Resistance measurement in ESD control

RTG, RTT and surface resistance are not interchangeable

Before testing, define the property that needs to be verified. Confusing methods can produce a precisely measured value that is nevertheless interpreted incorrectly.

• RTG – resistance to ground is measured between a selected point and ground or a designated grounding point. It can verify the path from a work surface or floor to ground.
• RTT – resistance point to point is measured between two points on a surface. It helps assess electrical continuity and uniformity.
• Surface resistance is a broader term for resistance measured across a surface with a defined electrode arrangement. It should not automatically be treated as the material's resistivity.

What is commonly measured in an EPA?

Checks may cover workbenches and mats, floors, racks, carts, chairs, handling equipment, storage systems and packaging. For each item, determine whether the requirement concerns resistance across the surface, the path to ground or the performance of the complete system including connections.

A practical measurement sequence

1. Define the item and purpose. Record what is being tested, the method and the requirement used for evaluation.
2. Check the instrument. Confirm its condition, calibration status, suitable range, electrodes and accessories.
3. Prepare the location. Follow the required surface condition and connection method. Clean only when required by the method or verification plan.
4. Apply the defined method. Observe electrode placement and pressure, test voltage, stabilization time and number of points.
5. Record the conditions. Include date, location, temperature and humidity, instrument, result and acceptance criterion.
6. Investigate deviations. If a result fails, verify the procedure and identify the cause instead of searching for another point that happens to pass.

Common measurement errors

• confusing RTG and RTT, or treating complete-system resistance as a material property;
• using unsuitable electrodes, voltage or measurement time;
• testing through contamination, protective film or coating, or without required conditioning;
• overlooking grounding connections, cables and contact points;
• recording a value without its method, conditions and acceptance criterion.

The result must be repeatable

A well-defined check allows another trained person to repeat the measurement and obtain a comparable result. Limits differ by control item and method, so use the current applicable standard, internal plan or product specification instead of a universal table without context.

Return to the ESD measurement and verification overview. For in-house checks, see resistance measuring instruments and audit equipment. For method verification or a wider site assessment, Stat-X provides measurement and calibration.