Sensoray 518 - Manual (Page 41)

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A three-wire RTD still has half the cable loss of the equivalent two-wire RTD. To eliminate
cable losses entirely, the four-wire circuit must be used. Like the three-wire circuit, separate
wires are run from the V- and I- terminals to the RTD. In addition, separate wires are run from
V+ and I+ to the RTD, where they are shorted together. This eliminates cable loss effects from
the V+ line, as well as the V- line.
Sensoray recommends the four-wire hookup for all installations. Regardless of the chosen
hookup scheme, you should always use shielded cable. The cable shield must be connected only
to the S terminal on the 7409TC.