Railway Signalling Cables-Part 3 (Last Part )

Electrical Characteristics

"Ohmical"Resistance (EN 50289-1-2)

According to  EN 50289-1-2 ,corrected ohms/km at 20ºC shall be less than ≤ 23.4 Ω/km for a 1 sqmm cable . Each manufacturer will mention this resistance in the respective cable data sheet.As a signaling engineer its our responsibility to calculate the voltage drop to decide the conductor thickness required for each application .Voltage drop more than 10% at the end gear would not be suitable as the end gear voltage tolerance might be within this range .As an example ,A point machine motor designed for 110V  shall minimum receive 99V to achieve the expected performance for the motor .Throw force and  thrust  required cannot be achieved with reduced voltage and point machine might take more time to make the detection and there by impacting even headway and life span of the motor.(A point machine suppose to finish the throw within 3 second might take 5 to 6 second ) .If the resistivity of the cable is mentioned as 20Ω/km in the data sheet  means a closed control circuit (with positive and negative of two conductors ) shall have 40Ω/km  and a detection circuit have 80Ω/km  for four wires (to and fro) During cable continuity test if the resistance is much greater than this value means poor conductivity and cable shall be discarded. Bigger conductor thickness shall be selected to cover the voltage drop until it reaches  less than 10% voltage drop (Check the end gear voltage tolerance ).Continuity test shall be performed with multimeter by joining each two conductors at one end and record the value.

Mutual Capacitance(EN 50289-1-5)

Mutual Capacitance between conductor shall be minimum for better electrical performance especially for audio frequency based track circuits.(Safety case of the product shall be checked for requirement )  . A good signaling cable  shall have  mutual capacitance ≤ 45 nF/km  as per standard ,especially cores are arranged in the form of star quads

Insulation Resistance (EN 50289-1-4)

When the conductor under test is separated from the rest of the conductors which are grouped together using a megohmmeter (Lovingly known as Meggar ) a 500 V DC voltage is applied for 1 minute having the direct reading of the insulation resistance and the reading shall be infinity (≥10.000 MΩ x Km)

Di-Electric Strength (DIN VDE 0479-509)

Using a power source, the each  groups of conductors are tested to each other at 3535 Volts DC for 30 seconds no perforation is expected .

Annealed Copper conductor (IEC 60228/EN 60811-203 )

Best copper used shall be compliant to either of the above standard

Class 1 ,Class 2 and Class 5

Depend on number strands on each core  cables are classified into Class 1 ,Class 2 and Class 5 .Class 1 constitute of  single conductor wires .More the class more the number of strands be !

Note :- I have covered all about designing a robust long lasting cable and prepare a specification for your requirement .By reading these three part you can design a cable as well as select a cable as per voltage drop ,load requirement and short circuit current for power circuits .I will cover Cat 5/Cat 6 and SM /MM Fiber Cable as part of another topic in commutation topic.