Electrostatic Solutions Ltd
Tel: +44 (0)23 8090 5600

Home
Site map
Seminars Training
ESD in electronics manufacture
Hazards & nuisance
Electrostatic Technologies
R&D services
Test and Measurements
ESD Answers
Links
Library
News & Events
Contact us
About us

This article explains how electrostatic charges build up on people to cause electrostatic shocks, damage to electronic parts or fire risks when handling flammable materials.

Why static charge builds up on people

See also Static shocks, and how to avoid them

Charge can often build-up on people and reach levels that give uncomfortable shocks, can damage sensitive electronic parts or give fire risks when handling solvents and other flammable materials. Dry air humidity encourages static charge build-up, and under dry external conditions the atmosphere in a building can become even drier. Electrostatic charge build-up can be far worse under these conditions. In the UK the worst case is often in winter (January to March) when cold and dry external air is warmed and brought into the building leading to very dry internal atmospheric conditions.

We can help you understand the causes and effects of static electricity in the workplace.

We can help you find the cause of static shocks, and possible solutions, with an Electrostatic Site Survey.

Please contact us to discuss a price and proposal.

There are a wide range of factors that can affect the amount of electrostatic charge that can build up as a voltage on personnel. Some common major factors include:

  • The floor material and its electrical resistance, and resistance-to-ground

  • Personnel footwear, especially the materials of the shoe sole and its electrical resistance

  • Atmospheric humidity

  • The manner in which a person walks, e.g. scuffing and friction of the shoes against the floor

  • Actions of brushing against furniture, sitting and rising from seats

Static electric charges are separated whenever two materials make and break contact. This occurs with regularity between the shoe sole and floor material in walking action. Charge tries to dissipate and recombine where possible, but this cannot happen if it is prevented from moving by insulating materials. Charge builds up if the charge is generated faster than it can dissipate, and a high voltage can quickly result.

Shocks are felt by people when they touch a discharge path (e.g. metal frame or another person), if the voltage on their body exceeds about 4 kV (4000 V).

Charge generation is affected by the types of materials in contact. A guide to this is given in a triboelectric series.

Rabbit fur

More positive

Glass

 

Human hair

 

Polyamide (nylon

 

Wool

 

Fur

 

Silk

 

Aluminium

 

Paper

 

Cotton

 

Steel

 

Wood

 

Rubber

 

Acetate rayon

 

Polyethylene (PE) and polypropylene (PP)

 

PET

 

PVC

 

Polyurethane

 

PTFE

More negative

Table 1. An typical triboelectric series

If a material in the series (e.g. wool) is rubbed against a material lower in the series (e.g.rubber) the higher material (wool) normally charges positive wheras the lower material (rubber) charges negative. The further the materials are apart in the table, the stronger the charging action is expected to be. So polyurethane soled shoes (quite common!) walking on a glass floor (not unknown!) is a recipie for static problems! Unfortunately a shoe sole (e.g. PU walking on PP) which gives low generation on one floor material may give high generation with another (e.g PU walking on glass or nylon)

Some shoe sole materials are commonly materials such as rubber and polyurethane (PU) are much lower in the table than wool and nylon which are high in the table. If a PU shoe is worn walking on a wool/nylon mix carpet, high charge generation is expected. Conversely if a PU shoe sole is walked on a polymer carpet or tile (such as PET or PVC) much lower charge generation is likely to occur. However in both cases, charge could build up under certain conditions.

The floor resistance-to-ground (Rg) gives a measure of how easily charge can dissipate to earth. Resistance values above about 109 Ω can be expected to promote electrostatic charge build-up on people and trolleys moving on the surface. Sometimes, if the floor material is identified as a major part of the problem, certain areas may be replaced or covered to provide lower charge generation and lower resistance to ground. In some cases a grounded mat can be provided. Unfortunately modification of the floor covering alone does not guarantee that charge build-up will be prevented, especially under dry atmospheric conditions. As previously explained this is due to many influential factors being outside control. If dry atmospheric conditions (less than 40% r.h.) are frequent it may be advisable to investigate humidification of the atmosphere. An optimum range is often found to be 40% - 60 % r.h.

Table 2 shows some floor materials that do, or dont, tend to give static problems in our experience. Please note that these general guidelines cannot be taken to apply to any specific material - if you want to know how your material really performs you have to test it under the conditions of usage! A lot may depend on whether the manufacturer has formulated the material with static electricity in mind - specially formulated polymer floors are used in static controlled areas in the electronics industry to reduce static electricity build-up.

Floor materials that often give static problems

Floor materials that don't so often give static problems

plastic tiles

concrete

polymer carpets

wood

wool carpets

cotton rugs

synthetic stone tiles

 

laminate imitation wood

 

glass

 

epoxy coatings

 

asphalt

 

Table 2. Floor materials that do, or don't, tend to give static problems

The same factors of charge generation and dissipation control whether furnishing materials cause static charge build-up. Again, highly insulating man-made fiber materials are common factors, but natural materials such as wool also can be. A person wearing a wool sweater sitting on a man-made fiber chair may charge to a high voltage when they leave their seat!

 

 

 

 

Stop ESD damage in electronics manufacture

ESD Seminars and training

 

Avoid fires and explosions in process industries

 

Prevent shocks to personnel in offices, retail and other workplaces

 

Develop electrostatic applications

 

Test and Measurement

 

The ESD Guide

Our on-line guide to achieving a successful ESD Program.