An exception to the need for certification is made in the case of “simple apparatus”. This is the term used for very low power or passive devices that will not cause ignition. Good examples are thermocouples and RTD’s.
Avoiding ignition entails minimizing both the available power and the maximum temperatures. Defining the maximum level of available power is complex, but in general terms can be considered as meaning voltage less than 29V and under 300 mA. A simpler view is to say that power must be less than 1.3 W. (Note that much instrumentation requires 24V and can often be designed to draw less than 500 mA; sufficient to meet IS certification in many situations).
Six classes define temperature levels. In general, equipment meeting the T4 designation is considered intrinsically safe because temperatures will not exceed 135°C (275°F) (equipment dissipating less than 1.3 W generally stays below this temperature).
A wide range of industrial equipment, such as flashlights, cameras, gas detectors and even radios, are available in intrinsically safe forms. In terms of instrumentation the biggest need is for pressure and weight measurement. Temperature measuremen t generally meets the “simple apparatus” rule although temperature transmitters may be needed to send thermocouple signals over longer distances.
Typically IS equipment is slightly more expensive than noncertified versions. This results more from obtaining and maintaining the approval than from t he cost of additional or uncommon components. The basic design of an IS barrier uses Zener Diodes to limit voltage, resistors to limit current and a fuse, none of which are expensive.
When electrical equipment and instrumentation must be placed in a hazardous environment, IS offers several benefits.
It helps ensure a safe work environment and protects those nearby from explosion risks.
It avoids the cost and bulk of explosion proof enclosures. Additional cost savings accrue from the ability to use standard instrumentation cables.
Maintenance and diagnostic work can be performed without shutting down production and ventilating the work area.
Insurance premiums may be lower as a result of the reduction in risk.
No, intrinsically safe apparatus cannot replace these methods in all applications due to the reliance on low power and temperature. Where it is possible i t often leads to significant savings in installation and maintenance costs.
No, the performance is the same as the as the non-certified with higher reliability. These devices use the same parts as the non-certified device but have been designed to limit the energy stored and heat generated in case of an internal fault condition.
Many industrial, chemical and process environments have significant explosion risks, either due to the presence, actual or possible, of flammable gases and vapors, dusts or fibers. Such environments are termed “hazardous” and it is essential that they are designed so as to eliminate the possibility of igniting the flammable material.
Often it’s necessary to incorporate instrumentation of an electrical nature in such environments. When this is unavoidable there are three possible app roaches: put the equipment in an explosion proof enclosure, purge the enclosure with inert gas, or adopt Intrinsic Safety design principles.
IS design minimizes power and heat creation. Equipment must be independently certified as IS, and the whole system must be designed to IS standards before entering service. However, adopting IS design can simplify installation, save money, enable maintenance on live equipment, and most importantly, makes for a safer workplace.