light. In most locomotives, it also idles the engine and disconnects the traction motors. In multiple-unit
operation, the indicating light is the only action extending to the other units. A reset button is used to
release the relay, and a cutout switch may be provided to make the relay entirely inoperative.
(2) Wheel slip relay. An indicating system using a wheel slip relay is frequently installed on
diesel-electric locomotives. The system's relays are connected to resistors in the traction motor circuit.
If a pair of wheels slip, unbalanced voltage causes current to flow through the relay. When this relay is
activated, the contacts energize a governor solenoid which reduces engine speed and operates a warning
buzzer, an indicator light, or both in the engineman's cab. Interlocks reduce generator excitation at the
same time. When slipping stops, balanced voltage again exists and power is automatically restored. The
buzzer or light is transmitted to all units in multiple, but engine speed is reduced only on the slipping
MAIN POWER SWITCHES
The power contactors connect the traction motors to the main generators. They are
pneumatically operated, heavy-duty, single-pole switches located in the high-voltage electrical-
equipment cabinet. The contactor, sometimes called a unit switch or pneumatic power switch, consists
of one fixed and one movable contact, a magnet valve, an arcing horn, an arc box, and a blowout coil.
When the throttle is advanced to the first notch, the magnet valve admits air to the cylinder,
moving the piston to close the contacts. When the magnet valve is no longer energized, spring action on
the piston opens the contactor. Contactors normally open when traction motor current has dropped to a
negligible quantity, but provisions must be made for interrupting heavy current when the contactors have
to open under load, as during an emergency brake application. Equipment for interrupting heavy current
consists of an arcing horn to carry the current, an arc box to confine the arc, and a blowout coil by which
the magnetic field of the coil forces the arc away from the tips. Arcing is a buildup of current at a
contact point. This concentration of current, caused by a collection of matter that conducts current,
burns the contacts, destroying their serviceability. With the interrupting equipment, the path of the arc
increases in length, its difficulty in jumping the gap increases, it is cut, and current stops flowing.