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5.0 Cable Requirement
Cable type : Two core or screened
Recommended size : 1.0 to 2.5mm
2
(see Volt drop calculation)
Maximum Resistance : 40 Ohms
Maximum Capacitance : 2.0 µF / km
Maximum cable length : 2 km (see Volt drop calculation)
5.1 Cable Length - Voltage drop
5.1.1 Spur Circuit.
Make sure that the minimum voltage at the furthest point of the spur exceeds 15 Volts
alternatively the maximum voltage drop on the cable should be less than 10 Volts.
Calculation for Voltage Drop
Vdrop = R x I x n (Volts) (1)
Re-arranged equation (1)
R = Vdrop/I x n Ω (2)
n= Number of fittings or addressable interfaces on the spur
I = Average Current of an addressable interface in the fitting = 0.0012 Amps
R = Cable cold resistance ( go and return) Ω
Vdrop not to exceed 10 Volts
This resistance will vary depending on the size of the cable as shown below;
Table 1.0
Therefore for worst case condition, the maximum cable resistance R (max) is;
R(max) = 10/ I x n = 10/0.0012 x n = 8333.33/n Ω (3)
Example 1 If a spur has 250 fittings , using equation (3) the maximum cable resistance is.
R(max) = 8333.33/250 = 33.3 Ω
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EASICHECK™
Size Cold Resistance Cold Resistance
Single Core Go and Return
1.0 mm
2
18.1 Ω/km 36.2 Ω/km
1.5 mm
2
12.1 Ω/km 24.2 Ω/km
2.5 mm
2
7.4 Ω/km 14.8 Ω/km