Quantcast Chapter 6 Power Cables

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TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
(2) Physical damage.
6-1. Components.
(3) Excessive tension.
Power cables are generally made up of three
(4) Cables laying under water.
components: conductor, insulation and protective
(5) Cable movement or dangling.
covering. The single most important component of a
(6) Insulation swelling.
cable is its insulation. The best way to ensure con-
(7) Soft spots.
tinued reliability of a power cable is through visual
(8) Cracked protective coverings.
inspection and electrical testing of its insulation.
(9) Damaged fireproofing.
The guidance provided here applies only to cables
(10) Poor ground connections or high imped-
rated 600V AC or less and, to the occasional appli-
ance to ground.
cations found in DC motor drives operating at 500,
(11) Deterioration of metallic sheath bond.
600, or 700v DC or less.
(12) Corrosion of cable supports or trays.
b. Raceway and cable tray installations. Since the
6-2. Visual inspection.
raceway or cable tray is the primary mechanical
A visual inspection of a power cable can be made
support for the cable, it should be inspected for
with power on. However, if the visual inspection is
signs of deterioration or mechanical damage. The
to include touching, handling or moving cables in
cable jacket should also be checked for abrasions or
manholes or at terminations, then all circuits in the
mechanical damage.
group to be inspected should be de-energized before
6-3. Cable insulation testing.
the work is started.
The electrical test most often conducted to deter-
a. Manhole installations. Manholes are not usu-
mine the quality of low voltage cable insulation is
ally located inside buildings. Terminations and
the insulation resistance test (para 14-2). It is per-
splices of non-lead cables should be squeezed in
formed as a routine maintenance test for cables
search of soft spots, and inspected for tracking or
already in service or as an acceptance test for new
signs of corona. The ground braid should be in-
cables. DC overpotential testing is another way of
spected for corrosion and tight connections. Inspect
testing cable insulation. This test is performed pri-
the bottom surface of the cable for wear or scraping
marily on medium and high voltage cables to test
due to movement at the point of entrance into the
their dielectric strength and is not recommended for
manhole and also where it rests on the cable sup-
routine maintenance testing of low voltage cables.
ports. Inspect the manhole for spalling concrete or
The insulation resistance test for low voltage cables
deterioration above ground. If the manhole is
is usually performed using a megohmmeter (para
equipped with drains, these may require cleaning
13-4). It is a simple, quick, convenient and nonde-
or, in some instances, it may be necessary to pump
structive test that can indicate the contamination of
water from the manhole prior to entrance. Do not
insulation by moisture, dirt or carbonization. Before
enter a manhole unless a test for dangerous gas has
testing any cable, the circuit must be de-energized.
been made and adequate ventilation gives positive
Once that is done, it is usually best to disconnect
assurance that entry is safe. High voltage cables
the cable at both ends in order to test only the cable,
may be present fireproofed with asbestos containing
and to avoid error due to leakage across or through
materials which pose additional health hazards.
switchboards or panelboards. For an acceptance
Potheads should be inspected for oil or compound
test, cable less than or equal to 300 V maybe tested
leaks and cracked or shipped porcelains. The
at 500 V and cable greater than 300 V but less than
procelain surfaces should be cleaned and if the con-
nections are exposed, their tightness should be
600 V maybe tested at 1,000 V. For a routine main-
checked. Since inspection intervals are normally
tenance test, test voltage should be restricted to 60
one year or more, comprehensive records are an
percent of the factory test voltage. The test voltage
important part of the maintenance inspection. They
should be applied from phase to ground on each
should be arranged so as to facilitate comparison
conductor with the shielding tapes and metallic
from one year to the next. Cables in manholes,
jackets also connected to ground (fig 6-l). While no
ducts or below grade installations should be in-
general standard exists for minimum acceptable in-
spected for the following:
sulation resistance values for cables in service, a
(1) Sharp bends in the cables.
"rule-of-thumb" of one megohm of resistance (mini-


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