Transfer Trip System

The SMC transfer trip system is used to trip a remote feeder breaker when the local substation feeder breaker detects a fault condition on the third rail or overhead trolley line. The system can also be used as an interlock. The SMC transfer trip system consists of two parts, a transmitting unit and a receiving unit. A typical installation may have the transmitter and receiver located up to 4 miles apart. The maximum distance is determined by the resistance of the wires used to connect the transmitter to the receiver.

The transmitter, located in the local substation, normally produces a 100 mA constant current that is sent to the receiving unit in the remote substation to energize the 74. Interruption of the normal 100 mA current to the receiver causes the 74 relay to drop out, providing a signal to an annunciator indicating a lack of transfer trip capability. When the fault trip contact to the transmitter unit is closed, a 300 mA current is sent to receiver unit which will energize the 85 trip relay in the receiver at the remote substation. After a short time delay, the transmitter will return to normal current. The relay can be configured to transmit a continues trip signal.

Features

• Solid-State Design
• 125 VDC Operation
• Transmitter / Receiver Input: Fuse Protected Surge and Transient Limited
• High efficiencey DC Chopper Constant Current Regulator

Description

Transmitter: The transmitter used in the SMC Transfer Trip System consists of input power protection circuitry, a high efficiency DC chopper constant current regulator, output protection circuitry, indicators for normal current output, transfer current output, and high line resistance. The transmitter is protected with a 0.5 amp fuse. Input power protection circuitry limits transients on the 125 VDC control power into the DC Chopper circuit and the voltage is clamped at 240 VDC.

Receiver: The receiver used in the SMC Transfer Trip System consists of one relay used for the type 74 function and has two Form C sets of contacts and one relay used for the type 85 function and has two Form C sets of contacts. The input is fuse protected at 0.5 amps and has a voltage suppressor to protect the semiconductors. Each relay has its own voltage snubber. The input from the transmitter is fed into a full wave bridge rectifier allowing the two wires from the transmitter to be hooked either way and still have proper operation.