T-305E User Manual Version 3.5 | 10
The stored energy is discharged into the cable with a steep-fronted, high energy surge.
Examples of these waveforms are shown in Figure 3.3.
Figure 3.3. Surge characteristics
This is a series of curves with axes; V (kV) and t (µs). The black line represents a typical fault
characteristic showing that it will not break down instantaneously.
It is a common misconception that the ability of a surge to break down the fault is solely
dependent on the magnitude of its voltage. Figure 3.3 shows that this is not always the case.
If the green 15kV (4µF) curve is considered, it only cuts the fault characteristic after 6 µs,
when it is past its peak. Meanwhile with twice the voltage but a much lower capacitance,
the blue 30kV curve fails to cut the fault characteristic at all and does not result in a
breakdown. Increasing the capacitance by a factor of two, as shown in the red curve causes a
breakdown after around 4µs. The increase in capacitance serves to increase the energy
delivered in the surge, allowing the breakdown to be achieved at a lower voltage, which may
be the difference between a successful and unsuccessful test.
A further benefit of increasing the capacitance is that the extra energy should result in a
louder sound and more vibration at the fault point which will enhance the pinpointing
process.
The T-305E is available in versions with either 1024J output or 2048J, the disadvantage of
the
higher value being an increase in the surge generator’s size and weight.
3.4
Low-voltage cable break-down
The term “l
ow voltage cable
” refers to
cables rated below 400V and hence, the insulation
rating is also low. In order to protect the healthy sections of the cable, the operator should
be careful to ensure the discharge voltage does not exceed 5kV. If it is difficult to break-
down the fault point at this level, it is better to increase the capacitor value. Typically, 5kV
with 10µF should be sufficient to break-down the fault.
