There is a very common misconception that if a person is wearing a wrist strap, an ESD garment (ESD smock, ESD shirt, ESD coat etc.) is unnecessary. Operators falsely believe that any charge on a person and their clothes will find its way to ground via the wrist strap.
Today’s blog post will explain the importance of ESD smocks and why they should be considered for your ESD Protected Area (EPA).
Purpose of ESD smocks
The main reason people wear ESD smocks is to shield their insulative clothing and minimize the electric fields generated from their clothing. As we learned previously, all process essential insulators should be kept at a minimum distance of 12 inches from ESD susceptible items. Clothing fabric, particularly when made from synthetic fibres, is a significant charge generator. Non-ESD clothing fabric is an isolated charged insulator which cannot be grounded and the resulting charges can threaten ESD control.
An insulator (like clothing) will not let charges flow and holds the charge until either neutralized naturally over time (hours or days) or with an air ionizer (artificially under a few seconds). Until the charges are neutralized, your clothing may have several thousand volts that could suddenly discharge and damage nearby static-sensitive items.
The ESD Standard does not require ESD smocks, however they are a very practical solution for minimizing ESD events from a person’s clothing. ESD smocks can be an important step to demonstrating commitment to an ESD control program.
“Garments are intended to attenuate electrostatic fields that may be present on personnel clothing. The need for ESD protective garments is generally determined based on the sensitivity of the ESD items being handled where ESD control is a requirement.
While a person may be grounded using a wrist strap or other grounding methods, that does not mean that insulative clothing fabrics can dissipate a charge to that person’s skin and then to ground. Personnel clothing usually is electrically separate or isolated from the body.” [ESD TR20.20 Clause 5.3.13 Garments]
ESD smock properties
Most smocks are constructed of a dissipative material which incorporates texturized polyester and carbon nylon fibres. The conductive nylon fibres are woven in a chain-link design throughout the material, providing continuous and consistent charge dissipation.
ESD Smocks are an ESD protective product that should possess the following ESD control characteristics:
- Antistatic low-charging so they minimize the generation of electrostatic charges;
- Dissipative so when grounded they will remove charges to ground;
- Shielding creating a “Faraday Cage” effect so they will restrict charges generated on the user’s clothing to the inside of the ESD Smock and
- Groundable so the user can easily and reliably connect them to ground.
The majority of ESD smocks on the market are single-wire ESD smocks meaning they provide one electrical connection to ground.
The all-new SCS Dual-Wire Static Control Smocks are designed for use with dual-wire grounding systems. The smock’s dual-wire circuit loop closes when its two conductive cuffs are bridged by the skin on the wearer’s wrists.
- One conductive cuff is electrically bonded to the dissipative garment and one snap stud at the hip.
- The second conductive cuff uses an isolated conductive path to bond to a second snap stud at the hip.
When used in conjunction with a dual-wire continuous monitor, SCS Dual-Wire Static Control Smocks provide assurance of a proper dissipative path from operator to ground at all times.
For more information on the differences of single-wire and dual-wire systems, please review this post.
Installation and grounding of ESD smocks
Follow the directions below for proper installation and grounding of the ESD smock.
- Put on the smock, and close the garment by fastening all of the snaps on the front. Verify that no clothing is exposed outside of the smock.
- Fit the conductive knitted cuffs over the wrists. Ensure that the cuffs make contact with the skin. They should never be worn over the shirt sleeves.
- Ground the ESD smock. A popular way to ground an ESD smock is with a coiled cord either attached to a snap on the waist area of the smock or via a wrist strap snapped to the inside cuff of an ESD smock. If none of these methods are suitable, the smock should be grounded via the person’s wrist removing charges via ESD footwear to ESD protected flooring.
“After verifying that the garment has electrical conductivity through all panels, the garment should be electrically bonded to the grounding system of the wearer so as not to act as a floating conductor. This can be accomplished by several means:
- Ground the garment to the body through a wrist strap-direct connection with an adapter
- Ground the garment through a conductive wrist cuff in direct contact with the skin of a grounded operator
- Ground the garment through a typical separate ground cord, directly attached to an identified groundable point on the garment
- Garments should be worn with the front properly snapped or buttoned to avoid exposure of possible charges on personal clothing worn under the garment.“
[ESD TR20.20 Clause 188.8.131.52.6 Proper Use]
ESD Garments are a conductor and therefore should be grounded. If not grounded, the ESD smock can be a potentially threatening isolated charged conductor. If an operator is wearing a smock but is not electrically connecting the smock to either their body’s skin or ground, then charges on the smock may have nowhere to go or discharge to.
Testing of ESD smocks
Panel-to-panel conductivity is essential to ensure portions of the smock are not left as isolated charged conductors. A Resistance Test Kit can quickly measure resistance of the fabric and ensure panel-to-panel conductivity by placing five pound electrodes on different fabric panels.
To ensure that the fabric is low tribocharging, a Static Field Meter can be used to measure charges generated by causing contact and separation with other materials. In addition, the Static Field Meter can demonstrate shielding by measuring a charged object and then covering the charged item with the ESD smock. Being shielded, the measured charge should be greatly reduced.
“Static control garments that electrically bond to the test subject and provide a path to ground for the test subject (Category 3) shall be evaluated by all three methods:
- the resistance point-to-point test method (Fig Sa, 5b and 5c);
- the resistance point to groundable point test method (Figures 4, 6a, 6b and 7); and
- the system test to determine the resistance from the person, through the garment
groundable point of the garment to the groundable point, including the ground cord (Figures 8a and 8b).” [ANSI/ESD STM2.1 Clause 1.3.2]
Cleaning of ESD smocks
Smocks must be laundered periodically for proper operation. The proper method to clean a smock is to:
- Wash the garment by hand or with a standard household washing machine in cold or warm water.
- Only use non-ionic liquid softeners and detergents when laundering. Do not bleach your ESD smocks!
- Tumble dry with low heat or hang dry.
Do not use:
- Hot water
- Dry detergent
- Fabric softener
- Industrial laundry machines
Please also note that smocks should not be altered in any way. The smocks effectiveness is in fully covering the human body and street clothes – especially at the wrists and front of the body. Altering the smock in any way will nullify its effectiveness.
The typical useful and effective life of an ESD smock under normal wearing and recommended washing conditions is a minimum of 75 washings. Under the same conditions, SCS smocks will maintain their usefulness and effectiveness for a minimum of 100 washings