Doc No. FM0804 issue 4 Page 13
9.
NETWORK LOAD CONSIDERATIONS
It is very important to take voltage-drop across the system network into consideration when planning an installation.
It is the cable resistance on the power supply wiring that gives rise to voltage-drop, the longer the cable run and the
larger the current requirement the larger the voltage-drop. To perform a detailed assessment of voltage-drop is
complex; you need to know the current requirement of each device, the distance between each device and the
resistance of the cable per metre. To estimate the worst-case scenario simply take the maximum cable run and
calculate the voltage-drop as if all devices were at the end of the cable.
Typical current requirements of devices connected to the Advent XT2 network are given below:
24V Network Devices
Standby
Intercom Unit
2mA
Ceiling Pullcord with 24V LED
-
Door Entry Telephone
2mA
Manager Callpoint
2mA
Lift Speech Unit
2mA
Network Receiver
6mA
Door Entry Panel
30mA
EXAMPLE
The estimated voltage-drop on an installation with 40 intercoms, 1 Manager callpoint, 1 lift speech unit and 3 network
receivers on 400 metres of cable would be;
(40 x 2mA) + (2 x 2mA) + (3 x 6mA) = total current 102 mA (0.102A) on 24V line
400 metres of CW1308 cable = 40
Ω
(10
Ω
per 100 metres)
using V = I x R: 0.102A x 40
Ω
= 4.08V drop on 24V line (leaving approx. 20V)
Devices on the 24V supply will work down to approx. 16V therefore the above drop is acceptable.
REDUCING VOLTAGE DROP
There are several means of reducing the voltage-drop; site conditions will dictate which is used;
1. The best solution is to split the network into several short spurs rather than one long run. This has the advantage of
a smaller voltage-drop per cable run and simplified fault finding.
2. On large installations install a 1.0mm power pair or a CW1308 with additional spare pairs to allow “doubling
-
up” on
power & 0V conductors. Increasing the cable cross section area will reduce its resistance therefore reducing the
amount of voltage-drop.
3. Looping back the power & 0V conductors from the last intercom to the controller will form a ring circuit which will
effectively halve the resistance.
4. An auxiliary power supply can be connected to the site wiring at a suitable point to boost the supply midway along
the cable run. The disadvantage of this being future Service E
ngineers may not be aware of PSU’s presence and the
network will remain powered when the controller is switched off.
IMPORTANT:
Max current rating per CW1308 0.5mm core is 0.25A, therefore 1 pair = 0.5A
THE RESISTANCE OF CW1308 CABLE IS
APPROX 100
Ω
PER 1000 METRES
