CABLES,
INSULATION RESISTANCE
(Adapted from:D.T. Hall:Practical Marine Electrical Knowledge)
Shipboard cables
 Apart from an IR (megger) test on a main cable run (e.g. along
the flying bridge of a tanker) the survey of cables and their
installation is largely based on a close visual examination.
 Inspection would search for any external damage of a cable's
outer sheath and wire or basket weave armouring (if fitted).
 The cable must, of course, be adequately supported along
horizontal and vertical runs by suitable cable clips or ties.
IR /Megger test equipment
 A flying bridge is a (usually open) area on top of, or at the side
of, a ship's pilothouse, or closed bridge, that serves as an operating
station for the ship's officers in good weather or when
maneuvering in port, where good views along the ship sides are
important. It is also a raised, usually second story cockpit on a
smaller boat, such as a sport-fisher.
Cable construction
Cable clip (brass)
 Where cable-runs along an open deck have expansion loops,
these must be examined for abrasion and wear.
 Where cables pass through fire check bulkheads they must be
correctly glanded or pass through stopper boxes which
prevent the passage of fire between compartments.
Cable expansion loop
Cable glands
EExd compound stopper box / adaptor
 Probably the most common ship-board cable insulations used
are EPR (ethylene propylene rubber) or butyl rubber which
is sheathed with either PCP (poly- chloroprene) or CSP
(chlorosulphonated polyethelene).
 EPR or butyl rubber are good electrical insulators but are
not mechanically strong or resistant to oil. This is why a
sheath of PCP or CSP (which is stronger and has greater oil
and fire resistance) is fitted around the inner insulation.
 Where EPR/butyl cable terminations may be subjected to oil
vapour it is usual to tape or sleeve the cable ends to prevent
deterioration of the insulation. Check that such taping is
secure.
Cable end-sleeves
 Flexible cables to light fittings, power tools, etc., should be
inspected for mechanical damage. In normal operation a
flexible cable may be repeatedly dragged and chafed so
reducing its safety. If in doubt replace flexible cables.
 A copper strap or flexible earthing braid/wire is used to
bond the steel frame of all electrical motors and other
equipment to the ship's hull.
Copper wire braided strips
 Without an earth strap, a loose internal wire may touch the
frame causing it to become live at mains voltage with obvious
danger to operators. The earth strap electrically anchors the
frame to the ship's hull (zero volts) to eliminate the shock
hazard to personnel.
Insulation Resistance
 The surveyor will require a list which shows the results of
recent insulation tests on all main 440 V and 220 V circuits.
 Such a list should also indicate the test dates, weather
conditions (hot, humid, etc.) together with any comments
relevant to the test conditions (e.g. machine hot or cold).
 For essential items such as generators and main motors, the
surveyor will be more interested in the IR trend, so a set of
past results showing the insulation history of such machines
may be requested.
Insulation resistance testing
 Insulation resistance is one of the important readings of marine electrical
equipment systems and serves as the best guide to indicate the health of the
equipment. Insulation resistance is measured between the insulated
conductors and earth and between conductors.
 The insulation resistance is measured by the equipment known as megger,
which is a high resistance meter with a test voltage of about 500 volts dc.
The 500 V is produced with the help of a hand driven generator or with the
help of batteries and electronic voltage charger.
 The 500 V test charge is suitable for testing equipments which are rated for
440 volts AC. The equipments to be tested for insulation resistance must be
disconnected from the live power supply and the supply to be locked down
to prevent any accidents.

Read more: http://www.marineinsight.com/tech/marineelectrical/importance-of-insulation-resistance-in-marine-electricalsystems/#ixzz1tAxdWcC7
 On ships, the insulation resistance of all the motors is
checked from time to time and values are logged as a part of
planned maintenance system. The insulation resistance of the
machinery reduces with increase in temperature. The reasons
for increase in temperature may be due to dust deposits on
the windings or improper ventilation. The resistance is
checked between the windings U-V, V-W, W-U and between
U & earth, V& earth, W& earth.
Consecutive readings
 The readings are logged down and the graph is plotted and
the trend of insulation resistance is checked. If the reading is
reduced to a very low value then the windings have to be
checked and cleaned and the readings are to be taken again.
Read more: http://www.marineinsight.com/tech/marineelectrical/importance-of-insulation-resistance-in-marineelectrical-systems/#ixzz1tAyKxORb
Readings logged
THE END!