ESD Personal Grounding

In our ESD FAQ’s section we cover in depth what static is and how it is generated but in simple terms static is caused by two surfaces rubbing together and transferring electrons from one surface to the other, adding too many electrons to one surface and taking them away from the other.

When the surface with more electrons (charge) meets another surface that has a different level of charge, the charged surface tries to balance out and rapidly discharges the excess electrons to the surface with less charge (fewer electrons)– creating a static spark.

Actions that can contribute to generating charges include walking across a carpet (this can generate around 1,500 to 30,000 volts), clothes rubbing on each other or on skin, air flowing over a surface, handling equipment (an operator working at a bench can generate around 6,000 to 7,000 volts) and more.

ESDS devices can be damaged by charges of less than 1,000 volts. Some modern components can be critically damaged by as little as 30 volts. A shock of 6,000 volts runs the risk of doing serious damage. We cover the damage ESD can cause in great depth in our ESD FAQs.

A “static-safe” area is commonly referred to as an ESD Protected Area, or EPA. EPA’s involve multiple protection systems including personal grounding, ESD safe furniture and equipment, ESD flooring, EPA signs, ESD packaging and storage and ESD testing points.

Personal ESD Grounding Minimum Requirements

In ESD control there is a level of minimum competency that is required and this is mainly about grounding the operator, as people can potentially produce the most charge. There are specific and in-depth international standards on effective ESD control set out in EN61340 that cover how to assess the risk of ESD and how to protect against ESD but there is a minimum level of competency required. The minimum is simply to ground the operator and provide a safe, grounded workspace.

As a minimum, this consists of an ESD wrist strap, connected to an ESD bench mat, connected to an earth bonding point, connected to the building.

An ESD work matt has a static dissipative top layer and a static conductive layer on the underside. ESD mats are also equipped with at least two 10mm connection studs.

The ESD Wrist band worn by an operator connects to one of these studs via a “curly cord” with an inbuilt 1 meg resistor in the curly cord and in the ESD wrist band. The ESD matt then usually connects to an Earth Bonding Point (EBP) via its second 10mm stud. The EBP is usually mounted on a bench and then connect to a special yellow ESD grounding plug via a link wire.

The yellow ESD plug has an inbuilt 1 meg resistor as well and plugs into the buildings mains power sockets, connecting the whole system to the buildings earth.

Grounding Operators Further

Grounding a person with a wrist band to a bench mat to an EBP is all well and good whilst that person is standing or sitting at the work station. But what if operators need to move around the EPA, such as to get more ESDS devices to work on. Once they disconnect from the work station then the operator is no longer grounded but could still come into contact with ESDS devices. Moving around can also generate a static field effect which could potentially harm ESDS devices if they are not properly protected.

There are two layers to further ESD protection around an EPA. One is packaging and equipment which we cover in our ESD Work Place Grounding page, the other is further personal grounding.

To aid in personal ESD grounding there is ESD clothing, ESD heel straps and ESD footwear.

ESD Heel Straps

ESD heel straps (or heel grounders) are very common in EPAs. They are inexpensive, easy to use and fit pretty much any shoe, plus shoes are pretty much always in contact with surfaces that can dissipate static charges safely. Disposable heel straps are great for visitors as they are one use and very cheap. More permanent heel straps can be issued to each member of staff.

Heel straps work by having one end tucked into the wearer’s sock so they have contact with the skin. The heel strap uses the body’s natural moisture as a conductor for the static charge. A thin conductive strip runs through the heel strap. The last section is placed under the heel of the footwear and discharges the conducted static when it makes contact with the floor.

Heel straps do however have one flaw, dirt is a very good insulator and can stop a static charge from dissipating. Heel straps can quickly become dirty and damaged being stuck under the heel of a shoe all day, meaning as they are used their effectiveness as a grounding point is reduced to the point it is ineffective. The heel strap then needs to be replaced with a new clean one.

Heel straps are most effective when using an ESD floor.

ESD Garments

Clothing is a big generator of ESD charges and ESD static fields that can easily damage ESDS devices. Clothing has lots of constantly moving surfaces and is insulated from the skin – the bodies normal discharge point – by other layers of clothing. ESD Garments work by containing the normal clothing’s electrostatic field and have conductive fibres throughout their materials. If one of these fibres touches the wearer’s skin, usually via a collar around touching the wearer’s neck or from a cuff on the wearer’s wrists, then any charge the garment is carrying discharges into the wearer’s body and grounds via either the operators ESD wrist strap or ESD footwear.

Commonly ESD garments are lab coats (or smocks) T-shirts, polo shirts, fleeces and work trousers. But they can also come in many other forms such as Hi-Viz jackets, smart trousers and skirts, business shirts, beanie hats, jumpers and more.

ESD Footwear

ESD footwear comprises of a wide range of shoes, trainers, clogs and safety footwear with inbuilt ESD grounding properties, removing the need for heel grounders. For a shoe to be classed as ESD safe it needs to have an on-state (when it is being worn on a foot) resistance value between 1.0 x 105ohms and 3.5 x 107ohms to be compliant with EN61340.

ESD footwear works on the same principles as heel grounders but gives you a longer-lasting path to ground as even as the shoe wears it retains its ESD grounding properties.

ESD Shoes come in a range of styles including trainers, safety shoes, clogs (rated for food or medical environments) and stylish smart shoes. It is important to clean the shoes regularly to avoid build-ups of insulative dirt that will reduce the effectiveness of the shoes.

It is also important to understand that ESD shoes (and heel straps) work best on ESD safe floors (which are covered more in our ESD Work Place Grounding page.

Personal ESD testing equipment

With all control systems, it is important to ensure that they continue to function as intended. This is no different with ESD control in an EPA.

To ensure operators personal grounding equipment is still working as intended testing stations should be used to check the operator before they enter the EPA. These can take the form of simple test stations with a wrist band and footwear tester with a green light pass and red light fail display to automated turnstile systems that only unlock if the test is passed.

There are also continuous wrist band monitors that can be put inline in the ESD control system and provide a real-time indication of the state of the path to ground, if the path is broken or has to much resistance then an alarm will sound.

Summary

Proper ESD control systems are laid out in EN 61430 and require multiple layers to be fully effective. From ESD wristbands, mats, shoes and clothing to ESD flooring, ESD safe furniture and equipment. In order to properly implement effective ESD control measures it needs to be understood:

• Why you have to have it
• What products are required
• How they control ESD
• How they all work together
• How to monitor their effectiveness and use

Without this, ESD will continue to be a constant threat in the electronics industry.