The incident which sparked the PJB development occurred on July 30 this year when an inappropriately located and unrestrained low voltage cable contacted with a rotating shaft. The FRAS (Fire Resistant Anti-Static) cable trunking hose, outer cable sheath and earth screen of the cable were worn through until the two conductors were exposed. A shower of electrical sparks resulted and no protection system operated to clear the fault.

The cable was part of a 12V DC lighting system on a diesel vehicle. The system was two wire and isolated from the earth. Each enclosure in the system was electrically bonded through the collective earth screen of the cable. The connection to the vehicle frame was through mounting of the various enclosures.

PJB said all Ex.d alternators had fields connected across output lines - meaning a short circuit on the two lines causes the fields to be shorted and the output drops to near zero.

“This effect makes it near impossible to provide any real electrical protection for the cables, particularly when the very high inrush current demanded by dichroic lights and the limited available fault current from the alternator are considered,” PJB said.

Besides making sure all cables are correctly restrained and appropriately located, there is no easy solution to the issue. Fuses of any rating or circuit breakers do not provide a solution and there does not seem to be any commercially available earth leakage devices that would tolerate the shock, vibration and electrical noise that is typical of an installation on a vehicle.

“While the probability of an electrical fault external to all enclosures that does not include an earth fault would seem to be remote in the extreme, the industry consensus seems to indicate that a device is required that will provide bother over-current and earth fault protection,” PJB said.

To this end, PJB have developed a low voltage cable protection relay that includes both elements of safety.

The relay has a logarithmic over-current characteristic with adjustable time delay discrimination and trip points. An over-current trip requires a reset with a key. The earth leakage part of the relay locks out and will not permit the connection of power to a faulty circuit.

PJB managing director Phil Berriman said he had received significant interest from the longwall sector in the device to date.