Typically, in a Metro Station, the Power from the transformer goes to the main panel of the station and is distributed further to the vital services or facilities. At times, booster transformers are provided to minimize interference with communication lines. In the case of DMRC, for instance, there are 2 Auxiliary Substations (ASS) located at each underground station. The number of ASS could be higher if there is an intersection. Generally, the 2 ASS are located at the extreme ends of the Station. If there is no possibility of finding land, or due to other construction requirements, the ASS might be located closer together.

The facilities at the underground station are divided into different categories starting from Emergency services to Normal services. The emergency services are provided with suitable UPS and Diesel Generator backup systems. Functions such as power supply, control and emergency require different type of cables suited for that particular application.

• Emergency lighting in underground stations and tunnels
• Ventilation fans
• Fire suppression systems

Fire Survival (FS) cables are used to power critical facilities within underground stations and tunnels. Further on, these cables are specialised to not generate halogen gasses in fire conditions. It is recommended that Fire Retardant LSZH type cables be used in underground metro stations.

British Standards BS 7846

The British standards prescribe different durations and different temperatures for FS cables and circuity integrity cables. Depending upon their fire-resistance characteristics, the British standard classifies cables into 4 categories: F2, F30, F60 and F120. The minimum requirement to meet for all cables is F2 when tested under CI.17.4.2 specifications.

F2 category cables must survive under 950 degrees for 3 hours. Provisions for mechanical shock and fire testing of F2 cables have also been laid down in the standard.

The tests for fire, fire and water, and mechanical shocks are carried out simultaneously on F2 cables while for other categories they are conducted separately.

Know the new Standard: IS 17505 (Part 1) for Fire Survival Cables

The Indian Standards for Fire Survival Cable which have been under consideration by the Bureau of Indian Standards (BIS) are now published by BIS in a Gazette notification dated 19 March 2021 - Standard Number: IS 17505 (Part 1) The standards set the specification for Thermosetting Insulated Fire Survival Cables for Fixed Installation having Low Emission of Smoke and Corrosive Gases when affected by fire for working voltages up to and including 1100 Vac and 1500 Vdc.

Until now, the domestic industry, including the Metro Railways have been following the British Standard BS 7846 for fire-survival cables. Such Cables must withstand temperatures up to 950°C for at least three hours. This is to ensure that emergency services continue to stay operational to ensure a safe passage. With the publication of IS 17505 (Part I), the new standards can now be used as a benchmark by manufacturers and users.

This Indian Standard IS 17505 (Part 1) has been developed keeping in mind the specific needs of the Indian industry and are aligned with other international specifications like BS 7846/IEC 60331

As of now, the Delhi Metro uses FS-type XLPE insulated, armored copper conductor cables as per BS7846 standard for emergency services. In near future, the Indian Standard for Fire Survival Cables IS 17505 (Part 1) will have to be followed for emergency and other critical applications with high fire risks.

Code requirements for the selection of cables

Regulations laid down in the NFPA 130 (2017) are followed in the selection of FS cables for underground stations. Key requirements with regard to such FS cables are: