A specification sometimes calls for an ‘anti-static’ or ‘static-conductive’ floor. The word ‘anti-static’ is sometimes used loosely and it is best to check with the specifier the reason for the specification and whether specific electrical resistance figures are quoted and if the floor needs to be connected to an earthing point. This can affect the installation procedures.
For example: if ‘anti-static’ performance is requested just to ensure that people do not receive a shock when touching metal after walking across a floor then ordinary linoleum and vinyl in the range of normal humidity levels will meet that requirement (EN1815 <2kv) with standard laying techniques and materials and no additional earthing.
Marmoleum Ohmex sheet has a further reduced electrical resistance of less than 108 ohm.
Measurements of electrical resistance are expressed either in terms of volume resistivity (between two points on opposite surfaces of the material) or surface resistivity (between two points on the same surface). Measurements for hospitals, for example, are usually of surface resistivity.
Static control floors are now categorised according to the following classification:
• Static Dissipative - Between 1 x 10 6ohm and
1 x 10 9 ohm.
• Static Conductive - Between 4 x 104 ohm and
1 x 10 6 ohm
Health Technical Memorandum No.2 calls for new floors to have readings between 5 x 104 ohm and
2 x 10 6 ohm.
Computer companies have varying requirements, usually up to 109 ohm.
Low electrical resistance figures are required when static is to be avoided by discharging it to earth before a build-up can occur. The total path between the charged object and earth needs to have low resistance: it is pointless having a floorcovering with a low resistance if the materials between it and earth have a higher resistance.
The adhesives and levelling compounds used therefore also need a low resistance and the contact between all the materials needs to be good, therefore a static conductive primer is often used.
Installation of conductive floorcoverings over a grid of copper tape is often specified to improve control of static discharge.