Electrostatic discharge (ESD) performance is influenced by several environmental factors, with relative humidity being one of the most important. This is particularly relevant in South Africa, where seasonal changes can result in significant differences in humidity levels.
For companies operating electronics manufacturing, assembly, clean rooms, laboratories and other ESD-sensitive environments, it is important to understand that an ESD system that performs well during the humid summer months may behave differently during the dry winter months.
Why does humidity affect ESD?
Static electricity is generated when two materials come into contact and separate. This can occur through everyday activities such as walking across a floor, moving on a chair, handling plastic materials, removing packaging, clothing rubbing against the body, or moving components across work surfaces.
When the surrounding air contains more moisture, electrical charge can dissipate more easily from many surfaces. When the air becomes very dry, materials tend to retain charge for longer, increasing the possibility of higher static voltages developing.
As a result, low-humidity environments generally represent a greater ESD risk than high-humidity environments.
Why is this particularly relevant in South Africa?
South Africa has significant seasonal variations in temperature, rainfall and relative humidity. This is especially noticeable in inland regions such as Gauteng, Mpumalanga, Limpopo, North West and the Free State.
During the summer months, these regions generally experience their rainfall season. The increased moisture in the atmosphere results in higher relative humidity and generally more favourable conditions for controlling static electricity.
During winter, however, the situation changes considerably. The inland South African winter is typically extremely dry, and relative humidity inside factories and warehouses can fall significantly.
How can this affect ESD flooring?
ESD flooring is designed to provide a controlled path for electrical charge to dissipate to ground. However, the electrical resistance of some flooring systems can be influenced by environmental conditions, particularly relative humidity.
A floor tested during summer at higher humidity may therefore produce different resistance readings when tested during a dry South African winter.
For example, a flooring system could theoretically measure:
• Summer conditions: Resistance to ground = 2 x 106 Ω
• Dry winter conditions: Resistance to ground = 2 x 107 Ω
This does not mean that every floor will change by this amount. Different flooring technologies and materials react differently to humidity. The point is that ESD readings should always be considered together with the environmental conditions under which the testing was conducted.
Footwear and personnel grounding can also be affected
The seasonal effect is not limited to flooring. ESD footwear and personnel grounding systems can also produce different results during very dry conditions.
Dry skin, socks, footwear materials and the interface between the person, footwear and floor can all influence the overall resistance of the personnel grounding system.
For this reason, an ESD flooring system should ideally be evaluated as part of the complete system: Person – ESD footwear – ESD floor – Ground
It is the performance of the complete system that ultimately determines how effectively charge is removed from the person.
Recording humidity when conducting ESD tests
When conducting ESD flooring tests, it is good practice to record the environmental conditions at the time of testing. A proper test record should therefore include the date and time, temperature, relative humidity, resistance to ground, point-to-point resistance, and personnel/footwear-floor resistance where applicable.
For example:
• Temperature: 21°C
• Relative Humidity: 19%
• Resistance to Ground: 4,8 x 107 Ω
This provides considerably more useful information than simply recording the resistance measurement. It also allows companies to compare summer and winter results and identify whether seasonal environmental changes are influencing their ESD control system.
Designing for the worst-case environment
When specifying an ESD flooring system in South Africa, consideration should be given to the lowest relative humidity that the facility is likely to experience during the year. This is particularly important for facilities situated on the Highveld.
A flooring installation tested during a humid January or February may produce excellent results. However, the same installation still needs to provide acceptable ESD performance during July or August when humidity levels may be considerably lower.
The ESD flooring and personnel grounding system must meet the specified ESD performance requirements under the lowest relative-humidity conditions reasonably expected within the facility.
It is important not to apply the same seasonal assumptions across the whole of South Africa. The lowest-humidity period experienced by a facility in Cape Town may differ significantly from that experienced by a facility in Johannesburg or Pretoria.
ESD specifications should consequently consider the actual environmental conditions of the individual facility rather than simply applying a national seasonal assumption.
Humidity should not be the ESD control system
While humidity can assist with reducing static charge accumulation, it should not be relied upon as the primary method of ESD protection.
A properly designed ESD Protected Area should use suitable controls such as ESD flooring, ESD footwear, personnel grounding, ESD work surfaces, grounding points, appropriate packaging and ionisation where required.
Seasonal changes can have a meaningful impact on ESD performance, particularly in South Africa’s inland regions. Therefore, ESD performance should not be judged from a single resistance reading without considering the environmental conditions at the time of the test.
For South African ESD installations, particularly in Gauteng and other inland regions, the system should be designed and evaluated for the driest conditions likely to occur during the year.
An ESD system that performs correctly during a dry South African winter provides far greater confidence that it will provide effective protection throughout the rest of the year.
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