Wrist Straps – Q&A – Part 2

Today’s blog post is a continuation of last week when we covered the basics of wrist straps – what they are, what different styles there are etc. Missed the first post? Catch-up here!

Today we’ll be taking a closer look at queries we receive in connection with using wrist straps. So without further ado, let’s jump right in!

What is the 1 megohm Resistor for?
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.

Can I connect to ground via a bench mat?
Many wrist strap users connect the 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 EN 61340-5-1 table 1.

Connecting a wrist strap to an Earth Bonding PointIt is recommended to connect a wrist strap to an Earth Bonding Point (EBP)

Can I buy a wireless wrist strap?
Passive “wireless” or “cordless” wrist straps have severe limitations. Assuming you were tribocharged to 10 KV and wearing the “wireless” wrist strap, it would take many hours (days even depending on the ambient relative humidity) to get you below 5 KV, nevertheless 10 Volts. Most (if not all) of the charge reduction would be due to natural recombination of the charges on your skin with the air molecules and the natural conductance of the air through water vapour content.
At this point there is no “cordless” wrist strap system on the market that works at all. In fact “cordless” wrist straps will damage ESDS devices because you will not be grounding your body.

Do I need to wear a wrist strap on both hands?
Good question, since the concept of grounding an individual is not very intuitive. The skin is one of the largest organs of the human body. The resistance of the human body is averaged to 1,500,000 ohms (or 1.5 x 103 ohms) from the ESD-STM5.1 [ESDA Standard for Human Body Model]. The resistance of an individual may vary from 1 kilohm to over 1 megohm. In either case, the skin is conductive in the sense it can conduct electrical current. Therefore, since the skin is (for the most part) continuous, i.e., the ball of your foot is electrically connected to your index finger, then grounding the skin at any point will in fact ground all of the skin. So you can in good conscious say that if you properly wear one grounded wrist strap, then both hands are grounded as well as other exposed skin areas.

Do wrist straps need to be tested?
Yes, 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 wrist band, 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.

Example of a Wrist Strap TesterExample of a Wrist Strap Tester

What should I do if I fail my wrist strap test?
If the wrist strap tester outputs a FAIL test result, stop working. Test the wrist band 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 coil cord with a new one and repeat the test.

If the operator and wrist strap system fails high:

  • Make sure the coil 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 coil 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-moisturise the skin thereby increasing its conductivity. Retest. Operators with dryer skin should wear metal banded wrist straps to minimise 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.

 

Did we miss any question(s)? Let us know in the comments!

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About Vermason

Vermason is a manufacturer of ESD protection products and was founded in Letchworth in 1979. Our mission is to maintain our position as the primary ESD solutions provider for the electronics industry in Europe. Vermason strive to manufacture quality ESD Control products with innovation, leadership and partnerships which deliver long term commercial benefits. We understand the many challenges our customers face regarding quality and reliability. We support these requirements with products and services of exceptional value which help them gain competitive advantages in their markets. We also appreciate that the control of ESD is one of many necessary links in a long chain required to bring a customer’s project to a successful completion. We endeavour to make that happen. We sustain our vision and mission by constantly seeking renewal via continuous education, application of new technologies and good business practices. Employees are expected to serve the customer with the highest level of technical knowledge in the industry.

Posted on 2016-05-05, in Articles, ESD Information, How-To, Question & Answers, Wrist Straps and Coiled Cords. Bookmark the permalink. 4 Comments.

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