In Railways, Electromagnetic compatibility (EMC) between power systems and other systems especially other communication, signalling systems is a necessary demand for the reliable and safe operation of devices. It is obvious that the traction power systems could interfere with signalling systems with probably serious consequences. In congested underground tunnels of Metro system, EMC is equally a risk to Signalling systems due to limited space available to accommodate power and Signals.
EMC is concerned with minimising electrical interference; something which potentially is a major safety issue within the railway. Therefore EMC is during all the necessities to be enclosed in a safety case for the introduction of latest wheeled vehicle, locomotives or track maintenance vehicles.
What is EMC?
Often when dealing with EMC it is necessary to ask: what is EMC; and to have a definition.
EMC definition: “EMC is defined as the ability of devices and systems to operate in their electromagnetic environment without impairing their functions and without faults and vice versa.”
Electromagnetic compatibility, EMC ensures that operation does not influence the electromagnetic environment to the extent that the functions of other devices and systems are adversely affected.
Aspects of EMC
Emission: The ability of a system to operate without interfering with other systems or equipment.
Immunity: The Ability of a system to operate within a specified electromagnetic environment.
What is EMI?
Electro Magnetic Interference (EMI) is an electromagnetic disturbance which may degrade the performance of equipment (device, system or sub-system) or causes malfunction of the equipment.
There are many forms of electromagnetic interference, EMI that can affect circuits and prevent them from working in the way that was intended. This EMI or radio frequency interference, RFI as it is sometimes called can arise in a number of ways, although in an ideal world it should not be present.
Types of EMI
EMI can arise in many ways and from a number of sources. The different types of EMI can be categorised in a number of ways.
Type of EMI is by the way it was created:
- Man-made EMI: This type of EMI generally arises from other electronics circuits, although some EMI can arise from switching of large currents, etc.
- Naturally occurring EMI: This type of EMI can arise from many sources – cosmic noise as well as lightning and other atmospheric types of noise all contribute.
Type of EMI is by its duration:
- Continuous interference: This type of EMI generally arises from a source such as a circuit that is emitting a continuous signal. However background noise, which is continuous may be created in a number of ways, either manmade or naturally occurring.
- Impulse noise: Again, this type of EMI may be man-made or naturally occurring. Lightning, ESD, and switching systems all contribute to impulse noise which is a form of EMI.
EMI by their bandwidth:
- Narrowband: Typically this form of EMI is likely to be a single carrier source – possibly generated by an oscillator of some form. Another form of narrowband EMI is the spurious signals caused by intermodulation and other forms of distortion in a transmitter such as a mobile phone of Wi-Fi router. These spurious signals will appear at different points in the spectrum and may cause interference to another user of the radio spectrum. As such these spurious signals must be kept within tight limits.
- Broadband: There are many forms of broadband noise which can be experienced. It can arise from a great variety of sources. Man-made broadband interference can arise from sources such as arc welders where a spark is continuously generated. Naturally occurring broadband noise can be experienced from the Sun – it can cause sun-outs for satellite television systems when the Sun appears behind the satellite and noise can mask the wanted satellite signal. Fortunately these episodes only last for a few minutes.
How to mitigate the EMC in Railways?
Method 1 : Segregation:
This method is simply consists of ensuring that electrically noisy items (i.e. those with high levels of emissions) are located in a separate place from sensitive electronic items (i.e. those with low levels of immunity). It is often beneficial to provide increased spacing between the segregated areas (or zones). The segregation will depends on how powerful the source is and how resilience of the victim. Segregation is sometimes called ‘system partitioning’ or ‘partitioning’ or ‘zoning’.
Segregation is one of the most powerful and lowest-cost EMC mitigation techniques if it is used early enough in a project. An installation’s apparatus and their supplies should be arranged in geographically separated groups right from the start of a project.
Apparatus should be classified
- High-voltage (HV, > 33kVAC rms)
- Medium voltage (MV, between 1 and 33kVAC rms)
- Low voltage (LV, < 1kVAC rms).Low voltage equipment should then be classified at least as ‘noisy’ or ‘sensitive’, with more sub-categories used where even greater control of EMC is required.
Where the separation distances between segregated items are insufficient to prevent radiated interference from occurring, one or both can be shielded.
Key issues with Segregation:
- EMC Directive compliance means nothing in practice : It is wrong to assume if a product is CE marked, then it must be compliant with the EMC Directive and its emissions and immunity standards – and this means that they will not interfere with each other or with other equipment. This is often a dangerous assumption.
- For low cost, segregation should be done at the earliest stage : If the appropriate amount of separation is not designed-in from the start, applying the segregation technique can involve moving the locations of large numbers of items and their conductors/cables, and this is usually very costly and time-consuming indeed.
Method 2 : Cable Routing:
Cable routing techniques are used to reduce the ‘unintentional antenna’ efficiency of cables, and also to prevent low-frequency crosstalk and RF coupling between cables in a system or installation.
Segregate cables into ‘classes’ according to the kinds of signals they carry, then route each class so as to maintain at least minimum spacings from the other classes. The spacings depend on the classes. Screening cables for EMC is of little use unless both send and return conductors are enclosed together in a single screen.One way to do this technique is, replace cables which use metallic conductors with metal-free fibre optic cables, infra-red, wireless, free-space laser or microwave communications.
Reducing the efficiency of the ‘unintentional antenna’ of a cable helps to reduce emissions and improve immunity. Reducing crosstalk and RF coupling helps to reduce the amount of electromagnetic noise that equipment in the same system or installation is exposed to.
Key issues with Cable Routing:
- Using non-metallic communications: Metallic communications (i.e. wires and cables) all suffer from being ‘unintentional antennas’, which means that some of their internal signals leak into the environment as RF emissions. At some frequencies, some cables leak all of their signal into the environment. Matched transmission-line cable types leak the least, but they still have emissions.
Method 3: Shielding
Cables behaving as ‘unintentional antennas’ are usually the main contributor to emissions and immunity problems at frequencies below 200MHz.
Cable shielding is used to reduce the unwanted emissions (‘leakage’) from the signals carried by a cable, and also to improve the immunity of the signals in the cables to ambient electromagnetic noise.
‘Cable screening’ is another term for cable shielding.
In this technique a conductive layer is applied all around the circumference of a cable, and all along its length. For flexible cables the layer is usually a metal braid or a spirally-wrapped metallised plastic foil, sometimes both together, and sometimes multiple layers of braid and/or foil or even metallic tape.
High-permeability metal tape is sometimes wound around the cable, along with one or more braids, to give a very high-performance (and costly) shielded cable known as ‘super-screened’. Cables which do not need to be flexible can be shielded with a solid metal outer shield, giving excellent shielding performance at a low cost.
Key issues with Cable Shielding:
- Continuity of the shield: Any holes or gaps in the shield compromise its shielding effectiveness.The cable shield needs to RF bond to the enclosure shield at both ends of Example of RF bonding a cable shield in a connector each shielded cable, otherwise there is a gap in the overall shielding and the RF emissions and immunity performance will suffer as a result.Where an item of equipment is small, it may be possible to RF bond the cable shield to its RF reference plane instead of needing an enclosure shield
- The need for shielded connectors and glands:To be able to use shielded cables effectively, connectors and glands used with them must carry and/or bond to the cables’ circumferential shields without creating holes or gaps in the overall shield. This is sometimes called 360° bonding.Shielded connectors and glands are sometimes called screened connectors and glands.
- Bonding shields at both ends:Many years ago it used to be normal practice in some industries to only bond cable shields at one end. This was because the poor circuit designs typically Photograph of some 360 degree shield-bonding cable glands used in those industries forced any cable shield noise currents to flow in their circuits.
- Never use pigtails: So it is now very important (and strongly recommended by IEC 61000-5-2) that cable shields are only bonded using RF bonding techniques appropriate to the highest frequency that is to be protected against.
- Cable shields have large amounts of stray capacitance, making any amount of inductance in the bonds at their ends have a large effect at surprisingly low frequencies by creating a series resonant circuit. Close to the resonant frequency the shield will amplify any voltages it picks up from internal cables, or from the ambient, instead of attenuating it.
- A length of twisted-braid, foil shield drain wire, other wire or connector pin used to electrically bond a cable shield to an item of equipment is often called a ‘pigtail’.
- Safety issues:Long shielded cables which only have their shields bonded at one end, or use capacitive bonding at one end, may need to have surge protection devices fitted at the unbonded or capacitively bonded end. These are to protect people and equipment from the high voltage surges that can occur during thunderstorms or faults in the power distribution network, and also to reduce the risk of fire. This particularly applies to cables that pass between buildings
Method 4: Meshed bonding (‘earthing’) networks:
Long ago, most of the interference problems in installations were at 50Hz and it was possible to control circulating currents (‘earth loops’) using star bonding techniques. Now that we cannot (in general) use star ‘earth’ bonding, we need to embrace its opposite: mesh bonding.This is mainly used in enclosed equipment rooms ..etc.
Mesh CBN: Common bonding scheme for a fixed installation, often called mesh earthing. CBN for the common bonding network that is often colloquially referred to as the earthing structure, and they refer to a 3-dimensionally meshed CBN as a MESH-CBN. Meshing the CBN helps protect equipment against the damaging effects of lightning surges, and surge protection devices (SPDs) function better when they are connected to a low-impedance CBN.
The bonding ring conductor: It is recommended that the MESH-CBN at each level of a structure incorporates a bonding ring conductor (BRC) with a substantial cross sectional area (CSA), at least capable of handling the maximum fault currents. This could be method adopted in Railway Stations with Overhead Power Lines. Incoming cables and metallic services such as air, gas and water pipes, ventilation ducts, etc. should be bonded to a single main earthing terminal.
Method 5: Parallel Earth Conductors(PEC)
Any part of a properly constructed MESH-CBN may be used as a PEC, and it is best to run a cable over the same type of PEC along its whole length. Of course, PECs must be bonded at each end to the equipment cabinets that the cables enter. They must also be effectively bonded at all joints all along their lengths.
If Cable Armours are acting as PEC then Cable armour should always be bonded at both ends, and acts as a PEC.
Another practice which is followed in the Railways is installing the Parallel Earth Cables(PEC) or Common Conductors along the Power cables which will mitigate the Electromagnetic emission.
What are the Regulations related to EMC in United Kingdom?
- Engineering Safety Management (The Yellow Book), Issue 3, RSSB
- Directive 2004/108/EC of the European Parliament and of the Council on the approximation of the Laws of Member States relating to electromagnetic compatibility, 15 December 2004, OJEU L/390, 24-37
- GE/RT8015: Electromagnetic Compatibility between Railway Infrastructure and Trains – Railway Group Standard, Issue 1, October 2002, RSSB 8
- .NR/L1/SIG/30040: EMC Strategy for Network Rail – Network Rail’s Company Standard, Issue 1, August 2008, NR
- NR/L2/SIG/30041: EMC Assurance Process for Network Rail – Network Rail’s Company Standard, Issue 1, August 2008, NR
About the Author :
Sujay is an experienced Railway Engineer. He worked in Major Railway projects in UK including Crossrail , London Underground and Great Western Modernisation. Sujay has good experience in Integrated Railway Design and Interface Management.