Your sole compound on your safety footwear can offer various and different grades of protection. Requesting safety footwear that simply conforms to ISO 20345:2014 is just the tip of the iceberg when assessing risk in your specific work environment.
When addressing the issue of anti-static safety boots, one clearly needs to understand the different compounds used to manufacture soles on safety footwear; Polyurethane (PU), Thermoplastic Polyurethane (TPU), Carbon Black (CB), and Rubber (RUB).
Each compound has different properties of slip resistance, heat resistance, chemical resistance and electro contact resistance.
One needs to understand ECR (Electro Contact Resistance). These graphs below will help understand OHMS and OHMS range.
The ohm is the standard international unit of measurement of electrical resistance – and covers 3 major areas : 1. CONDUCTIVE / 2. ANTI-STATIC & 3. NON-CONDUCTIVE (or INSULATING).
- PU (Polyurethane) is anti-static. There is also a very special parameter within Polyurethane that sits in a very specific range of anti-static requirement called ESD (electro static discharge).
- TPU is also anti-static.
- Rubber is non-conductive. Used in environments where no electrical conductivity is allowed – resistance to high voltage / electrical / shock and spark.
- Carbon black soled footwear is 100% fully-conductive and helps to protect the wearer in an environment where the accumulation of static electricity on the body can be a hazard. (Handling explosive or volatile materials). Conductive safety footwear is made with materials and cements that offer no electrical resistance.
There is no one safety boot that covers all 4 requirements in 1, nor can one be both non-conductive and anti-static at the same time.
So, it all has to do with the amount of charge build (static build up) up which is generated on the personnel and the effect that has in the working environment.
- Rubber is non-conductive meaning no risk of electrical charge being earthed through the shoe. This is why companies at risk of standing on high voltage (or contact with high voltage) use rubber. We supply rubber soled footwear to many electrical contractors and electricians for the same purpose.
- Carbon black however allows any and all electrical charge build up to continually and rapidly dissipate through the sole – allowing zero static build up.
There are a wide range of factors that can affect the amount of electrostatic charge that can build up as a voltage on personnel.
- The floor material and its electrical resistance, and resistance-to-ground.
- 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 (A person wearing a wool sweater sitting on a man-made fibre chair may charge to a high voltage when they leave their seat).
Furthermore, PU (Polyurethane) has the best slip resistance ratings due to its softer more flexible compound (providing better grip). Rubber has slip resistant properties, but is harder and therefore less flexible and won’t have as higher slip resistant ratings. Again, the need to assess liquids on various surfaces needs to be taken into account when choosing footwear for different work environments.
Always familiarise yourself with specific needs in your work environment and the associated risks.