“Our design firm has been asked to write a specification for a mission-critical data center floor covering. This data center will operate 24/7/365. The client has instructed us that they cannot afford any interruptions to their operation. What floor do you recommend and should we specify dissipative or conductive flooring?”

I think the most important issue you raise involves the difference between the words “static dissipative” and “conductive” or “static conductive.”As I said on the phone, all static control materials have certain electrical properties that allow electricity to flow through them and, hopefully, that electricity can flow onward to some form of ground connection. This process of electricity flowing could be described as any of the following: The electricity could be discharging, dissipating, grounding or conducting. In this sentence all 4 terms connote the same thing – the electricity is simply flowing somewhere. This electrical property is achieved by adding elements like carbon, silver or graphite to otherwise, ordinary flooring materials like carpet, vinyl, rubber and coatings. Once a flooring material has been developed for static control applications, that material needs to be rated for its ability to discharge electricity. The rating of the material is achieved by performing a test called an “electrical resistance measurement.” Resistance is measure using an ohm meter and the unit of measurement is the “ohm.” You can think of ohms as obstacles to the flow of electricity – hence their association with the word resistance.Most people get confused when they must differentiate a “static dissipative floor” with a “conductive floor.” Often they think, I don’t want to conduct static; I want to dissipate it – I want to get rid of static, not create it. They assume that conduct must mean create static. Unfortunately, static control terminology, like computer or medical language, can get a little esoteric. When we formally rate a flooring material and describe it as either “static dissipative” or “conductive” we mean two very different things – even though either material dissipates, conducts, discharges electricity. I guess you could say the confusion comes when we use the operative terms as nouns instead of as verbs.

• A static dissipative material is any flooring material that has an electrical resistance above 1 million ohms but below one billion ohms.
• A conductive material is any flooring material that has an electrical resistance below 1 million ohms but above 25,000 ohms.

So, which is the better material  - static dissipative or conductive - and when should one material be specified over the other?Let’s start by talking about static control performance. On this point, consensus is clear. ESD experts agree that conductive flooring materials eliminate static problems far more effectively than static dissipative materials for several reasons.

• Because they are less resistive, conductive floors discharge/dissipate/conduct static more rapidly than their more resistive counterpart,  static dissipative floors. Less resistive, AKA fewer ohms, means a more rapid discharge. A more rapid discharge of static means that charges cannot accumulate on a person in the first place. There isn’t time for static accumulation because the static gets discharged as fast as it is generated.
• Static dissipative floors allow for static to build up on a person’s body faster than the floor can discharge them. The higher resistance or ohms rating of a static dissipative material does not allow the discharge of static to keep pace with the generation of static. This means that a person walking on a static dissipative floor will accumulate some level of static before the floor is able to absorb the charge. If the person touches a piece of equipment before the floor absorbs the charge, the static could transfer to the computer instead of to the floor. This is obviously a serious limitation in performance.
• Static dissipative materials, particularly vinyl and carpet, do not prevent static build-up on people wearing regular footwear. Static dissipative vinyl and carpet should only ever be used in applications requiring ESD footwear and even in those applications, conductive flooring should be chosen over static dissipative flooring. Since regular footwear and not static control footwear, is the norm in a data center, this could become a problem. Most technical studies have shown that people wearing ordinary footwear while walking on a static dissipative vinyl can generate anywhere from 3000 to 5000 volts.
• Many static dissipative floors require some sort of anti-static rejuvenator. This is because the floor’s properties depend on the maintenance system and not just on the properties of the floor itself. Some static dissipative tile or SDT requires 3 to 5 special coats of antistatic polish before the floor can be used. These antistatic polishes need to be periodically reapplied. The question is when should they be reapplied and how is that determined? Without static electricity measuring equipment it would be difficult to know if the antistatic properties in the polish or wax are still performing.

The other concern is always employee safety. We are commonly asked if a conductive floor is less safe than a static dissipative floor. Based on NFPA regulations, a static control floor must have adequate electrical resistance to protect people from dangers associated with shocks from AC powered equipment. That is the reason conductive floors have a minimum resistance of 25,000 ohms. 25,000 ohms is actually a tremendous amount of resistance compared to the low resistance of a copper plate (less than 1 ohm) or other “pure conductors” like silver. This large electrical resistance protects people from electrical shock. At this point we are actually back we started – dealing with words that seem imprecise, confusing . Even though we call a floor a “conductive floor,” it’s really not all that conductive in the scheme of things. It’s just more conductive than a dissipative floor – and it works better as result of that fact.

On this link you can view a spec sheet with the visual graph of the ideal electrical resistance for ESD flooring.