Eaton SEFELEC 1000-M, Руководство пользователя, Страница: 190 / 209

User manual SEFELEC 5x Series 189 v1.06
that case the unit indicates its maximum value and takes a long time to come back to the
real measured value.
C) The insulation resistance value of a capacitor being a function following a time expo-
nential law, it is important to make sense to the measured value, to indicate the duration of
the measurement. The units of the MG range allow complying with this requirement with
the built-in timer, able to measure times going from 1 second up to 16 minutes.
D) Never disconnect a capacitive specimen before switching into DISCHARGE mode and
waiting the necessary time to discharge its capacitance through the 2.2 kohms built-in re-
sistance of the discharge circuit (about 1 second per 100 uF).
19.4.3. MEASUREMENT ON CABLES
The measurement on cables is similar to the measurement on capacitors
The measurement configurations on cables are very varied. The measurements have to be per-
formed either between each conductor for multi-wires cables, either between main conductor
and shielding for shielded cables, either between the cable and its environment for mono-wire
cables.
A) In that last case, the generally used way is to immerse the cable reel in a water tank (called
SWIMMING POOL), to wait for the water penetration in to the cable reel centre, and then to
perform the insulation resistance between the cable and the water. For safety and construc-
tion reasons the water tank is grounded. The insulation resistance measurement unit must be
able to measure a specimen with one grounded end. The units of the MG range allow per-
forming easily this type of measurement, because the hot point of the high voltage generator
is already grounded. You just have to connect the measurement input of the unit (with the HV
probe) on the cable to be measured and to trigger the measurement.
B) Another specific point, when measuring on cables, is that the specifications of the cable man-
ufacturers give resistance values for a standard length of cable equal to 1 km (1,000 meters).
When testing the reels of cable, those are never equal to the standard length, that forces the
operators to calculate the resistance as a function of the cable length and the number of
wires in parallel for the multi-wires cables. Consequently the built-in comparators of the
measurement units cannot be used, because they compare regarding to the total insulation
value and not regarding the standard value. The units of the MG range allow with the option
23 to display insulation resistance measurements reduced to 1 km and 1 wire, and therefore
allow the use of the built-in comparators. The operator can enter in a specific menu of the unit
the length of the cable under test as well as the number of wires. The result is given in Mohm
per km.
i.e.: the unit measures a value of 10 Mohms for a 10 km long mono-wire cable. Therefore, the
value reduced to 1 km will be:
(R.total / 1 km) x Length = 100 Mohm.km
For the same cable with 10 wires, the value for 1 wire will be: