Update Documentation
P521/EN AD/B21
MiCOM P521
Page 21/24
ACCESSORIES (P521/EN AC/B11)
5.1
Function of loopback test box
In the unlikely event that a P521 relay has a failure in the protection communications
hardware, it is desirable to determine if the fault is located in the active part
(withdrawable part) of the relay or in the protection communications card. If the fault
is within the active part only, this can be sent back for repair whilst leaving the relay
case in situ. If, however, the fault is on the protection communications card the whole
relay, including the relay case, needs to be sent back for repair.
The P520 loopback test box facilitates the fault location process by attaching to the
protection communications card in the back of the relay case (see Figure 6). For a
complete description of the fault location procedure please refer to the “Equipment
Failure” section of the “Commissioning and Maintenance” document (P521/EN CM).
P1250ENc
LO
O
P
B
A
C
K
B
AT
T T
S
T
O
FF
Relay Case
Protection Comms Card
Loopback
Test Box
Green Test LED
Fuse Holder
100MAF 250V
Test Mode
Switch
Figure 6:
Application of P520L loopback test box
