In general, the installation practice for HART communicating devices is the same as that for conventional 4-20mA instrumentation. Individually shielded twisted pair cable, either in single pair or multi-pair varieties, is the recommended wiring practice. Unshielded cables may be used for short distances, if ambient noise and cross-talk will not effect communication. The minimum conductor size is 0.51mm diameter (#24 AWG) for cable runs less than 1,524 meters (5,000 ft.) and 0.81mm diameter (#20 AWG) for longer distances.

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.

Grounding
To prevent interference by external signals, it is important to 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, at one point only, and 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, the control system).

Power supply
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, taking into account voltage drops in the cable and load resistor, and a passive IS barrier if one is present. Smart devices may take up to 22 mA to indicate an alarm condition; use this value to calculate the worst loop voltage drop.
There are additional communication-related specifications for the power supply for a HART loop; these are shown in the table below. The ripple and noise specifications are designed to prevent direct interference with the HART signals. The impedance limit ensures that HART signals see the power supply as a low impedance path, and prevents inadvertent coupling and crosstalk between multiple HART loops powered from a common supply. (The resistance of output fuses, if any, must be included, when measuring this value.)

Power supply specification

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 (mostly capacitance) and the combination of connected devices can affect the maximum allowable cable length of a HART network. The table below shows the affect of cable capacitance and the number of network devices on cable length. The table is based on typical installations of HART devices in non-IS environments, i.e. 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.

The allowable cable length depends upon the cable capacitance and the number of network devices.

Cable Length - feet (meters)

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