Wrist strap testing and care: why do snaps come loose and how do you test correctly?
A wrist strap only protects when the whole chain of skin contact, band, snap, coil cord with 1 MΩ resistor and ground point is intact. Test the system daily while worn, the pass window is 0.75 to 35 MΩ. Loose snaps are usually a fit problem between band and cord.
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What does a wrist strap system consist of?
A wrist strap system has four links: the band with a conductive inner surface, the snap as coupling, the coil cord with a built-in 1 MΩ resistor, and the plug to the ground point. All links are electrically in series. If one fails, the person is no longer grounded, even though the band is visibly on the wrist.
Fabric bands with silver-coated fibres are adjusted to size, metal expansion bands keep good contact even on dry skin. The resistor in the cord limits current if the wearer touches a live part. The strap is still not electrical shock protection and is not intended for equipment above 250 V AC.
4 mm or 10 mm snap, banana plug or clip?
The band carries a 4 mm or 10 mm snap, and the cord coupling must match exactly. At the earth bonding point (EBP) the 4 mm banana plug is common, but some European countries do not allow it on ground cords, so a 10 mm snap is used there. Alligator clips slip off and have an undefined contact resistance.
Why does the snap come loose from the cord?
Unintended release happens when the holding force between snap and coupling is too low. The cause is usually geometry: on stamped, riveted snaps head and neck diameter barely differ, so twisting and pulling drag the coupling over the head. Snaps machined from solid stock hold considerably better.
The risk is that a disconnect is often noticed only minutes later, and the person works ungrounded until then. The breakaway force should resist movement at the bench but give way when standing up. ANSI/ESD S1.1 specifies 1 to 5 lbf (about 4.5 to 22 N), and IEC 61340-4-6 also includes a breakaway test.
- Prefer bands with machined stainless steel snaps.
- Use band and cord from the same system, mixed combinations often fit only marginally.
- Choose a cord length that is not under tension during normal movement.
- Replace worn couplings.
Which faults typically occur on wrist straps?
Most failures affect the cord and the skin contact, the band less often. Typical faults are a cord break at the strain relief, a loose or dirty band, dry skin and incorrectly connected cords. Many only appear when the wearer moves.
| Symptom | Cause | Remedy |
|---|---|---|
| Cord break at strain relief | Repeated bending at coupling or plug | Replace cord, check routing |
| Reading high, band loose | Band too wide or dirty | Readjust, clean, replace |
| Reading high, band fine | Dry skin | Approved ESD hand lotion |
| Reading too low | Resistor bypassed or missing | Remove cord immediately |
| System high despite good parts | Cord clipped to mat edge | Connect to the ground point |
If the cord is clipped to the edge of a bench mat, the mat resistance adds up and 35 MΩ is quickly exceeded. The cord belongs in a socket at the ground point. Mats and wrist straps are each wired there individually, never daisy-chained.
How do you test a wrist strap correctly?
The complete system is tested while worn: band on the wrist, cord snapped on, plug in the tester, finger on the contact plate. The measurement thus covers all links together. EN 61340-5-1 requires less than 3.5 x 107 Ω for a person grounded via wrist strap, and its Annex A describes the method.
The upper tester limit of 35 MΩ matches the standard, the lower one, typically 0.75 MΩ, detects a cord without a working resistor. The instrument should cover at least 5 x 104 to 1 x 108 Ω.
- Put the band on snugly, snap on the cord, insert the plug into the tester socket.
- Press the contact plate and move the cord slightly to reveal intermittent contacts.
- Record the result with date and name.
What if the test fails?
Do not start work until a pass result is obtained. Check fit and plug, then test band and cord separately, replace the faulty link and test again. If both are fine individually, dry skin is usually the cause.
Footwear and wrist strap testersTesters for the daily check with upper and lower limits.
View productsHow do you look after band and cord, and how often are they checked individually?
Fabric bands lose conductivity through skin oil, sweat and incorrect washing. Wash them regularly with a mild liquid detergent at no more than 50 °C, without bleach and only with non-ionic softener. Cords and ground points are also measured individually at fixed intervals.
- Dry metal bands completely after cleaning, otherwise the links corrode.
- Wear a fabric band for a day with the excess end attached, then trim it so the end disappears under the cap.
- Issue bands to individual people, which makes wear traceable.
Cords and ground points are measured individually about every six months: the end-to-end resistance and strain relief of the cord, and the ground point against protective earth, since even a correctly installed socket can lose its earth conductor through corrosion. The tester is calibrated annually. See ESD test intervals and wrist strap or heel strap.
Frequently asked questions
How often must a wrist strap be tested?
A wrist strap used every day is tested every day while worn, usually at the start of the shift. Cords and ground points are also measured individually at intervals set by the ESD control programme.
Why does the test fail high even with a new band?
Usually the band is too loose or the skin is very dry. Tighten the band and measure again. If the reading stays too high, an approved ESD hand lotion helps. Only then test band and cord separately.
May the cord be clipped to the bench mat?
No, not to the edge of a dissipative mat. The mat resistance adds to the system and can exceed 35 MΩ. The cord belongs in a socket at the ground point; use the mat snap only if it is explicitly designated as the ground point.
Does a wrist strap protect against electric shock?
No. The resistor does limit current, but the strap is meant against electrostatic discharge. It is not used on equipment above 250 V AC, and electrical safety rules continue to apply.
Looking for wrist straps, cords or testers?
ESD wrist straps, coil cords and testers for the daily personnel check according to EN 61340-5-1.
Standards-based
Testing to IEC 61340-4-6 and EN 61340-5-1.
Measurable
Pass window of 0.75 to 35 MΩ.
Documented
Daily and individual tests recorded.
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