The detector is similar to that described previously for electrical detection of discharge
activity at the high voltage terminals of plant, i.e. a discharge-free capacitor coupled
to a low voltage detector impedance but, in this case, the low voltage impedance is
connected to instrumentation which can record both the magnitude and time of arrival
of discharge pulses. When a degradation site discharges, part of the discharge pulse
propagates towards the detector end and part to the open circuit end. When the latter
encounters the open circuit, it is reflected and propagates back towards the detector
end. In consequence, at the detector, a pulse arrives representing that part of the
discharge which originally propagated from the discharge site followed by a second
pulse representing that part of the pulse which propagated in the opposite direction,
was reflected at the open circuit, and then propagated back along the cable to the
detector (see Figure 4.17).