train overhead wire[OHE] insulation side

Sometimes on a larger electrified railway, tramway or trolleybus system it is necessary to power different areas of track from different power grids, without guaranteeing synchronisation of the phases. Long lines may be connected to the country's national grid at various points and different phases. (Sometimes the sections are powered with different voltages or frequencies.) The grids may be synchronised on a normal basis, but events may interrupt synchronisation. This is not a problem for DC systems. AC systems have a particular safety implication in that the railway electrification system would act as a "Backdoor" connection between different parts, resulting in, amongst other things, a section of the grid de-energised for maintenance being re-energised from the railway substation creating danger.

For these reasons Neutral section are placed in the electrification between the sections fed from different points in a national grid, or different phases, or grids which are not synchronized. It is highly undesirable to connect synchronized grids. A simple section break is insufficient to guard against this as the pantograph briefly connects both sections.

In countries such as France, South Africa and the United Kingdom, a pair of permanent magnets beside the rails at either side of the neutral section operate a bogie-mounted transducer on the train which causes a large electrical circuit-breaker to open and close when the locomotive or the pantograph vehicle of a multiple unit passes over them.[2] In the United Kingdom equipment similar to AWS is used, but with pairs of magnets placed outside the running rails (as opposed to the AWS magnets placed midway between the rails). Lineside signs on the approach to the neutral section warn the driver to shut off traction power and coast through the dead section.

A neutral section or phase break consists of two insulated breaks back-to-back with a short section of line that belongs to neither grid. Some systems increase the level of safety by the mid pint of the neutral section being earthed. The presence of the earthed section in the middle is to ensure that should the transducer controlled apparatus fail, and the driver also fail to shut off power, the energy in the arc struck by the pantograph as it passes to the neutral section is conducted to earth, operating substation circuit breakers, rather than the arc either bridging the insulators into a section made dead for maintenance, a section fed from a different phase, or setting up a Backdoor connection between different parts of the country's national grid.
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Section Insulator is a device installed along with the contact wire for insulating two elementary sections where the pantograph of the locomotive negotiates smoothly and maintaining the current continuity. It is used in crossover, turnouts, maintenance pits and yard line, etc. During movement of locomotive pantograph, it first passes over the runner and smoothly glides to the contact wire on the other side without the break of continuity. Section insulator restricts the speed to 70 Kmph therefore, its usage is limited on slow speed lines and avoided on the mainline.

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