Ground-Censor® Ground-Fault Protection System Type GC

System Overview

The Ground-Censor® Ground-Fault Protection System Type GC, Class 931, is an equipment ground-fault protection system consisting of a ground-fault relay, ground-fault sensor, and a nameplate for a monitor-test system. It is designed for use on grounded alternating current systems rated up to 600 volts ac, 60 hertz, providing protection in accordance with National Electrical Code Paragraphs 110-10 and 230-95.

The GC-100 Ground-fault Relay is rated 600 volts ac, 60 hertz and is suitable for equipment connected to a source with up to 200,000 RMS amperes available ground-fault current. It requires a 120 volt ac source and consumes 3 watts in standby and 12 watts during trip. The system supports zone selective interlocking (ZSI) by allowing each GC-100 relay to drive up to eight additional GC-100 relays.

The components are UL listed under File E-48368 as Ground-fault Sensing and Relaying Equipment.

Operation Principle

Under normal load conditions, the vector summation of all phase and neutral currents approaches zero. When a ground fault occurs, this summation becomes non-zero. A differential current transformer (sensor) encircling the phase and neutral conductors detects this current imbalance. The sensor's output, proportional to the ground-fault current magnitude, is fed to the ground-fault relay. When the ground-fault current exceeds a pre-selected level, the relay activates the circuit interrupting device.

Ground-fault Relay (GC-100)

The GC-100 relay continuously monitors the sensor's output and initiates circuit opening. The ground-fault current pick-up is field adjustable from 100 to 1,200 amperes. The time delay is adjustable for 0.1, 0.2, 0.3, or 0.5 seconds. Zone selective interlocking allows for instantaneous operation (0.03 seconds) and system coordination.

Sensor (GT Sensor)

The sensor features a laminated iron core with two main windings connected in parallel to minimize local saturation and enhance reliability. If one winding opens or shorts, the other continues to operate. The iron core is bolted at the corners, and a leg can be removed for easier installation around conductors. A special test winding simulates ground-fault conditions when energized.

Monitor-test System

The monitor-test system includes components for testing the ground-fault condition. For test with or without trip, a nameplate with instructions is provided with each GC-100 relay. The required components include:

• One green illuminated push button with contact block.
• One red illuminated push button with contact block.
• One miniature or general purpose relay with socket.

For test with trip only, the following components are required:

• Non-illuminated push button with contact block.
• Red illuminated push button with contact block.

Electrical Conditions and Circuit Interrupting Devices

Wires from the sensor to the relay should not exceed 25 feet and be no smaller than No. 14 AWG. Wires for test-monitor components should not exceed 50 feet and be no smaller than No. 18 AWG. Interconnecting wires between relays must comply with specific gauge requirements based on length (e.g., 1-150 feet requires #16 AWG, 651-1000 feet requires #8 AWG).

Any circuit interrupting device used must be suitably rated for the duty and incorporate a 120 volt ac shunt trip or electric tripping mechanism with specific inrush and continuous current requirements.

Molded Case Circuit Breaker

A molded case circuit breaker rated at a specified ampere rating with a specified RMS symmetrical ampere interrupting rating at a specified voltage is required. It should include thermal and magnetic elements for overload and fault protection, along with a ground-fault trip mechanism activated by the ground-fault relay.

Fusible Bolted Pressure Contact Switch (Alternate)

This switch, rated at specified amperes and volts, must be equipped with fuses and provide for opening the circuit during normal switching, overload, short circuit, or ground-fault conditions. It includes a ground-fault trip mechanism activated by the ground-fault relay, which, with an interposing relay and auxiliary power source, releases a stored-energy device to open the switch. The mechanism must operate satisfactorily at 55% of rated voltage.

Low Voltage Power Breaker (Alternate)

A low voltage power breaker rated at specified amperes and volts is required for opening the circuit during normal switching, overload, short circuit, or ground-fault conditions. It includes a ground-fault trip mechanism activated by the ground-fault relay, which, with an interposing relay and auxiliary power source, releases a stored-energy device to open the breaker. The shunt trip release must operate satisfactorily at 55% of normal operating voltage.

System Coordination

Figure 7 illustrates a typical coordination arrangement where Relay No. 3 restrains Relay No. 2, and Relay No. 2 restrains Relay No. 1 in a time-delay mode. For a ground-fault of 1500 amperes at Location No. 3, Relay No. 3 initiates a signal in 0.03 seconds to the Branch Device to open the circuit. If the Branch Device fails, Relay No. 2 initiates a signal 0.2 seconds after the fault. If both fail, Relay No. 1 initiates a signal 0.5 seconds after the fault.

If a 1500 ampere ground-fault occurs at Location No. 2, Relay No. 2 initiates a signal in 0.03 seconds to the Feeder Device. Relay No. 1 provides a restraining signal. If the Feeder Device fails, Relay No. 1 initiates a signal 0.5 seconds after the fault.

For a 1500 ampere ground-fault at Location No. 1, Relay No. 1 initiates a signal in 0.03 seconds to the Main Device. This coordination ensures the closest interrupting device operates instantaneously (0.03 seconds), with time delay only in the back-up devices. Clearing time of the circuit interrupting device must also be considered for complete system coordination.

Suggested Specifications

General

A ground-fault protection system shall be included as indicated in the switchboard schedule and on the plans. It shall consist of a current sensor enclosing all phase (and neutral, if present) conductors, appropriate relaying equipment for desired ground-fault current sensitivity and time-current response, and an appropriately sized circuit interrupting device (molded case circuit breaker, fusible bolted pressure contact switch, or low-voltage power breaker) equipped to function with the system. Installation must follow manufacturer's recommendations.

Current Sensor

A sensitive current sensor, sized to encircle phase and neutral conductors, shall be provided. Its output must be coordinated with the relay's input. A test winding shall be included to simulate ground-fault current for testing. The sensor frame should allow for easy installation around cables or bus without disturbing them.

Ground-fault Relay

The relay shall be solid-state for maximum reliability and provide zone-selective interlocking for instantaneous (0.03 seconds) operation. It requires 120 volt power. Adjustable pickup current sensitivity for ground-fault currents from 100 to 1,200 amperes shall be provided with a calibrated dial for field setting. Restrained time delay shall be nominally (0.1, 0.2, 0.3, or 0.5 seconds) for main, feeder, and branch circuits, and field adjustable.

• A self-contained test circuit utilizing the sensor's test coil shall be incorporated for testing with and without tripping of the circuit interrupting device.
• A self-contained test circuit utilizing the sensor's test coil shall be incorporated for testing with tripping and the circuit interrupting device.
• A molded case circuit breaker rated at specified amperes and interrupting rating at specified volts shall be provided. It should include thermal and magnetic elements for overload and fault protection, plus a ground-fault trip mechanism.

Fusible Bolted Pressure Contact Switch (Alternate)

A fusible bolted pressure contact switch rated at specified amperes and volts, equipped with fuses, shall be provided to open the circuit for various fault conditions. It shall include a ground-fault trip mechanism activated by the ground-fault relay, capable of operating at 55% of rated voltage. The switch's interrupting rating must be coordinated with the relay's time delay.

Low Voltage Power Breaker (Alternate)

A low voltage power breaker rated at specified amperes and volts shall be provided. It shall include a ground-fault trip mechanism activated by the ground-fault relay, capable of operating at 55% of normal operating voltage. The shunt trip release must operate satisfactorily at 55% of normal operating voltage.

Characteristic Tripping Curve

Figure 8 displays the characteristic tripping curve for the Ground-Censor Type GC system, showing time in seconds versus multiples of rated current for various curves (A, B, C, D) and time delay settings (e.g., 0.03 sec, 0.1 sec, 0.2 sec, 0.5 sec, 1 sec, 2 sec, 5 sec).

Dimensions

The document provides detailed dimensions for the system components, including the sensor and relay, with measurements in both inches and millimeters. A table lists various catalog numbers (Cat. No.) and their corresponding dimensions (A through G) for different sensor sizes.