The present impedance bonds are non-resonating and non transformer type.
They are used on d.c. electrified lines for carrying the traction return
current to electrical substations, through the rails, in cases where a.c. type
track circuits with isolating joints are used. A pair of impedance bonds are
placed astride the insulating joints and are connected to the rails through the
end terminals and between them through the mid-point of the respective
windings.
The alternating currents feeding the two adjacent track circuits shall be
confined in their respective track circuits, thanks to the high impedance
encountered by the a.c. current towards the impedance bonds, while the d.c.
return current will flow through the mid-point connection.Under these
circumstances, each rail will carry one half of the traction return current. In
cases when a difference between the two rails is verified, an unbalance is
produced between the return currents that flow in the two rails and in the two
halves of the impedance bond windings. This unbalance produces a drop in the
impedance value of the impedance bonds. In order to avoid the unbalance to
become excessive, thus introducing malfunctioning in th track circuits, and
hence, in the signalling systems, a suitable air core is introduced in the iron
core of the impedance bonds. New models 30/120/U and 50/220/U are standard
versions for the Italian State Railways, conforming to their standards I.S.
416.
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