Ground Fault Relay


DGF7-3

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Ground Fault Relay DGF7-3 front
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Ground Fault Relay DGF7-3 bottom
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Ground Fault Relay DGF7-3 back
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Ground Fault Relay DGF7-3 and Display
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Ground Fault Relay DGF7-3 Discription
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Ground Fault Relay DGF7-3 typical field connection using internal sensor
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Ground Fault Relay DGF7-3 typical field connection using external sensor
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Ground Fault Relay DGF7-3 typical field connection using interposing current transformer
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Ground Fault Relay DGF7-3 dimensions

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The DGF7-3 is a microprocessor based ground fault relay for use on solidly grounded or resistance grounded systems. This innovative digital electronic relay measures ground fault current using a built-in 28 mm zero sequence Current Sensor (CS), or an external Current Sensor. External Current Sensors with different cable windows, round or square, and various current ratios are available. The external current sensors must be connected to terminals T1 and T2. Once an external CS is detected this is logged in memory and is used to detect a short in the CS wires or an open connection. Both situations will call for a trip of the system, as it is unprotected from there on. All CS's will react to alternating current (ac) only and will reject direct current (dc) signals. Accuracy will be maintained over a frequency range of 45 to 450 Hz, making it suitable for variable frequency drive applications. The DGF7-3 is a Class A device as defined in the IEC 60755 standard; it is therefore fully characterised for operation with sinusoidal ac and pulsating dc currents.

The DGF7-3 houses a non-isolated universal power supply that ranges from 24 to 240 V ac or dc. The internal relay is equipped with form 'Z' (4-wire) isolated N.C and N.O. contacts. The following three functions are some of the available features, to be set on a front accessible dipswitch array:
  • The Trip Levels for ground fault current (ranging from 30mA to 3A in 8 steps).
  • The Trip Delays in 5 settable levels between 20 ms and 3 s.
  • The Relay Operating Modes (F/S (Fail Safe), pulsed F/S, pulsed non-F/S and non-F/S).
For all features, see the manual.
The dipswitch array is made 'tamper proof', meaning that manipulating the switches will not change the settings, unless a special procedure is followed. If the procedure is not followed correctly the red and green LED's will flash alternately, indicating an attempt to illegally change the settings. The original settings will stay valid until the entire procedure is followed.

The measured current is compared against two setpoints. The lower one is the Ground Fault (G/F) setpoint, the higher one is the High Current (H/C) setpoint. Using different ratio Current Sensors or interposing CT's will influence the setpoints, as outlined in the manual in table 2. When the current exceeds the G/F setpoint for the set delay, but does not reach the H/C level, a 'G/F' is diagnosed and the unit trips by operating its internal relay. Should a fault be approaching a dead short to ground, causing a residual current exceeding the H/C setpoint, then without any intentional delay the unit operates its relay, indicating a Short Circuit (S/C).

The user can select the operation of the relay to be 'Trip Inhibited'. Together with an upstream tripping device (fuses or breaker) it can protect local contactor contacts in the case of H/C faults. In this case the unit does not trip initially when the measured current exceeds the H/C setpoint, but waits for the upstream device to take care of current interruption and then trips. The latter trip flags the location of the fault to the user and also allows, after inspection on welded contacts, fast re-closure of the upstream tripping device. Should the upstream device fail to perform, then automatic backup protection makes the DGF7-3 trip -as a last resort- one second after the H/C occurred. This makes it possible to design a Type 2-coordinated installation, according to the IEC 60755 standard.

A single press of the RESET button, or an external, voltage free, momentary pushbutton connected to terminals C and S1, resets the relay after a trip. By double clicking the cover mounted RESET button a functional test of the DGF7-3 is invoked.

The unit shows it has sufficient control voltage applied on terminals L+ and FB by flashing the green RUN LED (1s on 1s off). The red TRIP LED shows that the DGF7-3 has sensed a fault and that it has operated it's output contacts. The blinking sequence of the red LED indicates the type of fault. When the power to the DGF7-3 is removed, pressing the cover mounted reset button will show the green LED when no fault was there before power-down. If a fault was present before power-down, the red LED will indicate the fault by the blinking sequence.

The DGF7-3 is encapsulated in polyurethane (PUR) to insure that it will function in all circumstances. This will make sure it is not vulnerable to weather, vibration or mechanical shock.

By using the internal CS of the DGF7-3 the maximum system voltage is 660 V. By insulating the busbars any GFS supplied external CS can be used on any system voltage, or using a 5 A interposing CT with the correct voltage rating will isolate the DGF7-3 from the system power lines.

DGF7 Display

Ground Fault protection Display
Ground Fault protection Display

The door mounted DGF7 Display Unit is connected to the DGF7-3 by up to 5 m of cable (must be shielded cable if longer than one meter). It provides the following remote indications and functions:
  • Continuous reading of actual, residual ground fault current in Amperes or as a percentage of the G/F trip level, employing auto ranging.
  • 'RESET' pushbutton with configurable 'Start Test' capability.
  • Display of the trip current setting, after a Test Trip has been activated.
  • 'SHOW TRIP' pushbutton. Remains functional even when supply power is switched off. Pressing the button will cause the LCD display to show whether or not the DGF7-3, to which it is connected, was in a tripped state at the instant of power loss. This way the user is able to verify which one of a group of units tripped their associated tripping device. This information remains accessible for at least 10 hours after power is lost.
  • Display of the pre-trip ground fault current, after a trip has occurred (flashing display).
  • Shows trip causes, after pressing 'SHOW TRIP'.
  • When the G/F current exceeds the system's measuring range, the display shows Out meaning 'Out of range'. Depending on various internal variables, the maximum value that can be represented (whether in relative or absolute format) before resorting to the 'Out of range' indication varies from around 400 to 800% of the G/F Trip Level.
  • Power over datalink, no external power supply needed.
The Numerical LCD window displays actual ground fault current in percentages or in A. 3 blank boxes to the right of the LCD display window are marked '%', 'A' and 'kA'. Use a permanent marker to check the appropriate box as follows:
      '%' - when using the relative format.
      'A' - when using the built-in CS, an external CS, or a 500:5 ratio interposing CT.
      'kA' - when using a 5000:5 ratio interposing CT.

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