ANSI/ESD

Wrist straps are generally straight forward with what they do and how they work, but when it comes to foot grounders there is still a lot of confusion out there – something we want to address in today’s post. So, let’s get started.

Introduction
An Electrostatic Discharge (ESD) flooring / footwear system is an alternative for grounding standing or mobile workers. Sitting personnel are usually grounded via a wrist strap, but this method is not feasible for operators moving around in an ESD Protected Area (EPA).
ESD foot grounders are designed to reliably contact grounded ESD flooring and provide a continuous path-to-ground by removing electrostatic charges from personnel. ESD foot grounders are easy to install and can be used on standard shoes by placing the grounding tab in the shoe under the foot.
Per ANSI/ESD S20.20 Clause 8.2 Personnel Grounding: “For standing operations, personnel shall be grounded via a wrist strap or by a footwear/flooring system meeting the requirements of”:

  • the total resistance of the Footwear / Flooring system shall be less than 1.0 × 109 ohms
  • the maximum body voltage generation shall be less than 100 V.

Structure of Foot Grounders
Foot grounders discharge static from a person to ground by connecting the person to a grounded walking surface. A conductive ribbon placed inside the wearer’s shoe or sock makes electrical contact with the skin through perspiration. The ribbon is joined to a resistor which limits electrical current should accidental exposure to electricity occur. The other end of the resistor is joined to a conductive sole. The sole contacts a grounded ESD floor mat or ESD flooring system.

Structure of a Foot Grounder
Structure of a Foot Grounder

Foot grounders must be worn on both feet to maintain the integrity of the body-to-ground connection.
Wearing a foot grounder on each foot ensures contact with ground via the ESD floor even when one foot is lifted off the floor. This will more reliably remove static charges generated by human movement and more reliably protect ESD sensitive devices (ESDS).

Installation of Foot Grounders
1. Standard Style Foot Grounders
Standard D-ring heel grounders are equipped with an elastic D-ring fastening system which provides adjustable cinching of an ankle strap and allows “flex” during walking. They are designed for use on most types of shoes and boots.

  • Place the grounding tab in the shoe so that it will lay under the heel. Once heel is repositioned inside tied shoe, tuck excess ribbon material into side of shoe.
  • Place heel cup onto the shoe. For models with a non-marking interior, install so that the lined cup surface is making contact with the shoe.
  • Pull the strap through the D-ring and cinch down for snug, comfortable fit.
  • Test each heel grounder to confirm proper installation.
Installation of Standard Style Foot Grounders – more information
Installation of Standard Style Foot Grounders – more information

2. Cup Style Foot Grounders
Cup Style Foot Grounders are heel grounders designed for use on standard shoes and can be easily adjusted to fit the individual wearer.

  • Place the foot grounder on the shoe so that the lining is contacting the shoe.
  • Insert the grounding tab inside of the shoe and under the foot. Make sure that solid contact is made between the sock and body. Cut contact strip to desired length.
  • Fasten hook and loop straps together, securing the foot grounder firmly on shoe.
  • Test each foot grounder to confirm proper installation.
Installation of Cup Style Foot Grounders – more information
Installation of Cup Style Foot Grounders – more information

3. D-Ring Toe Grounders
Toe Grounders with the elastic D-ring fastening system are designed for use with a variety of men’s and women’s shoes including high heels, cowboy boots, flat shoes, loafers and safety shoes.

  • Insert the grounding tab inside of the shoe and under the foot. Make sure that solid contact is made between the sock and body. Cut grounding tab to desired length.
  • Place rubber toe material under toe area of shoe sole. Pull hook-and-loop strap over top of shoe and cinch down until snug. Install so that the lined surface is contacting the shoe.
  • Pull elastic strap around the back of the heel. Adjust D-ring plastic loop for a comfortable fit.
  • Test each toe grounder to confirm proper installation
Installation of D-Ring Toe Grounders – more information
Installation of D-Ring Toe Grounders – more information

4. Disposable Foot Grounders
Disposable Foot Grounders are designed for applications where the use of permanent foot grounders are not economical or practical. They are constructed so that it may be used once and then discarded.

  • Remove shoe. Wipe any excess dirt from underside of heel. Remove release paper from heel grounder.
  • Apply the adhesive end of the heel grounder to the underside of heel of the shoe. Wrap the tape snugly around the outside of the shoe.
  • Insert the non-adhesive end of the heel grounder inside the shoe so that the black dot is over the middle of the heel area facing upwards.
  • Put the shoe on.
  • Test each foot grounder to confirm proper installation.

NOTE: This product is not recommended for use on equipment with operating voltage.

Installation of Disposable Foot Grounders – more information
Installation of Disposable Foot Grounders – more information

Advantages of ESD Foot Grounders
ESD foot grounders are often preferred over ESD shoes for several reasons:

  • One size fits many foot sizes, reducing stock holdings and simplifying operations.
  • ESD foot grounders usually pass the mandatory resistance test as soon as worn, whereas some ESD shoes require a ‘warm-up period’ in order for the operator’s Resistance to Ground (RG) to drop below 35 megohms.
  • The operator is allowed to wear their own footwear, increasing their comfort in the workplace and not limiting footwear selection to available ESD shoe styles.
  • Less initial investment cost in comparison to ESD shoes if outfitting all operators in an EPA.

Disadvantages of ESD Foot Grounders
ESD foot grounders have a useful life that is dependent on the floor and it’s surface roughness, which can make them seem like they have shorter useful life in comparison to ESD shoes. However, there are a few simple tricks to avoid a quick ‘burn-out’:

  • We recommend ESD foot grounders only to be used indoors where floors are usually smoother (and where the ESD foot grounder is less likely to become wet, thereby short circuiting the resistor). The rougher the floor the greater the wear.
  • The manner in which the wearer walks can also affect the life span of the grounder.

In summary, with reasonable care and if used only indoors, ESD heel and toe grounders can last several weeks.

Testing of Foot Grounders
Proper testing of your foot grounders involve testing:

  • the individual foot grounder
  • the contact strip
  • the interface between the contact strip
  • the wearer’s perspiration layer

There are personnel grounding testers on the market designed to properly test foot grounders. For more information, check out our selection chart.
If you obtain a fail reading from the tester you should stop working and test the foot ground and contact strip individually to find out which item has failed. Replace the foot grounder or replace the bad component if possible. Retest the system before beginning work.

Ensure your Foot Grounders are working before handling ESDs
Ensure your Foot Grounders are working before handling ESDs

Cleaning of Foot Grounders
Foot grounders are used to ground static charges, however dirt provides an insulative layer adversely effecting reliability. For proper operation, the foot grounder and its conductive strip must be kept clean.
The rubber portion of the foot grounder should be cleaned using an ESD cleaner. Ensure that your ESD cleaner is silicone free. This is critical as silicone is an insulator. An alternative would be to clean using isopropyl alcohol. ESD cleaners should not be used to clean the nylon polyester grounding tab. Foot Grounders can be safely hand or machine washed on gentle cycle. Mild detergents, such as Woolite® or a liquid dish washing product used with warm water are recommended for cleaning, however care must be taken to ensure that these detergents are silicone free.

Conclusion

  1. It is recommended that ESD foot grounders are worn on both feet to ensure that a continuous path to ground is maintained at all times (even when lifting one foot).
  2. Contact strips should be tucked inside the shoe with as much contact area as possible to the bottom of the stockinged foot. ESD foot grounders rely upon the perspiration layer inside of the shoe to make contact through the stocking.
  3. Foot grounders must be used with an ESD protected floor system (such as properly grounded ESD floor finish, carpet tiles or floor mats) to provide a continuous electrical path from the user directly to the ESD ground.
  4. A current limiting of one or two megohm resistor in series with the contact strip is recommended but not required.
  5. ESD foot grounders should be tested independently at least daily while being worn to periodically test for proper grounding.

As we have learnt in our last blog post, wrist straps are the most common personnel grounding device to ground operators. In today’s blog post, we will talk about the different types of wrist straps and also explain how continuous monitors can support you in your fight against ESD.

What is a Wrist Strap?
A wrist strap is arguably the best way to provide a safe ground connection for the operator to dissipate accumulated static charges with the purpose to prevent dangerous ESD exposure to sensitive ESD components.
Wrist straps must be tested to ensure that they are installed and working properly.
On-demand or “touch” testers have become the most common testing method.

  • On-demand testers complete a circuit when the wrist strap wearer touches a contact plate.
  • On-demand testers require a dedicated action by the wearer of the wrist strap to make the test.
  • Knowing that the wrist strap has failed after the fact may possibly have exposed a highly sensitive or valuable assembly to risk.

Continuous monitors eliminate the possibility of a component being exposed to ESD during the period that the wrist strap was not working properly.

Types of Wrist Straps
A wrist strap in general is a conductive wristband which provides an electrical connection to skin of an operator and, in turn, by itself is connected to a known ground point at a workbench or a tool. While a wrist strap does not prevent generation of charges, its purpose is to dissipate these charges to ground as quickly as possible.

  • A single-wire wrist strap is comprised of one conductive surface contacting the wrist of an operator and providing one electrical connection to ground.
  • A dual-wire wrist strap has two electrically-separate parts and two separate electrical connections to ground combined in one cord.

For more information on wrist straps, we recommend checking out our last blog post here.

Example of a Wrist Strap
Example of a Wrist Strap

Both types of wrist straps – when in good condition and properly worn – provide equally good connection of operator to ground.

Wrist Strap Monitors
Monitoring of single-wire and dual-wire wrist straps is fundamentally different:

  • Single-wire wrist strap monitors do not have a return signal path; the only physical parameter they can rely on is parasitic capacitance of the operator’s body to ground.
  • Dual-wire wrist strap monitors measure the resistance of the operator’s wrist between the two halves of the wrist strap.

Single-Wire Wrist Strap Monitoring
1. AC Capacitance Monitors
The first constant monitors developed made use of the fact that a person can be thought of as one plate of a capacitor with the other plate being ground. The ground and the person are both conductors and they are separated (sometimes) by an insulator (shoes, mats, carpet, etc.) thus forming a capacitor. The combined resistance of the wrist strap and person forms a resistor so that the total circuit is a simple RC circuit. A tiny AC current applied to this circuit will cause a displacement current in the capacitance to flow to ground providing a simple way to make sure the person (capacitor), resistor (wrist strap) and coil cord are all hooked up. Any break in this circuit results in a higher impedance that can be used to trigger an alarm.
This technology is still around today and is purchased by some because of its low cost. A big plus of this technology is the ability to use any standard single-wire wrist strap.

770075-UseExample of a single-wire capacitance monitor – more information

2. Wave Distortion Monitors
What the wave form distortion monitor looks at is not the impedance level, but at the waveform generated by the circuit. Current will lead voltage at various points due to the combinations of resistance and capacitive reactance. (There is a negligible amount of inductive reactance from the coil cord.) By monitoring these “distortions” or phase shifts the monitor will determine if the circuit is complete i.e. the wearer is in the circuit and the total equivalent DC resistance is within specifications given a range of installations. Essentially, the unit will monitor the operator by sending a “signature” signal down the coil cord to the operator’s wrist. The operator acts as a load and will reflect that signal back to the monitor with a different signature. The monitor will then compare the reflected signature to its factory pre-set signatures. If the signal is within the “good” range, the operator passes and the monitor will continue its work. If the signature is “not” good, the monitor will go into an alarm-state to warn the operator to stop working and fix the problem.

Dual-Wire Wrist Strap Monitoring
A number of issues can come up when using single-wire monitors, such as:

  • They do not provide a reliable way to know if the total resistance of the circuit is too low, i.e., if the current limiting safety resistor is shorted.
  • Simple AC capacitance monitors can be tricked into thinking the person is wearing the wrist strap when they are not. For example, laying a wrist strap and cord on a grounded mat will increase the shunt capacitance, which allows the monitor to show a good circuit even with the person out of the circuit. Forming the cord into a tight bundle or stretching it can also provide false readings.
  • Since the capacitance and therefore the impedance of the circuit will also vary with such things as the person’s size, clothing, shoe soles, conductance of the floor, chair, table mat, the person’s positions (standing or sitting), etc., these monitors often have to be “tuned” to a specific installation and operator.

Dual-wire resistance monitors were developed to overcome some of the problems with the AC capacitance types. By providing a second path to ground (without relying on the capacitor above), we can apply a tiny DC current to measure the DC resistance of the circuit. The monitor will alarm if that resistance goes too high (open circuit) or too low (the safety resistor is shorted).

  • If you are monitoring your dual-wire wrist strap and one wire fails, then the unit will alarm.
  • You will still be grounded by the other wire, so there will be a significantly reduced risk of damaging ESD sensitive components if you happen to be handling them when the wrist strap fails.
  • The wrist strap would still need to be replaced immediately if a wrist strap fails.

724useExample of a dual-wire pulsed monitor – more information

There have been some reports that a constant DC voltage applied to the wristband causes skin irritations. This has been addressed in some models by pulsing the test current and in others by lowering the test voltage.

Conclusion
While both single-wire and dual-wire wrist strap monitors help to dissipate accumulated charges on an operator, only dual-wire wrist strap solutions provide assurance of a proper dissipative path from operator to ground. Dual polarity technology provides true continuous monitoring of wrist strap functionality and operator safety according to accepted industry standards.

  • Dual-wire continuous wrist strap monitors ensure that the wrist strap is worn properly at all times. These units monitor proper connection of the operator to ground and alarm should this connection fail.
  • In critical applications, dual-wire systems have redundancy built-in to have a backup if the primary option fails. Two-wire monitors require two wires to create redundancy – this means that the wearer must wear a dual-wire two-conductor wrist strap / coil cord which are more expensive than standard single-wire wrist straps.
  • A two-wire monitor provides the same reliability as a touch tester and a simple, easy to understand measurement while eliminating the shortcomings of the AC capacitance monitors

For applications where sensitive components are being handled, the ability to guarantee that the wrist strap provides proper dissipation of charges on the operator is critical.

The share of dual-wire wrist straps in sensitive component handling is growing rapidly. Click here to view our range of dual-wire monitors.

 

If your company has an ESD Control Program per ANSI/ESD S20.20 in place, you need to define ESD protective packaging for ESD sensitive items (ESDs).
The Organization shall prepare an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include:
– Training
– Product Qualification
– Compliance Verification
– Grounding / Equipotential Bonding Systems
– Personnel Grounding
– ESD Protected Area (EPA) Requirements
– Packaging Systems
– Marking” [ANSI/ESD S20.20 clause 7.1 ESD Control Program Plan]

But where do you start? Don’t panic – we’re here to help and we’ll be following the guidelines set-out in the ESD Standard.

Definition and Purpose of ESD Protective Packaging
ESD Protective Packaging covers any materials coming into direct contact with ESD sensitive devices during handling, shipping and storage. You don’t need to worry about secondary or exterior packaging unless it’s used for ESD protection purposes.
Packaging for ESD sensitive items is commonly derived by modifying existing packaging to prevent the packaging itself from causing static damage. The packaging generally retains physical and environmental protective qualities. ESD protective packaging has been modified further to prevent other sources of static electricity from damaging a packaged item.“ [ANSI/ESD S541 Foreword]

The fundamentals of ESD control include grounding all conductors in the EPA. ESD packaging will have special material composition to lower the resistance so that when grounded, electrostatic charges will be removed to ground thus protecting your ESD sensitive devices inside.
Transportation of electrostatic sensitive devices requires packaging that provides protection from electrostatic hazards in the transportation or storage system. In the case of an EPA designed with continuous grounding of all conductors and dissipative items (including personnel), packaging may not be necessary.” [ANSI/ESD S541 clause 6. Packaging Application Requirements]

Example of ESD Packaging

Packaging is to be determined for all material movements inside and outside of the ESD Protected Area (EPA). Best practice is to define the required packaging or material handling item on the product’s bill of materials. Remember: the ESD packaging is just as important as a component part.

Customer contract packaging can take precedence, but otherwise “the organization shall define ESD protective packaging requirements, both inside and outside the EPA per ANSI/ESD S541 or in accordance with the contract, purchase order, drawing or other documentation necessary to meet customer requirements.” [ANSI/ESD S20.20 clause 8.4 Packaging]

Choosing your ESD Protective Packaging
Numerous factors need to be taken into consideration when choosing your ESD protective packaging including the “environment and device sensitivity.” [ANSI/ESD S541 Annex A.1 Environment and Device Sensitivity]
It is best recommended to follow these 6 steps:

    1. Understand the product sensitivity
      You can gather information about the ESD sensitivity of an item by either measuring it in-house, contacting the manufacturer of the product or by analyzing published ESD sensitivity data.
    2. Determine the distribution environment for the packaged product
      Knowing the environment in which the product is shipped and how it will be handled is extremely important. Humidity and temperature are the main factors to consider when it comes to choosing the right type of packaging for your ESD sensitive items. If items are susceptible to moisture, a barrier material should be chosen to prevent excessive humidity exposure. On the other hand, condensation may occur inside the packaging if temperatures vary around the dew point of the established interior conditions. In those instances, desiccant should be put inside of the package or the air should be removed from the package before shipment.

A Moisture Barrier Bag – click here for more information

  1. Determine the type of packaging system that is best suited for the intended application
    The first step is to choose low charging or static dissipative materials when in contact with ESD sensitive devices. Many companies also require the packaging to protect the contents from a direct discharge or exposure to electric fields. In addition to these requirements, there are further questions that need to be asked:

    • Returnable or reusable packaging?
    • Disposable or one-time only packaging?
    • Aesthetic requirements for packaging?
  2. Select and test packaging materials
    Test methods are explained in ANSI/ESD S541 and will classify packaging materials as conductive, static dissipative or insulative.
  3. Design a packaging systemOnce the ESD sensitivity and distribution environment have been evaluated and available materials have been selected, the design of the packaging system can begin. Per the ANSI/ESD S541, the following general rules apply:
    • Inside an EPA:
      Packaging used within an EPA (that satisfies the minimum requirements of ANSI/ESD S20.20) shall be:

      • Low charge generation.
      • Dissipative or conductive materials for intimate contact.Items sensitive to < 100 volts human body model may need additional protection depending on application and program  plan requirements.”
        [ANSI/ESD S541 clause 6.1 Inside an EPA]
    • Outside an EPA:
      Transportation of sensitive products outside of an EPA shall require packaging that provides:

      • Low charge generation.
      • Dissipative or conductive materials for intimate contact.
      • A structure that provides electrostatic discharge shielding.
        [ANSI/ESD S541 clause 6.2 Outside an EPA]

    Example of ESD Packaging

    In addition to these guidelines, there may be additional factors that should be considered, e.g.:

    • Cost/value relationship: The cost of the packaging compared to the total value of the contents is important. Some companies choose less expensive packaging for less valuable parts.
    • Handling: If rigorous handling is expected, cushioned packaging may need to be considered.
  4. Test the final packaging design for effectiveness
    It is highly recommended to subject packages to the type of hazards that can be expected during shipments. These tests can, for example, involve the following:

    • High voltage discharges to the exterior of the packaging
    • Simulated over the road vibration
    • Drop tests
    • Environmental exposure

Final thoughts on ESD Protective Packaging
Now that you have an understanding of the factors to consider when choosing your ESD Protective Packaging, you’re ready to implement the above guidelines. ESD packaging comes in all sorts of shapes and forms so bear in mind to not just look at bags when deciding what type of packaging to choose.
Also, remember that ESD packaging should be marked. We’ll cover the specifics in a later post.