A simple explanation of how a static charge may be discharged
Science may not be your strong point, but if you are interested in how a static charge may be discharged, read this very simple explanation.
When two materials come into contact with each other, static electricity is generated. Every material in the world is made up of electrical charges which are contained in the atoms of the material. There is a balanced amount of negative electrical charges (electrons) and positive charges (protons) in the universe which try to stay at an equilibrium state.
On the other hand, when you put two materials together, this will result in some of the charges moving from one of the materials to the other; therefore an excess of protons are left on one material and electrons on the other material. This means that one material will become negatively charged, and the other positively when these materials are moved apart.
When the charges are separated in this way, the static electricity is created.
You may also find that a static charge is created when it comes into contact with another charged object. Likewise, it may be induced by the proximity to other charged objects.
The principle of EN1149-5 is to ensure that there is sufficient conductivity in the material to allow charges to disperse hence why there is the requirement that the wearer is suitably earthed.
A fabric can discharge static electricity by dissipation due to its conductivity (slow non-hazardous discharge). This could be enhanced by conductive finishes applied to the fabric or by the presence of conductive fibres within the fabric. Otherwise, the discharge could arise if a sufficient charge is built up on the fabric. Moreover it can be discharged more rapidly when it comes into proximity with an earthed source across the airspace between the charged fabric and the earthed source.
The arcing (spark) may not originate from the fabric itself, but it may originate from unearthed conductive materials that comes into contact with the charged fabric. This is because it will act as a focus for the discharge – for example this could be a metal accessories or from the human body itself (the human body is an effective conductor); therefore it is important that persons are effectively earthed.
In moist conditions charge may dissipate to the atmosphere, faster than it can accumulate which is why the risk of static discharge is greater in dry conditions.
EN1149-1 measures that the fabrics have sufficient surface conductivity to allow charge dissipation.
EN1149-3 measures the time it takes for a charge to dissipate (T50), and the shielding factor (a percentage of the electric field strength which the fabric allows to pass through it).
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