There is often confusion between the terms ESD and antistatic, and not only in matters related to protective footwear. Although both terms refer to contact resistance, there are fundamental differences between them. Interesting? Now we will try to explain the difference to you.
Shoes usually represent the point of connection between the human body and the floor. Therefore, electrostatic energy and contact resistance are important concepts in this area. However, one should distinguish 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. He defines three gradations based on contact resistance: conductive, antistatic and electrically insulating footwear. For safety shoes marked S1, compliance with the basic and additional requirements for antistatic is mandatory. Footwear is 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 that they are electrically isolated.
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 parts under voltage, as well as from sparks, which are a dangerous factor in explosive environments.
Therefore, the main purpose of using antistatic footwear is to protect personnel from the dangers associated with the accumulation of static voltage.
What is ESD?
Electrostatic voltage and personal safety are not the only priorities in the industry, there is also a need to protect and control equipment components.
In this case, the second standard applies, which concerns electrostatic discharge (ESD): EN 61340-5-1 Protection of electronic devices from 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 antistatic, but not all antistatic footwear meets ESD requirements. For example, if the contact resistance is 100 mΩ, the shoe is antistatic, but does not meet the requirements of ESD. However, if the contact resistance is only 1 mΩ, then the footwear is both antistatic and meets the requirements of the ESD standard.
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, the 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 recognized as antistatic.
Measurement methods and factors influencing the results
Testing of antistatic properties of footwear for certification includes the method of testing in laboratory conditions. Before going through the test procedure, the shoes must first be kept for a certain period of time in certain conditions (temperature, humidity). Then the shoes are filled with a total mass of 4 kg of stainless steel balls, which are connected to the device for measuring contact resistance through a copper cable. The shoe is placed on a copper plate that serves as an external electrode. The test voltage of 100 V direct current is applied between the copper plate and the steel balls for one minute while fixing the value of the contact resistance of the shoe sample. It should exceed 100 kΩ, but be 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 value of contact resistance is measured for the system "man - shoes - ground". 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 is less than 35 mΩ, then the footwear meets the requirements of ESD.
The second standard, EN 61340-4-3, defines contact resistance in laboratory conditions. The tested shoe sample is preliminarily kept at a given 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 conducted by the client. This does not necessarily mean that the footwear does not meet the requirements of ESD, as there may be many reasons that can affect the result. For example, the temperature of the shoes can affect the contact resistance. Protective footwear left in the car during the night in winter will cool down so much that the contact resistance will be higher as a result. In the same way, the degree of wear can be an influencing factor, as the level of humidity inside the shoe increases over time. Moisture usually improves test results. There are other factors associated with changes in the sole or insole.
