EXP-PSU
Page 4
Bus Cabling Requirements
Cable Type
In general, the EXP-PSU requires standard
7/0.2 un-screened four core alarm cable for
wiring the bus.
For maximum performance in harsh
environments use twisted pair screened
cable with a characteristic impedance of
100-120ohms eg: Belden 8132 or other
cable designed for RS485.
Use one pair for data lines A and B. Use the
other pair for 12V and 0V. For optimum
performance the voltage at the keypads
and expanders should be greater than 12V.
Screened cable may prove necessary if the
installation site has equipment that
produces high levels of R.F. (Radio
Frequencies). For example, welding
equipment is known to produce a large
amount of radio interference. If screened
cable is required, you should keep to the
following guidelines:
1. Avoid earth loops by connecting the
screen on the cable to mains earth at
the control unit but not at the keypad,
expander or EXP-PSU.
2. The continuity of the cable screen is
most important and screens MUST be
continuous along the full length of the
cable.
3. Where the cable enters any metal
enclosure, ensure the screen is isolated
from the case.
Cable Segregation
Segregate the bus cabling from any other
wiring, such as mains supply cables,
telephone cables, computer network cables
and R.F. cables. Use cable ties to keep
cables separated.
Keep the bus cable clear of cables supplying
sounders, extension loudspeakers or any
other high current devices.
Voltage Drop
In order for the system to work correctly,
the voltage at each device must NOT drop
below 10.5V even when running on the
standby battery. Cooper Security
recommend that the voltage at each device
should stay above 12V.
Standard 7/0.2 alarm cable has a
resistance of 8 Ohms per 100m per core.
The voltage drop is calculated using the
following formula: V Drop = Current drawn
x cable length x 0.08 x 2.
Table 1 on the next page shows the voltage
drop against the current drawn and cable
length.
The shaded area shows where the voltage
drop would cause the bus voltage to fall
from 13.8V to below 12.0V when using a
single core.
Reducing Voltage Drop - Method 1:
Double up the supply connections (12V and
0V), which will halve the resistance on each
core and therefore halve the voltage drop.
When using Table 1 to calculate the
expected voltage drop, simply divide the
voltage drop for a single core by two.
Reducing Voltage Drop - Method 2:
Supply the detection devices from the Aux
output on separate cores. This is the
preferred method of reducing voltage drop
as detectors generally operate at lower
voltages (9.5V). When using this method,
the network cable must have two spare
cores.
Connecting A Remote Power Supply
Figure 4 shows the recommended method
of connecting the EXP-PSU power supply.
The bus cable from the control unit
terminates on the EXP-PSU Bus In
connector and a new cable starts from the
EXP-PSU Bus Out connector. See page 6 for
more details.
