Hochiki IFD-E(IS) - Manual (Page 8)

8 Hochiki Europe (UK) Ltd
2-3-0-809/ISS2/JUL07
Wiring and Cable Types
It is not permitted to connect more than one barrier circuit in the hazardous area to any other circuit.
Both separate and twin cables may be used. A pair contained in a type ‘A’ or ‘B’ multicore cable (as
defined in clause 5.3 of EN50 039) may also be used, provided that the peak voltage of any circuit
contained within the multicore does not exceed 60V.
The capacitance and either inductance or the inductance to resistance (L/R) ratio of the hazardous area
cable must not exceed the parameters specified in Table 6. 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.
To calculate the total capacitance or inductance for the length of cable in the hazardous area, refer to
Table 7, which gives typical per kilometre capacitance and inductance for commonly used cables.
Note: The flame detectors have zero equivalent inductance (L
i
= 0) and a 0.03
μF capacitance (C
i
=
0.03
μF).
Gas group IIA IIB IIC
Capacitance
μF
Inductance mH
L/R ratio
μH/ohm
2.15
33.6
440 0.65
12.6
165 0.083
4.2
55
Table 6 – 28V Barrier, Maximum Permissible Stored Energy in Cables
Capacitance µF/km
Cable Type Core
Size
mm²
Conductor
resistance
ohm/km/core
Inductance
mH/km
core
to
core core
to
sheath
Sheath
Resistance
ohm/km
MICC Pyrtenax
light duty
MICC Pyrotenex
heavy duty
Pirelli FP200
PVC sheathed and
Insulated to BS6004
2
2
all
all 1.5
1.5
1.5
1.5 12.1
12.1
12.1
12.1 0.534
0.643
0.77 0.19
0.13
0.08
0.09 0.21
0.17
0.15 2.77
1.58
Table 7 Examples of electrical characteristics of cables commonly used in fire protection systems
Maximum Loading of IS Circuit
Because of the finite resistance of the safety barrier, there will be a limit to the current drain which can be
tolerated before the voltage on the circuit falls outside the specified limits for the IS detector. The standing
current for the detectors can be calculated by the sum of the individual selected detector currents as
given in the detector data sheet. This may limit the maximum number of detectors per barrier to two or
three.