Eng. R. T.

Paramanayagam
B.Sc. Eng (Hons), CEng (SL), CMEngNZ, IntPE (SL), IntPE (NZ), MIE (SL)

PROTECTION RELAYS (EFR, ELR,PFR, ….)

9/6/2023 1
Earth Fault Protection using EFR Vs ELR
• EFR is used to detect “Earth Faults” situation in the Electrical System.

• ELR is also used to detect “Earth Faults” situation in the Electrical System.

• The ELR uses a special equipment known as “Core Balancing Current Transformer (CBCT)” to
detect the unbalance in the phases and neutral and hence to detect the Earth Fault.

• The EFR uses 4 nos. of “Protective Current Transformers (CTs)” and the vector sum of the
secondary currents of the CTs is connected to the EFR to detect an Earth Fault.

• The ELR is more sensitivity than EFR as it can detect the Earth Faults in milliampere (mA) ranges
where as EFR cannot detect Earth Fault in milliampere (mA) ranges.

• The accuracy of the EFR is dependent of the CT ratio. For example, let’s say we use a CT of
1000/5A for the EFR and there is an earth fault current of 2A in the system.

• So, the secondary current of the CT will be 2/200 = 0.01 A, which is very small for the EFR to detect
the earth fault.

• Generally, a minimum secondary fault current of 0.1A is required for EFR to be triggered. So, the
any earth fault that is below 19.99A may go un-detected. This is a dangerous
9/6/2023 2
situation especially in
TT system.
Earth Fault Protection using EFR Vs ELR
• The earth fault current in a TT earthing system, which is used generally except special situations
where there is a Client owned Transformer, the earth fault currents are very small due to the high
earth loop impedance.

• So, in general, the EFR is not recommended for TT system and ELR is recommended.

EFR ELR
Low set (I>): 0.1A to 2A. Sensitivity adjustment: 30mA to 30A.
Low set delay time (t>): 0.05s to 1.0s. Delay time adjustment: 0 sec to 3 sec.
High set (I>>): 1 to 10 times I. 9/6/2023 3

High set delay time (t>>): instantaneous.

An EFR is used with a CT ratio of 1600:5 to protect against earth faults in a TT system. The earth loop
impedance is 10Ώ. Please critically analysis whether the proposed EFR provides an effect earth fault
protection or not with necessary calculations.

Solution:
• Step-1: Earth fault current = 230/10 = 23 A.
• Step-2: Secondary current of the CT = 23/320 = 0.072 A (< 0.1 A, which is the minimum
current setting of the EFR).

• Step-3: So, the earth fault will not be detected by the proposed EFR.

• Step-4: The sensitivity of an ELR is 30mA to 30A. So, the earth fault current (23A) will fall
within this range and hence, an ELR can detect and disconnect the faulty circuit.
Conclusion:
• Do not use EFR instead of ELR in a TT system as the earth fault current is very less (in mA range)
due to the high earth fault loop impedance.

• EFR can be used in TN earthing system. 9/6/2023 4

(a) Complete phase loss.

Phase asymmetry

9/6/2023 5
Phase Failure Relay (PFR) Eng. R. T. Paramanayagam
B.Sc. Eng (Hons), CEng (SL), CMEngNZ, IntPE (SL), IntPE (NZ)

Complete phase loss - This is the condition where one or more phases are no longer present.

Under voltage - This is when the voltage of one or more phases is under the pre-set minimum limit. This
condition will make the motor to demand More current which may cause damages over
time. This situation can be arrested by the help of Thermal Overload Relay if installed.

Some Engineers say that PFR to detect the under voltage situation is not required for
Motors as the Thermal Over Load Relay if detect it and trip off the Motor.

Over voltage - This is when the voltage of one or more phases is over the pre-set maximum limit.
Over voltage itself can cause damage to the motor.

Phase sequence - This is when the three phases are not connected in the correct sequence. If a motor is
started with an incorrect phase sequence it will cause them to start in the reverse
direction which may cause damages.

Phase asymmetry- This is when the load on all three phases is not balanced properly. If the supply is
more than 5% out of balance, then this can cause damages to the AC motors and
pumps.

9/6/2023 6
 Typical Single Line Diagram of
MSB with ATS and PFRs

PFR

PFR
MANUAL CHANGE OVER
SWITCH

Eng. R. T. Paramanayagam
B.Sc. Eng (Hons), CEng (SL), CMEngNZ, IntPE (SL), IntPE (NZ)

9/6/2023 7
Typical Single Line Diagram of MSB
Eng. R. T. Paramanayagam
B.Sc. Eng (Hons), CEng (SL), CMEngNZ, IntPE (SL), IntPE (NZ)

ELR

CBCT

9/6/2023 8
 Typical Connection Detail of ELR with CBCT and Shunt Release

Eng. R. T. Paramanayagam
B.Sc. Eng (Hons), CEng (SL), CMEngNZ, IntPE (SL), IntPE (NZ) 9/6/2023 9