Basic Techniques of Intrinsically Safe

Circuit Assessment (Page 1)

Introduction

Intrinsic safety is a protection concept employed in potentially explosive atmospheres. Intrinsic safety relies on the electrical apparatus being designed so that it is unable to release sufficient energy, by either thermal or electrical means, to cause an ignition of a flammable gas. The energies required to ignite various gas groups have been proven by experimentation. Graphs of this data have been produced, and can be used to indicate safe levels of energy.

A very small amount of energy is required to cause an ignition, for example, a mixture of Hydrogen in air requires only 2OuJ of energy. In electrical circuits the mechanism for the release of this ignition energy is one or more of the following.

• Open circuit or short circuit components or interconnections in a resistive circuit

• Short circuit of components or interconnections in a capacitive circuit

• Open circuit components or interconnections in an inductive circuit

• Ignition by hot surfaces.

 This article is a brief appraisal of the basic electrical considerations of power limiting to a safe level. The article  is not concerned with many of the safety factors that are required during the design or certification process as it is intended as an overview to the subject.

The safety factors, graphs and tables mentioned are references to British and European published standards  (EN5O 020:1994 and  EN5O 014:1992) for intrinsic safety.

Resistive intrinsically safe circuits

In a circuit that is non-reactive (quasi non-inductive/capacitive) there is no stored energy to be released in an arc. The main consideration, therefore, is the amount of energy in the circuit. The power provided to intrinsically safe equipment is normally derived from batteries, a zener barrier or a galvanic isolator. Barriers are widely used in intrinsic safety to allow connection to power supply lines. They are used to limit both voltage and current (and hence power) into the hazardous area equipment. If a circuit can be proven to have no means of dissipating stored energy, and the voltage and current to it were proven to be safe, then the circuit will be safe electrically. (The thermal data of components would still need to be assessed so that ignition could not occur due to their heat in operation.)

A typical barrier is shown below.

In this form the shunt diode safety barrier will operate so that if a dangerous voltage is seen on the input. The diodes