An electric fence energiser is intended to reduce exposure of an animal tangled in the fence line to high power deterrent pulses for an indefinite period. The method of operating the electric fence includes sensing a characteristic associated with said evaluation pulse (s) that varies with load on the fence line, and set a threshold value (16) based on said sensed characteristic. This is proceeded by sending one or more further low power evaluation pulses along the fence line, sense a characteristic to the threshold value. When the sensed characteristic pulse (55) falls below the threshold value, indicative of an animal in contact with the fence line is detected, send one or more but not more than a predetermined maximum number of higher power deterrent pulses (56) along the fence line.