11
Example 1.
Protection of
industrial low-
voltage switchgear
6
8
7
3
2
4
5A
1A
5
SGR/1
SPAJ 131 C
U2
U3
U3
1
1
1
1
D
C
B
A
+ -
Uaux
E
+
-
(
~
)
(
~
)
5A
1A
5A
1A
5
81
68
66
1 2 3
4
6
7 8 9
10
11
61 62
70 71 72
77 78
80
69
65
SGR
IRF
START1
SIGNAL1
TRIP2
TRIP1
IRF
BS
+
SPA-ZC_
Rx
Tx
0
Ι
0
Ι
-
-
+
+
L1
L3
L2
TS2
TS1
SGB
SS1
SS2
U1
5
4
I/O
3I>
t >>
t >,k
3I>>
~
SERIAL
PORT
Fig. 5. Overcurrent relay SPAJ 131 C used to protect an outgoing feeder in industrial switchgear.
The switch settings are shown in the table on the next page.
The low-set stage of the overcurrent module
SPCJ 3C3 operates as overcurrent and short
circuit protection for the low-voltage switchgear
and as back-up protection for the outgoing
feeder of the distribution switchgear. The low-
set stage is set to extends to the next protection
stage. The setting of the low-set stage should be
selected so as to ensure that the protection
operates selectively together with the fuses on
the outgoing feeders. The high-set stage is set to
operate at close-by short circuits.
Separate protection for the low-voltage switch-
gear is important if there are several distribution
transformer along the same feeder. In particular
faults in low-voltage switchgear fed by the smaller
transformers of a system are not always capable
of starting the overcurrent relay of the distribu-
tion switchgear feeder.
Current asymmetry, if it appears, does not have
to be allowed for in the current settings, because
due to the peak-to-peak measurement method
employed by the overcurrent relay such asym-
metry does not affect the operation of the relay.