- Voltage
Rating. The
voltage rating of a new 7J46 cable is 1200 volts D.C. of any conductor with
respect to armor. All new cables are tested at twice the catalog maximum
voltage rating for 5 minutes before they leave the factory. The catalog
listed maximum voltage is conservative. This rating attempts to take
into account possible splices (in used cables) and some physical abuse the
cable will normally experience in field use.
Since
this catalog voltage rating is between any conductor and armor, +1200 volts DC
can be put on one set of the conductors, while - 1200 volts DC is applied to
adjacent conductors, without violating the voltage rating restriction..
The
catalog voltage ratings are not reduced by temperature within the
temperature rating of the cable. Also, the catalog voltage ratings
are conservative. The ratings should apply to used Camesa cables provided that
splices are done carefully, and physical abuse to the exterior of the cable is
not excessive.
-
Power Handling. The combination of cable maximum voltage rating and the
conductor electrical resistance are the factors that limit the conductor current.
One
exception to no limitation on current would be the situation of passing
several amps through one or several conductors for several hours, with a
portion of the cable tightly wound on the drum. Since
the heat buildup in the conductor can not dissipate, the cable on the drum
acts as a big heating coil. High currents in such situatuins can
cause sufficient heating t melt the plastic insulation around the
conductor.
-
Insulation Leakage.
All plastic insulation used in logging cables are such an excellent
dielectrics that in an unspliced cable there should be no measurable
leakage in any conductor. To check cable insulation be sure:
-
Cable is
disconnected from collector (slip rings)
-
Cable is
disconnected from the head or bridle cable
- The
insulation at both ends has been cleaned and all conductive coating
material
is removed.
Under
the above conditions there should be no measurable leakage once the conductor
is fully charged. This leakage will not
vary with surface temperatures.
The
insulation
resistance is so high, over 10 5 megohms/1000
ft, that even though it decreases slightly with temperature to the range
of 10 4 or 10 3 megohms/1000 ft., the decrease is of no significance. The
one exception might be Tefzel insulated conductors (7J46RZ) in the 425 500
degree F range. Teflon does not show the same amount of decrease in
insulation resistance that Tefzel does. For this reason Camesa suggests
use of the 7J46RTZ rather than the 7J46RZ. Most generally, low insulation
resistance is caused by rubber, neoprene, or similar type boots in contact
with the connector pins.
- Insulation
Defects. If any leakage can be observed after taking the above
precautions it will be due to:
-
a.
Manufacturing defects
-
b.
Mechanical damage to cable
-
c. Splice
in conductor
-
d. Z
kinks
Methods
of locating leaks will be discussed later under service, but experience
clearly indicates that most electrical failures are associated with
mechanical damage to the cable. One form of mechanical damage is caused by
shooting with multi conductor cables. This can result in the formation of
"Z" kinks in the conductors near the cable end.
- Conductor
Resistance. The maximum electrical resistance of the cable
conductors is listed in the catalog. For the 7J46, the maximum
electrical resistance is 10.6 ohms/Kft. at 68 degrees F. A 7J46 line
typically has 10 ohms/Kft. at 68 degrees F.
The
conductor is made of copper and therefore the resistance of the conductor
varies with temperature as
For
T in degrees Centigrade
| RT12
= RT1 |
(.9214 + .00393 T1) |
| (.9214 + .00393 T2) |
For
T in degrees Fahrenheit
| RT12
= RT1 |
(.8515 + .00218 T1) |
| (.8515 + .00218 T2) |
More
specifically for a 7J46 cable with a typical resistance of 10 ohm/1000 ft. at
68 degrees F,
For
T in degrees centigrade RT
= (9.21 + .0393 T) ohm/1000
ft.
At 274
degrees C. (526 degrees F) the resistance of copper is double its value at 20
degrees C (68 degrees F).
This
demonstrates the significant effect of temperature on conductor resistance. Of
course as the resistance increases, the cables ability to transmit power and
return m signals decreases!
As
a cable is lowered into the hole the total conductor resistance for the 7J46
cable will be
For
T in degrees Centigrade
RL
= (9.21 + .0393 TS )L + [9.21 + .0197 (TB
- TS )]D ohms
For
T in degrees Farenheit
RL
= (8.52 + .0218 TS )L + [8.57 + .0109 (TB
- TS )]D ohms
where
RL -total conductor resistance - ohms
L
- total length of cable on truck winch - units of 1000ft.
TS
- surface termperature
D
- depth of tool - units of 1000 ft
TB
- bottom hole temperature
Previous Page
| Print Page
