| 1) Why
choose intrinsic safety?
The topics on this page include:
1.1) Introduction
Intrinsic safety is a low energy technique that
prevents explosions by ensuring that the energy transferred to a
hazardous area is well below the energy required to initiate an
explosion. The two mechanisms considered are initiation of an explosion
by a spark or by a hot surface.
The useable energy level available is small,
but is more than adequate for the majority of instrumentation systems.
Return
to top
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 technique of intrinsic safety 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 equipment usually satisfies both
the dust and gas hazard requirements.
- Appropriate intrinsically safe apparatus
can be used in all Zones. In particular, it is the only solution
for Zone 0 instrumentation that has a satisfactory history of
safety. The use of levels of protection (‘ia’, ’ib’
and ’ic’) ensure 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).
- Usually intrinsically safe apparatus and
systems are 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 about area classification, gas grouping
and temperature classification in almost all circumstances and
becomes the universal safe solution.
- The ‘simple apparatus’ concept
allows many simple pieces of apparatus, such as switches thermocouples,
RTDs and junction boxes to be used in intrinsically safe systems
without the necessity of certification. This gives a significant
amount of flexibility in the choice of these ancillaries.
- The intrinsic safety technique is the only
technique, which permits live maintenance within the hazardous
area without the need to obtain ‘gas clearance’ certificates.
This is particularly important for instrumentation, since faultfinding
on de-energised 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 this is only
a problem on cables longer than 400m needed 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.
Return
to top
| 1.3)
Available power
Intrinsic safety is fundamentally a 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
applicable.
The blue and green curves are the accepted
design curves used to avoid spark ignition by resistance-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 metres corresponds
to 80nF which has a permissible voltage of 29V in ‘IIC
ia’ circuits) and the maximum current by cable inductance
(400metres corresponds to 400µH which has a permissible
current of 300 mA in IIC ia circuits). A frequently used limitation
on power is 1.3W, which easily permits a T4 (135°C) temperature
classification. These limits are all shown in Figure 1.1.
A simple approach is to say that if 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’ requirements.
The first choice, however, is always to
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).
In practice almost all low voltage instrumentation
can be made’ IIB ic T4’ as the limits are set
by the least sensitive of the ignition curves in Figure 1
(typically 24V 500mA).
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.
Return
to top
|
Click image for
greater detail
Figure 1.1 |
1.4) Conclusion
Intrinsic safety is the natural choice for all
low voltage instrumentation problems. Adequate solutions exist which
are compatible with all gases and area classifications. The technique
prevents explosions rather than retains them which must be preferable,
and the ‘live maintenance’ facility enables conventional
instrument practice to be used.
Return
to top
|
