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Wire Length & Capacitance

Wiring HART Devices
In general, the installation practice for wired HART devices is the same as for conventional 4-20mA instrumentation. Guidelines for a wired HART installation involve:

Using shielded twisted pair cabling with the proper conductor size
Grounding at one point only
Ensuring a properly specified power supply
Staying below the maximum allowable cable length which depends upon the cable capacitance and the number of network devices.

Typical Wiring Connection using a HART Field Device, a PC with a PC-based Application
and a Handheld Terminal. Also shown is a required HART Interface or Modem - either RS232 or USB version.

Cabling considerations
If possible, use individually shielded twisted pair cable, either in single pair or multi-pair varieties. Unshielded cables may be used for short distances, provided ambient noise and cross-talk will not adversely impact communication. The minimum conductor size is 0.51mm diameter (#24 AWG) for cable runs less than 1,500 meters (@ 5,000 ft.) and 0.81mm diameter (#20 AWG) for longer distances.

Grounding
To prevent external interference, ground the system properly. In particular, the signal loop should be grounded, if at all, at one point only. The cable screening must be connected to ground, again at one point only. It must not be connected to instrument or junction box cases unless these are isolated from ground. The single ground point will usually be at or near the host – for example, at the control system.

Power supply voltage
Power for a two-wire instrument loop is typically 24V d.c. As always, the voltage must be sufficient to provide the necessary lift-off voltage for the field device. Take into account voltage drops in the cable and load resistor, as well as from any passive intrinsic safety, or IS, barrier present. Smart devices may take up to 22 mA to indicate an alarm condition. Use this value to calculate the worst loop voltage drop.

Power supply ripple and noise
The table below shows additional communication-related specifications for the power supply for a HART loop. The ripple and noise specifications prevent direct interference with the HART signals. The impedance limit ensures that HART signals see the power supply as a low impedance path. This prevents inadvertent coupling and crosstalk between multiple HART loops powered from a common supply. Include the resistance of output fuses, if any, when measuring this value

Maximum ripple (47 to 125 Hz)	0.2 V p-p
Maximum noise (500 Hz to 10 kHz)	1.2 mV rms
Maximum series impedance (500 Hz to 10 kHz)	10 Ω

Cable length
Most installations are well within the 3,000-meter (10,000 ft) theoretical limit for HART communication. However, the electrical characteristics of the cable – primarily its capacitance – and the number of connected devices can affect the maximum allowable cable length. The table below shows the effect of cable capacitance and the number of network devices on cable length.
This is based on typical HART devices in non-IS, or intrinsic safety, environments, which means there is no miscellaneous series impedance. Detailed information for determining the maximum cable length for any HART network configuration can be found in the HART Physical Layer Specifications.

Cable Length - feet (meters)

Be sure to also review the wire specification and requirements of the specific device you are installing provided by your supplier.

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