45
Therefore, to calculate the distance to a short you must
first use a sample length of cable to determine the
inductance per foot value, as explained in the following
section. Record this amount in Table 12 for each type
and manufacturer of cable you encounter.
To determine the distance to a short:
1.
Zero the LC103 test leads.
2.
Connect the red test lead to the center conductor and
the black test lead to the braided shield outer
conductor of a shorted cable.
3.
Press the INDUCTOR VALUE test button and read
the total inductance of the cable.
4.
Divide the LC103 inductance reading by the cable's
inductance per foot value. This gives the distance to
the short in feet.
NOTE: To help pinpoint the short with greater
accuracy, measure the inductance from both ends of the
cable.
Determining Capacitance and Inductance Per Foot
The capacitance and inductance per foot values for a
particular type of coaxial cable can be determined by
measuring a sample cable of known length. After you
measure the amount of capacitance and inductance with
the LC103, simply divide the total amount by the length
of the sample. A sample length of at least 10 feet is
recommended for an accurate capacitance measurement,
and 25 feet for accurate inductance measurement.
To determine capacitance and inductance per foot:
1.
Zero the LC103 test leads.
2.
Connect the red test lead to the center conductor and
the black test lead to the braided shield outer
conductor at one end of the sample cable.
3.
Leave the other end of the cable open to measure
capacitance. Short them together to measure
inductance.
4.
Press the CAPACITOR VALUE or INDUCTOR
VALUE test button and read the total capacitance or
inductance of the cable.
5.
Divide the LC103 reading by the length of the sample
cable.
Identifying Aging Cable
All coaxial cables eventually degrade to the point where
they need to be replaced. The LC103 can be used for
preventative maintenance checks of coaxial cable to
determine if deterioration is beginning to occur. As a
cable begins to fail, the dielectric separating the
conductors becomes contaminated causing a change in
the cable's capacitance and the DC leakage through the
dielectric.
All cable has a normal amount of capacitance per foot
and any significant change that occurs over a period of
time indicates a developing problem. The best check for
aging cable is to measure and record the total
capacitance of the installation when it is first installed. If
the initial value is not known, you can multiply the
length of the cable by its nominal capacitance per foot.
Then compare periodic capacitance measurements back
to the initial amount and look for any changes. As the
dielectric becomes contaminated, the LC103 capacitance
reading will increase.
The LC103 leakage power supply also provides a good
test of a cable's condition. Simply measure the amount of
leakage through the dielectric between the conductors.
Most cables have a maximum operating voltage of 1000
volts or more and should be tested with the LC103
leakage supply set to 999.9 volts. A few “air space”
dielectric types of coaxial cable, such as RG37, RG62,
RG71, and RG72 have a maximum operating voltage of
750 volts and should be tested at this lower voltage.
———————— WARNING —————————
This test should only be performed by a qualified
person who understands the shock hazard of up to
1000 volts applied to the test leads and open ends on
the coaxial cable.
————————
ATTENTION
————————
Le LC103 est fait pour être utilisé par une personne
qui est techniquement capable et qui comprend les
chocs que peut produire l’équipement (jusqu’à 1000
V) à sortie de la sonde ou du cable coaxial.
A good piece of cable should have no leakage when the
voltage from the LC103 is applied between the center
conductor and outside shield. The length of the cable
being tested will make no difference on the leakage
reading. Any leakage reading indicates the dielectric is
breaking down.
