The importance of Personnel Grounding Testers
We already know that in an ESD Protected Area (EPA) all surfaces, objects, people and ESD Sensitive Devices (ESDs) are kept at the same potential which is achieved by using ‘groundable’ materials that are then linked to ground. We have also learnt that the most common personnel grounding device to ground people to ground are wrist straps. People who are moving around should instead wear ESD footwear. So how do you know if your wrist straps and ESD footwear are working properly? Excellent question! And one we’ll answer with today’s post so let’s jump right in!
1. Purpose of Personnel Grounding Testers
Wrist Straps and ESD footwear should be part of your Verification Plan. Each component in an EPA plays a vital part in the fight against electrostatic discharge (ESD). If just one component is not performing correctly, you could damage your ESD sensitive devices potentially costing your company thousands of pounds. The problem with wrist straps and ESD footwear is that you can’t always see the damage. Just by looking at the items you would not know if they still provide sufficient protection. That’s where personnel testers provide feedback. They verify the functionality of an operator’s wrist strap and/or footwear and can determine if a person’s wrist strap and/or footwear function correctly.
Your Personnel Grounding Checklist
Your Personnel Grounding Checklist:
- Both wrist straps and footwear need to be tested at least daily before handling ESD sensitive devices and should be worn while checking.
- Verify your personnel grounding system using a wrist strap and/or footwear tester.
- Remember that a record of each test should be kept for quality control purposes.
- ONLY handle ESD sensitive components if your wrist strap and/or footwear pass(es) the test.
2. Types of Personnel Grounding Testers
Broadly speaking, personnel grounding testers can be purchased in two configurations:
- Wrist strap tester and
- Wrist strap and footwear tester.
As wrist straps are the most commonly used personnel grounding device to ground operators, you will find a lot of testers on the market that check wrist straps only.
As the name suggests, combined wrist strap and footwear testers will verify your wrist straps AND footwear.
In addition to WHAT they test, you will also be faced with a wide range of devices differing in HOW they test. Below you will find a (by no means complete) list of options:
- Continuous and split footplate: You will find testers with a continuous footplate which require each foot to be tested separately one after the other. Dual-footplate or independent footwear testers feature a split footplate which allows the unit to verify both feet independently at the same time. This can be a huge time-saver if you have a number of operators in your company who are required to check their personnel grounding devices.
- Portable, wall-mountable and fitted testers: Portable battery-powered (predominantly) wrist strap testers are perfect for small labs or for supervisors to spot-check workers and ensure compliance. Wall-mountable units are generally supplied with a wall plate which attaches to a wall; the tester is then mounted on to the wall plate. Some personal grounding devices are accompanied by a stand (and built-in footplate) which allow for a more freely positioning of the unit within a room.
- Relay terminal: A few testers on the market are fitted with a relay terminal that can be integrated with electronic door locks, turnstiles, lights, buzzers, etc. This can be of advantage if companies only want to allow personnel in an EPA that have passed their wrist strap and/or footwear test.
- Data acquisition: A growing number of personnel grounding devices allow for test activity data to be logged in a database. The units link to a computer which records operator identification, test results, resistance measurements, time and more. Paperless data can enhance operator accountability, immediately identifying problems while reducing manual logging and auditing costs.
Example of a Data Acquisition Tester – more information
3. Operation of Personnel Grounding Testers
Wrist strap testing:
If you are not using a continuous or a constant monitor, a wrist strap should be tested while being worn at least daily. This quick check can determine that no break in the path-to-ground has occurred. 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 should be tested periodically. The frequency of testing, however, is driven by the amount of usage, wear and ESD risk exposure that can occur between tests. For example, what is the quantity of product handled between test periods? Typical test programs recommend that wrist straps that are used daily should be tested daily. However, if the products that are being produced are of such value that a guarantee of a continuous, reliable ground is needed then continuous monitoring should be considered or even required.” (CLC TR 61340-5-2 User guide Wrist Strap clause 220.127.116.11.4 Test frequency)
“The operator shall wear the wrist strap in the normal position and plug the free end of the cord into the test apparatus. The hand contact plate shall be pressed to verify that the wrist strap system resistance is within acceptable parameters. The test apparatus can be an integrated, commercially available tester or other Instrumentation that is capable of measuring resistance from 5,0 x 104 ohms to at least 1,0 x 108 ohms. The tester open-circuit voltage is typically between 9 V d.c. and 100 V d.c.” (EN 61340-5-1 Annex A Test method A.1 Measurement method for wrist strap testing).
Example of a Wrist Strap Tester – more information
If the wrist strap tester outputs a FAIL test result, stop working and test the wrist band and cord individually to find out which item is damaged. Replace the bad component and repeat the test. Obtain a PASS test result before beginning work. For more information on troubleshooting failed wrist straps, check this post.
If using a flooring / footwear system as an alternative for standing or mobile workers, ESD footwear should be tested independently at least daily while being worn. Proper testing of foot grounders involves the verification of the individual foot grounder, the contact strip and the interface between the contact strip and the operator’s perspiration layer.
“The operator shall stand with one foot on the conductive footwear electrode. The hand contact plate shall be pressed to verify that the person footwear system resistance is within acceptable parameters. The test shall be repeated for the other foot. The test apparatus can be an integrated, commercially available tester or other instrumentation that is capable of measuring resistance from 5,0 x 104 ohms to at least 1,0 x 108 ohms. The tester open-circuit voltage is typically between 9 V d.c.and 100 V d.c.” (EN 61340-5-1 Annex A Test method A.2 Measurement procedure for footwear testing).
Example of a Wrist Strap and Footwear Tester – more information
If the footwear tester outputs a FAIL test result, stop working, and test the foot grounder and contact strip individually to find out which item is damaged. Replace the foot grounder. Obtain a PASS test result before beginning work.