We offer “Anti-Static” non-marking wheels in 75mm (3”), 100mm (4”) and 125mm (5”) diameters that can be assembled into our M series castors. These wheels are designed for use in ESD (Electrostatic Discharge) environments as part of an effective ESD Control Program. Our wheels have a conductivity of < 1099 Ohms as recommended by ANSI/ESD S20.20-1999 for Mobile Equipment in Protected Areas (section 6.2.3)

When used with plastic expanding adaptors (codes ER/ES - see page 80-81 for details) provisions for electrical continuity between the castor and trolley will be required.

Further technical notes follow. These notes are summaries of complex matters originally prepared as background material for the internal use of Fallshaw staff. In allowing others to access them without charge we assume no legal liability.

Background:

Nearly everyone has experienced a static shock when touching a metal object (eg: motor car, door handle) after walking for a while, particularly if dressed in synthetic fibres and wearing synthetic shoe soles. That shock comes from a sudden flow of static electricity to earth. This is know as Electrostatic Discharge (ESD). If it is of a sufficiently high voltage, it can either ignite flammable vapours (eg. ether or chloroform) or destroy sensitive electronic equipment. This can be prevented by ensuring there is always a pathway for the static electricity to leak slowly to earth rather than being restrained until it builds up to dangerous voltages and then surges to earth.

What causes charges to build up?

Basically when electrons are brushed off a moving surface, leaving it electrically unbalanced. Air can cause this on moving motorcars, but within buildings the normal cause is clothing fibre brushing against itself, or nearby furniture or flooring.

How high does the static voltage get?

This depends on the balance between how much is being added in, and how much can leak away. But as an example, a person clothed in a nylon uniform and wearing synthetic shoe soles, and on a hot day with low humidity, has been measured at in excess of 50,000 volts.

What can be done to reduce static being formed?

To reduce the build up it is better if natural fibres can be used. If material used for uniforms, clothing, sheets etc, can have (al least) the majority of natural rather than synthetic fibres, this will reduce the build up, and because they are open fibres containing moisture, they allow the static charges to slowly move away rather than building up in a restricted area. The same applies to leather soles on shoes. All electrical equipment should be properly earthed.

 

What can be done to allow the safe dispersal of static charges (ESD)?

The two main types of danger from ESD are areas that contain (albeit only occasional or briefly) explosive vapours (eg. hospital operating areas or chemical laboratories) or areas containing electronic equipment (eg. computer rooms, control rooms etc.). Such rooms should have conductive floors installed so that the charges on people or trolleys can be dispersed. Specific work areas on benches etc. should have conductive mats installed. These are directly connected to earth. If the floors are adequately conductive, then as we move around, the static charges are constantly dispersed and do not build up to dangerous levels. Most conductive floors are made with either copper sheet or strip under conductive PVC or other plastic sheeting, which allows for adequately fast, but not surging, dispersal.

The Standard: ANSI/ESD-S20.20-1999

In this Standard the ESD Association recommends (section 6.2.3) that in a Protected Area the floor should have a resistance of less than 109 ohms.

If suitably conductive floors are installed do they need special maintenance?

Modern conductive floors if properly installed have a working life in excess of 20 years. BUT - they depend on the ‘earthing’ being maintained. Any changes to the surrounds of the floor that might disrupt the earth path must be guarded against and the floor must not be ‘waxed’ or treated with ‘protective layers’ that will increase the resistance. Cleaning is best done with ammoniated water, as most commercial cleaners contain ‘polish’. NEVER wax the floor, or apply a ‘protective coating’.

What about the castors and wheels for use in ESD Protected Areas?

Once you have a conductive floor you need conductive wheels on the trolley. To ensure the wheels have a reasonable life the Standard recommends they have a resistance of of less than 1099 Ohms. Until recently the only way to ensure sufficient conductivity was to use activate carbon in rubber tyres. Unfortunately this marked floors and lost its conductivity over (about) 3 years. Recently additives have been developed that retain their conductivity for longer, and hold it at the recommended standard. Our ESD wheels have been developed to provide the necessary conductivity and provide a non-marking tyre with enhanced durability.

Installation of ESD wheels:

ESD wheels must be properly installed to ensure a conductive path from the trolley leg, through the stem, and the castor and the axle and the bearing, and through the centre and tyre. Solid metal pintles will provide such a conductive path. If plastic expanding stems are selected, they will need to be installed with a copper wire attached to the stem and wrapped between the adaptor and tube inner to ensure a conductive path.

What ESD wheels do Fallshaw offer?

We offer 75mm (3”), 100mm (4”) and 125mm (5”) wheels that can be assembled into our M series castors. Our ESD wheels are non-marking and have resistance of less than 1099 ohms as recommended by the Standard.