ELECTRICAL HAZARD

One problem that has recently been identified is the potential deadly threat posed by the electric circuits after pole impact by an errant vehicle. There are many documented deaths of motorists who survived the impact with a luminaire pole only to be subsequently killed from the resulting explosion and fire. The explosion and fire are usually caused when the fuel tank ruptures, the vehicle having been caught on an improperly constructed foundation, and the electrical system sparks repeatedly until the fuel explodes. In other incidences, medical personnel have been delayed from attending victims because of the risk of electrical shock from exposed conductors near or under a vehicle.

Past research efforts have concentrated on evaluating the structural breakaway characteristics of luminaire poles. In addition to the need for the pole itself to have breakaway ability, it is recognized that the underground wiring system should also be capable of properly separating. There are a number of reasons for requiring proper separation of the wiring system. One of these reasons is that the size, and associated tensile strength, of the wire cable is sufficient to significantly increase the deceleration rate of impacting vehicles and to also change the trajectory of the falling pole. Another reason is that improper separation of the electrical cabling can result in bare conductors that are still energized, posing an electrical and a possible fire hazard at the accident scene.

Early efforts to reduce electrical hazard concentrated on providing line fuses placed in a breakaway device. However, these widely used “breakaway fuse holders,” which for years have been the standard, have not been certified by testing. Prior expe­rience indicates that they frequently perform improperly during an accident situation. Rather than properly separating, the device frequently pulls off the wire, leaving an exposed end that is potentially deadly. Part of the problem with the breakaway fuse holder is the location of the device in the pole or T-base and the 24 to 36 in (610 to 910 mm) of distribution cable inside the base. This extra length of wire is placed in the pole to allow service crews the ability to pull the wire out of the pole and make the connections to the luminaires. Upon impact this extra length of wire obstructs proper separation of the breakaway fuse, and allows the wiring insulation to be damaged by the fractured pole. The resulting bare electrical conductor poses a safety hazard because of the relatively large voltages used in underground roadway illumination systems.

Most luminaire underground wiring systems operate on 480 V. The reason for using 480 V is that the voltage drop in the copper conductors that supply a given load is only one-fourth the value of the voltage drop when using 120 V and one-half that of 240 V. In addition, luminaires are designed to perform within a certain percent of the rated voltage. Thus for a given percent, such as 10 percent, the allowable drop would be 4 times greater for a 480-V circuit than for a 120-V circuit (48 versus 12 V) or twice that of a 240-V circuit. These factors are additive, so a 480-V circuit requires a much smaller copper wire to deliver the necessary amount of energy over a long dis­tance. Using 480 V is desirable, but proper precautions and installation techniques must be used to reduce the inherent hazard on the public right-of-way.

A modular cable system initially developed by MG2 Inc. and Duraline Inc. elimi­nates a number of problems presented by the current wiring method [14]. This cable system is a submersible, modular plug and cable system that allows the circuit compo­nents (i. e., the low-amperage, fast-acting, current-limiting fuses; the surge arrester where desired; and the conductor splices) to be placed in an underground junction box adjacent to the pole foundation. The circuit breakaway connector can be positively positioned at the top edge of the conduit inside the pole base. Since the stiff, typically no. 4 or no. 6 copper, conducting cables never enter the pole, the system unplugs at ground level. The impact that knocks down the pole will not put stress on the electri­cal cables and will not weaken splices in adjacent poles. Most important, with the modular cable assembly, there is no exposed electrical hazard upon knockdown as can exist with the conventional wiring method. When this system (Fig. 7.67) is combined with a properly installed foundation, the possibility of fire and explosion or electrical shock is significantly reduced if not eliminated. Recent developments have shown that the splices, the surge arresters, the fuse holders, and the ground rod must be placed underground in a junction box adjacent to the pole base to provide the greatest possible degree of safety. This requires that all components be submersible. This design will positively place a breakaway connector in the wiring system at the top edge of the foundation; the fuses are underground, where no damage can occur on the supply side. The Modular Cable System developed by MG2/Duraline was the first of these sub­mersible wiring systems on the market and has proven to be very reliable [14]. By using fast-acting, current-limiting fuses installed below ground, the potential for elec­trical shock and fuel explosions is greatly reduced if not eliminated.

The AASHTO Standard Specification for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, 4th ed. (2001) has included some positive statements strongly encouraging the use of this submersible-type wiring system for all breakaway poles.