5.3.2 - The importance of loop impedance
The earth fault
loop impedance can be used with the supply voltage to calculate
the earth-fault current.
|
IF =
|
Uo |
|
|
Zs |
|
where
|
IF = fault current, A |
|
|
Uo = phase voltage, V |
|
|
Zs = loop impedance |
For
example, if a 240 V circuit is protected by a 15 A semi-enclosed
fuse and has an earth-fault loop impedance of 1.6 Ohms,
the earth-fault current in the event of a zero impedance
earth fault will he:
|
IF =
|
Uo
= |
240
A
|
= 150 A |
|
|
Zs |
1.6
|
|
This
level of earth-fault current will cause the fuse to operate
quickly. From {Fig
3.13} the time taken for the fuse to operate will be
about 0.15 s. Any load current in the circuit will be additional
to the fault current and will cause the fuse to operate
slightly more quickly. However, such load current must not
be taken into account when deciding disconnection time,
because it is possible that the load may not be connected
when the fault occurs.
Note that there
is no such thing as a three-phase line/earth fault, although
it is possible for three faults to occur on the three lines
to earth simultaneously. As far as calculations for fault
current are concerned, the voltage to earth for standard
UK supplies is always 240 V, for both single-phase and three-phase
systems. Thus the Tables of maximum earth-fault loop impedance
which will be given in {5.3.4}
apply both to single- and to three-phase systems.
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