1. What is Intrinsic Safety ?
Introduction
Intrinsic Safety is a concept that came out of the mining industry in the
early 19th century. At that time, a lot of mining accidents used to happen,
many of which were traced to sparks that were generated by electrical
circuits, that were used in mine lighting, signalling equipment and the
like. These mining accidents were explosions resulting from the ignition of
a mine gas called Firedamp (now known to be mainly composed of
Methane). Due to the nature of the area (underground mines without
ventilation), the gas used to be trapped inside and explode on contact
with sparks. These mining explosions also had larger number of fatalities
than say an above ground explosion would because of the confined area
and the spreading of the fires and debris through underground passages.
To reduce the number of these accidents, it was proposed to use electrical
equipment that could not generate sparks that could ignite the Firedamp.
Thus was born Intrinsic Safety.
What is Intrinsic Safety?
Intrinsic Safety is a technique used to prevent explosions, which could be
caused due to sparking of electrical apparatus in hazardous areas. For the
uninitiated, hazardous areas, are those areas of a plant or facility, that
have a probable presence of explosive mixtures of gas, vapor or dust-
present either for long periods of time above a certain duration-generally
10 hours or more in a year. For more information on hazardous areas,
please click here.
Intrinsic Safety is used to design and implement electrical circuits (mostly
Instrumentation or measurement 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.

2. The use of barriers and isolators
In Intrinsically safe circuits, a safety barrier is used between the "safe
area" and the "hazardous area", so that any fault that generates a high
energy level (and sparks), would not get carried over to the hazardous
area; it gets stopped at the safety barrier itself. Barriers are of infinite
varieties, but broadly they can be classified as Zener barriers (passive
devices) and Isolators (active devices). Functionally, they achieve the
same objective, i.e. prevention of high energy levels from reaching the
hazardous area.
The use of simple apparatus
There are certain simple devices that are classified as low power devices,
these can be used without any safety barriers at all. They are called as
"simple apparatus". Examples are thermocouples and LEDs. This is a
great concept and it makes designing measurement loops simpler.
Preventive techniques versus mitigative techniques
All of us know that "prevention" is better than cure. This principle is
embodied in the principle of intrinsic safety, since the sparks are
prevented from ever causing an explosion. In contrast to this, the
explosionproof method of protection is mitigative as in case sparking
happens in a hazardous area, the explosion does occur- only the effects
are mitigated to some extent. Therefore, some engineers prefer Intrinsic
Safety over Explosionproofing or similar methods.
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 that is
allowed to be used under Zone 0 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 0. 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 use
weatherproof junction boxes too, with IS (intrinsically safe) loops.
The main disadvantage of this method is that, it can be used for only low
power circuits; thus you cannot have heavy duty intrinsically safe motors
for example. Typically the circuits are powered by 24 V dc power supplies.
So Intrinsic Safety is mainly used for only measuring and control
instruments like pressure transmitters, control valve positioners, small
capacity solenoid valves and so on.
Some people feel that Intrinsic safety is complicated because it requires
more engineering effort as one needs to select the instruments, select

3. appropriate barriers and isolators, design proper grounding schemes and
even select the right type of cable. However it is not complicated at all.
You do need a step by step method, with examples, to understand how
the process works and you're done.
More information on Intrinsic Safety and Intrinsically Safe circuits
We 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. The course explains much more than just intrinsic safety, of
course. It covers the concepts of hazardous area instrumentation like no
other!
Symbol
The IEC symbol for Intrinsic Safety is Ex-i.
(Ex-i circuits are of either of two types Ex-ia OR Ex-ib).In the near
future, the IEC will be adding a third type, known as Ex-ic. Look out for
another White Paper on this topic from us!