7
N93-561-85 Issue 0
© Copyright Protec Fire Detection plc 2009
Equation 2 – Calculating the maximum cable resistance of a spur fed off a loop.
Where:
η
total is the total number of fittings on the total loop (loop and all spurs)
η
spur is the total number of fittings for a particular spur
Rloop is the go and return resistance of the main loop
Rspur is the go and return cold resistance of a particular spur
To calculate the maximum go and return cold resistance for a particular spur the following equation
may be used.
R
spur (max)
= 8 – ( R
loop
x
η
total
x 0.0004 )
(
η
spur
x 0.0004 )
Worked Example 2
The following example shows a situation where the main loop resistance is 25
Ω
and there are four
spurs fed off the loop, each with 70 fittings connected.
The maximum go and return cold resistance for a spur may be calculated as follows.
R
spur (max)
= 8 – ( 25 x 280 x 0.0004 )
( 70 x 0.0004 )
R
spur (max)
= 185
Ω
Worked Example 3
The following example shows a situation where the main loop resistance is 16
Ω
and there are four
spurs fed off the loop, two with 100 fittings connected and two with 70 fittings connected.
The maximum go and return cold resistance for a spur with 100 fittings may be calculated as follows.
R
spur (max)
= 8 – ( 16 x 340 x 0.0004 )
( 100 x 0.0004 )
R
spur (max)
= 145
Ω
Digilite®
control panel
Loop broken
at one end
η
spur
η
spur
η
spur
η
spur
Rloop (
Ω
)
Rspur (
Ω
)