Ensuring safety beneath the surface - A deeper role for Power Cables in Metro Rail Infrastructure

Metro Rails are all set to take the centre stage of mass urban transportation in India’s fastest-growing cities. But the performance and safety of the new and fast-emerging metro rail infrastructure in India will only be as good as its supporting infrastructure and systems. Safe and reliable electrical power is at the core of keeping Metro Rails running. And Power Cables have a key role to play in keeping the safety and reliability of Metro Rail infrastructure on track.

Metro rail systems are growing across the length and breadth of India. Presently, there are 13 metro rail systems running across 678.52 kms of operational metro lines and over 540 stations. A further 557.34 kms are under construction.

Safety of stations

Metro Rails Stations are used by millions of people every day, and the safety of such buildings is critical. As of now, the Metro Rail Stations are classified into three broad categories: Elevated, At-Grade, and Underground stations. With tough operating conditions the Metro infrastructure in general carries a high risk of electrical fires. The challenges are even more complex in the case of underground stations where smoke cannot easily pass into the open air. Power Cables are key to arresting the occurrence of electrical fire and reducing the risks, in case of an instance of fire.

Recently, an incident of fire was reported at the ongoing Metro-3 line in Mumbai Metro Rail Corporation. The incident was caused by short-circuit, damaging 12 Power Cables. While no injuries were reported, the incident highlights the risks to human safety in such facilities

Cabling in underground metro stations is critical to arrest the spread of fire and smoke.

Setting the standard

Standards set the benchmark and show the way. Delhi Metro Rail Corporation (DMRC) is the biggest metro network in India, carrying over 3 million passengers per day. Over the years, cabling practices and specifications of Delhi Metro have become benchmarks for most other upcoming metro networks in India. The standards followed by DMRC, especially the use of Fire Survival (FS) cables, must be understood and followed further in ensuring the safety of underground stations.

Understanding Power Supply in Underground Metro Station

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.

Fire Survival Cables to safeguard underground stations

Underground stations present complex challenges in terms of fire safety. The underground stations and tunnel areas do not allow free passage for smoke and gasses in an instance of fire.

Critical facilities in the underground stations must be kept working under fire situations.

  • 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:

  • The cables should have at least 1 hour of fire-resistance rating when tested in line with ANSI/UL 2196
  • Emergency power should be provided to the enclosed trainway in accordance with article 700 of NFPA 70 and chapter 4 of NFA 110

Article 700 of NFPA further states that circuit integrity cables should have a fire rating of minimum of 2 hours when tested in accordance with ANSI/UL 2196 standards for testing of fire cables.

The Indian National Building Code also provides regulations for FS cables. It uses the IS 16246:2015 standard for reference and states that the circuit integrity of FS cables should be 750 degrees for 3 hours. DMRC and follows BS 7846 which specifies FS cable tolerance to be at 950 degrees for 3 hours. It is now important that the IS 17505 (Part 1) Standards are widely adopted by Metro Rail Corporations across India

Important safety standards for Power Cables

  • IS 17505: Indian Standards for Fire Survival Cable)
  • BS 6387/8491: Circuit integrity test
  • BS 7629-1/2: Fire resistant multicore and paired cables (300/500V)
  • EN50525: Halogen-free single core cables
  • IEC 60331-21: Circuity integrity test (750 degrees C)
  • IEC 60754: Determination of amount of halogen gasses.
  • IEC 61034: Measurement of smoke density
  • IEC 60332-1: Smoke density
  • IEC 60332-3 Vertical flame test on bunched cable
  • BSEN 50288-7: Sectional specification for instrumentation cable

Ensuring complete safety - Cable selection and installation considerations

Compliance with critical requirements needs to be ensured at every stage: from choosing, manufacturing, to the installation of cables in Metro stations. The DMRC closely monitors the production process of the cables at every stage, meeting various challenges that emerge along the way.

Tender stage: Specifying the correct manufacturing and testing standards must be choosing the cables for various applications

Manufacturing: The rail corporation must correctly specify the requirement of cables to the manufacturer. Ensure strict compliance with material and manufacturing requirements. Conduct diligent supervision of testing and quality control.

Factory Acceptance Tests:A person knowledgeable about cable testing and standards must be closely involved in the testing of the cables. Strict adherence should be ensured withprocedures for sample preparation, test apparatus, assemblies, and test methods and guidelines

Third-party testing:11Scheduling of third-party testing is a major challenge in cabling. Sufficient time needs to be allowed for actions that are an essential part of their-party testing.

  • Preparation of samples
  • Calibration of test equipment
  • Interpretation of codes and guidelines
  • Accommodation of witnessing agencies

Testing procedures: There is a need to refine test procedures to minimize variance in test results due to human errors and other factors.

Risk assessment: Time is a very limited resource in metro systems. Projects run on extremely compact timelines, Many months cannot be spared for assessment of cable and feedback from manufacturers or third parties. This creates some complications. Many times, third-party testing is conducted after the cables have been laid. Procedures need to be developed to quantify the risks and marginal shortcomings that can emerge

Challenges in BMS control cables: There is no single standard available for screening of BMS cables. Control cables for BMS need to be procured after careful consideration of three standards. These cables are chosen as per BS 7846 and 6724 standards. But in these standards have nothing mentioned about the screening of the cable. Hence, a third standard, BS 5038 is also applied, at times.

Policy for testing

Considerable challenges are faced by Metro railways in cable testing, especially during phase III. These are related to testing procedures, plans, and the question of who should foot the bills and what should be done if the cables fail. Based on its experiences, the DMRC has put in place a testing policy for cables in phase III. These tests are conducted in addition to routine and acceptance tests conducted in the factory before dispatch. More importantly, the policy also fixes the responsibilities of the manufacturer, contractor, and client in situations where the cable fails

Fire Survival Cables key to establishing a reputation for safety

An efficient, safe, reliable and relatively inexpensive mass rapid transit system (MRT) is pivotal to transforming India's smart city infrastructure. With rapid urbanization, the Underground has gradually become a very important part of public transportation. From reducing traffic collapses, air pollution to saving the city of valuable space, Metro Rails have the potential to unleash new growth for cities. But with confined spaces with limited escape capabilities, underground metro railways and tunnels pose safey concerns. Due to the special nature of underground operations in Metros evacuation of trains and stations in case of a fire incident takes a significantly longer time. This significantly raises the risks for injuries or fatalities, more so, if a fire incident happens during crowded rush hours.

In case of a fire incident, thousands of travellers could be trapped, hard to reach, and are likely to panic. Fires also cause damage to the infrastructure, lead to the disruption of service resulting in economic damage. The prevention of such instances is therefore critical to the reputation and reliability of Metro Rails

Greater adoption of Fire Survival Cables, in an emergency, critical as well as other applications, holds the key to ensuring the safety of underground Metro Rails. The newly launched IS 17505 shows the way and set the benchmark in achieving a high level of safety for Metro Rail infrastructure. As India's cities adopt Metros, let's ensure speed doesn't overtake safety!

Author: Amol Kalsekar, Chief Manager - Safe & Green Buildings Program, ICA India.
This article was featured in IEEMA Journal, July 2021 issue

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