There is often confusion between the terms ESD and antistatic, and not just in safety 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 junction point of a person's body and gender. Therefore, electrostatic energy and contact resistance are important concepts in this field. However, a distinction must be made between the antistatic properties of footwear and their electrostatic conduction (ESD) capability.
What are antistatic properties?
The EN ISO 20345 standard defines various requirements for safety footwear, including requirements for electrostatic properties. It defines three grades based on contact resistance: conductive, antistatic, and electrically insulating footwear. For safety shoes marked S1, it is mandatory to meet the basic and additional requirements for anti-static. Shoes are considered antistatic if the measured contact resistance is 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 calls for the use of anti-static shoes to prevent the build-up of static electricity and ensure efficient 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 atmospheres.
Therefore, the main purpose of using antistatic shoes is to protect personnel from the dangers associated with the accumulation of static electricity.
What is ESD?
ESD 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, another standard applies, which 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 threshold of contact resistance is 100 kΩ and the upper threshold is 35 mΩ (3.5 x7Ohm). This means that ESD-compliant shoes are always ESD-compliant, but not all ESD-compliant shoes are ESD-compliant. For example, if the contact resistance is 100 mΩ, the shoe is antistatic but does not meet ESD requirements. However, if the contact resistance is only 1 mΩ, then the shoe is both antistatic and ESD compliant.
Since ESD is a product protection standard, 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 they are marked as S1, they are usually only antistatic.
Methods of measurement and factors influencing the results
Testing the anti-static properties of shoes for certification includes a test method in the laboratory. Before passing the test procedure, footwear must first be aged 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 a contact resistance measuring device via a copper cable. Shoes are placed on a copper plate that serves as an external electrode. A test voltage of 100 V DC is applied between the copper plate and the steel balls for one minute while fixing the contact resistance value of the shoe sample. It must be greater than 100 kΩ, but less than or equal to 1 gΩ.
Testing the ESD capabilities of shoes is a little trickier because there are many different measurement procedures for both standards. For EN 61340-5-1, the contact resistance value is measured for the man-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 is less than 35 mΩ, then the shoe is ESD compliant.
The second standard, EN 61340-4-3, defines contact resistance in the laboratory. The tested sample of footwear is preliminarily kept at a given temperature and air humidity in the conditioning chamber.
What factors can affect the contact resistance for shoes?
Sometimes it may happen that shoes that have been labeled 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 ESD requirements, as there can be many reasons that can affect the result. For example, shoe temperature can affect contact resistance. Safety shoes left in the car overnight in winter will cool down so much that the contact resistance will be higher as a result. Similarly, the degree of wear can be a contributing factor, as moisture levels inside the shoe increase over time. Moisture usually improves test results. There are other factors associated with changes in the outsole or insole.