Megger BTDR1500 Guia Del Usuario página 14

7.2.3. Auto test – Fault Finding
The Auto test sequence then moves to the Fault Finding phase where the Bridge principle will
be used to locate the fault. Here, the resistance to fault can be derived by using the digital
bridge and making use of the loop resistance calculated in the step above. The insulation
resistance is assumed to be at its lowest at the fault location and so is used to inject current
into the bridge circuit at that point. However, if the insulation resistance at the fault is too high,
(>20 MΩ), there will be insufficient current drive into the digital bridge circuit to be able to
resolve the reading accurately. If this is the case, an error message: FAULT RESISTANCE
OUT OF RANGE will be given.
Having measured the fault resistance and knowing the loop resistance or leg resistance
(2- or 3-wire mode) then the RTS and STF can be derived. When testing in 2-wire mode,
selected during the Loop Test detailed in 7.2.2 above and shown below, the RTS is assumed
to be half the loop resistance. If the measured RTF indicates that the fault is on leg B, then
the error message: FAULT NOT FOUND IN LINE 'A', WIRES MAY BE CROSSED is
displayed.
C
AU TO TEST SE QUE N CE:
B
I N SULATIO N TEST
LO OP TEST A
DTF
→ FAU LT FIN D I NG ←
E
2 W f au l t t es t- pl ea se w a i t
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If the reading is within range, you will
be asked if the cable under test is
made up of one type of wire along its
length (SINGLE) or multiple sections
STF
with different types of wire (MULTI).
DTS
Single
If SINGLE is selected, this then generates the result of the distance to fault, distance to strap
and the strap to fault distance using the calculated resistances scaled by the Ω/m of the gauge
and type of wire selected. (See the section on calculated factors for more information).
From the display you can modify the temperature or selected wire gauge or can display the
result in m, ft or Ω. Simply select the required parameter with the MENU key (#4, table1) and
alter with the cursor keys (#3 & #5, table 1).
D TS: 4.464
FAU LT FI N D I NG
SI N GLE M U LTI EXIT
D TF: 3. 571 k m
Pr ess M ENU t o
Tem p:
Pr ess t o p r oce ed
Uni t :
BELOW MIN OF the selected range is displayed.. To do this, select the wire gauge parameter
(as shown above) and step through the available options until "USER GA" is displayed. Select
this with the Up / Down gain key (#7, table 1). You can now define the resistance per metre
of the cable by selecting the digit using the Menu keys (#4, table 1) and changing the value
of each digit using the cursor keys (#3 & #5, table 1). Press the Up / Down gain key to return
to the results screen.
If the gauge of the cable under test
is not included in the built in table,
STF: 0.903
you can define your own gauge of
wire by specifying its resistance per
metre (or ft). This must be within the
range 0.01000 Ω/m to 0.32500 Ω/m
Cu 0.60
(0.10000 Ω/ft to 0.00300 Ω/ft) or a
EXIT
warning of ABOVE MAX OF or