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11.2 Q & A
■
General
Q
To what degree of overvoltage and overcurrent does the unit withstand? Is it necessary to connect a protective circuit
out of the unit?
A
Instantaneous
*
:
Up to 20 times the rated current and up to twice the rated voltage
*
Instantaneous: Current is applied 9
times for 0.5 sec at intervals of 1 min and once for 5 sec.
Continuous
: Up to 1.1 times the rated current and rated voltage
Q
Can this unit be used as a watt-hour meter?
A
The unit can be used to measure the electric energy and control the consumption of electric energy.
However, it should not be used for the purpose of trading or certification of electric energy regulated by Measurement Law.
Q
Is it easy to check for wiring failures?
A
Check the state of measurement of electric energy.
If MEASURE LED on the front panel of the main body (
see page 5) is on, the electric energy is being measured. Make sure
that the electric energy value is approximate to the value obtained by the following formula to confirm more reliably that the unit is
wired correctly.
√
3
×
current
×
voltage
×
(0.8 to 1.0)
Q
Is it secure to open the secondary terminal of the current sensor?
A
The secondary side of each of Models EMU2-CT5 (-4W), EMU-CT50, EMU-CT100 and EMU-CT250 has a built-in protective
circuit against opening of the secondary terminal. No problem occurs by opening the terminal during wiring work. However, for
safety, do not continuously apply current while the terminal is open.
Although the secondary side of each of Models EMU-CT400 and EMU-CT600 has a built-in protective circuit against opening of
the secondary terminal, keep the secondary short-circuiting switch of the product in “Short” during wiring work. After the
completion of the work, set the secondary short-circuiting switch to “Open.” If the switch is not set to “Open,” measurement will not
be performed correctly.
Q
When a load, such as a welder, is used, current flows into the unit for only a short time (for example, 2 cycles). Can it
measure the current correctly?
A
This unit measures the parameters in a sampling period of 4340 Hz (at 50 and 60 Hz).
The display updating period for display of measurement results on the display unit (EMU2-D65 or EMU2-D65-M) and the updating
period for communication data output through communication, such as CC-Link, are 500 msec at the shortest.
To determine the electric quantities, such as current, voltage, electric power, power factor, frequency, high-frequency voltage and
high-frequency current, the results of measurement in a 500 msec period are processed by 2 second moving-average method (to
be concrete, the moving average of four measurements obtained in a 500 msec period is calculated). Therefore, the mean values
of these parameters are displayed. The maximum and minimum values of these electric parameters are calculated from the mean
values. Accordingly, the unit cannot measure the instantaneous current correctly.
The electric energy (integrated value) is continuously measured in a sampling period of 4340 Hz without intermission of
measurement by processing separately from the above-mentioned instantaneous data processing. Therefore, the unit can
measure the load in a short period.
Q
The measurements obtained by this unit differ from those obtained by another instrument. Why?
A
There are various possible causes. Check the following points.
○
1
Check the wiring (particularly the polarity of the current sensor and connection of the voltage circuit).
○
2
Check that the parting faces of the split type current sensor are engaged without gaps and are not separated.
○
3
Check that there is no foreign matter between the parting faces of the split type current sensor.
■
Q&A relating to specifications
Q
To what is the measurement accuracy designed?
A
For the electric energy, the measurement accuracy indicates the allowable range of error of readings. When the reading is 10
kWh, the error is
±
0.2 kWh. For the measuring parameters other than the electric energy, the measurement accuracy indicates
the allowable ranges of error for the rating input. When the rated current is set to 250 A, the allowable range for current is 250 A
±
1%.
Q
Is the accuracy of the current sensor included?
A
The accuracy of the current sensor is not included in the accuracy of this unit.
The maximum allowable error is the sum of the allowable error of this unit and the error of the current sensor.
Q
To what range is minute current measured?
A
The measurement of current is indicated starting from the range in which the current exceeds 0.5% of the rated current. In the
range in which the current is less than 0.5% of the rated current, the measurement result is indicated as 0. However, also in this
range, the electric energy is measured. Even when the indicated value of current is 0, the measurement of electric energy will
increase if such a condition lasts for a long time.
The electric energy is measured at a load of about 0.4% or more of the full-load power.
