Intrinsic Safety

When using control devices in hazardous areas such as explosive or flammable atmospheres special care must be taken to reduce or eliminate the possibility of heat or an electrical arc or spark causing damage to personnel or equipment. Because of this a set of standards for equipment and devices have been developed for these environments.

From the Primer:

Intrinsic safety (IS) is a protection technique for safe operation of electronic equipment in explosive atmospheres and under irregular operating conditions. The concept was developed for safe operation of process control instrumentation in hazardous areas, particularly North Sea gas platforms. As a discipline, it is an application of inherent safety in instrumentation.

The theory behind intrinsic safety is to ensure that the available electrical and thermal energy in the system is always low enough that ignition of the hazardous atmosphere cannot occur. This is achieved by ensuring that only low voltages and currents enter the hazardous area, and that all electric supply and signal wires are protected by Zener safety barriers. Sometimes an alternative type of barrier known as a galvanic isolation barrier may be used.

Intrinsically Safe Barriers

In the use of these barriers a low voltage (typically under 8vdc) device or sensor is connected to the field side of the barrier and the other side is wired into the standard inputs of a controller (typically 24vdc). The barriers are then located outside of the hazardous area.

In normal uses, electrical equipment often creates internal tiny sparks in switches, motor brushes, connectors, and in other places. Such sparks can ignite flammable substances present in air. A device termed intrinsically safe is designed to not contain any components that produce sparks or which can hold enough energy to produce a spark of sufficient energy to cause an ignition. For example, during marine transfer operations when flammable products are transferred between the marine terminal and tanker ships or barges, two-way radio communication needs to be constantly maintained in case the transfer needs to stop for unforeseen reasons such as a spill. The United States Coast Guard requires that the two way radio must be certified as intrinsically safe.

Another aspect of intrinsic safety is controlling abnormal small component temperatures. Under certain fault conditions (such as an internal short inside a semiconductor device), the temperature of a component case can rise to a much higher level than in normal use. Safeguards, such as current limiting by resistors and fuses, must be employed to ensure that in no circumstance can a component reach a temperature that could cause autoignition of a combustible atmosphere.

No single field device or wiring is intrinsically safe by itself (except for properly designed battery-operated, self contained devices), but is intrinsically safe only when employed in a properly designed IS system. Such systems are usually provided with detailed instructions to ensure safe use.

Many sensor and device manufacturers have products designed for use in hazardous areas either with a barrier or as a self contained unit. A well known manufacturer of both intrinsically safe barriers and devices is the German company Pepperl+Fuchs. Further information can be found in their product literature.

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