2–24
M-0420 Instruction Book
51N
Inverse Time Overcurrent, Neutral
Circuit
This is identical in function to 51V except that,
since it is intended for use in the neutral, it will
operate by detecting zero sequence current that
flows during ground faults, and can usually be
set to a more sensitive value than those on the
individual phases. (Voltage control or voltage re-
straint are not provided in the neutral circuit since
they are not needed.) When used for generator
ground fault protection, 51N provides alternative
or backup ground-fault protection to 59N. This
relay must be set above the maximum zero se-
quence current that normally flows in the gener-
ator neutral. A typical setting would be 1.5 to 2
times the expected maximum zero sequence cur-
rent value.
If the 51N function is not used at the generator
neutral circuit, it can be used to detect ground
faults on the system by monitoring the current
through the ground connection of a wye-connected
power transformer.
N
O
I
T
C
N
U
F
E
G
N
A
R
T
N
I
O
P
T
E
S
T
N
E
M
E
R
C
N
I
L
A
I
T
I
N
I
G
N
I
T
T
E
S
-
)
N
1
5
(
l
a
r
t
u
e
N
,t
n
e
r
r
u
c
r
e
v
O
e
m
i
T
e
s
r
e
v
n
I
s
e
v
r
u
C
ci
t
si
r
e
t
c
a
r
a
h
C
y
r
e
V
,
e
s
r
e
v
n
I
,
e
ti
n
if
e
D
y
l
e
m
e
rt
x
E
,
e
s
r
e
v
n
I
e
s
r
e
v
n
I
e
s
r
e
v
n
I
y
r
e
V
g
n
it
t
e
S
p
a
T
A
5
4
.
1
o
t
0
5
.
0
A
0
9
.
2
o
t
0
5
.
1
A
0
8
.
5
o
t
0
0
.
3
A
0
0
.
2
1
o
t
0
0
.
6
A
5
0
.
0
A
0
1
.
0
A
0
2
.
0
A
0
5
.
0
A
0
.
6
g
n
it
t
e
S
l
ai
D
e
m
i
T
0
.
1
1
o
t
5
.
0
1
.
0
0
.
5
-
e
h
t
y
b
d
e
m
m
a
r
g
o
r
p
e
r
li
t
n
u
y
r
o
m
e
m
t
n
i
o
p
t
e
s
n
i
s
i
e
u
l
a
v
e
h
T
:
g
n
it
t
e
S
l
a
it
i
n
I
.r
o
t
a
r
e
p
o
Table 2-8
Inverse Time Overcurrent Neutral (51N) Setpoint Ranges
Summary of Contents for M-0420
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