Signalling Power Supply Part -3

In this chapter we will look into various earthing system utilized for Signalling Power Supply ,Insulation Monitoring ,Earth Leakage Detection ,Local and Remote Monitoring.

Please read part 1 and Part 2 for the complete series and you will be in a position to design a power distribution sub system for the signalling system

Earthing Systems

There are Functional Earthing and Protective earthing carried out at the electrical installations .A functional earth connection serve the purpose other than electrical safety and might carry electrical current as part of normal operation .For the functional earthing cases a special terminal is provided for the installer to connect external earth usually for the purpose of noise reduction .Screen earthing of a cable can be considered as functional earth .

A protective earth is used to protect the operator by means of reliable ground connection to make sure the touch current wont exceed certain values .Protective earthing is known as 'dirty' earthing where as Functional earthing is also known as Clean earthing .In nutshell Protective earth is intended to protect personals from electric shock during an earth fault .This earthing is done from any metallic exposed part upto Main earth terminal in the Signalling Room (depends on type of earthing system implemented ) /Fault current will flow through this conductor to earth which in turn on to the protective devices such as RCD (Eg residual Current Device Fitted MCB) to safely open the circuit within 0.4 seconds,where as functional earth is used to reduce radio frequency noise .Fuctioanl Earthing and Protrective Earthing must be connected to separate earthing system and can be tied together through Potential Equalisation Clamp.(PEC) .In General Power Earth and Signalling Earth are separate earth .

Example :- Cabinet case earthing is a type of Protection Earthing ,Shield /screen earthing of a cable is a type of functional earthing in a signalling installations.

Type of Protective Earthing System.

There are few types of earthing system (Protective Earthing ) implemented as per country practices.These can be broadly classified as TNS Earthing System ,TNC Earthing System ,TNC-S Earthing System ,TT Earthing System and IT Earthing System.

T(Terre or earth) Denotes that the Power Distribution System at SER is solidly earthed independently of the source earthing method.

N(Neutral) Denotes that a low impedence conductor is taken from earth connection at the source and directly routed to the Power Distribution System at SER (Signalling Equipment Room Signalling Power Supply) for the specific purpose of earthing of the PDC system

S (Separate) Denotes that the neutral conductor routed from the source is separate from the protective earthing conductor ,which is also routed from the source (Upstream council/Rail supply )

C(Common ) Denotes that the neutral conductor and the protective earthing conductor are one and the same conductor used

TNS (Terre ,Neutral, Separate ) Earthing System.

Here Terre stands for Earth .In this type of earthing system neutral conductor routed from the source is separate from protective earthing conductor ,which is also routed from the source .Upstream supply normally tapped from Council (Government Electricity Authority ) or Railway exclusive supply which is the source for power distribution system .This is used as supply source in the Power Distribution Cabinet or UPS depends on the power supply distribution requirement at the downstream on a signalling power distribution cabinet .For a three phase source ,will have five wires (Phase 1 ,Phase 2 ,Phase 3 , Neutral and Exclusive earth ) and a single phase source have three wires (Phase ,Neutral and Earth ) leading into your Signalling Power Distribution Cabinet or UPS .Here Neutral and Earth are separate conductors and earthed at source.No separate dedicated earth used at Power Distribution Cabinet End in the Signalling Equipment Room.Refer Figure 1 for details

TNC (Terre Neutral Common ) Earthing System .

Here C denotes that the neutral conductor and the protective earthing conductor are ONE and same conductor is used .For a three phase source will have four wires (Phase 1 ,Phase 2 ,Phase 3 ,Combined Neutral and Earth ) and two wires for a single phase system (Phase and combined Neutral and Earth) In this type of system joined Neutral and Earth are earthed at source(upstream) end and destination end (down stream ) on separate earth electrodes. .There could be additional electrodes at source end as shown in the Figure 1

TNC-S System .

This earthing system is an enhanced version of TNC system .In this type of system a three phase source(Upstream) will have four wires (Phase 1 ,Phase 2 ,Phase 3 ,Combined Neutral and Earth ) and two wires for a single phase system (Phase and combined Neutral and Earth) .That is,Joined Neutral and Earth at source(Upstream) end but separate Neutral and Earth conductor at down stream(PDC) end .In nutshell the difference for TNC-S from TNC system is that there are five wires used at down stream end(PDC ) joined into four wires towards source(Joined Neutal and Earth toward Source) for a three phase system and three wires (Phase ,Neutral and Earth ) joined earth and Neutral towards source and joined Neutral and Earth in a single phase system .Refer the Figure above .

TT Syetm

In this type of earthing system ,there are four wires (Phase 1 ,Phase 2,Phase 3 and Neutral ) for a three phase system and two wires for a single phase system .Both source (Upstream ) and load (Downstream ) have exclusive earth and neutral is tied to earth at source end not at load end (down stream /PDC) Please refer the Figure 2 below

IT Earthing System

This is similar to TT earthing system .However neutral is floating compared to other types of earthing system .Neutral is not earthed at source (upstream ) or Load (Downstream ) .That is your Power Supply Cabinet . Both upstream and Down stream has exclusive earth ,however source earth is routed via Insulation Monitoring Device which can use for monitoring the insulation fault

Power Earth ,Signalling Earth ,and Communication Earth

It is always advisable to have separate dedicated earthing system for Power supply subsystem ,Signalling System and Communication System . Communication and Signalling system exclusively requires functional earthing for EMC purposes where as Power Distribution System requires Protective earthing .Signalling Earthing System can be used to earth all the exposed metal part for the protective earthing of the signalling system and communication earthing system can be used for functional earthing and protective earthing of the communication cabinets respectively .

All these earthing systems can be tied together with a potential equalization clamp which is an open circuit between earthing ssytems and which conducts when there is an earth fault .

This practice is adopted in Sydney Practice

Earth Leakage Detection

An earth Leakage System and Insulation Monitoring system shall be implemented in the signalling system at primary of the isolation transformer for the Power Distribution System and Secondary side of the isolation transformer .Earth Leakage Relay contact can be wired on to the trip coil arm of the transformer to isolate the downstream from source when leakage exceed a certain set value .This set value shall be based on the maximum leakage current possible from the end equipement and the current that can electrocute the person comes in contact .For an example .If leakage current from a signalling cabinet is 40mA and above this current can electrocute a person ,ELR shall be set for 40mA and if it exceed this value the incoming MCB will isolate the source by tripping the system .

Insulation Monitoring Device (IMD)

This is another protective mechanism ,especially in IT earthing system which is used to monitor the leakge on the insulation of the conductor and can be locally and remotely monitored

Lightning Protection

Its advisable to implement lightning protection for the conductors from the source for a distributed power system when conductors are exposed to lightning and thunder .There are protective devises available to connect across the conductors which are available in the market .It shall be implemented based on your operator practice

Local and Remote Monitoring

Its also vital to read the status of various group of supply ,incoming power availability ,UPS availability ,UPS on battery , Earth Leakage /Insulation Monitoring Status ,Current ,Voltage ,Frequency ,Synchronization ,ATSw status to be locally monitored (through local indication on the cabinet ) and remotely monitored (for maintainer to rush ) throgh Automatic Train Supervision system .Telemetry could be used to transmit thease status remotely