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.
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Table 9. 7 Arc energy and damage to devices
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.
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| Fig. 9. 22 Example of MCCB breaking for phase to phase arc short circuit caused by arc ground fault |
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