A protection apparatus, for protecting a multi-terminal electrical power network, comprising: a measurement apparatus to measure the respective terminal current (I L, I R1, I R2, I R3, I R4, I R5) flowing at each terminal (L, R 1, R 2, R 3, R 4, R 5) in a multi-terminal network including a plurality of electrically interconnected terminals (L, R 1, R 2, R 3, R 4, R 5); and a control unit, the protection apparatus being characterised in that the control unit is programmed to process the measured terminal currents (I L, I R1, I R2, I R3, I R4, I R5) to obtain first and second summed time-varying waveforms (I POS (n), I NEG (n)) defined over the same time period, the first summed time-varying waveform (I POS (n)) being the sum of the or each terminal current (I L, I R1, I R2, I R3, I R4, I R5) flowing in a first direction (D 1) relative to a corresponding terminal (L, R 1, R 2, R 3, R 4, R 5), the second summed time-varying waveform (I NEG (n)) being the sum of the or each terminal current (I L, I R1, I R2, I R3, I R4, I R5) flowing in a second direction (D 2) opposite the first direction (D 1) relative to the corresponding terminal (L, R 1, R 2, R 3, R 4, R 5), and the control unit is further programmed to carry out a magnitude comparison algorithm which includes: establishing a discriminative ratio (R INT) based on the norm of a sum of the first and second summed time-varying waveforms (I POS (n), I NEG (n)) and the norm of a difference between the first and second summed time-varying waveforms (I POS (n), I NEG (n)), and comparing the discriminative ratio (R INT) with a threshold value (R THR) to determine whether a fault in the multi-terminal network is internal or external to the multi-terminal network and thereafter to produce an internal fault output signal upon the determination of an internal fault. a measurement apparatus to measure the respective terminal current (I L, I R1, I R2, I R3, I R4, I R5) flowing at each terminal (L, R 1, R 2, R 3, R 4, R 5) in a multi-terminal network including a plurality of electrically interconnected terminals (L, R 1, R 2, R 3, R 4, R 5); and a control unit, the protection apparatus being characterised in that the control unit is programmed to process the measured terminal currents (I L, I R1, I R2, I R3, I R4, I R5) to obtain first and second summed time-varying waveforms (I POS (n), I NEG (n)) defined over the same time period, the first summed time-varying waveform (I POS (n)) being the sum of the or each terminal current (I L, I R1, I R2, I R3, I R4, I R5) flowing in a first direction (D 1) relative to a corresponding terminal (L, R 1, R 2, R 3, R 4, R 5), the second summed time-varying waveform (I NEG (n)) being the sum of the or each terminal current (I L, I R1, I R2, I R3, I R4, I R5) flowing in a second direction (D 2) opposite the first direction (D 1) relative to the corresponding terminal (L, R 1, R 2, R 3, R 4, R 5), and the control unit is further programmed to carry out a magnitude comparison algorithm which includes: establishing a discriminative ratio (R INT) based on the norm of a sum of the first and second summed time-varying waveforms (I POS (n), I NEG (n)) and the norm of a difference between the first and second summed time-varying waveforms (I POS (n), I NEG (n)), and comparing the discriminative ratio (R INT) with a threshold value (R THR) to determine whether a fault in the multi-terminal network is internal or external to the multi-terminal network and thereafter to produce an internal fault output signal upon the determination of an internal fault. establishing a discriminative ratio (R INT) based on the norm of a sum of the first and second summed time-varying waveforms (I POS (n), I NEG (n)) and the norm of a difference between the first and second summed time-varying waveforms (I POS (n), I NEG (n)), and comparing the discriminative ratio (R INT) with a threshold value (R THR) to determine whether a fault in the multi-terminal network is internal or external to the multi-terminal network and thereafter to produce an internal fault output signal upon the determination of an internal fault.