Protection from arcing ground fault

Actual condition of arcing ground fault

General ground faults are caused by contact of electric circuits or electrical equipment with the earth at a resistance value lower than the normal insulation resistance value and include faults caused by electric shock and earth leakage and ground fault, and the ground fault current ranges from several mA to thousands A depending on the earth voltage and ground impedance. For prevention of these ground faults, high speed type ELCB or earth leakage relays are used at branches, and a time delay type ELCB or earth leakage relay is used on the main line for coordination of operating time. This method is gradually coming into use. The method ensures protection from general ground faults and limits damage to the faulty parts, and the damage is relatively minimal. To the contrary, upon occurrence of arcing ground fault, an electric circuit or device is connected with the earth through arc; the fault current value is restricted to about 30% of the 3-phase short circuit current owing to arc voltage drop and earth resistance. Therefore, on a circuit using only general circuit breakers, the arcing ground fault current does not reach the instantaneous tripping area of any circuit breaker, and the arc may be kept for a long time. When a time delay type ELCB is used, since its operating time is long, the arc is kept during the operating time. If arcing ground fault continues, the electric circuit and device will be significantly damaged by a large amount of thermal energy generated by the arc, and other sound devices may be affected because the arc point moves. The best-known accident caused by arcing ground fault is the fire accident of a large apartment house in New York. In this accident, arcing ground fault continued for 1 hour on a 480/277V distribution circuit, the distribution board is burned out, and the 5000A bus bar burned out to the starting point. As the result of this, the lifts, lamps and feed pumps were stopped, and approx. 10,000 residents of the apartment house were forced to live rough, and several days were taken to restore the facilities. As stated above, when arcing ground fault continues, unlike in the case of general ground fault, electric circuits and devices are considerably damaged, and there is a possibility of damage to other devices.

Damage to electric circuits and devices by arc

(1) Arc energy and damage to devices

The scale of arcing ground fault is indicated not only by the ground fault current value, but also by the arc energy determined by the ground fault current, arc voltage and sustaining cycle and can be determined by the following formula.

Table 9. 7 shows the arc energy obtained by the above formula and the degree of damage to devices determined according to the results of various tests. The limit of arc energy is 2000kW-cycle.

Table 9. 7 Arc energy and damage to devices

(2) Minimum sustaining current of arcing ground fault

The minimum sustaining current of arcing ground fault is up to about 350A although it significantly changes depending on the gap, pole arrangement at arc point and shape. If the sustaining current lowers below this value, the arc will become unstable and spontaneously disappear.

(3) Transition from arcing ground fault to phase-to-phase arcing short circuit

In most cases of normal metallic panels, if arcing ground fault does not spontaneously disappear, arcing ground fault changes to phase to- phase arcing short circuit. The transition time varies significantly from several ms to hundreds ms depending on the device layout, gap and arcing ground fault current. Fig. 9. 22 shows the oscillogram in a case of transition from arcing ground fault to phase-to-phase arcing short circuit. In this case, the ground fault arc energy is 215kW-cycle, and the phase-to-phase arc energy is 2250kW-cycle. The damage caused by the phase-to-phase arc is larger. According to the above, it is found that:

a) The arcing ground fault can occur on electric circuits on which ground fault current of 350A or more flows.

b) It is necessary that electric circuits must be broken quickly while the arc energy is within 2000kW-cycle.

Fig. 9. 22 Example of MCCB breaking for phase to phase arc short circuit caused by arc ground fault