PCE Instruments Xgard Bright Manual Del Usuario página 22

To minimise cable voltage drops (and to maximise the potential total cable length and
detector network quantity) large cross-sectional area (c.s.a.) cable must be used for
the 24V/0V power connection. Crowcon recommends cable with 1.5mm
used for the power.
Twisted pair and screened cable is recommended for the RS485 signals. The screen is
to be earthed at the control panel only, but continuity must be maintained through the
detectors extending to the end of line detector. The end of line detector also needs a
terminating resistor link fitted to the top PCB (the terminals labelled RT).
Specialist cables are available combining large c.s.a. conductors for power and twisted-
pair signal cables for RS-485 communications, however in some cases it may be nec-
essary to run separate cables to the detector network. In this instance it may be most
practical to terminate the two cables within a junction box near to each detector, and
drop and single/combined cable with smaller power conductors locally to the detector.
On large networks, or where long cable runs are required, it may be necessary to power
groups of detectors via separate power supplies placed locally around the installation.
Where this method is deployed, the 24V/0V cables for each group of detector must be
isolated to their dedicated local power supply.
3.7.1 Calculating acceptable cable length and detector quantities
It is essential before attempting installation to calculate the voltage to each detector
given the power supply voltage, cable resistance and cable lengths required. The more
detectors connected to the linear bus, the greater the power required to run the system.
To calculate the power required for a particular setup, it is necessary to know the cable
resistance between each pair of detectors. A current of a maximum 0.07A (toxic) must
be allowed for each 'hop' between each detector (this assumes the highest power con-
figuration for each detector: pellistor sensor). The voltage to be applied can be calculated
by estimating the voltage drop across each detector 'hop' – at the end at least 10V must
remain to ensure that the last Xgard Bright detector functions correctly.
Follow the steps outlined below and the sample calculation shown in the next section to
calculate for specific applications.
1. The voltage must not fall below 10V, so start the calculation by setting the voltage
at the last detector in the line at that value.
2. Each detector may draw up to 0.070A. Calculate the cable voltage loss of the first
'hop' between detectors by taking the current 0.070A and multiply this by the cable
resistance of the 'hop' between the last and the last but one detector.
3. Add this voltage drop to the initial 10V to get the lowest acceptable voltage at the
last but one detector. Add 0.070A to the value for the 'aggregate current' to get
to 0.14A, the minimum current running through the last but one 'hop' of the bus.
Multiply this by the cable resistance for the last but one 'hop' to get the next volt-
age drop.
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Xgard Bright
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