Furuno FELCOM251, Installation Manual

Page: 50 / 122

3. CONNECTING POWER
3-3
There are several ways of turning the Communication unit ON/OFF:
• ON/OFF switch on the back of unit
• ON/OFF switch on the front of unit (Only 19" version)
• Ignition ON/OFF by Discrete I/O pin 5 (must be configured in Web interface first)
Calculating the maximum power cable extension
For 24 V DC operation , the total impedance must be max. 500 m

, including the
source impedance in the ship installation.
For 12 V DC operation , the total impedance must be max. 85 m

, including the
source impedance in the ship installation.
The total impedance is made up of the following:
• the source impedance in the ship installation.
• the cable impedance of the supplied power cable, including the impedance in the
joint of the two cables. In the following example, the impedance of the cable and
joint is set to 10 m

(1 m power cable). Note that if the cable length or type is
changed, the impedance will change accordingly.
• the extension cable impedance.
To calculate the maximum cable extension, do as follows:
1. First research the source impedance in the ship.
2. Then find the resistance per meter for the cable type you are going to use:
These numbers are typical cable-impedances for copper cables.
Installer must consult datasheet for the specific cable he choose to use.
3. Calculate the maximum allowed impedance in the extension cable as follows:
Max. allowed impedance in extension cable = max. total impedance - (measured
source imp impedance of the supplied cable).
4. Then calculate the max. extension cable length as follows:
The length is multiplied by 0.5 above because there are two conductors in the ca-
ble.
Max. length = 0.5 x
Max. impedance in extension cable (from step 3)
impedance/meter (from step 2)