BECKWITH ELECTRIC M-3425A, Instruction Book

Page: 232 / 482

4–101
System Setup and Setpoints – 4
Calculate the total current measured by the current input I
N
as follows:
Where:
1 + (Z
S
) sin
θ
8
R
N
8
R
N
8 + 1 +
(Z
S
) cos
θ
ArcTAN
φ
=
Calculate the real component of the current measured by
the current input I
N
with respect to the neutral voltage input
as follows:
Re(I
T
) = I
T
COS(
φ
)
Re(I
N
) = I
T
COS(
φ
)
80
I
T
=
25
8
R
N
8 + 1 +
Z
S
θ
= -90 0 - tan -1
-X
CS
R
I
N
= I
T
80
Z
S
=
R
S
X
CS
θ
R
S
2 + X
CS
2
e
Re(Z
S
) is the real component of Z
S
and Im(Z
S
) is the imaginary component.
Calculate the total current when the system is faulted and unfaulted to determine if there is adequate sensitivity
for this pickup setting. Use the following two assumptions for the insulation resistance to calculate the current
during normal operating conditions and a ground fault:
R
Stator
= 100 kilo-Ohms (normal operating conditions)
R
Stator
= 5 kilo-Ohms (ground fault)
There maybe only 2 to 3 milli-amps or less in difference for the total current when the system is faulted
and unfaulted for applications that have a large value of capacitive coupling to ground (C
O
greater than
1.5 micro-Farads) when combined with a low value for the grounding resistor (R
N
less than 0.3 Ohms). Use
the real component of the total current for these applications as there will be a larger margin in difference
when the system is faulted and unfaulted.