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IP2046/IM
Nov 2006
3.3.3 Relays
The MSP400RH transmitter has 2 integral relays which may be used for control purposes. These relays
are light duty and should be used as signal relays only, with control functions being performed by external
control relays.
Relay 2 is defaulted as a ‘fault’ relay, normally energised, but may be reconfigured on site as a set point
relay if required.
3.4 Additional components in the two wire loop.
3.4.1 Lightning / surge protection and other loop devices
It is allowable to fit loop powered or separately powered devices in the two wire loop provided that the
transmitter receives a minimum voltage of 12V dc at 21 mA loop current.
If the area is prone to lightning strikes or voltage surges, fitting of a suppressor device is desirable
between the transmitter and the control unit.
3.5 Wiring to allow HART communication
If it is intended to use HART digital communications with the MSP400RH transmitter, a 250 Ohm 0.25W
load resistor must be installed in the loop.
When used with the Mobrey MCU900 family of Control Units, there is no need to install an
external load resistor in the loop as there is a suitable resistor built in to the Control Unit.
If the transmitter is being supplied through a safety barrier, ensure the type chosen will pass HART/
SMART information.
Once installed, a HART communicator can be connected across the load resistor, or across the loop at
any point downstream of the load resistor.