17
18
Fault localization and trouble shooting for voltage drop
Measurement
Item
Normal
measurement
result
Fault
measurement
result
Possible
cause
Trouble
shooting
Voltage drop
The
voltage drop
is
too high.
Redistribute
the circuit load
The circuit is
overloaded.
Rearrange
the wire in
compliance
with the
relevant
standard.
High
impedance
point exists
between the
circuit and
switchboard.
Repair or
replace the
parts
generating
high
impedance
<5%
Compared
with power
transmission
length,
the wire
is small.
Cable impedance (Z) measurement
If the voltage drop exceeds 5%, analysis on the
impedance of live line and zero line shall be made.
If one datum is obviously larger than the other, it
indicates that the high impedance conductor goes
wrong. Under these circumstances, check all the
conductor connection behind the switchboard. If
impedances are all high, this indicates that the lead
wire diameter is too small for the power transmission
length or the quality of equipment, parts or
connector is poor.
Usually, the earth line impedance is less t
han 1Ω
to provide a free discharge route for failure current.
According to IEEE, the earth line impedance shall
be less than 0.25Ω to ensure the earthing conductor
to discharge the failure current which threatens all
the equipment. The surge suppression sy
stem shall
be grounded reliably to protect the equipment when
this system suffers transient overvoltage.
ASCC is the data calculated based on this formula:
phase voltage/line impedance (Live line + zero line):
ASCC= Phase voltage/
(Live line imp Zero line impedance)
Notes:
1. As the test of earth line impedance will trigger
the residual current device due to test principle,
similar device shall be removed from the circuit
before testing.
2. Check the circuit to see if there is heavy load
over the circuit before testing and, if necessary,
turn off the load to avoid wrong test result.
3. Earth connection is required when the cable
impedance is tested in the 2-wire system
(without earth line).
