What is intrinsic safety?
With reference to IEC 60079-11, an intrinsically safe circuit is one which during normal operation and under specific fault conditions does not generate a spark or have sufficient thermal energy to cause ignition of an explosive atmosphere.
As such intrinsic safety or Ex i protection method is based on the principle of limiting the current, voltage and stored energy within an electric circuit.
Intrinsic safety is different from other protection methods in that all devices in the “loop” must be considered. A simple intrinsically safe loop would typically consist of simple or intrinsically safe apparatus located in a hazardous area, connected via intrinsically safe wiring to associated apparatus located in the non-hazardous safe area.
What is Simple Apparatus?
Simple apparatus are generally electrical components with well-defined electrical properties that are compatible with the intrinsic safety of the circuit in which it is used.
RTD’s, thermocouples, potentiometers and switches are typical devices that can be classified as simple apparatus.
Simple apparatus does not typically need to be certified.
What is Intrinsically Safe Apparatus?
Intrinsically Safe Apparatus are devices such as temperature transmitters, solenoid valves and I/P converters, which are designed for installation in hazardous areas. As these devices are capable of storing energy, they must be certified as intrinsically safe apparatus.
The certification, usually by a notified body, will include hazardous area classification and intrinsically safe entity parameters specific to the device, such as voltage, power and current limits. This information is required to complete the necessary I.S loop calculations.
What is Associated Apparatus?
An intrinsically safe loop requires an interface between the devices in the hazardous area and the safe area devices in the control room. A device capable of limiting the energy to the hazardous area and ensuring that even under fault conditions there is insufficient energy to cause an ignition of the explosive atmosphere.
Typically, this interface takes the form of an intrinsically safe or I.S. barrier.
Two main types of I.S. barriers are used, either a "Zener barrier" or the increasingly more popular "intrinsically safe galvanic isolator".