HART is a very small, low-frequency signal riding on top of the traditional 4–20 mA current loop. Poor shielding and earthing are among the most common reasons for HART communication failures. Since the HART signal is only about ±0.5 mA, electrical noise can easily corrupt the communication while the 4–20 mA analog signal still appears normal.
HART uses Frequency Shift Keying (FSK) communication at 1200 Hz and 2200 Hz. Any electrical interference around these frequencies may result in checksum failures, communication retries, or complete communication loss.
Symptoms of Poor Shielding and Earthing
- Intermittent HART communication: Communication breaks randomly.
- AC Drive Noise: Communication stops when a nearby AC drive starts running.
Why Shielding and Earthing Matter
Prevents ground loops: Proper shielding and earthing prevent ground loops. If the cable shield is connected to earth at more than one point, circulating currents can flow through the shield and introduce 50 Hz noise, harmonics, and switching interference into the communication pair.
The recommended practice is to use shielded cable with the shield connected to earth at only one point, normally at the control room or power supply side. The shield should remain continuous throughout the cable length while remaining insulated and floating at the field end.
Reduces electrical noise: Proper shielding protects HART communication from interference generated by variable frequency drives (VFDs), motors, contactors, and radio equipment. Without proper earthing, the shield becomes ineffective.
Controls common-mode voltage: Good earthing keeps common-mode voltage within acceptable limits. Poor grounding can make HART gateways and analog input modules fail to demodulate the HART signal correctly.
Enables surge protection: Surge protection devices rely on good earthing. High-resistance earth increases the risk of damage to HART transmitters and communication modules.
Installation Checklist for Reliable HART
- Use shielded communication cable.
- Use a separate instrument earth.
- Verify that earth resistance is in a single-digit ohm value.
- Connect the cable shield to earth at only one point (power supply/control room side).
- Keep the field side shield floating.