In schematics, ground is a single triangle symbol — a reference point assumed to be 0V everywhere. In real systems, ground is copper wire with resistance, and current flowing through that resistance creates voltage differences between points that are all supposedly "ground." When two pieces of equipment share a ground connection through multiple paths, you create a ground loop: a closed conductive loop that picks up magnetic interference and carries stray currents you never intended.

How they form: Imagine a sensor mounted on a machine, wired back to a PLC in a control cabinet 30 meters away. The sensor's signal ground connects to the PLC's analog ground via the cable shield. But both the machine and the cabinet are bonded to building ground through their power cords. Now there are two paths between the sensor ground and PLC ground: through the signal cable, and through the building's safety ground. That loop is an antenna.

Why it matters: A 60 Hz magnetic field from a nearby motor passes through the loop and induces a circulating current (Faraday's law). Even a few millivolts of difference between "grounds" gets injected into your signal path. For a 4-20 mA loop you might not notice. For a thermocouple reading microvolts, or audio equipment, you hear it as a 60 Hz hum.

Rule of thumb: Copper wire is roughly 1.7 mΩ per foot for 12 AWG. If 10 A of fault or return current flows through 50 feet of ground conductor, the voltage drop is 10 × 50 × 0.0017 ≈ 0.85 V. That's not "zero" — and any sensitive instrument referencing the far end sees that 0.85 V as offset.

Real-world example: Recording studios and broadcast facilities are obsessive about this. A guitarist plugs an amplifier into one wall outlet and a pedalboard into another on a different circuit. The two outlets' ground references differ by a few hundred millivolts at 60 Hz. The signal cable between them completes the loop, and the speakers buzz audibly. Fix: plug everything into one outlet strip (single-point ground), or use an isolation transformer on the signal line.

Common mitigations:

• Single-point grounding (star topology): all grounds converge at one node, no loops possible
• Differential signaling: RS-485, 4-20 mA, balanced audio — receivers ignore common-mode ground offsets
• Optical isolation: optocouplers or fiber break the conductive path entirely
• Isolation transformers: for analog signals or AC power
• Ground the shield at one end only: common practice for instrumentation cable