FAQ on Flooring

 

 

Q:        What is surface resistivity?

A:         Is the measurement in Ωs of the electrical resistance over the surface of a body (not across the bulk) by two electrodes placed at the two ends of a square (a "sq" is clarified in the FAQ Miscellaneous page), where the surfaces are homogeneous the size of square is non-relevant.

 

Q:        What is understood to be a conducting surface?

A:         Is a solid surface of a material showing a resistivity  < 100 KΩ/sq (or a vol resistivity <10KΩ-cm  ANSI/ESD S11.11).

 

Q:        What is defined as a static dissipative surface?

A:         Is a solid surface of a material showing a resistivity =/> 100 KΩ/sq and =/< 1000GΩ/sq (or a vol resistivity in between 10 KΩ-cm to 100GΩ-cm  ANSI/ESD S11.11) [see ESD Assoc. Glossary]. 

 

Q:        How can I measure static charges on conductive surfaces?

A:         This is equivalent to the measuring of an electric potential or field strength.  May be used an electrostatic voltmeter  which is based in the principle of attraction of forces in between two vanes or electrodes, one fixed and the other movable and which are at different potentials. Issues as safety of the location and other influential parameters during measurement suggest that the services of a qualified NATA specialist may be required.

 

Q:        What is the best recommended solution to fight ESD in a room where people moves in all directions?

A:         The best solution would appear to be the laying of antistatic flooring and the provision of mandatory use of foot-straps on at least one of the two shoes. Maintenance of the antistatic flooring must be planned as to maintain it effective.

 

Q:      If a long roll of dissipative material is spread for persons to walk on, this in an aisle in between two rows of ESD sensitive equipment, how do I ground it?

A:    Usually the following of the manufacturer instructions is sufficient. However every location is distinct  due to local conditions like ambient temperature variations, relative humidity, etc... Makes common sense to use a surface megohmmeter with one surface type probe and a clip type, the clip is to be connected to ground. Proceed to measure away from the one earth point and see how the resistance to ground increases, should the resistance increase in a abrupt manner then multiple earthing points may need to be effected.   

 

 Q:        Are antistatic wrist straps unnecessary where there is an antistatic dissipative floor and foot straps are in use.

 A:         No. Antistatic wrist straps must be used always that an electronic device is being handled. There is a possibility of charges accumulation as only one foot strap could be used or if antistatic shoes are used as there is could be a moment when a person sitting may not be in touch with the floor.

 

Q:    Would wearing ESD dissipative footwear help when on an uncontrolled surface?

A:    No. Even when the body charges will make contact with such a floor there will be no trustworthy conductive path to dissipate the charges, hence there will be, possibly, a small spot charged on the floor surface with each step on the contact area, however the triboelectric effect of walking may recharge the body again. Use of ESD footwear on surfaces must be considered on their inter-dependability where both parts must be in harmony as to achieve elimination or control of the charges eg bad footwear on ESD uncontrolled surfaces is useless and vice-versa.

 

Q:    I need to provide conductive flooring on an aisle of ESDS equipment. What's available in floor mats?

A:    The most common conductive mats or floor runners are made of conductive rubber or polyethylene and are available from different manufacturers and in different design and shapes, both are to be grounded as specified by the manufacturer. They are physically quite resilient and their conductive resistance to ground is usually under 50KΩ (under 10KΩ is preferred, depending on application). Some rubber mats can be interlocked in between sections conserving their conductive properties,  you can make a walking path or some other shape with them.    

 

Q:        How can we reduce de nuisance of ESD discharges to staff of a carpeted small office?

A:        ESD manufacturers produce sprays that can be used on carpets which often will reduce dramatically the effect of static discharges on people. This is usually a known antistatic compound with some water mix added to thin it. Application to carpets must be strictly adhered to as per the manufacturer instructions as there will be different types of products for the many different types of carpets.

 

Q:    Given that raw or unsealed  concrete flooring exhibit a resistance to ground in the vicinity of 100MΩ, can this be categorised as a dissipative flooring and then proceed to use ESD footwear on it?

A:    No. Even though the concrete dissipating range, on average, may fall in the surface dissipating flooring definition it must be considered not different to an untreated floor. It cannot be used to control static discharges as concrete will not exhibit a constant resistance value, it may dry up or absorb moisture from the ambient and ground changing its electrical characteristics probably many times a day, or in the long term from, quite conductive initially to very resistive after a few years. Also is susceptible to to surface depositions of dirt and grime which will make it difficult for footwear to make appropriate constant to the floor.   

 

Q:        How to maintain a conductive or antistatic dissipating floor?

A:        Using mop and clean water every day should be enough adding a visual inspection for surface deterioration. However there are products which protect de electrical dissipative qualities of a floor surface allowing for more in-between days distanced cleaning. Never use common detergents or waxes as an insulating film will be deposited over the surface and the dissipative qualities will be rendered ineffective.

 

Q:        Which would be the approximate figure of a range of resistivity required on the surface of a floor where ESD sensitive equipment is to be installed?

A:        An appropriate range of surface resistivity would be under 1GΩ/sq

 

Q:    Can I just let staff to use normal shoes on a conductive or dissipative mat or floor?

A:    No. You must provide antistatic shoes or foot straps to go with such surfaces, otherwise there will be no control of the charges dissipating if any and the ambient will have a great influence, most common street shoes are manufactured of insulating materials and the leather sole ones provide not trustworthy control as humidity makes them vary widely in its conductivity.    

 

Q:        What type of ESD products are available for flooring work?

 A:        There is a product which is a spray to use while buffing a floor, clean the surface from dirt and more-or-less make good small surface defects like scratches, also protects the surface. Some are restorers, meaning that they can bring back the dissipative properties of a floor and also extend their useful life. Also there is a normal floor cleaner  with static dissipative properties which is usually indicated for tiling conductive flooring as in false floors. Also there is a floor stripper that you can use to get rid of previous ESD dissipative applications so you can start anew.

 

Q:    How antistatic floor sprays or floor finishing work?

A:    These compounds work in the principle of dramatically reducing the triboelectric phenomenon when a shoe separates from the surface during walking and consequently reducing the ESC process. 

 

Q:        Can we simply use a digital meter to measure the resistance of a surface to earth?

A:        Absolutely NO. Even tough a digital meter has a range from about 1MΩ to 1GΩ it will provide values in error as the connection points are inadequate to make surface contact and it can only use low voltages typically 3,5V to around 9V. The appropriate meter to use is a megohmeter which can provide 10V, 100V and 500V as to do an appropriate measurement. Also has got special contacts that look like small solid cylinders of some 6.4cm in diameter and weighting some 2.5Kg each.

 

Q:    Is the term antistatic exchangeable with dissipative?

A:    No. Antistatic is a material resilient  to charge generation due to triboelectricity, this term does not represent resistance or resistivity as the term dissipative does.

 

Q:    Why we use 2MΩ resistors in wrist straps and foot straps instead of the usual 1MΩ?

A:    There is a known reason. If a person wears one wrist strap with 1MΩ resistor and simultaneously one heel strap also with a 1MΩ resistor on a conductive type of flooring, the person a some point (standing on floor AND wrist strapped) is grounded with a net parallel resistance to ground of 0.5MΩ, this is considered marginal for a person's protection where mains is available. There can be many combinations of 1MΩ and 2MΩ resistors in operation but normally there is an aim for standardisation. The situation where a person uses two heel straps, one on each foot, and is standing with both feet on the surface, a 1MΩ resistor on each one, then the two resistors in parallel will give the effect of 0.5MΩ to ground. If we use 2MΩ resistor the end value of the parallel resistance will be 1MΩ. Further on, if we add the effect of a 2MΩ wrist strap to the equation the net resistance will be around 660Ω, which is not too bad of a protection. The foot wear resistance values are or not much significance when a dissipating type of flooring is used.

 

Q: How a dissipative or conductive shoe should be built in order to provide protection to the user against main's chocks?

A: If the surface flooring is in the dissipative ESD range then does not matter whether the making of the shoe has got too conductive qualities as the current limiting for main's chocks will be provided by the flooring, to be effective the shoe must be at least in the dissipative range as well. However where the flooring surface characteristics are in the conductive range then close attention to the construction of the shoe must be paid. The sole of the shoe should be as conductive as possible, say with a surface resistance under 10MΩ. However the inner lining of the shoe must be the one to provide the main's chock protecting function.