ESD smocks are required to be both static dissipative as measured on the Ohms Scale and grounded with other materials (like mats) at a workstation.
1. Surface Resistivity vs. Surface Resistance
A garment is considered “static dissipative” if it measures above 1 million ohms (or 1 x 106 ohms) per square, and below 1 trillion ohms (or 1 x 1012 ohms) per square.
However, we cannot physically measure surface resistivity per square, so we apply electricity through ESD smocks to measure their surface resistance in just ohms.
Theoretically, surface resistivity is ten times greater than resistance. For example, a material that measures 108 ohms in surface resistance should measure 109 ohms / square in surface resistivity. For testing purposes, per ESD TR20.20 , the correct unit of measurement is surface resistance (just ohms).
To measure surface resistance, we use what is known as an ESD meter. Meters can measure surface resistance from electrodes (the yellow cylinders) placed on a garment we are testing.
ANSI / ESD S20.20 2014 calls for us to perform different types of tests using the meter to determine how to classify ESD Smocks.
According to S20.20 garments are either “Static Control Garments,” “Groundable Static Control Garments,” or “Groundable Static Control Garment Systems.”
Meters typically display surface resistance in ohm values with a mantissa and exponent (power) of the number. For example, if the #8 exponent LED illuminates and the meter displays “9.87”, the measurement is 9.87 x 108 ohms of surface resistance.
2. Static Control Garments according to ANSI / ESD S20.20
A “Static Control Garment” has < 1 x 1011 ohms surface resistance measured point-to-point on the garment.
To perform this test the garment should be electrically isolated, (laying on an insulative surface). Then, place the electrodes connected to the meter on the sleeve panels of the garment. The meter will measure the lowest resistance path.
A “Groundable Static Control Garment” has < 1 x 109 ohms surface resistance measured point-to-groundable point. To perform this test, you will replace one of the electrodes that were on one of the sleeve panels with a grounded object.
A “Groundable Static Control Garment System” has < 3.5 x 107 ohms surface resistance measured point-to-groundable point with a person wearing it while connected by the wrist strap.
In ANSI / ESD S20-20 2007 this is mentioned in section “8.2 Personnel Grounding” in Note 2 from Table 2: “For situations where an ESD garment is used as part of the wrist strap grounding path, the total system resistance including the person, garment, and grounding cord shall be less than 3.5 x 107 ohms.”
ESD TR53‘s “Garments” section and “Personal Grounding with Garments” section covers the specifics of performing these tests.
3. Isolated Floating Conductor
ESD smocks are “groundable” because they are woven with conductive black carbon threading.
If an ESD Garment is not electrically bonded to mats, wrist straps, or floors that are grounded it becomes a floating lightning rod- an “isolated floating conductor” essentially.
The floating conductor issue is acknowledged by the ESD Association in ESD TR20.20-2008 section 5.3.13 ‘Garments’:
“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.”
4. Bonding an ESD Garment Electrically to a Grounding System
The standard used for creating a grounding system is “ANSI / ESD 6.1 ESD Association Standard for the Protection of Electrostatic Discharge Susceptible Items – Grounding”. As they explain in the foreword:
The single most important concept in the field of static control is grounding. Attaching all electrically conductive and dissipative items in the workplace to ground allows built-up electrostatic charges to equalize with ground potential. A grounded conductor cannot hold a static charge.
To satisfy 6.1, you have to connect yourself to ground. This typically means you are connected to an electrical outlet. Wearing a wrist strap bonded with an anti static mat together at a common point ground is the most common way to build an ESD safe workstation where you and the work surfaces are electrically bonded.
The key thing for you to remember is that the garment you are wearing cannot be isolated from the ground when you are handling electronics. If you are not wearing a wrist strap (say because you are wearing ESD gloves), you should have a connection for the garment through your body to ground by wearing foot grounders over your shoes. If your floor is not conducting static to ground you can lay mat rolls over it and connect mat grounds to the matting so that your floor mat is grounded to an electrical outlet.