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Reference Manual
00809-0100-4021, Rev FB
August 2011
2-15
Rosemount 3144P
Surges/Transients
The transmitter will withstand electrical transients of the energy level usually
encountered in static discharges or induced switching; however, high-voltage
transients, such as those induced in wiring from nearby lightning strikes, can
damage both the transmitter and the sensor.
The integral transient protection terminal block (option code T1) protects
against high-voltage transients. The integral transient protection terminal
block is available as an ordered option, or as an accessory. Refer to
“Transient Protection (Option Code T1)” on page A-6 for more information.
Grounding
Sensor Shielding
The currents in the leads induced by electromagnetic interference can be
reduced by shielding. Shielding carries the current to ground and away from
the leads and electronics. If the ends of the shields are adequately grounded,
only a small amount of current will actually enter the transmitter.
If the ends of the shield are left ungrounded, voltage is created between the
shield and the transmitter housing and also between the shield and earth at
the element end. The transmitter may not be able to compensate for this
voltage, causing it to lose communication and/or go into alarm. Instead of the
shield carrying the currents away from the transmitter, the currents will now
flow through the sensor leads into the transmitter circuitry where it will
interfere with the circuit operation.
Shielding Recommendations
The following are recommended practices from API Standard 552
(Transmission Standard) section 20.7, and from field and laboratory testing. If
more than one recommendation is given for a sensor type, start with the first
technique shown or the technique that is recommended for the facility by its
installation drawings. If the technique does not eliminate the transmitter
alarms, try another technique. If all of the techniques do not eliminate or
prevent the transmitter alarms because of high EMI, contact an Emerson
Process Management representative.
Ungrounded Thermocouple, mV, and RTD/Ohm Inputs
Option 1: Recommended for ungrounded transmitter housing
1. Connect the signal wiring shield to the sensor wiring shield.
2. Ensure the two shields are tied together and electrically isolated from
the transmitter housing.
3. Ground the shield at the power supply end only.
4. Ensure the shield at the sensor is electrically isolated from the
surrounding fixtures that may be grounded.
Connect shields together,
electrically isolated from the
transmitter
4-20 mA loop
Transmitter
Sensor Wires
Shield ground point
Aotewell Ltd
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