I-Gard MGFR Series, Instruction Manual

3.3 MGFR-SE-ZB

These relays are designed to operate with I-Gard Type RZ R=rectangular zero sequence sensors, all of which are 
electrically interchangeable.

A test winding is built into each sensor and four screw type terminals are provided, two for the sensing winding (W1 
and W2), and two for the SELF-TEST winding (R3 and R4). The test winding has an equal number of turns (1000) 
to the sensor winding, thereby providing primary injection testing with relatively little test current. (For example, if 
the relay is set for 1200A, the relay can be tripped with 1.2 Amperes in the test winding, i.e. 1000 x 1.2 = 1200A). 
Application of 120V, 60 Hz. to the test winding causes a current of about 8 to 10 Amperes to flow in it providing test 
current to trip the relay on all settings.

CAUTION

Risk of equipment damage

The test winding current should not be continous, or overheating may result, with 

permanent damage to the winding.

Failure to observe this precaution may result in equipment damage

The types of RZ sensors that are to be used depend only on size requirements. The size is implicit in the catalog 
number (in inches) as follows:

RZ5-11, RZ5-21, RZ5-31, RZ5-35, RZ10-11, RZ10-21, RZ10-31.

4. OPERATION

The functional block diagram of Fig. 1 illustrates the basic operation of MGFR relays.

Three phases and the neutral of a 3-phase 4-wire power system pass through the window of a zero sequence 
sensor described in section 3. The secondary of the sensor is connected to X and XI (W1 and W2 in case MGFR-
SE-ZB) terminals of the relay. Analog current signals from the sensor are converted to proportional voltage outputs 
and digitized by an A/D converter in the relay. The microprocessor collects the digital samples approximately at 
2 msec intervals, in a 60 Hz system, and computes the RMS value from the fundamental of the waveform. Under 
normal operating conditions, the output of the ZSCT is zero and therefore the RMS computed by the relay is zero. 
The display ideally should show ‘0.00’ under this condition. In practice, the vector sum of all phase currents may not 
be exactly to zero for several reasons and thus, the display may not show exactly ‘0.00’. Any value showing less than 
or equal to ‘0.10’ may be ignored.

Under a condition of ground fault, the vector sum of all three phase currents will not be zero, and the component 
of the current not returning through sensor will be reflected in the output of the ZSCT. The RMS of that current will 
be computed by the relay. When that value exceeds the pick-up setting, the relay begins routines of time delay 
computation depending on curve and time selections on the faceplate described in earlier sections. After the 
desired time delay is elapsed, the relay issues a trip signal to energize the trip coil of the circuit breaker associated 
with the relay.