Protec Digilite Installation Manual page 7

Equation 2 – Calculating the maximum cable resistance of a spur fed off a loop.
Digilite®
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Where:
ηtotal is the total number of fittings on the total loop (loop and all spurs)
ηspur 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 = 8 – ( R
spur (max)
Worked Example 2
The following example shows a situation where the main loop resistance is 25Ω 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 = 8 – ( 25 x 280 x 0.0004 )
spur (max)
R = 185 Ω
spur (max)
Worked Example 3
The following example shows a situation where the main loop resistance is 16Ω 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 = 8 – ( 16 x 340 x 0.0004 )
spur (max)
R = 145 Ω
spur (max)
N93-561-85 Issue 0
Rloop (Ω)
x η x 0.0004 )
loop total
( η x 0.0004 )
spur
( 70 x 0.0004 )
( 100 x 0.0004 )
Rspur (Ω)
η
η
spur spur
7
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Loop broken
at one end
η η
spur spur