Ø All marine electrical equipment must have a breaker or fuse to protect the wiring as well as the product. Don't use breakers or fuses larger than those recommended by the manufacturer, or you may find yourself with 'fried' equipment.
Ø Don't tap into other wires, no matter how tempting! Go directly to a terminal block and circuit breaker, as well as directly to a grounding bus.
Ø Turn off all power before starting any work.
Ø Maintaining good "grounds" is critical. Avoid unnecessary stray current.
Cable Insulation and Ingress Protection
Class A Insulation
- These are materials or combinations of materials such as cotton, silk, and paper when I suitably impregnated* or coated or when immersed in a dielectric liquid such as oil. Other materials or combinations of materials may be included in this class if experience or accepted tests can reveal their capability to operate at 105C (221F).
Class E Insulation
There are materials or combinations of materials which, by experience or accepted tests, can reveal their capability to operate at 120C (248F) (materials possessing a degree of thermal stabilit y allowing them to be operated at a temperature 15C (27F) higher than Class A mat erial).
Class B Insulation
These are materials or combinations of materials such as mica, glass, fibre, etc, with suitable bonding substances. Other materials or combination of materials, not necessarily imorganic, may be included in this class, if experience or accepted tests can reveal then capability to operate at 130C (266F).
Class F Insulation
These are materials or combinations of materials such as mica, glass, fibre, etc, with table bonding substances. Other materials or combination of materials, not necessarily imorganic, may be included in this class if, experience or accepted tests can show them shown can be capable of operation at 155C (311F).
Class H Insulation
These are materials or combinations of materials such as silicone, elastomer, mica, glass, fibre, etc, with suitable bonding substances such as appropriate silicone resins. Other materials or combination of materials, not necessarily inorganic, may be included in this class, if experience or accepted tests can reveal their capability to operate at 180C (356F).
Conductor Insulation
To be useful and safe, electric current must be forced to flow only where it is needed. It must be channelled from the power source to a useful load. In general, current-carrying conductors must not be allowed to come in contact with one another, their supporting hardware, or personnel working near them.
Cable Gland
Cables are insulated, mechanically protected and watertight. They may be armoured and suitable for installation in a hazardous / explosive area. A cable gland maintains these properties where the cable is terminated at an appliance, e.g., at a motor terminal box. It prevents ingress of any foreign material, liquids even at pre-determined pressures andsometimes gases too.
|
|
|
The cable gland comes in three separate parts i.e., the gland body which has a standard electrical thread, usually 20 mm for the smaller cables and 25mm for the larger, an olive or compression ring, which tightens on to the cable thus affording good continuity and the lacking nut which tightens down on the olive when the gland has been positioned correctly to the appliance terminal box. Nuts on the gland while compressing sealing rings maintain watertight seals on the inner and outer sheaths and clamp the armour braiding. The gland must suit the type and size of cable (Refer Figure 15.5). MICC cables require special sealing points to prevent ingress of moisture into its hygroscopic insulation. A simple gland clamps the copper outer sheath to earth it and make a watertight seal. A non-combustible, pressure tested scaling compound may also be used in addition. Sometimes rubber-based compounds are used. The standard seals for MICC cables are designed for continuous operating temperatures up to 105C. High temperature sealing kits can be used to allow operation up to 210C.
Figure 15.5 - Cable Glands
In most cases earthing of the cable is done on the cable gland armouring. Where cables piss through watertight bulkheads and fire-stop barriers they must be especially "glanded" to maintain the integrity of such bulkheads.
Cable terminal sockets can be soldered to the conductors but are more frequently crimped onto the cable by a compression tool. Cable sockets must be securely attached to theappliance terminal screw by nuts and shakeproof (or split) washers. A loose terminal will invariably become a source of localised overheating and may also lead to severing of He I cable at that point due to undue stress. Periodic maintenance should always include checking the tightness of terminal connections. Small cables are terminated in terminal blocks of various designs. (Refer Figure 15.6).
SHORE SUPPLY
Where arrangements are made for the supply of electricity from a source onshore or other external source, the following requirements apply.
Connection Box and Cable
A shore connection box is to be provided on the vessel for the reception of the flexible cable from an external source. Fixed cables of adequate rating are to be provided between the shore connection box and the main or emergency switchboard.
The cable is to be protected by fuses or a circuit breaker located at the connection box. Where fuses are used, a disconnecting means is also to be provided. Trailing cable is to be appropriately fixed to avoid its imposing excessive stress on the cable terminal.
Interlock Arrangements
An interlocking arrangement is to be provided between all generators, including the emergency generator, and the shore power supply to prevent the shore power from being inadvertently paralleled with the shipboard power.
Instrumentation
An indicator light is to be provided at the main or emergency switchboard to which shore power is connected to show energized status of the cable. Means are to be provided for checking the polarity (for DC) or the phase sequence (for three-phase AC) of the incoming supply in relation to the vessel's system.
Earth Connection
|
|
|
An earth terminal is to be provided for connecting the hull to an external earth.
Information Plate
An information plate is to be provided at or near the connection box giving full information on the system of supply and the nominal voltage (and frequency if AC) of the vessel's system and the recommended procedure for carrying out the connection.
Shore supply is generally required so that the ship's generators and their prime movers can be shut down for major overhaul during a dry docking period. It must also be remembered that the engine generator cooling is in most cases impossible whilst the ship is in a dry dock.
The connection box to connect the shore supply cable is often located at the entrance to the accommodation or in the emergency generator room.
The connection box must have suitable terminals to accept the shore supply cable, including an earthing terminal to connect the ship's hull to the shore's earth point (ReferFigure 4.4). It must have a circuit-breaker or a switch and fuses to protect the cable linking the connection box to the main switchboard, with a data plate depicting details of the ship's electrical system (voltage and frequency), including the method for connecting the shore apply cable.
