Dräger Flame 5000, Техническое руководство

Страница: 18 / 35

18
Installation of the Dräger Flame Detector 5000
Equation 2: Characteristic Impedence Calculation
L = Cable Impedance (mH)
C = Cable Capacitance ( —
F)
Zo = Characteristic Impedance (Ohms)
C L ) ( Zo y
ȍ
The RS485 communications 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:
RS485 Communications Cable Characteristics
Cable
Characteristic
Characteristic
Impedance
Capacitance Conductor
Resistance
Attenuation @ 1MHz Inductance
Nominal 120 Ω 50nf/Km - - - - - -
Absolute Limit 90 Ω to 120 Ω 100nf/Km 120 Ω 12dB 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 the same as for the video
above.
Installation Check-Points
Experience has shown that poor installation and commissioning practice may result in an unreliable fi re detection system
that is prone to malfunction, unwanted alarms, and at the same time fails to meet the site performance targets. Before
installing the detector it is important to take into account where it is to be located and how it is to be mounted.
Mechanical Installation
When locating the Dräger Flame Detector 5000 consideration should be given to maintenance access to the •
detector.
The detector mounting should be secure and vibration free.
•
It is advisable to check the detection locations, prior to fabrication of the mounting supports, as changes are •
frequently made during construction at site which can a ff ect detector coverage.
The installation should allow for easy detector removal for maintenance or repair. •
The detector should be fi xed to a stable supporting structure using the mounting bracket provided. The supporting 1.
structure must allow for horizontal adjustment of the detector orientation. The support structure should be in place
prior to detector installation.
The threaded
fl ame path of the enclosure cover and body must be protected from damage during installation. Any 2.
such damage can destroy the validity of the enclosure.
The detector electronics shall be protected from mechanical damage and external sources of EMI such as X-rays,
3.
RFI and electrostatic discharge.
Fit the mounting bracket to the support structure using 8mm bolts (not provided). The detector (bracket) should
4.
be oriented to provide the desired coverage.
The hex head bolts should be
fi tted to the enclosure body prior to mounting to the bracket. The detector enclosure 5.