Earth fault and Phase loss Protector
The Earth fault and Phase loss Protector (EPP) applies a simulated Earth fault (E/F) current on a 525V system when either (a) the phase voltages are very unbalanced or (b) a test button is pressed.
For safety of the operator the E/F current is only applied when the voltage on local Earth which is caused by this current, does not exceed 25V as would be the case when the resistance of the Earth wire is too high.
This voltage can be described as the voltage between the local earth and the transformer star point minus the voltage on a 27Ohms NER (Neutral Earthing Resistor).
When the resistance of the Earth wire is found too high, no E/F current is applied and the red LEDs light up.
A simulated E/F current is considered to be negligible if the duration is not longer than 0.12 sec.
When the resistance of the Earth wire is found within limits, the green LEDs will light up and an E/F current of about 400/200mA will flow for more than a second. This current can be used to trip an E/F protection unit rated 250mA/125mA, when installed.
The unit monitors the voltages of the 3-phase supply and when one phase is lost it starts an activation cycle. A phase loss is also simulated when the test button is pressed.
In the activation cycle the EPP starts with an evaluation cycle in which a current pulse is applied for a maximum of 6 cycles. During the evaluation cycle the voltage on the earth connection is monitored. The voltage reference is an internal star point.
When the activation cycle is initiated by the test button and the voltage between the earth connection and the transformer star point exceeds 25Vac plus the voltage on a 27Ohms NER, the EPP will stop any further excitation of the earth connection and indicate by a Red LED that the earth path resistance is unacceptably high.
As long as the voltage between the earth connection and the transformer star point does not exceed 25Vac plus the voltage on a 27Ohms NER, the unit will keep up the current in the earth connection and thereby simulate an E/F for about 1 second. Also a green LED will light up to show the earth path resistance is within limits.
By applying the E/F current an associated E/F Protection Relay or -monitor can be operated. When that E/F Relay trips it implies that the earth bonding, E/F protection and circuit breaker shunt trip coil are all functional.
525Vac 3-phase system with a NER of value 27Ω or 63Ω
The test button is external and supplied seperately
Supply (continuously connected): 525V ±20% , 3-phase 50 Hz
burden: capacitive, 100nF per phase
Excitation by test button:
N/O switch contacts to carry about 0.1 mA (average) from a 2.5Vdc source.
The voltage between the EPP internal star point and transformer start point minus the voltage on a 27Ohms NER, above which the simulated E/F current will be cut off after the evaluation cycle, is 25V ±10
Operating temperature range: -5 to +70°C
Storage temperature range: -40 to +85°C
Connection diagram example:
Flying leads: 6 x ±500mm long
Connect according to the connection diagram example above,
making sure the earth fault Relay is rated the same as or lower than the rating on the EPP
An ammeter can be connected in the (green) earth wire if desired
Apply 525Vac and watch the green LED blinking
If the red LED also blinks it means the previous trip was due to a phase loss
Press the test button for at least ½ sec and watch the green and red LED’s
If the green LED illuminates solidly and the red LED is extinguished,
the earth bonding is healthy and the earth fault Relay will receive a trip pulse
If the red LED illuminates the resistance of the earth bonding is too high
and the earth fault Relay will not receive a trip pulse.
Remove one of the phases an watch the green and red LED’s
The effect of this is the same as pressing the test button