Surges/Transients; Ground The Transmitter - Emerson Rosemount 644H Reference Manual

Reference Manual
00809-0100-4728, Rev HA
December 2002

Surges/Transients

Ground the Transmitter

The transmitter will withstand electrical transients of the energy level
encountered in static discharges or induced switching transients. However,
high-energy transients, such as those induced in wiring from nearby lightning
strikes, welding, heavy electrical equipment, or switching gears, can damage
both the transmitter and the sensor. To protect against high-energy
transients, install the transmitter into a suitable connection head with the
Rosemount Model 470 Transient Protector. Refer to the Model 470 Transient
Protector Product Data Sheet (document number 00813-0100-4191) for more
information.
The transmitter will operate with the current signal loop either floating or
grounded. However, the extra noise in floating systems affects many types of
readout devices. If the signal appears noisy or erratic, grounding the current
signal loop at a single point may solve the problem. The best place to ground
the loop is at the negative terminal of the power supply. Do not ground the
current signal loop at more than one point.
The transmitter is electrically isolated to 500 V ac rms (707 V dc), so the input
circuit may also be grounded at any single point. When using a grounded
thermocouple, the grounded junction serves as this point.
NOTE
Do not ground the signal wire at both ends.
Ungrounded Thermocouple, mV, and RTD/Ohm Inputs
Each process installation has different requirements for grounding. Use the
grounding options recommended by the facility for the specific sensor type, or
begin with grounding Option 1 (the most common).
Option 1:
1. Connect 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 shield at the power supply end only.
4. Ensure that the sensor shield is electrically isolated from the
surrounding grounded fixtures.
Sensor Wires
Connect shields together, electrically isolated from the transmitter
Models 644H and 644R
Transmitter
Shield ground point
4–20 mA loop
2-13