Page 30 - FL-20A Instruction Manual
Model FL-20A Cable Fault Locator
pathway. With enough arcing, the entire area becomes carbonized. If
the resistance between the shorted conductors is high enough, arcing
may stop entirely at a given test voltage.
However, while arcing may stop, current can continue to flow. In some
cases, enough current will flow to cause the FL-20A to continue trying
to locate the fault. In these situations the results will be inaccurate. In
severe cases, the FL-20A will show an error message.
The reason the results can be inaccurate are best understood by looking
at the schematic diagram of a typical fault.
Figure 1
All Clinton cable fault locators require a high degree of consistency of
resistance per unit of measure of the various conductors in order to
locate short circuits. During the bridge measurement, the total resistance
of Segment A plus the value of R1 equals the resistance of Segment B.
This is referred to as “balancing the resistances” and is the basis of cable
fault location of short circuited conductors.
When there is significant arcing and carbonization, there may be mul
-
tiple current pathways, each with a different resistance. This is illustrated
below in Figure 2.
Figure 2
The FL-20A will try to balance the resistance of Segment A to Segment
B first using one resistance, (R1) but then the arc will move to another
site (R2) with lower resistance. The unit will then try to balance these
two segments.
Regardless of which of the resistances the FL-20A balances, the result
will likely not be very accurate because the carbon at the fault site adds an
unknown level of resistance, making balancing the resistances difficult.
For this reason, the location of shorts revealing themselves at voltages
above 15 kVDC or 10.5 kVAC should be suspect unless the FL reports
“Standard” or “Exceptional” accuracy.
