Page 396 - IRSEM_Main Book

(h) As an interim measure, length of DC track circuit may be retained at the
existing level at higher catenary current of 800 Amperes on single-track
section and 1000 Amperes. On double track section by providing one
additional „B‟ type choke in series with track relay. Audio Frequency Track
Circuit or any other approved type of device shall be used on sections
where catenary current exceeds 300 Amperes on single- track section and
600 Amperes on double track- section.

(i) Before installation of single rail DC track circuits in AC Electrified areas,
measurements of stray DC currents shall be taken in accordance with the
instructions in Annexure: 17-A1. The total stray current as measured, shall
not exceed,

(i) 10 milli-amperes, if length of track circuit is less than 100 meters.

(ii) 100 milli-amperes, if length of track circuit is 100 meters and above.

(iii) If measured value of current exceeds the specified limit as above, other
type of track circuit suitable to work in A.C. traction area shall be used.

(j) To protect the equipment from the effects of the rail voltage, a choke coil of
approved type shall be provided in series with the feed resistance.

17.5.3 Audio Frequency Track Circuit for RE Area

(a) In double rail track circuits, since both rails are used for traction return
current, impedance bonds shall be provided at the IRJs.

(b) No O.H.E mast or any other structure shall be connected to either of the
rails. The Electrical Department shall run separate earth wire for earthing
the O.H.E masts.

(c) Connections at the feeding points and from booster transformers and
return conductors shall be made to the center points of impedance bonds.

(d) Only approved type Audio Frequency Track Circuit shall be used.

(e) Audio Frequency Track Circuit shall be configured only as double rail track
circuit and like any double rail track circuit it requires both rails to be
balanced with respect to earth and with respect to current flow in each rail.

(f) Impedance bond shall be used where there is no provision in design of
Audio frequency track circuit to balance the traction rail return current.

(g) Electric Traction Units shall be critically examined for harmonics generated
by them before they are brought into service.

Chapter 17: Train Detection Track Circuits & Axle Counters Page 344 of 535