Intrinsic Safety (Ex i)

Introduction #

Intrinsic Safety (I.S) is a low energy signalling technique that prevent explosion by ensuring that the energy transferred to a hazardous area is well below the energy required to initiate ignition. It can only be used for very low power applications, typically in control and instrumentation.

The safe area equipment limits the available energy into the hazardous area to below that which could ignite the most easily ignited gas/air mixture.

The field equipment surface temperature will not cause ignition, even under specified fault conditions.

Intrinsic safety is a system concept and applies to the whole system – not just to the individual parts of the system.

Standards #

Definitions #

IEC & BS EN60079-11 defines intrinsic safety as:

“A type of protection based on the restriction of electrical energy within equipment and of interconnecting wiring exposed to the explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects.

Enclosures

The minimum ingress protection for enclosures in IS systems is given in the standards as IP20. However it will generally be greater (typically IP54), according to the intended use and environmental conditions.

Definitions of terms used in Intrinsically Safe (IS) system #

Intrinsically safe circuit #

A circuit in which any spark or thermal effect, under normal operation and in specified fault condition, is not capable of causing ignition of a given explosive atmosphere.

Intrinsically safe equipment #

Electrical equipment in which all the circuits are intrinsically safe circuits.

Associated apparatus #

Electrical equipment which contains both intrinsically safe circuits and non-intrinsically safe circuits and is constructed so that the non-intrinsically safe circuits cannot adversely affect the intrinsically safe circuits.

Simple apparatus #

An electrical component or group components of simple construction and well-defined electrical parameters, compatible with the intrinsic safety of the circuit in which it is used.

Simple apparatus includes passive devices such as switches, junction boxes, resistors, potentiometers and simple semiconductor devices.

Simple apparatus may also include sources of stored energy, components such as capacitors and inductors in simple circuits, having well defined parameters the value of which must be considered when determining the overall safety during the design stage of an IS installation.

Sources of generated energy (e.g. LED, thermocouple and photocell), can be described as simple equipment provided they generate not more than 1.5V, 100mA and 25mW.

Simple Apparatus is not required to be certified. The details of any simple apparatus used in a system will have been considered by the system designer as part of the assessment of the system, and the apparatus must appear in the documentation.

Infallible component #

An infallible component is a component or assembly of components that can be considered not to be subject to the fault modes specified in the standards, because the probability of the fault modes occurring is extremely low. Some types of transformers, resistors, diodes and opto-couplers can come within this definition. Infallibility is assumed provided that the components are operated well within their designated voltage, current and power ratings.

Advantages of I.S. #

• Safe – low voltage not harmful to personnel. Usually less than 50V, making live maintenance possible (under a PTW).
• Cost effective – certain uncertified (simple apparatus) may be used
• Cabling types may be less restricted
• Ex ‘is’ can be used in Zone 0, or other area requiring EPL Ga