scope:

Introduction

In order to reduce the risk of bypass paths causing wrong side failure of the track circuit, section 4.1.3 of the EBI Track 400 Technical Manuals [1][2] requires that the 48V DC supplies to the transmitter and receiver are earth free. The EBI Track 400 Transmitter contains a measuring circuit which has an impedance of 270kΩ connected to the chassis of the unit. Since the chassis is generally connected to the local equipotential busbar, earth leakage testing of the 48V supply lines to prove that they are earth free will fail. Note that the impact of the measuring circuit impedance on bypass paths was assessed as part of the EBI Track 400 Safety Case and was found to be negligible. The Receiver also contains capacitors to chassis, which may cause failure if an AC test is used.

This application note defines the method to be used for proving that the 48V supply lines are earth free.