Archive for July, 2009

Intrinsic Safety- A brief overview

In today’s blog post we will review a much-misunderstood explosion protection concept, which is Intrinsic Safety. We will present a very brief overview below.

As usual intelligent comments are always welcome through the Comments form!

Introduction
In the early 19th century the mining industry was a major employer. Unfortunately,  it also had the largest number of fatal accidents, many of which were traced to sparks caused by electrical circuits, that were used in signaling equipment and lighting. These mining accidents were explosions due to the ignition of a mine gas called Firedamp ( comprised of mainly Methane plus some other inflammable hydrocarbons). Due to the nature of the area (underground mines without ventilation), the gas used to be trapped inside and explode, which caused these large number of fatalities.
To solve this burning problem (literally), electrical equipment that could not generate sparks (that was capable of igniting the Firedamp) was designed. This was known as the technique of Intrinsic Safety and such equipment was called Intrinsically Safe equipment or Intrinsically safe apparatus.

So what is Intrinsic Safety really all about?
Intrinsic Safety is a technique used to prevent explosions that may be caused due to sparks generated by electrical apparatus in hazardous areas. In case you didn’t know (highly unlikely, eh)  “hazardous areas”,  are those designated areas of a plant or facility, that have the possibility of explosive mixtures of gas, vapor or dust present either for long periods of time or at least sometimes during the year. They are alos referred to as Classified locations in North America. For more information on hazardous areas, please click here.

Intrinsic Safety is used to implement circuits (mostly Instrumentation circuits), that will not have any sparking at all. There are other kinds of non sparking techniques of explosion protection, but these will not be covered here.

The use of barriers and isolators
In an Intrinsically safe circuit, a safety barrier is used between the control room or designated “safe area” and the “hazardous area” (in other words the plant or production area that ), so that any fault that generates a high energy level (and sparks) would not get carried over to the hazardous area; it is prevented from doing so by the safety barrier. Barriers are of many types, but broadly they can be classified as (passive) Zener barriers and (active) Isolators. The zener barriers have several zener diodes in them that carry away fault currents to ground. In the case of isolators, the devices provide transformer like isolation between the safe area circuits and the hazardous area circuits, so that large currents from the safe area do not enter the hazardous area.

Prevention is better than cure
All of us know that “prevention” is better than cure. This is the principle on which the concept of intrinsic safety is based, since the sparks are prevented from ever causing an explosion. As regards the explosionproof method of protection, one can consider it to be mitigative as the explosion does occur (say in case of a spark ina junction box located in a hazardous area), only the effects are mitigated to some extent. Therefore,  many engineers and safety professionals prefer Intrinsic Safety over Explosionproof or similar methods. However, many engineers and technical personnel who are not familiar with Intrinsic safety,  do not prefer it as they do not understand it completely. To be fair,  to use explosionproof equipment is simple, use everything that is explosionproof!  Designs using Intrinsic Safety is not so simple however, as the design engineer needs to select the right kind of instrument, barriers, cables and other accessories,  in order to meet the intrinsic safety requirements. A mistake means that the design is not safe.

Advantages and Disadvantages of Intrinsic Safety

The advantages of using intrinsic safety as a protection technique are many, but the most important one is that it is the only technique allowed to be used under Zone O  of the IEC Classification system for hazardous areas.  One cannot use any other technique like explosionproof (or increased safety, or non incendive methods, or any of the many other methods of protection) in Zone O. Other advantages are, that since it uses the entity concept, the designer can mix and match various compatible components to make the circuit intrinsically safe. It gives him/her greater flexibility. Also, using this technique eliminates the need for explosionproof junction boxes in the hazardous area, one can safely use weatherproof junction boxes too, with IS (intrinsically safe) loops.

The main disadvantage of Intrinsic Safety as a protection method, is that it can be used for only low power circuits; thus you cannot have intrinsically safe motors, for example. So Intrinsic Safety is used for mostly instrumentation  like pressure transmitters, control valve positioners, small capacity solenoid valves and so on.
The entire measurement loop needs to be designed as Intrinsically Safe and this can be a daunting task for many, but it need not be really so. You do need a step by step method, with examples to understand how the process works and you’re done.  I would recommend you to download the Hazardous Area Instrumentation e-learning course to really understand how intrinsic safety works and how intrinsically safe instruments can deliver value for your plants.