Intrinsic safety (IS) is a low-energy signaling
technique that prevents explosions from occurring by ensuring that the energy transferred
to a hazardous area is well below the energy required to initiate an explosion.
The energy levels made available for signaling
are small but useable and more than adequate for the majority of instrumentation
The two mechanisms being considered
that could initiate an explosion are:
- A Spark
- A Hot Surface
1.2 The advantages of intrinsic safety
The major advantage of intrinsic
safety is that it provides a solution to all the problems of hazardous areas
(for equipment requiring limited power) and is the only technique which meets
this criterion. The significant factors are as follows:
The IS technique is accepted
throughout the world. There is an increasing acceptance of international
certificates issued under the IEC Ex scheme but this has some way to go.
Intrinsic safety is an acceptable technique in all local legislation such as
the ATEX Directives and OSHA. The relevant standards and code of practice give
detailed guidance on the design and use of intrinsically safe equipment to a
level which is not achieved by any of the other methods of protection.
The same IS equipment usually satisfies the
requirements for both dust and gas hazards.
Appropriate intrinsically safe apparatus can be
used in all zones. In particular, it is the only solution that has a
satisfactory history of safety for Zone 0 instrumentation. The use of levels of
protection (‘ia’, ‘ib’ and ‘ic’) ensures that equipment suitable for each level
of risk is available (normally ‘ia’ is used in Zone 0, ‘ib’ in Zone 1 and ‘ic’
in Zone 2).
Intrinsically safe apparatus and systems are
usually allocated a group IIC gas classification which ensures that the
equipment is compatible with all gas/air mixtures. Occasionally, IIB systems
are used, as this permits a higher power level to be used. (However, IIB
systems are not compatible with acetylene, hydrogen and carbon disulfide.)
A temperature classification of T4 (135°C) is
normally achieved, which satisfies the requirement for all industrial gases except
carbon disulfide (CS2) which, fortunately, is rarely used.
Frequently, apparatus, and the system in which
it is used, can be made ‘ia IIC T4’ at an acceptable cost. This removes
concerns Why choose intrinsic safety? About area classification, gas grouping
and temperature classification in almost all circumstances and becomes the universal
The ‘simple apparatus’ concept allows
many simple pieces of apparatus, such as switches, thermocouples, RTD’s and
junction boxes to be used in intrinsically safe systems without the need for
certification. This gives a significant amount of flexibility in the choice of
The intrinsic safety technique is the only
technique that permits live maintenance within the hazardous area without the need
to obtain ‘gas clearance’ certificates. This is particularly important for
instrumentation, since fault-finding on de-energized equipment is difficult.
The installation and maintenance
requirements for intrinsically safe apparatus are well documented, and
consistent regardless of level of protection. This reduces the amount of
training required and decreases the possibility of dangerous mistakes.
Intrinsic safety permits the use of
conventional instrumentation cables, thus reducing costs. Cable capacitance and
inductance is often perceived as a problem but, in fact, it is only a problem on
cables longer than 400 meters, in systems installed in Zones 0 and 1, where IIC
gases (hydrogen) are the source of risk. This is comparatively rare and, in
most circumstances, cable parameters are not a problem.
1.3 Available Power
low energy technique and consequently the voltage, current and power available
is restricted. Figure 1.1 is a simplified illustration of the available power
in intrinsically safe circuits and attempts to demonstrate the type of
electrical installation in which the intrinsically safe technique is
accepted design curves used to avoid spark ignition by resistive limited
circuits in Group IIC and IIB gases. The ‘ic’ curves are less sensitive because
they do not require the application of a safety factor in the same way as for ‘ia’
and ‘ib’ equipment. In general the maximum voltage available is set by cable
capacitance (400 meters corresponds to 80 F which has a permissible voltage of
29V in ‘IIC ia’ circuits) and the maximum current by cable inductance (400 meters
corresponds to 400μH which has a permissible current of 300 mA in IIC ia circuits).
A frequently used limitation on power is the 1.3W, which easily permits a T4
(135°C) temperature classification. These limits are all shown in Figure 1.1.
the apparatus can be operated from a source of power whose output parameters
are within the (blue) hatched area then it can readily be made intrinsically
safe to ‘ IIC ia T4’ standards. If the parameters exceed these limits to a
limited degree then it can probably be made intrinsically safe to IIB or ‘ic’
choose ‘IIC ia T4’ equipment, if it provides adequate power and is an economic
choice, as this equipment can be used in all circumstances (except if carbon disulfide
(CS2) is the hazardous gas, in which case there are other problems).
instrumentation can be made ‘IIB ic T4’ as the limits are set by the least
sensitive of the ignition curves in Figure 1.1 (typically 24V 500 mA). The ‘IIB
ic’ specification does restrict application to Zone 2 and where the hazardous
gas is not hydrogen, acetylene or carbon disulfide but is still applicable to a
large range of installations.