DIFFERENCE BETWEEN ESD AND ANTI STATIC SAFETY SHOES

DIFFERENCE BETWEEN ANTISTATIC & ESD - SAFETY FOOTWEAR

THERE IS GREAT CONFUSION IN USER BETWEEN THE ESD & ANTISTATIC SAFETY FOOTWEAR. WHILE ALL ESD FOOTWEAR ARE ANTISTATIC BUT REVERSE IS NOT TRUE. THERE ARE FUNDAMENTAL DIFFERENCES BETWEEN THE TWO. LETS CLEAR THIS CONFUSION.

Safety footwear is generally the sole point of contact between the body & the floor. Electrostatic energy & contact resistance are therefore extremely important in this area. However, a distinction must be made between the antistatic properties of shoes & their electrostatic discharge (ESD) capability.

The applicable standard for safety footwear, EN ISO 20345 {Indian Equivalent IS 15298 (Part 2):2011}, specifies a variety of requirements, including for antistatic properties. It defines 3 areas, based on contact resistance: conductive, antistatic & electrically insulating (EH) footwear. For safety shoes to be labelled S1, they must fulfil the basic requirements plus the additional requirements for antistatic.

The same applies to all subsequent safety classifications, for both industrial & occupational shoes. Shoes are antistatic if the measured contact resistance is in the range between 100 Kohm (105 ohm) & 1 gigaohm (109 ohm or 1000 mega ohms). As per standard, if contact resistance falls below 100 Kohm, they are considered to be conductive, while a value higher than 1000 mega ohms means

The EU standard stipulates that antistatic footwear should be worn to prevent electrostatic build-up & ensure it is discharged effectively. This is essential to eliminate the risk of electric shock from electrical equipment or live parts as well as that posed by sparks, igniting flammable substances or vapours. The aim is therefore to protect those wearing safety shoes & coworkers from dangers related to electrostatic build-up.

What ESD denotes?

Electrostatic build-up & personal safety are not the only important considerations, as controlled discharge is also needed to protect components & equipment. Here, another standard comes into play that deals with electrostatic discharge (ESD) : EN 61340-5-1, protection of electronic devices from electrostatic phenomena. The ESD area defined in this standard represents a delimitation of the antistatic range indicated in safety footwear standard EN ISO 20345. The lower threshold of contact resistance is at 100 Kohm & the upper threshold is at 35 megaohm (3.5 x 107 ohm). This therefore means that all ESD shoes are always antistatic at the same time, but not all antistatic shoes are ESD. For example, if contact resistance is measured as 100 megaohm, the shoes are antistatic but do not meet the ESD requirements. However, if the shoe contact resistance is just 1 megaohm, then the footwear is both antistatic & ESD capable. As ESD relates to meeting the requirements for the product protection standard, the labelling must be separate from the CE marking. Consequently, safety footwear which complies with the standard features an additional yellow ESD symbol. If the shoes do not feature the special ESD symbol but are labelled S1, they are antistatic.

Testing methods & factors influencing results

Testing the antistatic properties of shoes for certification involves a test method under laboratory conditions. Before undergoing the test procedure, the shoe must first be conditioned over a defined length of time (in both dry & moisture-regulated atmospheres). The shoe is filled with a total mass of 4 kg of 5 mm stainless steel balls, which are connected to the contact resistance measurement device via a copper cable. The shoe is placed on a copper plate as the external electrode. The test voltage of 100 V DC is applied between the copper plate & steel balls for 1 minute while recording the resistance. This must exceed 100 Kohms, but be less than or equal to 1 gigaohm/100 megaohms

Testing procedure for the ESD capability of shoes differs for the two standards. For EN 61340-5-1, the contact resistance value is measured for the person-shoeground system & is tested by an employee standing on a ESD shoe testing Machines electrode while wearing the safety footwear. The resistance is measured when they place their hand on a metal plate. If the resistance is less than 35 megaohm but more than 100 Kilo Ohms, the shoes are ESD capable.

The 2nd standard, EN 61340-4-3, determines the contact resistance in a laboratory process. The test object is preconditioned at a predefined temperature & air humidity in a conditioning chamber. Measurement is carried out once the test object has been conditioned. The shoe is placed on a stainless steel plate (1st electrode) & a 2nd counter electrode is placed inside the shoe on the insole. Weight of 12.5 kg (+/- 2.5 kg) is then applied. A device measures the contact resistance between the two electrodes. It must be less than 100 megaohm for ESD certification.

Factors influencing contact resistance for shoes at user site?

Sometimes footwear which has been labelled as ESD fails the test carried out by the user on their ESD testing machine. However, this does not necessarily mean that the shoe is not ESD, as there may be a number of reasons that have affected the result. For example, the temperature of the shoe can impact the discharge capacity. Safety shoes which are left in the car overnight in winter would cool down so considerably that the contact resistance would be higher as a result. Similarly, the duration of wear can be an influencing factor, raising the moisture levels inside the shoes. Moisture generally raises discharge capacity. Alterations to the outsole or insole & whether contact areas are in any way dirty or painted or insulative may result in failure.