Dräger Flame 5000, Technical Manual

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Installation of the Dräger Flame Detector 5000
17
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 su
ffi 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
2
Vd Vs
) V 4 ( V
min min
pd
d
¸
¸
¸
¸
¹
·
¨
¨
¨
¨
©
§
¸
¹
·
¨
©
§
u
y
km
min
pd
km
L
Vs
N Pd
V
R
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
Use the value of R
km
calculated above to select a suitable gauge of conductor, alternatively, to calculate the maximum
cable length from a known conductor resistance swap R
km
and L
km
in the above equation. The supply voltage and cable
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 su ffi cient allowance
for the e ff 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
Capacitance Conductor
Resistance
Attenuation @
1MHz
Inductance
Nominal 150 Ω 50nf/Km - - - - - -
Absolute Limit 90 Ω to 150 Ω 100nf/Km 150R 6db 0.7mH/Km
The maximum cable length is dependent on the cable manufacturer’s attenuation speci fi 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: