Verifying that grounding and bonding systems meet a specific resistance threshold—such as the common 10 ohms target for flammable liquid processes—requires a systematic approach to ensure both accuracy and technician safety. The following procedure outlines the steps for performing a static ground-resistance test using a dedicated ground-resistance tester (Ohmmeter). This procedure applies to the periodic verification of grounding points for tanks, piping, and loading stations that process or transfer flammable liquids.
- Required Equipment: A calibrated Ground Resistance Tester (e.g., Clamp-on meter or a three-point “Fall-of-Potential” tester).
- Safety PPE: Standard industrial PPE plus flame-resistant clothing (FRC) and static-dissipative footwear.
- Permit Requirements: Ensure a Hot Work Permit is obtained if the testing equipment is not intrinsically safe and will be used in a classified (Division 1 or 2) area.
Before taking electrical measurements, perform a visual and mechanical assessment of the grounding system:
- Physical Integrity: Check for broken conductors, frayed wires, or loose lugs.
- Corrosion: Inspect connection points for oxidation or “green” corrosion, which can significantly increase resistance.
- Contact Surface: Ensure the grounding clamp or lug is connected to clean, unpainted metal. Paint and coatings act as insulators.
The two (2) most common methods for verifying resistance in an industrial setting are
- the Clamp-On Method, and
- the Two-Point (Dead Earth) Method
Clamp-On Meter (Preferred for Closed Loops)
This method is ideal for systems that are already part of a multi-grounded system.
- Identify the Ground Lead: Locate the conductor connecting the equipment to the grounding electrode system
- Verify Cleanliness: Ensure the area where the clamp will be placed is free of debris
- Clamp the Meter: Place the jaws of the meter around the grounding conductor. Ensure the jaws close completely
- Record the Reading: The meter induces a voltage and measures the resulting current to calculate resistance (R = V / I)
- Target: The reading must be <10 Ohms
Two-Point Method (For Isolated Systems)
Used when a loop is NOT present or for a single point of ground.
- Select a Known Ground: Identify a “known good” ground (e.g., a main building steel column or a cold water pipe).
- Connect Leads: Connect one lead of the ohmmeter to the equipment being tested and the other to the known ground.
- Compensate for Leads: If the meter allows, “null out” the resistance of the test leads themselves to avoid a false high reading.
- Take Measurement: Record the resistance between the two points.
| Result | Action Required |
| 0.1 to 10 Ohms | Pass. System is adequate for static dissipation. |
| > 10 Ohms | Fail. Inspect for loose connections, corrosion, or insufficient rod depth. Remediate and retest. |
| Open Circuit | Critical Failure. Stop flammable liquid transfer immediately. |
Log the following data in the facility maintenance system:
- Date and time of the test.
- Specific identification of the grounding point (e.g., “Pump P-101 Grounding Lug”).
- The exact resistance reading in Ohms.
- Initials of the qualified person performing the test.
While NFPA 77 provides guidance on static electricity, it is industry best practice to test grounding systems ANNUALLY, or following any significant maintenance or construction activity near the grounding site. High-vibration areas (like pumps) should be inspected more frequently for mechanical loosening.