Fault current
Fault current
(OP)
Recently we had an incident involving a falling branch on an LV power line which energised a colour-bond fence.
Our (protection) fuses are generally set to operate above specific load currents in order to facilitate overload tripping. Faults involving vegetation are often high resistive earth faults that are accompanied by arcing. In this case the fault current was considerably lower than the normal system load current. As a result, the protection system was unable to pick up low residual earth fault current. In this instance the fallen power lines came to ground and remained live constituting a major hazard for the general public.
I am just a tad confused as to why the current level was considerably lower than the normal system load current if it was a short circuit fault and the resistance to ground is reduced due to the conductivity of the colourbond fence (even more if the fence is earthed).
Our (protection) fuses are generally set to operate above specific load currents in order to facilitate overload tripping. Faults involving vegetation are often high resistive earth faults that are accompanied by arcing. In this case the fault current was considerably lower than the normal system load current. As a result, the protection system was unable to pick up low residual earth fault current. In this instance the fallen power lines came to ground and remained live constituting a major hazard for the general public.
I am just a tad confused as to why the current level was considerably lower than the normal system load current if it was a short circuit fault and the resistance to ground is reduced due to the conductivity of the colourbond fence (even more if the fence is earthed).






RE: Fault current
RE: Fault current
Yet in this case the current level was below the pickup of the fuse. A sustained vegetation ground fault should have a greater chance at reducing the current level below the pickup prior to any arcing ?
RE: Fault current
RE: Fault current
Or said another way, a 25 amp fuse may not blow at less than 50 amps.
A tree branch fault that does not clear is a common problem. It dosen't happen every day, but it is common.
Fuses don't offer a lower neutral current protection, but they are reliable and cost effective. You may want to investigate if you need a lower neutral current protection, and if your load can stay within the limitations of a lower neutral current protective device.
RE: Fault current
For intentionally grounding fences near electrical substation, the paint is always removed to allow tight metal to metal contact. Substation fencing installers pay careful attention to make sure there every piece of conductive material is bonded together to ensure the fence is tied solidly to the earth.
RE: Fault current