ESD Personnel grounding

Still up to today, people remains as the main culprit contributing the most in destroying the sensitive electronic chips used in electronic equipments and gadgets. Because of this reason, ESD Association established an ANSI/ESD S20.20 standard to provide guidelines to organizations across the globe on the protection of their ESD sensitive items from ESD damage greater than or equal to 100 Volts Human Body Model (HBM).

When a person walks, electrostatic charges continuously build up on the person body through the contact and separation process that takes place between the person’s shoe sole and the flooring material.

The electrostatic charges start to discharge from the person when he touches a conductive material. That explains the reason why a person would feel a jolt when he touches a conductive door knob in a cold and dry weather.. Static jolt is felt when static electricity of at least 3000 Volt build up in a person body. But a mere 10 Volts can kill a Tunneling Magnetoresistive head(TMR)straight away without the person knowledge.

In any organization, ESD personnel grounding are given the most attention.
There are two common ways where ESD personnel grounding are controlled:
• Wearing an ESD wrist strap
• Wearing an ESD shoes

ESD wrist strap
When wearing an ESD wrist strap, the wearer has to ensure that the wrist strap band is making an intimate contact with his bare skin. This is to ensure the wearer is really bonded to the ground via the wrist strap.
A good wrist strap usually comes with a 1 Megohm +/- 10%, ¼ Watt resistor to protect the wearer from the hazardous current or electrical shock.
The ESD wrist strap is used to drain the electrostatic charges from the wearer body to the ground.
The lower the resistance measured from the wearer to the ground when the ESD wrist strap is in use, the lower the body voltage would be. This is because the electrostatic charges flow faster to the ground when the total wrist strap resistance is lower.

The total wrist strap system resistance when wrist strap is in use has to be less than 35 Megohm to get a body voltage less than 100 Volts.

ESD shoes
Before buying any ESD shoes, the organization has to ensure that the esd shoes are evaluated with the presence of the ESD flooring. This is because the esd shoes and esd flooring has to work hand in hand like husband and wife to ensure full effectiveness.
To get a body voltage less than 100 Volts, the total system resistance from the wearer through the esd shoes and esd flooring to the ground shall be less than 35 Megohm.

If the organization has a total system resistance greater than 35 Megohm but less than 1000 Megohm, this condition is still acceptable as long as the body voltage of a person is less than 100 Volts.

When an organization is dealing with a more sensitive device which can be easily damaged by a 50 Volts human body voltage, then a total system resistance of the ESD personnel grounding device when in use to ground must be less than 10 Megohm.

Related Posts :