standard connector can be used. At that point, there are two options for taps. The tap can be covered (insulation and BIL fully restored), or left bare (which is necessary in the case of a connector with a stirrup). In the latter case, two precautions are necessary. The first is that a 4’ section of Line Duc be installed on top of the messenger and secured in place with a tie. The second is that a surge arrester be used on the tap. When a 3 phase tap is made, the taps also need to be staggered 24” longitudinally, so that they are not all lined up in a row.
Ampacity – the messenger is also used as a shield wire and either all or part of the neutral system. AW or AWA alumoweld can be designed with a much higher ampacity than what EHS provides.
Corrosion resistance – Spacer cable is often used in seaside and/or industrial sites where corrosion is a concern. Alumoweld has a much higher corrosion resistance than EHS galvanized steel, which has a notoriously low corrosion resistance.
Spacer Cable has not yet been used for HVDC applications although that is something worth discussing further, as industry interest increases in DC.
In a Spacer Cable configuration, should an animal make oral contact with the energized conductor, the messenger is only inches away. The argument has been made that this 60 Hz “buzz” is the reason we have never heard of nor seen chew marks on the spacer cable configuration. This argument would be absent in a tree wire configuration, where there is no neutral nearby. We have heard rumors of animal chewing on covered conductor, but believe this was on the old thin-walled weatherproof wire from the 60’s and 70’s. We have not heard of chewing incidents on tree wire systems using the Hendrix High Density PE covered conductor. It is important to point out that the old thin-walled weatherproof wire was never a Hendrix product.
Insulation is a 3-layer system. A Semicon shield is applied over the bare conductor, followed by a High Molecular Weight inner layer, then a High Density Polyethylene outer layer. The total thickness of the 3-layer covering system varies with kV class.
Typically 5,000’ on standard reels. However, larger, customized reels are available for specific project lengths. Limiting factors are typically physical size, weight, available lifting and handling equipment on the job site.
Yes. The conductor is turned around two insulators, so it only sees a maximum 45 degree angle. It should be noted that in heavy angles (greater than 60 degrees) the messenger is double dead ended on the pole.
In a Spacer Cable configuration, should an animal make oral contact with the energized conductor, the messenger is only inches away. The argument has been made that this 60 Hz “buzz” is the reason we have never heard of nor seen chew marks on the spacer cable configuration. This argument would be absent in a tree wire configuration, where there is no neutral nearby. We have heard rumor of animal chewing on covered conductor, but believe this was on the old thin-walled weatherproof wire from the 60’s and 70’s. We have not heard of chewing incidents on tree wire systems using the Hendrix High Density PE covered conductor.
In companies we have talked to, we have never heard a change in relay/recloser/breaker/sectionalizer settings for use of spacer cable compared with their bare wire protection scheme settings.
ACS can be and is used on systems configured in delta. The messenger would be recommended to be grounded in the event it was intended to be used as a lightning shield.
The messenger can be used as a neutral. In express lines (no taps or transformers), the messenger is used as the only neutral. On lines where there is a transformer on many of the poles, it is common to also run a system neutral below the spacer cable line. The neutral is bonded to the messenger.
The fiber can be run as OPMW, that is, inside the messenger. It can also be run as a separate ADSS line, either above, below, or beside the spacer cable line.
Substation exits are a common application of spacer cable and interest has increased with the added attention of the Metcalf event and the CIP-014 regulations to mitigate substation damage. Hendrix, as a leader in reliability solutions, is looking to support this industry effort.