CHAPTER 3: INSTALLATION TYPICAL WIRING
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL 3–17
3.2.13 Solid State Trip Output
A high-speed solid state (SCR) output is also provided. This output is intended for
applications where it is necessary to key a communications channel.
3.2.14 Analog Outputs
The 745 provides 7 analog output channels whose full scale range can be set to one of the
following ranges: 0 to 1 mA; 0 to 5 mA; 0 to 10 mA; 0 to 20 mA; and 4 to 20 mA. Each analog
output channel can be programmed to represent one of the parameters measured by the
relay. For details, see Analog Outputs 1 to 7 on page 5–43.
As shown in the typical wiring diagram, the analog output signals originate from Terminals
A6 to A12 and share A5 as a common return. Output signals are internally isolated and
allow connection to devices which sit at a different ground potential. Each analog output
terminal is clamped to within 36 V of ground. To minimize the effects of noise, external
connections should be made with shielded cable and only one end of the shield should be
grounded.
If a voltage output is required, a burden resistor must be connected at the input of the
external measuring device. Ignoring the input impedance, the burden resistance is:
(EQ 3.1)
If a 5 V full scale output is required with a 0 to 1 mA output channel, R
LOAD
= 5 V / 0.001 A =
5KΩ. Similarly, for a 0 to 5 mA channel this resistor would be 1 KΩ; for a 0 to 10 mA
channel, this resistor would be 500 Ω; and for a 4 to 20 mA channel this resistor would be
250 Ω.
FIGURE 3–16: Analog output connection
3.2.15 RS485/RS422 Communications
The 745 provides the user with two rear communication ports which may be used
simultaneously. Both implement a subset of the AEG Modicon Modbus protocol as outlined
in publication GEK-106636A: 745 Communications Guide.
With order code E, ports COM1 and COM2 are available. With order code T, ports COM1 and
Ethernet 10Base-T are available. COM2 and Ethernet are not both available on the same
unit.
R
LOAD
V
FULL-SCALE
I
MAX
-----------------------------
=
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