SEFELEC1500M Instructions
151
V1.06
The contribution of the capacitance to the measured leakage current decreases exponen-
tially with time. It is, therefore, important to specify the time taken for the test in order for
the result to be significant. SEFELEC1500M devices can satisfy this requirement thanks to
the built-in timer, able to time from
0.1 s to nearly 3 hours
.
Never disconnect a capacitive sample without switching to
DISCHARGE
and waiting for
the time required for the capacitance to discharge into the resistor of
2. 2 kΩ
built into the
device (allow about
1s
per
100 µF
).
16.4.3.
CABLE MEASUREMENTS
Measurement on cables is similar to the measurement on capacitor samples (see the pre-
ceding paragraph for the basic precautions).
The configurations for measurements on cables are very diverse. The measurements must
be take either between conductors for multi-core cables, or between the core and the
shielding for shielded cables, or between the cable and its environment for single-
conductor wires.
In this latter case, the method that is generally used is to plunge the cable reel into a res-
ervoir of water (called POOL), allow the water to penetrate to the core of the reel, then
take the insulation resistance measurement between the conductor and the water. For
construction and safety reasons, the reservoir is grounded. The insulation resistance
measurement device must therefore be able to measure a sample of which one of its
points is the ground. SEFELEC1500M devices are able to take this type of measurement
very easily since the top point of the voltage generator can be connected to the ground.
Simply connect the input of the measuring device (with the high voltage accessory) to the
conductor to be measured and trigger the measurement.
Another specificity when measuring insulation resistance on cables is that the manufac-
turers' characteristics give resistance values for a standardised cable length of
1 km
(kilo-
metre).
On the platform when controlling cable reels, the latter are never exactly the standardised
length, which requires the operators to perform a calculation that is a function of the length
of the cable and of the number of conductors in parallel for multi-core cables. On the other
hand the comparators built into the measuring devices can no longer be used, since they
compare with respect to the global insulation value, not with respect to the standardised
value.
SEFELEC1500M devices can display insulation resistance measurements brought to a
length of
1 km and with 1 conductor
, which also allows for the use of built-in comparators.
The operator has the possibility to enter in a menu on the device the length of the cable
being tested as well as the number of conductors that it is comprised of. The result is then
expressed in
MΩ per km
.
For example: the device measures a value of
10 MΩ
for a single-core cable
10 km
long. Its
value brought to 1 km would therefore be:
(Rtotal/1 km) x Length = 100
MΩ. km
For the same cable with
10
conductors, the value for 1 conductor will be
1,000
MΩ.km
.
