Installation 27
○
Our current transformers can only measure AC currents. Strong DC currents will saturate
the magnetic core of the CT, preventing an accurate measurement of the AC current.
The overwhelming majority of AC powered electric devices do not draw significant DC
current, so this is a rare occurrence.
○
Loads with a high current crest factor (ratio of the peak current to the RMS current) can
cause clipping in the measurement circuitry, resulting in lower than expected readings.
You can check for this with a handheld power quality analyzer that can measure crest
factor (CF) or by trying a CT with a higher rated current, which should allow the meter to
measure the peak current accurately.
○
The CTs may be malfunctioning. If possible, use a current clamp to verify the current,
then use a DMM (multimeter) to measure the AC voltage between the white and black
wires from the CT (leave them connected to the meter during this test). At rated current,
the CT output voltage should equal 0.333 Vac (333 millivolts AC). At lower currents, the
voltage should scale linearly, so at 20% of rated current, the output voltage should be
0.20 * 0.333 = 0.0666 Vac (66.6 millivolts AC).
○
If possible, verify the expected power with a handheld power meter. Current clamps can
be useful to very roughly estimate the power, but since they measure current, not power,
the estimated power (voltage times current) may be off by 50% or more.
Power Factor and Reactive Power
The measured power factor and reactive power are very useful in determining if there is a phasing
mismatch between the voltage and current measurement phases on the meter. For example, if
the phase A CT is around the phase B wire.
However, this troubleshooting is complicated because different loads have different typical power
factors and the power factor can vary significantly for some devices, like motors, as a function of
the mechanical load on the motor. Here are some general guidelines:
●
Motors, idling or with a light load
: power factor from 0.1 to 0.6, positive reactive power.
●
Motors, normal or heavy load
: power factor from 0.5 to 0.8, positive reactive power.
●
Motor with VSD
: power factor between 0.5 and 0.9.
●
Incandescent lighting
: power factor near 1.0, small negative reactive power.
●
Florescent lighting
: power factor between 0.4 and 1.0.
●
Electrical heating
: power factor near 1.0.
●
Office equipment
: power factor between 0.6 and 1.0, reactive power may be positive or
negative.
Negative power factor values either indicate you are generating power (as with a PV system) or
that the CTs are reversed.
If the measured power factor or reactive power appears to be outside the normal ranges, this
most commonly indicates that the voltage and current phases on the meter are not connected
properly, although some loads fall outside the normal ranges. Check the following:
●
The CT connected to the
Ø
A CT
terminal is installed around the line wire being measured by
the
Ø
A
Vac terminal (green terminal block).
●
The CT connected to the
Ø
B CT
terminal is installed around the line wire being measured by
the
Ø
B
Vac terminal (green terminal block).
●
The CT connected to the
Ø
C CT
terminal is installed around the line wire being measured by
the
Ø
C
Vac terminal (green terminal block).
If this doesn’t solve your problem, contact technical support for more assistance.
