Electric Cables

Electric Cables

Application of such more so­phisticated cables like for in­stance the fire resistant variety will reduce the consequences and damage of a fire contrary to the commercially attractive PVC-insulated types. These pvc cables generate toxic and corrosive gases during a fire, resulting in a lot more dam­age to the installation than the parts which are directly dam­aged by the fire.

A disadvantage, however, of the low smoke types of ca­bles is that their mechanical properties, as strength against mechanical stress while being pulled, is considerably less with the possibility of damage with installation.

Cables

Cables for ship's installations differ from those for on-shore installation by the way the conductor is built up. Instead of a solid conductor as in most on-shore or industrial ca­bles, a marine cable consists of a stranded conductor consisting of 7 or more wires to cope with the vi­brating environment.

This does not mean that a MARINE cable is flexible enough for a non­fixed or a moving installation. A further difference with on-shore installations is that cables in a ma­rine environment must be fixed to the cable supports.

Flexible cables for moving installa­tions such as cranes or telescopic supported wheelhouses are fixed to movable cable trays.

                                                                 1.Core of twisted copper conduc­tors
                                                                 2. Mica wrap
                                                                 3. Core insulation
                                                                 4. Filler
                                                                 5. Inner sheath
                                                                 6. Copper wrap
                                                                 7. Braiding copper or galvanized steel
                                                                 8. Outer sheath

Cables with solid conductors up to 2.5 mm² can be used in ship's ac­commodations. See for details of shipboard cables the relevant IEC standards.

Screened power cable consisting of, in addition to the above cable, 4 copper sheath, 5 and 6 galvanised steel wire braiding. Screened single core AC power cables shall have a non-magnetic screen, because a steel braiding will heat up by the magnetic field resulting from the current in the cable.

The same is applicable when single core AC power cables pass through a steel bulkhead penetration. The sum of the currents through such a penetration shall be zero. Also gland plates for single core cables shall be of non-magnetic materials.

Screened multicore control ca­ble consisting of laid up twisted pairs.

Fire resistant cable

Cables which should remain func­tioning under fire conditions have a similar construction as other ca­bles, but are provided with an ad­ditional layer of mineral insulation around the conductors, in this case mica tape. It is amazing to see how this simple measure makes the cable fire re­sistant, not only in straight lengths but also in bent parts of the cable run.

Tests have been performed at sev­eral cable manufacturers' works where straight and bent pieces of cables have been subjected to a standard fire test up to 1000 °C for a period of one hour. These cables remain in service, with acceptable megger readings between the con­ductors and between conductors and earth.

The cores are found still capable to transport electrical power, which means that no wire is interrupted. When fire resistant cables are used all other parts of the system like junction boxes involved, should also be fire resistant.

Application fire resistant cables

Fire resistant cables are applied when the circuits have to remain in operation under fire conditions. This is mainly limited to safety and fire fighting circuits such as emergency lighting, fire detection, alarming circuits, communication circuits and fire safety shutdown circuits. Fire resistant cables shall be used to ensure continuity of service in spaces adjacent to the space which could be damaged by fire. For example emergency lighting circuits routed through an engine room supplying a steering-gear room. The same is applicable to a public address circuit running through a fire zone servicing loudspeakers in a next fire zone.

Another example is a fire door, which requires electric power to close, has to get its supply by a fire resistant cable from a safe area. If the door would close by itself when the power supply is inter­rupted, a normal cable would be acceptable.

The same is applicable to any sort of safety equipment or essential propulsion equipment. Duplicated essential propulsion equipment shall not be powered from the same source or be pow­ered by cables routed along a com­mon cable run other than protected individually against mechanical and fire damage.

Cable selection tables

The table on the next page shows the cable ratings for various types of cables for an ambient tempera­ture of 45° C. When cables are installed in an area with a different ambient tem­perature, the correction factors as per table on the top of the page should be applied.

Example:

A cross linked PE cable of 3 x 4 has a current rating of 27A. When this cable is installed in an area with an ambient temperature of 60° C a correction factor of 0.79 must be applied. The current rating then will be 0.79 x 27 = 21.33A

Note: correction factors for bunch­ing of cables may also be applied and class rules must be consulted for the corresponding values

To indicate the quality of the cable, codes are printed on the outside, ac­cording to the production standard

   Correction factors for cables 

In order to determine the neces­sity for fire resistant cables and the cable routing, the approved Safety Plan showing the watertight bulk­heads, fire resistant bulkheads and decks, the A-60 insulation and the fire zones, is required.

Larger cross-sections are consid­ered unsuitable for installation on ships because of their size and as­sociated bending radius.

Parallel cables have to be routed in such a way that sufficient air can circulate for cooling. If this is not the case, de-rating fac­tors must be applied.

AWG in the above table re­fers to American Wire Gauge which is the cross section as per American standards.

When a cable is damaged due to a too high ambient tempera­ture, and has to be replaced, the proper quality cable has to be chosen. Refitting using the same quality cable will result in the same damage, or the allowed current has to be re­duced as per table above.