Railway Signalling Cables -Part 1

Ever growing availability requirements for the operational rail lines and the decreasing MTBR (Mean Time Between Repair) for most of the railways ( in some cases its even less than an hour to rectify the fault) making signaling components requirement  more robust and 'maintenance free' as much as practically possible  during signaling life span .Signalling Cables are one of the major component requires longer replacement time compared to any other signaling components during failure or first order or second order maintenance .These cables run from Signalling Equipment Room to the end gear which are few Kilometers away end to end .Requirements shall be separately considered for cables used with in tunnel application (Stringent Fire Retardant ,Fire Resistant ,Low Smoke Zero Halogen ) and outdoor application which are exposed to UV rays (consideration to be given to as per the level of UV intensity in the region its applied for) and rain exposure. Due to seepage reasons tunnel also could be flooded to make the cable requirement similar to the cable in open trenches with submersible conditions. Additional factors to be considered are pest attack (Termites and Rodent ) ,presence of carbon /Iron ,Greece ,Oil ,wash plant washing solution with in the depot area. If the cables are for tunnel application without direct sunlight exposure ,can eliminate requirement of UV booster on the outer sheath.Similarly there is no carbon dust /Iron dust/Oil /wash plant cleaning solution protection are needed on the non depot cable and there by make it economical. Its always better to ensure cable have fire retardant ,fire resistant ,LS0H (Low Smoke Zero Halogen ) ,lateral and longitudinal water protection irrespective tunnel environment or non tunnel outdoor application. Based on Application cable can be classified as Main Cable ,Tail Cable and Indoor cable and can further classified into Control Cables ,Power Cables ,and communication cables (including Multi Mode/Single Mode Fiber Cable ,Cat 5/Cat 6 ) .More classifications for control/power cables are made based on number of conductors in each wire of the cable (Class 1 ,Class 2 and Class5 ) .Class 1 cables are single copper wire cables .It ranges from 1core  upto 50 Core in general .More the class ,the conductors have more strands (tiny conductors to form one single core)

Main Cables

As defined by the British practice ,Main cables are the cables originating from Signalling Equipment Room to the Trackside boxes next to the end gear .These cables shall be armoured single stranded (class 1) /Multi stranded (Calss 2 ) based on application to protect from abrasion

Tail Cables

Cables from disconnection boxes /junction boxes near the end gear (Signal Mast base box for Example ) to the end gear cables are tail cables .This can be non armoured ,Class 1,class 2 or class 5 based on flexibility requirement .(It might be class 1 based on application signa frequency)

Indoor Cable

Cables which are "clean " (surge free ) originating from cable frame (Intermediate Termination Box) to the signaling cabinets .

Cable Frame segregate outdoor "Dirty" cables (with the help of Surge Arrestor and Lightning arrestor ) to Indoor cables .Indoor cables are usually multi stranded (Class 5) with or without braided shield or aluminum shield depends on application. Its out of scope for this article .

Control Cable

Control cables are used for controlling the end gear ,it could be carrying 24/48V DC or audio frequency depending on the application .Cables carrying low voltage or medium voltage and are class 1 or class 2 or class 5 depends on application .Main cables shall be armoured where as tail cables could be non armoured

Power cables

Cables carrying low voltage or medium voltage and are class 1 or class 2 or class 5 depends on application .Main cables shall be armoured where as tail cables could be non armoured.

Communication Cables

Data transfer cables falls under this category which include cat 6 copper cable ,Fiber Optic cables etc .Again Main cables shall be armoured where as tail cables could be non armoured

IEC Standards

IEC stanadrds are more popular among many countries ,however American and British and even German stanadrds are also applied depends on countries.(Philippines follow NFC stanadrds for Anti Fire Properties )

Water Blocking Properties

Longitudinal and Transversal water blocking properties are well suited for cable vulnerable to flooding .Two layers of Swellable Water Blocking Tapes are encircled around cable construction with the first layer of tapes around wires and second layer around cable armour .The protecting factors includes ,XLPE (cross linked poly ethylene) outer jacket ,two layers of water blocking tape , insulation of the conductors and armour in some case (if its corrugated steel armour  which are thermos welded around the cable formation ) .Swellable tape act as a expandable bed and the jelly absorbs water from the moisture there by protecting moisture absorbed into the bare conductor. Standard EN 50289-4-2A defines the requirements and test for Longitudinal and Transversal water tightness test alternatively its defined in EN 50288-1 and EN 50288-7 for Longitudinal water tightness and IEC 61156-1 for transversal water tightness .Requirements for water absorption are defined in IEC 60811-402 .Satisfactory test according to these standard will ensure a water fail proof signaling cable. Standards cannot be shared as it is and for informative purpose I try to describe the test procedures in a brief

Water Absorption on conductor insulation Test (EN60811-402)

1.-Samples of insulation, 300 mm length are taken off form the cable

2.-Samples are weighted.

3.-A length of 250 mm is immersed in deionized water at 80 oC for 14 days at 80oC.

4.-All the water drops are removed from the insulation by drying the surface.

5.-The samples weighed again to compared with the original shall be <5mg/cm

Water Absorption on Outer Jacket Test (EN60811-402)

1.-Samples (5 test pieces) are taken form the cable outer jacket and weighted.

2.-Samples are immersed in deionized water at 80 oC for 14 days at 80oC.

4.-All the water drops are removed from the samples by drying the surface.

5.-The samples are weighted again and the results compared to the original ones shall be<5mg/cm2

Armour Long Water Tightness Test EN50289-4-2/A

1.-Samples of 3-6m of final cable cut from the drum

2.- Circumferential portion of sheath 20 mm wide shall be removed of cable and a watertight sleeve shall be applied over the exposed area so as to bridge the gap in the sheath. The closer cable end shall be sealed.

3.-1 meter height water column is applied over the jacket gap tape for 24 hours.

4.-No water shall be detected at the unsealed end of the sample

Transversal water tightness Test IEC 61156-1 /EN50289-4-2/A

1.-Samples of 3m of final cables are cut from the drum.

2.-The samples are immersed in a pool having both ends out of the water for 24 hours.

3.-A high-voltage test (3,535 Volts DC – 1 min) is carried out between the armour and the water.

4.-Samples are open to check any trace of water within the cable core.

5.-No HV failure, No water penetration after the test.

Next Chapter ,I will cover Zero Halogen , Fire Resistant ,Fire Retardant Properties and the requirements with respect to IEC and NFPA standards followed by rest of mechanical features and electrical properties to design a good signaling Cable.Every week end I prepare a topic for my young aspiring signaling friends ranging all topics related to signaling ,including Interlocking ,Signalling Principles , Signalling a Track Layout ,Signalling Control Table ,Automatic Train Supervision ,Automatic Train Protection (Automatic Train Control ) ,CBTC system ,ERTMS and Cab Signalling ,Incremental Train Contriol System (American ) ,High Capacity Signalling for High Speed Lines and sub urban lines ,DCS systems ,Primary and Secondary Train Detection .Axle Counter Vs Track Circuits ,Insulation and Bonding Plan andSignalling Power Supply Systems in detail. Once I finish my series I voluntarily retire from Railways to follow my dreams !