Technical Guide P92x/EN IN/E11
Installation
MiCOM P921-P922-P923 Page 9/10
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It is recommended that the auxiliary supply wiring should be protected by a 16A high
rupture capacity (HRC) fuse of type NIT or TIA. For safety reasons, current transformer
circuits must never be fused. Other circuits should be appropriately fused to protect
the wire used.
6.2 RS485 Port
Connections to the RS485 port are made using ring terminals. It is recommended that
a 2 core screened cable is used with a maximum total length of 1000m or 200nF
total cable capacitance.
A typical cable specification would be:
- Each core: 16/0.2 mm copper conductors PVC
insulated
- Nominal size: 0.5 mm
2
per core
- Screen: Overall braid, PVC sheathed
Refer to chapter P92x/EN CO, paragraph 2.2 for references of RS232/RS485
converters.
6.3 Earth Connection
Every relay must be connected to the local earth bar using the M4 earth studs in the
bottom left hand corner of the relay case. The minimum recommended wire size is
2.5 mm
2
and should have a ring terminal at the relay end. Due to the limitations of
the ring terminal, the maximum wire size that can be used for any of the medium or
heavy duty terminals is 6.0 mm
2
per wire. If a greater cross-sectional area is
required, two parallel connected wires, each terminated in a separate ring terminal at
the relay, or a metal earth bar could be used.
NOTE: To prevent any possibility of electrolytic action between brass or
copper earth conductors and the rear panel of the relay,
precautions should be taken to isolate them from one another.
This could be achieved in a number of ways, including placing a
nickel-plated or insulating washer between the conductor and
the relay case, or using tinned ring terminals.
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