Auxiliary Terminal Wiring Length Guidelines; Maximum Cable Capacitance - Fisher FIELDVUE DVC6200 Instruction Manual

Wiring Practices
February 2017

Auxiliary Terminal Wiring Length Guidelines

The Auxiliary Input Terminals of a DVC6200 with instrument level ODV can be used with a locally‐mounted switch for
initiating a partial stroke test. Some applications require that the switch be installed remotely from the DVC6200.
The length for wiring connected to the Auxiliary Input Terminals is limited by capacitance. For proper operation of the
Auxiliary Input Terminals capacitance should not exceed 100,000 pF. As with all control signal wiring, good wiring
practices should be observed to minimize adverse effect of electrical noise on the Aux Switch function.
Example Calculation: Capacitance per foot or per meter is required to calculate the length of wire that may be
connected to the Aux switch input. The wire should not exceed the capacitance limit of 100,000 pF. Typically the wire
manufacturer supplies a data sheet which provides all of the electrical properties of the wire. The pertinent parameter
is the highest possible capacitance. If shielded wire is used, the appropriate number is the "Conductor to Other
Conductor & Shield" value.
Example — 18AWG Unshielded Audio, Control and Instrumentation Cable
Manufacturer's specifications include:
Nom. Capacitance Conductor to Conductor @ 1 KHz: 26 pF/ft
Nom. Conductor DC Resistance @ 20 Deg. C: 5.96 Ohms/1000 ft
Max. Operating Voltage - UL 200 V RMS (PLTC, CMG),150 V RMS (ITC)
Allowable Length with this cable = 100,000pF /(26pF/ft) = 3846 ft
Example — 18AWG Shielded Audio, Control and Instrumentation Cable
Manufacturer's specifications include:
Nom. Characteristic Impedance: 29 Ohms
Nom. Inductance: .15 μH/ft
Nom. Capacitance Conductor to Conductor @ 1 KHz: 51 pF/ft
Nom. Cap. Cond. to other Cond. & Shield @ 1 KHz 97 pF/ft
Allowable Length with this cable = 100,000pF /(97pF/ft) = 1030 ft
The AUX switch input passes less than 1 mA through the switch contacts, and uses less than 5V, therefore, neither the
resistance nor the voltage rating of the cable are critical. Ensure that switch contact corrosion is prevented. It is
generally advisable that the switch have gold‐plated or sealed contacts.

Maximum Cable Capacitance

The maximum cable length for HART communication is limited by the characteristic capacitance of the cable.
Maximum length due to capacitance can be calculated using the following formulas:
Length(ft) = [160,000 - C
Length(m) = [160,000 - C
where:
160,000 = a constant derived for FIELDVUE instruments to ensure that the HART network RC time constant will be no
greater than 65 μs (per the HART specification).
C = the capacitance of the control system or HART filter
master
C = the capacitance of the cable used (see table 2‐1)
cable
12
(pF)] [C
(pF/ft)]
master cable
(pF)] [C
(pF/m)]
master cable
Instruction Manual
D103605X012