Railway Signalling Cables Part -2

Low Smoke Zero Halogen (LS0H) ,Fire Resistance and Fire Retardant Properties

National Fire Protection Association of United States of America (NFPA-130)

Potential Fire on Tunnel based cable can be disastrous ,when the inert gas emitted from the smoke due to fire suffocate the passenger ,including in an emergency  evacuation situation.Various railways have made stringent on anti fire characteristics of installation materials.Cable has significant role hazard when fire occurs. In North America ,National Fire Protection Association (NFPA) has come up with dedicated standards for Fixed guide way transit and passenger rail systems ,popularly known as NFPA -130 standard 2017 version along with NFPA 262, Standard Method of Test for Flame Travel and Smoke of Wires and Cables for Use in Air-Handling Spaces, 2015 edition. It defines "all wires and cables shall be resistant to the spread of fire and shall have reduced smoke emissions. Wires and cables listed as having adequate fire-resistant and low-smoke producing characteristics, by having a flame travel distance that does not exceed 1.5 m (5 ft) and generating a maximum peak optical density of smoke of 0.50 and a maximum average optical density of smoke of 0.15 when tested in accordance with NFPA 262, shall be permitted for use instead of the wires and cables specified " There are many countries follow NFPA requirements and test conducted as defined on UL 1685 "Vertical-Tray Fire-Propagation and Smoke-Release Test for Electrical and Optical-Fiber Cables". Philippines is one of the Asian country follow as per NFPA norms and tests are conducted as per UL 1685 and FT4/IEE1202.

IEC standards and requirements for Low Smoke Zero Halogen (IEC 60754-1-2)

According to IEC 60754-1 :Determination of the amount of Halogen acid gas and  IEC 60754-2(Smoke Corrositivity)defines the inert gas emission shall not exceed not more than 0.5% . 

1)     Test sample subjected to flame combustion for 40 minutes : Light transmittance of smoke emissions shall not be less than 60% and Halogen acid gas evolved during combustion shall be less than 5 mg/g.

IEC standards and requirements for Fire Retardant ,High Ignition Temperature,Low Flame Spread, Low Fuel Content  (IEC 60332-3)

  1)     Test sample subjected to flame combustion for 40 minutes and  Length of charred portion shall not exceed 2.5m from the burner.

Abrasion and Crush Load

Cables could be damaged during installation and abrasion test and crush load resiatnce test according to IEC 60229  shall be conducted .

1. Test sample subjected to 25/25 passes over test path with applied force 15 N and visual inspection shall prove  No crack or splits observed

UV Resistance Test (UL 1581)

1.-Dumb bell test pieces are prepared  from the outer jacket

2.-Samples are exposed to a xenon-arc lamp ASTM 155 for 720 hours.

3.-The color in the aged samples is compared to the original ones to verify there is no fading.

4.-The tensile strength and elongation at break values in the aged samples are determined as per test Nº24 and Nº25 and compared to the original ones to determine the variation.

No color fading Mechanical properties shall be  ± 20 % original

Anti Pest Properties  (Rodent ,Termites )

Armour of the cable provide protection from pest especially for the outdoor cables resisting on moisture .There are three types of armour used 1) Steel wire 2) Steel Tape and 3) Corrugated Steel Tape .Steel tape are widely used these days for better bending radius .Approved pesticides could also be used along with the XLPE compound but over a period of time pest get used to the "taste " and became immune.Tests are conducted on live pest environment .

Longer Service Life (EN 60811-401(Thermal Aging Test for Tensile Strength and Elongation ) On Outer Jacket and Inner Jacket)

Thermal aging tests are performed as defined in EN 60811-401 to ensure cables can long last atleast life span of the signaling system

Cleaning Solution ,Iron Dust ,Carbon dust ,Greece Protection

These tests are conducted after exposing to each of these materials for 180 hours and tensile strength variance (unaged vs aged) shall not be : ±30%

Types of Armour

There are three types of armour available

1) Corrugated Steel Armour :- These are steel sheet 0f 0.2mm to 0.3mm wrapped around the cable and thermionically welded at the joints ,There are pros and cons for this tupe of armor .This gives better water blocking and poest resistance protection but bending radius will go more than 15D ,where D is the outer Dia of the cable .

2) Steel Tape Armour :- Widely used armour formed like a creepr on a tree with Steel tape

3) Steel Wire Armour :-Steel wires are covered around the cable just below the outer sheath.

Types of Insulation and Outer Jack

1) PVC (No Ls)H property)

2) XLPE

3)HDPE

are the popular compound used for outer sheath and insulation of the conductor .PVC is ruled out due to its inert gas emission properties .

Types of EMC shield

1)Aluminum Foil with Drain wire attached to it :- Used for low frequency application such as control cable for dry contact reading ,control of signals ,point detection etc

2) Braided Shield :- Used for high frequency application

Its always recommended to earth the armour along with shield at one end for DC traction and two ends for AC traction .A non earthed shielded cable is more vulnerable than a non shielded cable !

These are the Mechanical Properties for designing a better signaling cable and in last part I will cover the Electrical Properties of the cable