6
Hochiki Europe (UK) Ltd
2-3-0-809/ISS4/SEP14
System Design
Engineers familiar with codes of practice for hazardous area systems should only undertake the design of
an intrinsically safe fire detection system. In Europe the standard is EN 50014, Electrical apparatus for
potentially explosive atmospheres
– General requirements.
The fire detector performance is the same as the standard none intrinsically safe counterparts.
Performance information given in standard product guides is therefore applicable to the intrinsically safe
range.
The BASEEFA certification of the intrinsically devices covers their characteristics as components of an
intrinsically safe system. This indicates that the flame detectors can be used with a margin of safety in
such systems.
In safe area (standard) applications it is some times desirable to connect the wiring as a loop, with both
ends terminated at the control panel. In the event of an open-circuit fault it is then possible to drive both
ends simultaneously. In a hazardous area it is not possible to use a loop configuration because the
potential to feed power from each end of the loop would double the available energy in the hazardous
area and contravene the energy limitations of the intrinsically safe certification. All circuits must therefore
be connected as spars from the safe area or as radial connections from the control panel.
Types of Safety Barrier
The system configuration can for three types of safety barrier, each of which has its own advantages and
disadvantages. A brief outline of the characteristics is given below.
Single Channel 28V/300Ω Barrier
This is the most basic type of barrier and therefore the lowest cost. Being passive devices, they also
impose the minimum of restrictions on the operation of the flame detectors. Thus, single channel barriers
are available either as positive or negative polarity where the polarity refers to the polarity of the applied
voltage
relative
to
earth
.
The significance of this is that one side of the barrier must be connected to a
high-integrity (safety) earth.
Although this connection has no effect on the operation of the flame detector
and is not needed for their correct operation, it may not be acceptable to the operation of the control and
indicating equipment. This is particularly true if the control equipment incorporates earth-leakage
monitoring and even without this feature the earthing of the loop may cause unwanted cross-talk between
loops.
If the earth connection is not acceptable then the A.C. or isolating barriers should be used.
Star-connected A.C. Barrier
A.C. barriers are also passive devices and must still be connected to a high-integrity safety earth.
However, they are designed to allow either positive or negative voltages with respect to earth and under
normal conditions provide a connection to earth via a reverse-diode, rather than directly.
The disadvantage of this type of barrier is that the end-to-end resistance is nominally 1200ohms
compared with the 300 ohms of the single channel type. This high resistance results in an extra voltage
drop in the circuit. This type of barrier is not recommended for general use
Galvanically Isolated Barrier
Galvanically isolated barriers (also know as transformer isolated barriers) differ from conventional shunt
zener barriers in that they provide electrical isolation between the input (safe area) and the output
(hazardous area). This is achieved by the use of a D.C./D.C. converter on the input side, which is
connected to the hazardous area through a voltage and power limiting resistor/zener combination similar
to a conventional barrier.
The galvanic isolation technique means that the circuit does not need a high integrity (safety) earth and
that the intrinsically safe circuit is fully floating.
Earth leakage problems for control and indicating
equipment are therefore eliminated if this type of interface is used.
