BONDING AND EARTHING FOR 25 KV ac 50 Hz SINGLE PHASE TRACTION SYSTEM

BONDING AND EARTHING FOR 25 KV ac 50 Hz SINGLE PHASE TRACTION SYSTEM

In the case of 25 kV, a.c. 50 Hz single phase traction system, the traction current is drawn from the overhead equipment by the electric rolling stock, operating in a section and passes through the traction rail.

The return current flows mostly through the earth leaving the traction rail except in a zone extending over a few hundred metres on both sides of the electric rolling stock in operation in the section or in the vicinity of a feeding station and returns to the traction sub-station.

Bonding of all rails is, therefore, not absolutely essential unlike in the case of d.c. traction where practically the whole traction return current flows through the rail and hence bonding of rails is essential.

However bonding of rail facilitates passage of the traction return current from the earth to the traction rails which are connected to the rail to earth and vice versa and is, therefore, provided in the vicinity of traction substation- feeding posts where the traction return current has to flow back from the earth to the traction rails with are connected to the earthed leg of the traction transformer at the sub-station, through a buried rail opposite the feeding post.

Bonding of rails also ensures the spread of flow of return current into the earth and, therefore, reduces the voltage between rail and earth. So bonding of rails is done wherever it is essential to keep the rail voltage low to ensure safety of personnel.

The traction rail of a wired track when bonded or laid on metallic sleepers provides generally an earth better than an earthing station with earth electrode. If, therefore, any non-current carrying part of an equipment or metallic structure is required to be earthed, it shall ordinarily be connected to a traction rail.

?Track Circuited Sections

In sections equipped with single rail-track circuits, the traction rail shall be bonded to ensure that:

The ac voltage drop along its length is reduced so as to minimize the risk of ac voltage being applied to the track relays

As low a resistance path as possible is provided both for traction return as well as signaling currents as fish plate joints cannot be relied upon for low resistance.

In sections with double rail-track-circuits, both rails are longitudinally bonded to ensure a low resistance path for traction return and signaling currents; and also to distribute the return current more evenly in both the rails. Impedance bonds are installed at insulated joints to provide a continuous path to the traction return current. All track-circuited-rails are, in addition, provided with signal bonds.

Bonds

Bonds mean an electrical connection between two or more conductors or noncurrent carrying metallic parts of traction masts or structures or supports and rails.

?Types of bonds:

1. Longitudinal Bond (Rail Bond)

2. Transverse Bond (Cross Bond)

3. Structure Bond

4. Signal bond

5. Impedance bond

?Rail Bond/ longitudinal bond

Connection across Rail joint between consecutive lengths of rails.

Cross Bonds /transverse bonds

?Bond between two rails of track or two rails of adjacent tracks.

Structure bond

Connection from non-current carrying metallic parts of a traction mast /structure/support to the traction rail. Size:- M.S Flat 40mm x 6mm cross sectional area not less than 200 sq. mm

All the structure/masts supporting the traction overhead equipments shall be connected by means of a structure bond to the nearest traction rail. In case of ‘Portal”and ‘Head span’ structure, both leg shall be provided with structure Bond.

Where traction mats/structure are located on railway platforms and are bonded to the nearest traction rail, a cross-bond shall be provided at the location of the structure bond to connect the rail to the adjacent traction Rail.

If the nearer Rail from structure is track circuited rail then structure bond shall be connected to non-track circuited rail providing ‘PVC hand rail covering (40mm x 6mm size) on circuited rail. Suitable bending shall be given at Platform edge so that bond goes along with the platform wall. The hole centre (for fitting structure bond) should be minimum 100mm away from ‘alumino-thermic welds’

Impedance bond

Bond installed by the S&T department, which provided low impedance path for traction return current and a relatively high impedance path for track circuit current

Signal bond- Connection across a rail joint, provided by the S&T department, to facilitate flow of track circuit current

Mode of connection of Bond- Structure bond :- shall be rigidly connected by means of galvanized steel fasteners to the traction Rail.

Track bonding

In station yards or elsewhere, where a track is not wired for its entire length, it shall be deemed to be wired for a distance of upto 50metres beyond the traction mast at which the overhead equipment has been terminated. Rail-bond and one cross-bond shall be provided for a distance of upto 50metres beyond the last traction mast.

?Mode of Connection of Bond

All types of bond i.e. rail-bond, cross-bond and structure-bond shall be of mild steel of not less than 200 sq.mm cross sectional area.

A structure-bond shall be rigidly connected by means of galvanized steel fasteners to the traction rail and the metallic part of traction mast or structure or support.

A rail-bond shall be rigidly connected by means of galvanized steel fasteners longitudinally across the fish plate joint of the traction rail and the track-circuited-rail in a track circuited section except at the insulated joint of the track circuited rail.

A cross-bond shall be rigidly connected by means of galvanised steel fasteners between two traction rails of a track or non-track-circuited rails of an adjacent track.

Where it is not possible to provide a rail-bond a welded bond shall be used. The bond shall be connected to the rails by electric or gas welding.

The bond for connecting return conductor to the traction rail through the buried rail shall normally be made with GI nuts and bolts with spring washer and check nuts.

The cross-section of an earth wire used for bonding traction masts or structures or supports or the metallic parts supporting the traction overhead equipment in a tunnel or in double rail-track-circuited section shall be not less than 50 sq.mm copper equivalent.

?Bonding in single Rail –track-circuited sections

The traction rail shall be provided with rail bonds over the entire length and also for a distance of 50m on both sides of the track circuited length. In addition the traction rail shall be cross bonded to the traction rail of the adjacent track at interval not less than 100m. (In case the length of track circuited rail is not more than 350m, a cross bond shall be provided between the rails of the track immediately outside the track circuited length at both of its end.)

?In single line section, provided with single rail track circuit, the non-track circuited rail shall be provided with rail bonds over the entire length and for a further 50 meters on both side. It shall also be connected to an earth at distances not exceeding 100metres from each other. ( the connections of non-track circuited rail to each of the earths shall be made by two separate mild steel strips/flats each of cross section not less than 200 sq.mm.)

?Bonding in double Rail –track-circuited sections ?

Both the rails shall be provided with Rail-bonds. An Impedance bond shall be provided at insulated joints. Since no traction rail is available for structure bonding, an ‘earth wire’ shall be run on the traction masts/structure/supports. If the length of the earth wire exceeds 1000m, it shall be made electrically discontinuous by providing a cut-in-insulator. Each such section of the earth wire shall be connected to an earth at two traction masts/structures/supports at a distance not exceeding 500m apart.

?Bonding of Rails on Sleepers

A wired track shall be deemed to be on wooden or concrete sleepers. The traction rails shall be provided with ‘cross-bond’ at distances of not more than 350mts apart.

?Bonding of track in Loco Sheds and Loco/EMU stabling sidings

All the traction rails shall be provided with ‘Cross-bonds’ at distances of not more than 100m apart.

All sidingsand/or dead ends shall be connected by ‘Rail-bonds’ The rails on wooden/concrete sleepers/supports in Loco/EMU inspection pits shall be provided with ‘rail-bons’ for entire length of the pit and also upto a length of 50m on both sides and connected to an earth.

?Bonding of Rails in a tunnel

All the traction rails shall be provided with ‘Rail-bonds’ not only over the entire length inside tunnel but also for a length of upto 50m on both sides outside the tunnel. A ‘cross bond’ shall be provided between the traction rails at either ends of tunnel.

?Bonding at a level crossing

All the traction rails shall be provided with ‘cross-bonds’ at only one location within 5m from either of transverse edges of the level crossing.

?Bonding of rails on a Weigh Bridge

Both the rails of a wired track on a weigh bridge shall be provided with ‘Rail-bonds” for a length of upto 50m on both sides of the weigh-bridge. If the rails are on wooden/concrete sleepers/supports, they shall be connected to an earth.

?Bonding of Girder Bridge

Steel structure of a girder bridge shall be connected to a traction rail or to an earth by means of two mild steel strips/flats of cross section not less than 200 sq. mm each. The traction rails on the bridge shall be connected by ‘cross-bond’ at distances not exceeding 100m apart. In a single rail track circuited section, the non-track-circuited rail which is the traction rail shall be provided with ‘Rail-bond’ and connected to an earth at either end of the bridge.

?Bonding at Oil Depot sidings

An insulated joint in the rails of the track shall be provided to avoid risk of propagating stay current.

Setting up neutral zones/sections in the contact and catenary wire.

Both the rails of the siding/ secondary loop line shall be provided with ‘Longitudinal Bonds’

Besides, ‘Transverse-bonds’ shall be provided between the Rails at distance not exceeding 30metres apart.

The rail of the siding/secondary loop line shall be connected to an earth at both ends immediately out side the neutral zone. The value of earth resistance of an earth electrode shall not exceed 10 Ohms.

Each and every non-current carrying part of traction mast/structure/support and other metallic structure in the vicinity of the siding/secondary loop line shall be provided with ‘structure-bonds’. Only copper rivets shall be used for connection between the non-current carrying metallic part or rail and the bond.

?Bonding of Over Line Structure (FOB/ROB/ etc)

The metallic parts of foot-over-bridge/ road-over bridge/other over-line structures over wired tracks shall be connected either to attraction rail or to an earth by means of two MS Strips/flats of cross section not less than 200 sq.mm each.

?Bonding adjacent to TSS/FP

Opposite to a traction sub-station/feeding post, all the traction rails shall be provided with ‘Rail bonds’ for a distance of 1000metres on either side of TSS/FP. In addition these traction rails shall be ‘cross-bonded’ at approximate distances of 300m, 500m, 700m and 1000m from the TSS/FP

?Bonding of Earthing Heel of Isolators

Shall be connected by two MS flats of cross section not less than 200 sq.mm each to the supporting metallic traction mast/structure/support, which, in turn, be connected to a traction rail/an earth wire and in addition to an earth.

?Bonding of exposed Metallic parts like platform structure/sheds, metallic fencing, wires pipes and such other items not likely to come in to direct contact with the 25kV ac OHE and located with a distance of 20m from nearest electrified track and running parallel to it for a distance of more than 20m but less than 350m.

?Shall be connected to an earth or traction rail. If parallelism with nearest electrified track exceeds 35m, all such exposed metallic parts shall be connected to a separate earth a distance not exceeding 350m apart. No special precaution is required in case such metallic parts fitted on metallic supports directly buried in the ground if the earth resistance of such metallic support is less than 10 Ohms.

?Earthing Conductor size Calculation Formula & calculator

Earthing conductor size calculator:

Earthing conductor is used to carrying the electrical fault current to the ground. Here our earthing size calculator is used to calculate the exact required earthing conductor size in sqmm.

Enter the fault current in kilo Amps and fault clearing time in Sec. Then press the calculate button to get the required earth conductor size in Sqmm.

Earthing conductor size calculation Formula:

Earth conductor size PE(Sqmm)?in sqmm is equal to the square root of maximum operating time t(S)?in Seconds times of fault current square I(fault-A)?divided by material constant k. Hence we can write the formula for finding earth conductor size as below,

?Earth conductor size PE(Sqmm)?=?√?(I(Fault-A)2?x t(s)) / k

Material Material Constant (k)

GI 80

Copper 205

Aluminum 126

?But by considering Safety factor as 1.5

Earth conductor size PE(Sqmm)?=1.5 x?√?(I(Fault-A)2?x t(s)) / k

Here safety factor is your choice