YOKOGAWA WT332E User Manual page 135

Appendix 1 How to Make Accurate Measurements
Effects of Stray Capacitance
The effects of stray capacitance on measurement accuracy can be minimized by connecting the
current input terminal of this instrument to the side of the power supply (SOURCE) that is closest to its
earth potential.
The internal structure of this instrument is explained below.
The voltage and current measurement circuits are each enclosed in shielded cases. These shielded
cases are contained within an outer case. The shielded case of the voltage measurement circuit is
connected to the positive and negative voltage input terminals, and the shielded case of the current
measurement circuit is connected to the positive and negative current input terminals.
Because the outer case is insulated from the shielded cases, there is stray capacitance, which is
expressed as Cs. Cs is approximately 40 pF. The current generated by stray capacitance Cs causes
errors.
V
±
C
±
As an example, we will consider the case when the outer case and one side of the power supply are
grounded.
In this case, there are two conceivable current flows, i
current that flows through the stray capacitance. i
through the load, and returns to the power supply (shown with a dotted line). i
current measurement circuit, the stray capacitance, and the earth ground of the outer case, and then
returns to the power supply (shown with a dot-dash line).
Therefore, the current measurement circuit ends up measuring the sum of i
objective is just to measure i
V s (common mode voltage), i
C
of i s is ahead of the voltage by 90°, the effect of i
C
power factor gets smaller.
i s = V
C
i
Cs
App-2
. Only i
L
s can be found using the equation shown below. Because the phase
C
s × 2πf × s
C C
i
L V
±
C
i
L
±
SOURCE
Shielded case of the voltage
measurement circuit
Outer case
Cs
Grounding
Cs
Shielded case of the current
measurement circuit
and i
L C
flows through the current measurement circuit, then
L
s reduces measurement accuracy. If the voltage applied to Cs is
C
s on the measurement accuracy increases as the
C
s. i is the load current, and i
L
s flows through the
C
and i s, even if the
L C
Cs
i
L
i
Cs
s is the
C
LOAD
IM WT310E-02EN