Fall Of Potential
The Fall-of-Potential (FOP) method is a non-destructive field test that measures grounding system resistance by injecting a known current between the electrode under test and a remote probe, then measuring voltage drops at intermediate points to calculate resistance via Ohm's Law. It is the industry standard for verifying electrical safety and equipment reliability.
In shop floor maintenance, the Fall-of-Potential method is used to verify grounding system integrity for sensitive equipment like CNC machines, robotics, and PLCs. CMMS platforms like ServiceGrid integrate FOP test results as inspection tasks, triggering alerts when resistance exceeds thresholds (e.g., >5 ohms). This ensures compliance with NEC and IEEE Standard 81, preventing voltage surges that damage assets. Trending resistance data over time enables predictive maintenance, identifying gradual degradation from corrosion or loose connections before functional failure occurs.
Why is the potential probe ideally placed at 61.8% of the distance to the current probe?
This point corresponds to the theoretical Courant's point where the voltage gradient stabilizes, minimizing the effect of soil resistivity variations and providing the most accurate resistance calculation for a uniform earth model.
How does FOP differ from the Stakeless (Clamp-on) method?
FOP requires disconnection of the ground and is suitable for isolated grids or precise impedance mapping; Clamp-on methods test without disconnection but are less accurate for large, complex grids with multiple parallel paths.
What does a steep voltage gradient near the electrode indicate?
A steep gradient close to the electrode followed by a gradual decline indicates a well-functioning grounding system with effective current dissipation; a steep gradient persisting far out suggests poor soil contact or high resistance.