The Physics of Arcing Faults and How They Impact on Workers

Part 1 of this series of articles introduced the requirements of the Australian Work Health and Safety Act and Regulations to identify reasonably foreseeable electrical hazards including arcing hazards. It continued to show how the Australian WHS Regulations requires management of these risks so far as is reasonably practical using a hierarchy of hazard control measures.

It then explained that whilst arcing faults normally last less than a second, they develop over time from an insulation breakdown to single-phase, three-phase and two-phase-and-earth faults with amplitudes much higher than load currents that move away from the source of supply.

Parts 2 and 3 of this series described how an arcing fault consists of up to three live electrodes with plasma jets ejected from near where the arc columns attach to the electrodes. The plasma arc jets are then inter-connected by filamentary constricted arc columns. All the constricted filamentary columns lie inside a conducting region carrying diffuse currents that are made up of Copper (Cu), Oxygen (O) and Nitrogen (N) atoms and ions. The nearly invisible extremity of the conducting region is referred to as the molecular boundary where the radiation density falls to a level where molecules can survive and conductivity collapses. This leaves a decaying plasma cloud that causes most of the severe burns during arcing incidents. Then Part 4 described what happens when human beings with certain personal protective equipment initiate an arcing fault with an electrocution.

Part 5 then provides an engineering introduction on how the Law interacts with performing normal un-energised work. Part 6 extended the discussion on the interaction between the PPE and the decaying plasma cloud. Part 7 described how to identify the hazards and then what is involved in mitigating them. Part 8 discussed how to determine the characteristics of short-circuit currents that are required to select equipment and set protection. This Part 9 discusses the impact of the system minimum voltages and arc voltage on arcing fault currents and therefore the required protection settings.

What is needed to set protection

IEC 60909, Short-circuit currents in three-phase AC systems,
calculates the prospective maximum and minimum values of the short-circuit current at a particular location in a power system independent of the continually varying load currents and transformer tap positions but taking into account variations in system configurations. The results of IEC 60909 calculations are reported in terms of the RMS value of the symmetrical component of the short-circuit current Ik” at the start of the short-circuit. As well as balanced three-phase short-circuit currents, IEC 60909 provides procedures to calculate unbalanced currents (Ik1”, Ik2”, Ik2EL2”, Ik2EL3”, IkE2E”), which are particularly important in setting protection and are discussed in Part 8.

Arcing hazards and minimum short circuit currents

The minimum short circuit current values are required in order to avoid minimum breaking and critical currents in protection equipment and to ensure the protection is set to open on all faults and in particular during arcing faults, where people are often involved. With the normal time graded protection on distribution systems, the largest arc energy is normally produced by the minimum arcing fault current because, the influence of the increased time for the protection to trip exceeds the influence of the smaller arcing current. This is the reason that minimum short-circuit currents including the arc voltage influence is an essential calculation. The minimum short circuit current must be calculated using the minimum voltage allowed in the voltage standards together with the maximum impedance up to the fault location as discussed in Part 8. In this part we will look at the influence of either the highest voltage for equipment (HV) or the rated voltage of the system (LV) supplying the fault current and the lowest allowable system voltage plus the impact of the arc voltage on the fault current. IEC 60909 is yet to take into account arcing voltage, which will be shown to be a minor issue at high voltage but has a considerable impact at low voltage.