Panel Protective relays - Earth leakage relays (ELR), Earth fault relays (EFR) and Phase Failure Relay (PFR)


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When manufacturing a panel board it is essential to have some protective methods other than breakers which provide additional protection to the panel board, equipment that is connected to the panel board and the user.

For this protective relays are used. As panelboard is the heart of the distribution of the system it is vitally important to have protective methods.

ELR:

ELR (Earth Leakage Relay) ensures the protection of electrical installations and person against direct and indirect contacts. ELR is designed on an electronic basis, which ensures the monitoring of earth fault currents.

Earth leakage occurs due to reasons such as normal equipment wear or moisture around the terminals, which can cause partial failure of the insulation between the supply and the ground. Leakage currents to ground are dangerous since they can cause the heat generation of the cable and insulation faults.

To operate an ELR you must be connected to a CBCT (Core Balance Current Transformer). It is known that at any moment the algebraic sum of the currents in the balanced three-phase supply is equal to zero. Therefore, under normal conditions, the total algebraic sum of the currents in four cables (3 phases and neutral) must be zero.

When a leak occurs, there will be a leakage current and, ultimately, the algebraic sum of current through CBCT will not equal zero and, as a result, the current will be induced in the CBCT. This current provides a signal to ELR and begins to operate and closes its normally open contact.

EFR:

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EFR (Earth Fault Relay) is used for protecting from earth faults and use with MCCBs with a current rating greater than 250A. The function of EFR is as same as ELR, but more sensitive than ELR. Instead of a CBCT, four separate CT’s are used to connect an EFR.

EFR is used to Protects control panels and switchboards from earth fault, Protects motors/ transformers/feeders/generators etc., from earth fault, Hazardous and sensitive industries like oil refineries/pulp industries / electrical distribution etc., can be protected.

PFR:

PFR (Phase Failure Relay or Phase Voltage Balance Relay) is a three-phase voltage sensing device that trips on phase loss, phase reversal, over voltage, or under voltage. Voltage unbalance trips the device when any voltage drops certain percentage (around 10%) below the average.

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Generally, a PFR is used along with a UVT coil. But a disadvantage of this is when incoming supply cuts off, PFR considers it as a fault and trips the circuit. Then somebody has to switch on the breaker after power comes.

To avoid this disturbance sometimes shunt coil is used instead of a UVT. But it has some disadvantages also. When a failure of a phase which provides voltage to the shunt coil, occurs then PFR cannot send the tripping signal.