1. How Do ESD Smocks Work?
Answer: ESD Safe Smocks present a conductive, static shielding barrier between electrostatic discharges (ESD) emanating from your body and any electronics that you may be near. Made with static dissipative fabric these garments also resist becoming charged with static electricity.
In this article, I review the static shielding and static dissipative properties of ESD garments. For further information about testing or washing your ESD garments, please check out the articles below:
2. Static Shielding Properties of ESD Smocks
First, let’s look at the “static shielding” properties. Anything that is a conductor potentially serves a “static shielding” function. Conductors shield static because they attract electrons. Once the electron is on the conductor, it can be grounded by the conductor. Grounding electrons is what prevents static from accumulating on surfaces and ESD (electrostatic discharges of electrons). Conductors commonly used for grounding electrons are aluminum and carbon.
On ESD smocks carbon fabric is interwoven with cotton or polyester to make the garment act as a Faraday Cage. Much like a fence protects a yard, the grid pattern of the black carbon-loaded conductive fiber prevents static electrons on your body from leaping through the smock as electrostatic discharges (ESD) and zapping nearby electronics.
However, you have to be grounded for it to be “static shielding” instead of an isolated floating conductor.
The best way to ground the smock is to wear a wrist strap with it.
The wrist strap collects electrons off your skin and can be grounded by directly connecting it to a coil cord which is then clipped to a grounded surface or plugged into a wrist strap ground, common point ground (for mat and wrist strap), or a monitor.
“Hands-Free” Grounding Knit Cuff garments come with a dedicated ground sleeve that allows a wrist strap to be connected through the smock to a ground.
Once the smock is grounded, it is no longer an “isolated floating conductor.”
3. Static Dissipative Properties of ESD Smocks
ESD smocks are made of either cotton or polyester.
Cotton is preferred in warmer environments where its looser weave lets more air through to the wearer.
Polyester is preferred in cleanroom environments where its tighter weave prevents micron particles from slipping through it.
For determining if a garment is static dissipative we test it using an ESD Meter that shoots charges through the fabric and then measures the “ESD resistance” or “surface resistivity” of the fabric in ohms / square.
To meet the ANSI / ESD S20.20 standard an ESD smock has to meet 1 of 3 definitions (which are defined using ranges of “surface resistivity”). This is outlined in the ESD Association’s Standard Test Method (STM) 2.1.
Per ANSI / ESD S20.20, an ESD Garment is defined as either a Static Control Garment (surface resistivity of < 1 x 1011 ohms square); a Groundable Static Control Garment (surface resistivity of < 1 x 109 ohms square); or a Groundable Static Control Garment System (surface resistivity of < 3.5 x 107 ohms square).
If it is genuinely a Groundable Static Control Garment System it should be measured with the wrist strap. Per Section 8.2 of ANSI/ESD S20.20-2007 “Personnel Grounding” Note 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.”