Fire Fighting Enterprises Talentum IR3 Flame Detector - Manual (Page 7)

10
Interconnecting Cable Parameters
It is not permitted to connect more than one barrier circuit in the hazardous area to any other circuit.
There are four different intrinsically safe circuits shown on the system drawings. The stated cable
parameters are based on each Power Supply circuit, Remote Test circuit and Relay Output circuit
being isolated from each other with no other apparatus (other than the optional resistor across the
Relay Output circuit) being connected.
For each set of parameters shown below the cable capacitance and either the cable inductance or
the cable inductance to resistance ratio (L/R) must not exceed the values shown in table 7. The
reason for this is that energy can stored in a cable and it is necessary to use cable in which energy
stored is insufficient to ignite an explosive atmosphere.
Cable Parameters
When two Flame Detector Power Supply terminal pairs (terminal 1 & 2) or Remote terminal pairs
(terminals 3 & 4) are connected to a single circuit from the barriers or isolators listed in table 5 the
permitted cable parameters are as shown in table 7.
Group Capacitance - µF Inductance - mH L/R Ratio - µH/ohm
IIC
IIB
IIA 0.023µF (0.053µF)
0.59µF (0.62µF)
2.09µF (2.12µF)
4.2mH
12.6mH
33.6mH
54µΩ
217µΩ
435µΩ
Table 7 Maximum Permissible Stored Energy in Cables – [Two detectors (Single detector)]
When only a single Flame Detector Power Supply terminal pair (terminals 1 & 2) or Remote Test
terminal pair (terminals 3 & 4) is connected to a single circuit from the barrier or isolator listed in
table 5 then the capacitance may be increased to the values shown in brackets. The increase in
cable capacitance is not affected by the number of Flame Detectors Fire Relay connect terminal
pairs (terminals 5 & 6) or Fault Relay contact terminal pairs (terminals 7 & 8) connected to a single
circuit.
The above cable parameters are also valid for any non-isolating zener barriers with terminal
parameters not exceeding:-
Uo = 28V
Io = 93mA Po = 650mW
Note: The flame detectors have zero equivalent inductance (L
i
= 0) and a 0.03µF capacitance
(C
i
=0.03µ).
7
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 60079-0, 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.
The 016XXX series of Intrinsically Safe
Flame Detectors (Sensors) has been
assessed as an Intrinsically Safe System by
Baseefa as defined in EN 60079-25 for
Category ia, Group IIC and Temperature
Class T4. See system certificate number
Baseefa08Y0078.
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.
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.