a large amount of current on re-closure, if prior to the pull-out torque is achieved, resulting in a shut down. The in-rush current during re-closure will depend on many factors, however. As an example, a 250 kVA motor generator operating at 300 ampere of full load current will require 1550 ampere of in-rush current during a re-closure after 5 seconds. This example used a fixed mounted flywheel sized to result in a Hz per second slew rate. The motor–generator was a vertical type two-bearing machine with oil-bath bearings. Motors and generators may be coupled by a non-conductive shaft in facilities that need to closely control electromagnetic radiation, or where high isolation from transient surge voltages is required. Modern use of motor–generators Motor–generator sets have been replaced by semiconductor devices for some purposes. In the past, a popular use for MG sets was in elevators. Since accurate speed control of the hoisting machine was required, the impracticality of varying the frequency to a high power AC motor meant that the use of an MG set with a DC hoist motor was a near industry-standard solution. Modern AC variable-frequency drive electronics with compatible motors have increasingly supplanted traditional MG-driven elevator installations, since electronic AC drives are typically more efficient by 50% or more than MG DC-powered machinery. Another use for MG sets was in the southern region of British Rail. They were used to convert the 600 V DC – 850 V DC line supply voltage from the third rail into 70 V DC to power the controls of the EMU stock in use. These have since been replaced with solid state converters on new rolling stock. MG locomotives have also commonly been used for long distance rail Diesel locomotive transmission throughout the world, due to reliability and wear issues with mechanical and fluid transmissions, but this is generally being replaced with smaller engines with conventional or MG transmission on each carriage. Long distance electric locomotives with high voltage overhead power supply used MG transmission, but this is generally being replaced with distributed motor drive on each carriage with electronic power control and conversion. Similarly, MG sets were used in the PCC streetcar to produce a 36VDC output from the 600VDC traction supply. The low voltage output charges the streetcar's batteries and supplies current for control and auxiliary equipment (including headlights, gong ringers, door motors and electromagnetic track brakes). Motor-generator sets were