Fisher FIELDVUE DVC6200 Instruction Manual page 10

Wiring Practices
February 2017
As shown in figure 2‐2, the voltage available at the instrument depends upon:
D the control system compliance voltage
D if a filter, wireless THUM adapter, or intrinsic safety barrier is used, and
D the wire type and length.
The control system compliance voltage is the maximum voltage at the control system output terminals at which the
control system can produce maximum loop current.
The voltage available at the instrument may be calculated from the following equation:
Voltage Available = [Control System Compliance Voltage (at maximum current)] - [filter voltage drop (if a HART filter is
used)] - [total cable resistance  maximum current] - [barrier resistance x maximum current].
The calculated voltage available should be greater than or equal to 10 volts DC.
Table 2‐1 lists the resistance of some typical cables.
The following example shows how to calculate the voltage available for a Honeywellt TDC2000 control system with a
HF340 HART filter, and 1000 feet of Beldent 9501 cable:
Voltage available = [18.5 volts (at 21.05 mA)] - [2.3 volts] - [48 ohms  0.02105 amps]
Voltage available = [18.5] - [2.3] - [1.01]
Voltage available = 15.19 volts
Figure 2‐2. Determining Voltage Available at the Instrument
COMPLIANCE VOLTAGE
CONTROL
SYSTEM
+
-
Calculate Voltage Available at the Instrument as follows:
Control system compliance voltage
– Filter voltage drop (if used)
– Intrinsic safety barrier resistance (if used) x maximum loop current
– Smart Wireless THUM adapter voltage drop (if used)
– Total loop cable resistance x maximum loop current
= Voltage available at the instrument
NOTES:
Obtain filter voltage drop. The measured drop will be different than this value. The measured filter voltage drop
1
depends upon control system output voltage, the intrinsic safety barrier (if used), and the instrument. See note 3.
The voltage drop of the THUM adapter is linear from 2.25 volts at 3.5 mA to 1.2 volts at 25 mA.
2
The voltage available at the instrument is not the voltage measured at the instrument terminals. Once the instrument is
3
connected, the instrument limits the measured voltage to approximately 8.0 to 9.5 volts.
10
INTRINSIC SAFETY
HART FILTER
BARRIER
(if used)
(if used)
1
3
TOTAL LOOP
THUM ADAPTER
CABLE RESISTANCE
(IF USED)
R
Example Calculation
18.5 volts (at 21.05 mA)
– 2.3 volts (for HF300 filter)
– 2.55 volts (121 ohms x 0.02105 amps)
2
– 1.01 volts (48 ohms x 0.02105 amps for
1000 feet of Belden 9501 cable)
= 15.19 volts, available—if safety barrier (2.55 volts)
is not used
Instruction Manual
D103605X012
VOLTAGE
+
AVAILABLE AT THE
INSTRUMENT
-