VON MODEL SST15-832 ARC REFLECTION SECTIONALIZING SYSTEM Page 17
equation:
K = dielectric constant based on the insulation material
r
1
= inside radius of insulation
r
2
= outside radius of the insulation
Thus the characteristic impedance of the cable varies with the diameter of the cable,
thickness of the insulation and the type of insulation. A few common values of Z
0
are 20
ohms for 35kv 1000MCM polyethylene cable, 42 ohms for 35kv 1/0 polyethylene cable, and
74 ohm for RG59U polyethylene cable.
Any change in Z
0
along the length of the cable to the fault will cause reflections. The size of
the blip will be based on the reflection coefficient whose maximum value is 1 or -1. The
equation for the reflection coefficient p is:
At the far end terminals with the following impedances:
Z = 0 (short circuit)
p = - 1
Z>> Z
0
p = 1
Z = Z
0
p = 0
Z = 1/2 Z
0
p = -.33
Thus when the fault impedance exactly equals the cable impedance it will not show up on
the screen. Fortunately this almost never occurs in the field.
Distance is figured by a radar using the time delay which is based on how fast the radar
pulse travels along the cable. The distance to the fault is related to the time the pulse takes
to get to the fault and return. The accuracy of the speed of the pulse in the cable (called the
velocity of propagation) determines how accurately the distance to the fault can be
calculated.
XI
THEORY OF ARC REFLECTION METHOD
The arc reflection method utilizes the low resistance path to ground (less than 50 ohms)
created at the cable fault by an arc. The arc is provided by a capacitor discharge fault
locator (thumper) to temporarily display the fault on a standard radar. Using the low
resistance of an arc overcomes the main limitation in the past of radars which alone could
not see the high resistance faults most common in underground primary power cable. The
arc reflection method does not overcome the limitations of radar itself. The operator must
become proficient in the use of the radar especially in recognizing faults near the ends of the
cable. On cable with missing neutral, a cable radar may not even show the far end of the
Summary of Contents for SST15-832
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