Published on 25 Oct 2019 by ES Staff
This lesson learned reveals that the arcing between the bus and the metallic braid across the Fluoropolymer jacket was sustained for some period of time. The fault current was sufficient to generate temperatures well above the temperature limit of the heating cable insulation but not sufficient to trip the circuit breaker.
A new unit of a chemical required high temperature, zone-type constant wattage heat tracing operating at 208 VAC. The heating cable included a metallic braid and was installed on a stainless steel pipe.
After commissioning and operation for a period of time, the heat traced line required some maintenance. Following the completion of the work, low temperature alarms were activated. On further investigation a high ground leakage current was reported and detected by a solid state single-pole control device with ground leakage monitoring functions.
After manually turning the two-pole circuit breaker off, a visual inspection of the line was made and the thermal insulation was removed. Over 50 ft. of the zone type cable had burned under the insulation and at the power connection junction box. Closer inspection revealed that one of the 16 AWG bus conductors had fused to the metallic braid causing a short through the metal braid to the stainless steel pipe. The balance of the circuit was apparently unaffected.
The ground leakage monitoring function for the circuit in question was set up to disconnect only one side of the two-pole voltage supply but the short occurred on the untripped power supply lead.
This lesson learned affirms the need for ground leakage equipment protection and the need to have the controller switch all ungrounded conductors of the heat tracing circuit.