An Evaluation of Old Armored Cables in Building Wiring Systems

摘要

Armored cable has been used in building wiring systems since the early 1900s and remains an accepted wiring method today. The original product developed in the early 1900s and that which is commercially available today have significant differences in construction and performance. Likewise the demand and expectation of building wiring systems has changed. This paper explores some of those differences as well as concerns and hazards presented by the continued use of older constructions of armored cable. Testing was performed on samples of aged cables removed from service to determine impedance and temperature rise of the armor when conducting AC current. The results were compared to the DC resistance of the armor as well as the applicable Underwriters Laboratories standard. The performance of cable samples was also tested in construction configurations likely to be encountered in buildings. Results indicate that ground fault currents conducted via the armor of older cable constructions can result in fire and shock hazards where the impedance and circuit characteristics limit the fault current to below the trip level of the circuit overcurrent protective device. Temperature rise of the armor can easily exceed the rating of the conductor insulation contained therein and in some cases present a fire hazard to combustible material in contact with the cable.