Intrinsic Safety - Operating Principle

Operating Principle

The principle 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 no significant energy storage is possible.

One of the most common methods for protection is to limit the current by using multiple series resistors (assuming that resistors always fail open); and limit the voltage with multiple zener devices to ground (assuming diode always fail shorted). Sometimes an alternative type of barrier known as a galvanic isolation barrier may be used. Approval standards for intrinsic safety barriers may require that the barrier maintains approved levels of voltage and current with specified damage to limiting components.

For example, a measuring instrument in a hazardous area may be designed to operate with low voltage and current, and will be designed without any large capacitors or inductors that could discharge in a spark. The instrument will be connected, using approved wiring methods, back to a control panel in a non-hazardous area that contains safety barriers. The safety barriers ensure that, no matter what accidental contact occurs between the instrument circuit and outside power sources, no more than the approved voltage or current enters the hazardous area.

In normal use, 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 and maintenance.