The difference between antistatic and ESD is an example of safety footwear

There is often confusion between the terms ESD and antistatic, and not only in matters relating to safety footwear. Although both terms refer to contact resistance, there are fundamental differences between them. Interesting? Now we will try to explain the difference.

Shoes are usually a point of connection between the human body and gender. Therefore, electrostatic energy and contact resistance are important concepts in this field. However, a distinction should be made between the antistatic properties of shoes and their ability to conduct electrostatically (ESD).

What are antistatic properties?

    The EN ISO 20345 standard defines various requirements for protective footwear, including requirements for electrostatic properties. It defines three gradations based on contact resistance: conductive, antistatic and electrically insulating footwear. For protective shoes marked S1, compliance with the basic and additional requirements for antistatic is mandatory. Shoes are considered antistatic if the measured contact resistance is in the range between 100 kΩ (10⁵ Ohm) and 1 gOhm (10⁹ Ohm). According to the standard, if the contact resistance falls below this value, the shoes are considered conductive, while a higher value means they are electrically insulated.

  The standard provides for the use of antistatic shoes to prevent the accumulation of electrostatic voltage and ensure its effective discharge. This is necessary to eliminate the risk of electric shock from electrical equipment or live parts, as well as from sparks, which are a dangerous factor in explosive environments.

Therefore, the main purpose of using antistatic shoes is to protect personnel from the dangers associated with the accumulation of static voltage.

What is ESD?

     Electrostatic stress and personal safety are not the only priorities in industry, there is also a need to protect and control equipment components.

In this case, another standard applies that deals with electrostatic discharge (ESD): EN 61340-5-1 Protection of electronic devices against electrostatic phenomena. The ESD zone defined in this standard differs from the antistatic range specified in the safety standard EN ISO 20345. The lower contact resistance threshold is 100 kΩ and the upper threshold is 35 mΩ (3.5 x 10⁷Ohm). This means that ESD compliant footwear is always anti-static, but not all anti-static footwear is ESD compliant. For example, if the contact resistance is 100 mΩ, the shoe is antistatic, but not ESD compliant. However, if the contact resistance is only 1 mΩ, then the shoe is both antistatic and ESD compliant.

    Since ESD refers to the requirements of the protection standard of various types of products, the marking must be different from the CE marking. Therefore, safety shoes that comply with the standard have an additional yellow ESD symbol. If the shoes do not have a special ESD symbol but are marked as S1, they are usually only anti-static.

Measurement methods and factors affecting results

     Testing of antistatic properties of footwear for certification includes a method of testing in laboratory conditions. Before undergoing the test procedure, the footwear must first be exposed for a certain period of time under certain conditions (temperature, humidity). The shoes are then filled with a total mass of 4 kg of stainless steel balls, which are connected to the contact resistance measuring device via a copper cable. The shoes are placed on a copper plate, which is the external electrode. A test voltage of 100 DC is applied between the copper plate and the steel balls for one minute while recording the contact resistance value of the shoe sample. It should be greater than 100 kΩ but less than or equal to 1 gΩ.

     Testing the ESD capabilities of footwear is a bit more complicated because there are many different measurement procedures for both standards. For EN 61340-5-1, the contact resistance value is measured for the human-shoe-ground system. This is due to the fact that the employee has to stand on the electrode during the test. The resistance is measured at the moment when he puts his hand on the metal plate. If the measured contact resistance does not exceed 35 mΩ, the footwear meets the ESD requirements.

     The second standard, EN 61340-4-3, defines contact resistance under laboratory conditions. The tested shoe sample is pre-existed at the specified temperature and air humidity in the conditioning chamber.

What factors can affect contact resistance for shoes?

     Sometimes it can happen that shoes that have been marked as ESD do not pass the control test carried out by the customer. This does not necessarily mean that the shoe does not meet the requirements of ESD, as there can be many reasons that can affect the result. For example, the temperature of the shoes can affect the contact resistance. Safety shoes left in the car overnight in winter will cool so much that the contact resistance will be higher as a result. Likewise, the degree of wear can be an influencing factor, as the level of moisture inside the shoe increases over time. Moisture usually increases test results. There are other factors associated with changes in the sole or insole.