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It is not necessary for the DC power cable to be a twisted pair or individually screened, a 2-core stranded cable with
an overall screen is suffi cient. The minimum conductor size is determined by the cable length, the number of Flame
Detectors on each loop, and the maximum allowed voltage drop at the last detector.
To prevent RS485 and Video common mode problems this is limited to a maximum of four volts (4V) on the negative
supply (0V).
Equation 1 : DC Supply Conductor Resistance Calculation
Use the value of R
calculated above to select a suitable gauge of conductor, alternatively, to calculate the maximum
km
cable length from a known conductor resistance swap R
cross-sectional area (which equates to its resistance) limits the maximum cable length, increasing the supply voltage
(up-to a maximum of 32V) can dramatically increase cable length.
Prudence dictates that a cable is selected with a lower resistance than calculated above, with suffi cient allowance
for the eff ects of crimps, terminals and ageing which can increase overall resistance. Where a single cable cross
sectional area cannot be found to satisfy both the needs of the power and signal conductors consideration should be
given to using multiple paralleled conductors of a smaller cross section for the power.
Video (Twisted Pair)
The video cabling should be a twisted pair stranded cable with an overall screen. Where multi-core cables are used
then individual screened twisted pairs are recommended. The cable should have the following characteristics:
Video (Twisted Pair) Cable Characteristics
Cable
Characteristic
Characteristic
Impedance
Nominal
150Ω
Absolute Limit
90Ω to 150Ω
The maximum cable length is dependent on the cable manufacturer's attenuation specifi cation, which is approximately
proportional to conductor size.
The characteristic impedance of a transmission line is a function of the physical dimensions of the conductor and the
permittivity of the dielectric (the insulation), at high frequencies this is approximately equivalent to:
Vs
Vd
min
min
V
(
4
) V
pd
2
Pd
N
V
pd
Vs
R
min
km
L
km
Capacitance
50nf/Km
100nf/Km
Vpd
= Potential across each conductor (limited to 4V)
Vsmin
= Minimum Supply Voltage
Vdmin
= Minimum Detector Voltage (18V)
Pd
= 18 watts per Flame Detector (inc. Heater) or 6
watts excluding Heater
N
=Number of Detectors
Lkm
= Cable Length in Kilometres
Rkm
= Maximum Conductor Resistance per Kilometre
and L
in the above equation. The supply voltage and cable
km
km
Conductor
Attenuation @
Resistance
1MHz
- -
150R
Installation of the Dräger Flame Detector 5000
Inductance
- -
- -
6db
0.7mH/Km
17
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