Page 266 - Electrical Safety of Low Voltage Systems

Applications of Electrical Safety 249

FIGURE 15.6 Typical traction electrification system.

In TT systems, the bond to the running rails prevents the circu-
lation of the d.c. fault current through the train station’s grounding
system R G , which earths all the alternating current ECPs. Because of
this link, most of the fault current is returned to the source via the
rails, with the result to lower both the perspective touch voltage ap-
pearing over the ECPs and the fault clearing time of the direct current
protective devices.
A negative aspect of this arrangement is the establishment of per-
manentd.c.straycurrentsthroughthesoil,which,aswealreadyknow,
maycausecorrosionofmetalburiedparts.Therails,infact,areearthed
through the ECPs grounding system of the station. The return current,
in the ordinary operation of the train, in fact, will go back to the source
not only through the rails, the legitimate return circuit, but also via
the parallel path constituted by the actual earth.
In order to minimize the stray currents in a d.c. traction system,
therefore, the direct bonding connection ECP, rails should be avoided
and voltage-limiting device (e.g., diodes) should be used (Fig. 15.8).
In the regular operation of the train, the diode is an open circuit
and prevents the d.c. traction current to circulate through the earth.