Plastic contains explosion hazards

By making the ignition volume in the connection box sufficiently small, by choosing suitable plastics, by carefully designing the path to atmosphere and by encapsulating electronics and coil, it is possible to do away with the need for the more usual stainless steel housings normally considered necessary for these products. The plastic flame proof technology has been developed by Parker engineers in Geneva. The words 'explosion proof' or 'flame proof' refer not to the effects of an explosion or flame outside but mean that any ignition inside cannot be transmitted to any explosive atmosphere which might be outside. Rob Bentley, area sales manager for Parker Hannifin Fluid Controls explained that propagation of a flame or explosion from within the terminal box is not possible because its volume is 9cc. Being less than 10cc, there is not enough space to allow a proper explosion to get going. Even so, the regulations require that the enclosure must be able to resist 13bar pressure without leakage and it does this, being able to withstand 20bar. Furthermore, the joint under the lid and that between the cable and lead in are so made that they have a safe gap of 0.1mm and a flameway of 7mm, 1mm more than the 6mm minimum. This is achieved by making the cover of the terminal box out of polyarylamide (Solvay's 'IXEF'), which offers the required amount of strength and rigidity to ensure the dimensional stability of the safe gap. The coil and electronics are encapsulated in polypropylene sulphide - PPS ('Ryton' made by Chevron Phillips Chemical Co). The coil is ATEX certified EEx dm IIC T4, T5 and T6. The 'dm' designates the technologies used. The 'd' refers to the enclosure round the terminals and the 'm' to the encapsulation. The IIC means that the devices are suitable for atmospheres other than coal mines containing hydrogen or gases of an equivalent hazard. T4, T5 and T6, refers to the temperature in deg C at which a gas will ignite spontaneously without another source of ignition. T6 is the most stringent, imposing a maximum temperature limit of 85 deg C. The devices are further protected by incorporating fuses to prevent any possibility of the coil over heating and varistor surge suppressers. Power levels of coils are 'low power' 2W to 2.5W or 'standard' 8W, suitable for a wide range of 'Lucifer' valves with 1/4" and 1/8" ports and operating pressures up to 10bar. Parker Hannifin Pointers * Plastic containment is sufficient to ensure compliance with ATEX requirements because the space in the terminal box is so small and the plastic chosen is sufficiently strong to resist the required 13 bar pressure and maintain the shape of the safe gap. * Further protection is ensured by encapsulating coil and electronics in a particularly heat resistant polymer and incorporating fuses and varistors