Surface Resistivity Testing

BS EN 61340-2-3 / BS EN 1149-1

Surface resistivity is a calculated value based on the measured surface resistance using a test cell with known geometry. It is an inherent property of the surface of a material and is independent of sample surface area. Testing is conducted by applying a range of voltages from one electrode in intimate contact with the specimen’s surface and measuring either a current or direct resistance across the surface to a second electrode.

The values of either measured or calculated resistance are multiplied by a cell constant, generated from the test cell’s geometric size and configuration to derive a Surface Resistivity value. The unit of surface resistivity is the ohm, however, to differentiate between surface resistance (dependent on sample size) and surface resistivity, often the value is displayed in Ohms per Square (Ω/□).

Humidity can affect this parameter or characteristic, so testing is performed in controlled environmental conditions.

Surface resistivity indicates the ability of a material to allow charge to migrate across its surface.

The test classifies a material using the following standardised boundaries:

The following definitions are given as guidance by PD CLC TR 60079-32-1 “Explosive atmospheres – Part 32-1: Electrostatic hazards, guidance”.

A material with a high surface resistivity would be considered to be insulating and therefore stop the migration of charge. With a charging mechanism present, the accumulation of potentially hazardous levels of charge may occur, producing brush discharges, of sufficient energy to ignite most flammable solvent and gas atmospheres.

An insulating surface will provide an electrical barrier between conductors. If a conductor becomes electrically isolated from earth and charged, a spark discharge can be the outcome. This is an electrostatic discharge capable of igniting flammable vapour, gas AND dispersed powder atmospheres.

A material with a low surface resistivity would be considered to be static dissipative or conductive and freely allow charge to migrate, therefore, providing it is adequately connected to the earth, charge accumulation will not likely occur.

There usually is a close relationship between surface and volume resistivity, so is not common for a material or product to have these parameters differing. 

The test is predominantly conducted on liners, hoses, sheet materials and laminates (BS EN 61340-2-3 “Methods of test for determining the resistance and resistivity of solid planar materials to avoid electrostatic charge accumulation”) or items of clothing / personal protective equipment (PPE) components (EN 1149-1 “Protective clothing – Electrostatic properties – Part 1: Test method for measurement of surface resistivity”).

The test can be used to ensure that materials intended to promote sufficient electrical connection to avoid charge accumulation are in line with the standards guidance on resistivity boundaries. Often materials advertised as such fall short of the mark, so its always wise to get this checked before committing to a large order that may jeopardise safety, where having a dissipative material is intrinsic to ensuring low charge levels or sufficient electrical connection.