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mospheres can arise due to dusts are divided into Ex zones installations in a potentially explosive atmosphere without ad-
20 to 22. versely affecting explosion protection.
Depending on the ignitability of the ignitable substances oc- Intrinsic safety is therefore of paramount importance, particu-
curring in the relevant field of application, a distinction is larly in measuring and control systems, not least due to the
made between explosion groups I, IIA, IIB and IIC which have increased use of electronic automation systems. However, in-
different ignition limit curves. The ignition limit curve, which is trinsic safety demands more of the designer or installer of an
a function of the ignition behaviour of the ignitable substance installation than other types of protection. The intrinsic safety
under consideration, indicates the maximum values for the op- of a circuit does not only depend on compliance with the build-
erating voltage and operating current. ing regulations for the individual pieces of equipment, but also
Explosion group IIC contains the most easily ignitable sub- on the correct interconnection of all pieces of equipment in the
stances such as hydrogen and acetylene. When heated, these intrinsically safe circuit and on the correct installation.
substances have different ignition temperatures which are
classified into temperature classes (T1 ... T6). Transients in hazardous areas
To avoid that electrical equipment represents ignition sources The intrinsic safety type of protection considers all electrical
in explosive atmospheres, it is designed with different types of energy storage systems present in the system, but not surges
protection. One type of protection, which is used in measuring injected from outside e.g. resulting from atmospheric dis-
and control systems all over the world, is intrinsic safety Ex(i). charges.
Injected surges occur in large-scale industrial installations
Intrinsic safety type of protection mainly as a result of nearby and remote lightning strikes. In
Intrinsic safety is based on the principle of current and voltage the event of a direct lightning strike, the voltage drop across
limitation in a circuit. The energy of the circuit or a part of the the earth-termination system causes a potential rise between
circuit, which is capable of igniting an explosive atmosphere, some 10 and 100 kV. This potential rise acts as a potential dif-
is kept so low that neither sparks nor intolerable surface heat- ference on all pieces of equipment connected to distant equip-
ing of the electrical components can ignite the surrounding ment via cables. These potential differences are considerably
explosive atmosphere. Apart from the voltage and current of higher than the insulation strength of the equipment and can
the electrical equipment, the inductances and capacitances in easily cause sparkover. In case of remote lightning strikes,
mainly the injected surges in conductors have an effect and
the complete circuit which act as energy storage systems must as differential-mode interference (differential voltage between
be limited to safe maximum values.
the cores) they can destroy the inputs of electronic equipment.
To ensure safe operation of a measuring and control circuit, for Classification of electrical equipment into protection
example, this means that neither the sparks which arise during levels ia, ib or ic
the operational opening and closing of the circuit (e.g. at a An important aspect of the intrinsic safety type of protection
switch contact in an intrinsically safe circuit) nor those arising as far as explosion protection is concerned is the reliability
in the event of a fault (e.g. short-circuit or earth fault) must with respect to the maintenance of the voltage and current
cause ignition. Moreover, both during normal operation and limits, even in the event of certain faults. There are three dif-
in the event of a fault, heat ignition as a result of an excessive ferent protection levels (ia, ib and ic) concerning the reliability
temperature rise of the equipment and cables in the intrinsi- and safety of the intrinsically safe electrical equipment.
cally safe circuit must also be excluded. Protection level ic describes the undisturbed operation without
This basically limits the intrinsic safety type of protection to faults. In this case, intrinsic safety must be maintained during
circuits with relatively low power levels such as the circuits operation.
of measuring and control / data systems. Intrinsic safety, which Protection level ib requires that intrinsic safety must be main-
can be achieved by limiting the energy available in the circuit, tained if a fault occurs in the intrinsically safe circuit.
does not relate to individual devices – as is the case with other Protection level ia requires that intrinsic safety must be main-
types of protection – but to the complete circuit. This provides tained if two independent faults occur.
many advantages over other types of protection. Figure 8.2.4.1 shows the basic use of SPDs for a measuring
On the one hand, no expensive special constructions are re- and control circuit.
quired for the electrical equipment used in the field, for exam-
ple flame-proof enclosure or embedding in cast resin, which Maximum values of current I 0 , voltage U 0 , induct-
mainly leads to more cost-effective protection solutions. On ance L 0 and capacitance C 0
the other hand, intrinsic safety is the only type of protection Safety barriers or measuring transducers with Ex(i) output
which allows the user to work freely on all live intrinsically safe circuit are used at the interface between the hazardous and

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