Electronics Product Design for ATEX safety

A. Kumar
Honeywell,
India

Keywords: ATEX, intrinsically safe, capacitor, energy, rating

Summary:

In the digital era when electronics rule the world and it has been become part and parcel of life, it is very important to design a safe product. Safety shall be of prime importance when the product will be used in explosive environment where a spark can create explosion and be disastrous. Electrical circuits used in ATmospheric EXplosive (ATEX) environments shall be designed intrinsically safe so that it restricts the electrical energy within the product and its interconnecting wiring exposed to the potentially explosive atmosphere to a level below which it can ignite/explode by either spark or heat effects. Ignition capable electrical circuit can also be made intrinsically safe by employing various design techniques (using infallible safety components) which limit the permutation/combination of component connection. Few major ways to achieve Infallibility are as follows: 1.Using duplicate safety components and limiting their operation to 2/3 of their rating considering worst case limits 2.Three blocking diodes linked in series between two voltage sources will infallibly block the impact of a voltage source on another. 3.Enough creepage and clearance shall be kept around infallible components as per the standard to remove the chances of interconnections becoming open or short circuit. 4.ATEX approved insulation or enclosure shall be used to avoid interconnections from short circuit conditions and limit the ignition within the enclosure 5.Even a wider trace of width 2mm can be infallible if it has proper conformal coated. 6.Zener diodes at the output of voltage generation shall be used within two-third of its specified ratings to fix the maximum limit of voltage tolerance in case of any mal-function of the circuit. 7.Resistors would be used in series to limit current and dissipate power. They are also used to limit the instantaneous release of charge from discharging capacitors. 8.Capacitors can be used as safety components for blocking DC voltages but must be duplicated in series. Electrolytic and tantalum capacitors shall not be used at all and they are required to pass specific high voltage tests. 9.Fuses are used to determine the wattage of the product. Where fuses are used to protect other components, 1.7 times In shall be assumed to flow continuously. 10.Fuses must be potted if it is used in a hazardous area. It must have a breaking capacity of 1500A (so as not to conduct after breaking) if it is used to protect a main's transformer. It limits power only and is not used to limit instantaneous current. As per ATEX standard IEC 60079-11, there is allowed permitted capacitance limit for various voltage area and apparatus group. If there are multiple voltages on a PCB then equivalent capacitance of a voltage are with respect to other is calculated as per the following formula. Cequ. = C (V1 (Max Volt Area) / V2 (Min Volt Area))2 Where V1 is Maximum Voltage determined by positive tolerance of Zener across that voltage output and V2 is Minimum Voltage determined by subtracting the tolerance from Zener across that voltage output