Wrist Strap Tester

An Introduction to Wrist Straps

Image scsstaticcontrol November 30, 2017 3

We get a lot of inquiries regarding wrist straps: what they do, why there are different types, how they are used, etc. So, the purpose of today’s blog post is to answer all those questions for you. If there is something we did not cover in the blog post make sure you ask us in the comments!
Let’s get started!

Introduction
The ESD Standard S20.20 requires “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]

The most common personnel grounding device is a wrist strap which is used to connect people to ground.
A wrist strap in general is a conductive wristband which provides an electrical connection to skin of an operator and, in turn, 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. Wrist straps are required if the operator is sitting. They are not necessary if an operator is wearing two foot grounders on a conductive grounded floor and doesn’t lift both heels/toes at the same time. As some people lift both feet off the ground while seated, wrist straps are essential for sitting personnel.
A wrist strap is made up of two components:

a wristband that is worn comfortably around your wrist and
a coil cord that connects the band to ground.

Wristband and coil cord of a wrist strap

The key to the wrist strap is the intimate contact of the conductive band to the skin and of course the coil cord connecting to ground. It doesn’t matter if the contact point to your body is on your wrist, finger, forearm, ankle, etc., as long as it is in direct contact with your skin. The skin is electrically continuous over your whole body. The wrist is just a convenient place to couple the band to.

Styles of Wrist Straps
Operators can choose between elastic and metal wristbands:

Elastic wristbands are the most popular wristband as they are comfortable to wear and easy to adjust. Compared to metal wristbands they are also less expensive.
Some people prefer metal wristbands as they are generally longer lasting and easier to clean.

The key to personnel grounding is to have an adequate path to ground so that there is never a potential difference with respect to ground on the human body for longer than 150 milliseconds (ms) body movement time. Such rapid grounding is accomplished well by elastic or metal wrist straps. So, in terms of their effectiveness to protect against ESD, there is no difference between elastic and metal wristbands.

Both elastic and metal wristbands are (to a certain degree) adjustable. Metal wristbands offer less adjustment, so you will find those are generally available in different sizes depending on the circumference of your wrist. However, you are still able to adjust metal wristbands if you need a tighter/looser fit.
To adjust your wristband, follow the below steps:
1. Elastic wristbands:

Open the clasp by pulling upward on the “tail” of material that extends out from the clasp.
Tighten or loosen the elastic material through the clasp until the wristband fits snug but comfortably.
We recommend that you close the clasp and wear the band with the excess tail extended for a day to be sure the adjustment is snug, comfortable, and has the proper electrical contact with the skin before cutting.
Test the wrist strap system to be sure of proper electrical resistance and skin contact.
When you are ready to cut off excess material, mark with a pencil where excess material is to be trimmed.
Remove band from wrist. Open clasp. Cut off strip excess material about 1/4″ short of pencil mark so that the end of material is concealed by cap. This will eliminate the possibility of frayed ends.
Close clasp and use as a fixed elastic wristband.

Adjusting an elastic wristband

2. Metal wristbands:

Insert the link end of the wristband into the slotted opening on the cap. Insert it at a downward angle to allow the links to slide inside the channel in the backplate.
Change the size of the band by sliding the links in or out of the stainless steel backplate. For extra small size, you can cut off excess links with cutters.
Lock the links into place by pulling down on the band, seating the band securely over the lip on the edge of the backplate.
Test the wrist strap system to be sure of proper electrical resistance and skin contact.

Adjusting a metal wristband

1 megohm Safety Resistors
The purpose of the 1 megohm resistor found in series with wrist straps is solely to provide safety to the human body by limiting the amount of current that could be conducted through the body. The 1 megohm resistor is designed to limit the current to 250 microamps at 250 Volts rms AC. This is just below the perception level (and a bit before the nervous system goes awry) of most people. Physical perception of current traveling in/on the body varies depending on size, weight, water content, skin conditions, etc. Remember that the termination of the coil cord with the 1 megohm resistor must always be connected to the operator.
Such safety resistors are built into the wrist straps themselves and also in such wrist strap monitors as WS Aware, Iron Man® Plus and Ground Man Plus manufactured by SCS.

Typical Problems with Wrist Straps
Some of the typical problems with proper grounding of an operator using a wrist strap are:

worn out wrist strap which no longer has good electrical properties
stretched out wrist strap which doesn’t make good electrical contact with the skin
loosely-worn wrist strap which doesn’t make good electrical contact with the skin either
dry skin of an operator increasing electric resistance of a contact beyond specification
improper placement of a wrist strap, such as over the cuff of the garment

Also, another issue we often see is that wrist strap users connect their wrist cord to a stud on their ESD protective mat. This process is not recommended as it can increase the total system resistance to ground to over the 35 megohm limit required by ANSI/ESD S20.20 table 2.

Testing of Wrist Straps
Wrist straps need to be checked regularly to ensure they are faultless and ground the operator properly. Wrist straps should be worn while they are tested. This provides the best way to test all three components: the wristband, the ground cord (including the resistor) and the interface with the operator’s skin.
Wrist straps need to be checked before each use. Periodic testing is not required if continuous monitors are used. They provide instant feedback should the wrist strap fail while handling ESD sensitive devices.

Verifying a wrist strap using a wrist strap/footwear tester

If the wrist strap tester outputs a FAIL test result, stop working. Test the wristband and cord individually to find out which item is damaged. There are some methods to troubleshoot your wrist straps. First make sure your tester is properly adjusted and calibrated.

If the operator and wrist strap system fails low:

Make sure that the person is not directly connected to ground via another path, i.e., touching a grounded metal structure.
The most common cause of a fail low is a shorted resistor in the wrist strap coil cord. Replace the coiled cord with a new one and repeat the test.

If the operator and wrist strap system fails high:

Make sure the coiled cord has a secure connection both the banana jack/socket to tester and the stud snap to wrist strap buckle.
Ensure there is continuity in the coiled cord (you can test with an ohmmeter).
Remove the wrist strap and hold the bottom part of the band tightly between the operator’s thumb and index finger and test. If the test fails high, the band may be soiled and needs cleaning or the buckle to band connection may be suspect. Either replace the band or clean and then retest.
If the above test is okay, then the skin of the operator’s wrist may be too dry. Apply ESD lotion to the wrist to re-moisturize the skin thereby increasing its conductivity. Retest. Operators with dryer skin should wear metal banded wrist straps to minimize the contact resistance. If their skin is very dry, application of an ESD lotion may be required as part of their donning process.

You need to obtain a PASS test result before beginning work.

Now that we’ve covered the basics of wrist straps, we will dive into the different types of wrist straps – but that will have to wait until next time as this post is already very long. Stay tuned!

Checking your ESD Control Products

Image scsstaticcontrol July 27, 2017 0

Last time we explained how to easily create a compliance verification plan and why it’s important to have one in place. Today’s post will elaborate on the subject of periodic verification and highlight common products in your EPA that should be regularly verified and more importantly how they should be checked.

Why periodic verification
Compliance verification is a requirement of ANSI/ESD S20.20:
“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]

Installed ESD Control products must be checked regularly to ensure they meet the required limits per the ESD Standard. “Compliance verification records shall be established and maintained to provide evidence of conformity to the technical requirements. The test equipment selected shall be capable of making the measurements defined in the Compliance Verification Plan.” [ANSI/ESD S20.20 clause 7.3 Compliance Verification Plan]

Below, you will find a list of the most common ESD Control Products in your EPA and how to test them:

Worksurface Matting
The purpose of ESD workbench matting is to ensure that when charged conductors (conductive or dissipative) are placed upon the surface, a controlled discharge occurs and electrostatic charges are removed to ground. However, this only occurs if the ESD worksurface is connected to ground. If the matting is out-of-spec, not grounded at all, or the stud on the mat has become loose or if the ground cord has become disconnected, charges cannot be removed.
Many companies use a daily checklist, which includes the operator having to verify that ground cords are firmly connected.
Remember to regularly clean your workbench matting to maintain proper electrical function. Do not use cleaners with silicone as silicone build-up will create an insulative film on the surface.
The company’s compliance verification plan should include periodic checks of worksurfaces measuring:

Resistance Point-to-Point (Rp-p) and
Resistance-to-ground (Rg)

A surface resistance tester can be used to perform these tests in accordance with ANSI/ESD S20.20 and its test method ESD TR53; if these measurements are within acceptable ranges, the worksurface matting and its connections are good.

Wrist Straps
As discharges from people handling ESD sensitive devices cause significant ESD damage, the wrist strap is considered the first line of ESD control.
Before handling ESD sensitive items, you should visually inspect the wrist strap to see if there are any breakages etc. The wrist strap should then be tested while worn using a wrist strap tester. This ensures all three components are checked: the wrist band, the ground cord (including resistor) and the contact with the operator’s skin. Records of each test should be kept. Wiggling the resistor strain relief portion of the coil cord during the test will help identify failures sooner. Analysis and corrective action should take place when a wrist strap tester indicates a failure.

It is recommended that wrist straps are checked at least daily. An even better solution to daily wrist strap checks is the use of continuous monitors. They will alarm if the person or worksurface is not properly grounded.

A note on worksurface matting and wrist straps: if you are using common ground points to ground the operator and/or work surface matting, remember to measure resistance to ground regularly as well (every 6 months for example).

Floor Matting
A flooring / footwear system is an alternative for personnel grounding for standing or mobile workers. Foot grounders quickly and effectively drain the static charges which collect on personnel during normal, everyday activities. Foot grounders should be used in conjunction with floor surfaces which have a surface resistance of less than 10¹⁰ ohms.
As ESD floors get dirty, their resistance increases. For optimum electrical performance, floor matting must be cleaned regularly using an ESD mat cleaner. Do not use cleaners with silicone as silicone build-up will create an insulative film on the surface.
Dissipative floor finish can be used to reduce floor resistance. Periodic verification will identify how often the floor finish needs to be applied. As the layer(s) of dissipative floor finish wear, the resistance measurements will increase. So, after some amount of data collection, a cost-effective maintenance schedule can be established.
Floor matting can be checked using a resistance meter. A surface resistance meter is designed to measure resistance point-to-point (Rp-p) or surface to ground (Rg) in accordance with ANSI/ESD S20.20 and its test method ESD TR53.

Footwear
ESD Shoes or foot grounders play an essential part in the flooring/footwear system.
Before handling ESD sensitive devices, visually inspect your ESD footwear for any damage. Just like wrist straps, footwear should be checked while being worn using a wrist strap/footwear tester.

Records of each test should be kept. Analysis and corrective action should take place when a footwear tester indicates a failure. Footwear needs to be checked daily.

ESD Packaging
Re-using shielding bags is acceptable as long as there is no damage to the shielding layer. Shielding bags with holes, tears or excessive wrinkles should be discarded.

An operator packing an ESD sensitive item into a Shielding Bag — *Make sure your ESD shielding bags are un-damaged*

It is up to the user to determine if a shielding bag is suitable for re-use or not. The testing of every bag before re-use is not practical. Many companies will discard the shielding bag once used and replace it with a new one. Others will use a system of labels to identify when the bag has gone through five handling cycles:

Non-reusable labels are used that require the label be broken to open the bag.
The bag is then resealed with a new label.
When there are five broken labels, the bag is discarded.

The same principle applies to other ESD packaging, e.g. component shippers.

Ionizers
Ionizers are intended to neutralize static charges on insulators thereby reducing their potential to cause ESD damage. However, poorly maintained ionizers with dirty emitter pins and out-of-balance ionisers can put a charge on ungrounded items.
Remember to clean ionizer emitter pins and filters regularly. You can now even purchase ionizers that will alarm when emitter pins need to be cleaned or the ionizer is out of balance.

Charge plate monitor and static decay measurements using 963E ionized air blower

Static neutralization (the ability to reduce or eliminate a charge on a surface) is an important quality for ionizers. Static decay time is defined as the time interval needed to reduce a defined voltage potential on an object to a defined lower potential by means of applied ionized air. Another important aspect for ionizers is the ability to produce a balanced stream of positive and negative ions. A charged plate monitor or equivalent can be used to accurately measure both of these parameters.
For more detailed information on measuring the performance of ionizers refer to the ESD standard ANSI/EOS/ESD-S3.1 for Protection of Electrostatic Discharge Susceptible Items-Ionization.

Wrist Strap/Footwear and Resistance Testers etc.
So, you check your wrist straps and/or footwear and workbench and/or floor matting regularly. But have you remembered the testers themselves? What good do all the checks do, if the testers you use are out-of-spec and show you incorrect results?
Yearly calibration is recommended – many manufacturers offer a calibration service or alternatively you can purchase calibration units from them and perform the calibration yourself.

There you have it – a list of the most commonly used products in your ESD Protected Area (EPA) that you should check on a regular basis.
Questions for you: Do you have a verification plan in place? If so, how often do you check your ESD protection products?