What Is Electrical Surface Resistivity?

Electrical surface resistivity is the electrical resistance of the surface of a material, expressed in Ohms per Square (Ω/sq). It is independent of the material’s size or thickness. Surface resistivity is calculated based on the surface resistance of the material and the known geometry of the electrodes. It represents the material’s resistance to current flow when an electrical potential difference is applied across its surface. Unlike volume resistivity, surface resistivity specifically addresses the electrical characteristics of a material’s surface layer, affecting its behavior in applications such as coatings, films, and insulating layers.

Factors influencing surface resistivity include material composition, surface treatments, moisture, temperature, contamination, and mechanical stress. These factors impact the surface’s conductivity or insulation properties.

Testing Principle and Methodology

Place two electrodes of equal length on the material’s surface, spaced at a known distance. Apply a direct current voltage to one electrode and measure the resulting current at the other. Alternatively, you can use a concentric test cell with a known cell constant. Use a megger-ohmmeter to measure the resistance directly. Then, calculate the surface resistivity using the formula:

Surface Resistivity Calculation Formula Where:

ρ = Resistivity in ohms per square (Ω/sq)

R = Measured resistance

L = Length of electrode

D = Distance between electrodes

Testing is typically conducted on solid materials in their commercial form and under varying humidity conditions.

Factors such as temperature, relative humidity, and surface cleanliness can affect measurement accuracy. It is critical to ensure the surface is clean and free from contamination to avoid skewed results.

Applicable StandardSurface resistivity testing is performed according to these standards: 
ASTM D257: Standard Test Methods for DC Resistance or Conductance of Insulating Materials
BS 5958: Part 1: 1991
BS 7506: Part 2: 1996
NFPA 77

Data InterpretationSurface resistivity values are classified as follows:
< 10⁵ Conductive / Low Resistivity
10⁵ to 10⁹ Static Dissipative / Medium Resistivity
> 10⁹ Non-Conductive / High Resistivity

Knowing your material’s surface resistivity enables safe handling measures. Materials should be handled in a well-earthed environment to prevent electrostatic charge buildup. Sudden discharge of accumulated charge can ignite flammable atmospheres, so understanding electrostatic properties is crucial to preventing such hazards.

 We also offer:
Volume Resistivity
Surface Resistivity
Charge Decay (Relaxation) Time
Electrostatic Discharge (ESD) Testing
Breakdown Voltage
Liquid Conductivity Testing
Flexible Intermediate Bulk Containers (FIBC)
Electrostatic Chargeability Testing