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!
The ESD Standard S20.20 requires “an ESD Control Program Plan that addresses each of the requirements of the Program. Those requirements include:
- Product Qualification
- Compliance Verification
- Grounding/Equipotential Bonding Systems
- Personnel Grounding
- ESD Protected Area (EPA) Requirements
- Packaging Systems
[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!
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