Electrical Distribution

AC switchboards

If voltages exceed 250 volts d.c. or 55 volts A.C. then the switchboard must be dead front (no exposed live parts at the front) of the metal clad type.

Bus bars

High conductivity copper rated to withstand the thermal and electromagnetic forces which would arise in the event of a short circuit at the bus bars with all the generators in parallel. The bus bars will withstand these conditions for the length of time it takes for the alternator circuit breakers to trip or back up fuse to blow.

Certain instruments and controls require a feed direct from the bus bars. Any connection between the bus bars and protecting fuses must be capable of withstanding maximum fault level. Standard practice is to provide a three phase set of fuses, known as 'Back Up' fuses, as near to the bus bars as possible. Connections are then led to the racks of the many instruments fuses fitted.

Circuit breakers

Must be capable of making and breaking under normal conditions and also abnormal conditions such as a short circuit. As the circuit breaker must be able to withstand closing onto a fault conditions without sustaining damage, it is of heavy construction. Fitted with an over current release and overloads with time lags, a circuit breaker can be used as follows;

On modern switchboards 'draw out' circuit breakers may be fitted. In the open position the whole circuit breaker can be wound clear of the bus bars, thus full inspection and maintenance can be achieved without the necessity of de-energising the bus bars so providing a separate isolating switch.

The 'plug in' contacts joining the circuit breaker to the bus bars are not capable of taking the breaking load and it is essential that the circuit breaker is in the open position before any attempt is made to withdraw it. A mechanical interlock is fitted arranged to trip the circuit breaker before the winding handle can be inserted,

The breaker also has a mid position, in this position the control circuits are still connected with the bus bar connection isolated. The electrical operation of the breaker can then be tested.

Circuit breakers are normally fitted with under voltage protection and tripping is accomplished by shorting or open circuiting the no-volt coil which releases the latching in mechanism. The no-volt coil may also be open circuited by a reverse power relay and an overload trip fitted with a time delay


The following instruments are the minimum that must be fitted;

Additional instruments that may be fitted

When a check synchroniser is fitted it is there to prevent connecting an incoming machine to the bus bars whilst out of phase, it is not there as aid to synchronising. In an emergency the 'in synch' light may be used to indicate when the breaker may be closed.

When an incoming machine is selected, its no-volt coil and circuit breaker contactor relay coil are connected in series with contacts on the check synchroniser. These contacts must be closed, that is the machine in phase with the bus bars, before the breaker contactor relay may be energised. If starting from a dead ship the check synchroniser must be switched to off before the first generator is put on the board.


Automatic voltage regulators

Shall be supplied separately from all other instrument circuits. Protection should be by fuses mounted as close to the supply connections as possible.

Shore supply connections

Circuit breakers

When selecting a circuit breaker for a particular application the principal factors to consider are; system voltage, rated load current, and fault level at the point of installation

Voltage rating

At medium voltages the phase to neutral voltage may be 250v but the potential difference between two phases with the neutral insulated would be 440v. At these voltages no difficulties should arise in selecting the circuit breaker equipment. However, on a 3.3kV insulated neutral system the phase to neutral voltage is 3.3kV/ж 3 = 1.9kV. If an earth fault develops on one phase the potential of the other two phases to earth is 3.3kV. To ensure the insulation is not subject to excessive stress a circuit breaker designed for a normal system voltage of 6.6kV may be fitted. Also on insulated neutral systems high over voltages may be caused by arcing faults. Medium voltage systems switch gear insulation should be able to withstand such voltages, but 3.3kV and above, the margin of safety is reduced. When a high voltage system is installed both the voltage rating of the circuit breaker and the method of earthing must be considered.

Current rating

Consider three factors;

Maximum permissible temperature of circuit breaker copperwork and contacts

temperature due to LOAD CURRENT

Ambient temperature

In industrial use the ambient temperature considered is usually 35oC. If uses in a marine environment temperature of 40oC (Restricted areas) and 45oC (unrestricted areas) are used, therefore the circuit breaker rating may be 'free air' value and this does not consider the degree of ventillation, the number and position of the circuit breakers or the layout of the bus bars. The final switchboard arrangement could be only 80 to 90% of the free air rating

Fault rating

Breakers should be rated to accept a breaking current of about 10 times the full load current. The breaker should also be able to make against a fault condition where the making current may be 25 times the full load current when the contact first make. Circuit breakers must remain closed for a short time when a fault occurs in order to allow other devices which are nearer to the fault to trip first. The breaker should be capable of carrying its breaking current for a specified time of usually about one second.

Arc suppression

Blow force at right angles to arc and field.

The blow out coils, which are connected in series with the circuit breaker contacts, form an electro-magnetic field which reacts with the arc to give a deflecting force which tends to bloe the arc outwards. The increase in effective length of the arc causes it to extinguish more quickly. The blow out coils are protected form the arc by arc resistant material which may be in the form of an air shute

Hot ionised gases around the arc and contacts are displaced by cold air forming eddy current air flow. This helps to increase resistance between contacts.


Attention should be paid to all contacts likely to deteriorate due to wear, burning, inadequate pressure, the formation of a high resistance film or becoming welded together. Faulty contacts are often indicated by overheating when loaded. Different contact materials may need different treatment.

Copper is widely used but is liable to develop a high resistance film, and copper contacts may become welded together if the contact pressure is low and the contents have to carry a high current. Copper is commonly used for contacts which have a wiping action when closing and opening., this action removing the film. Copper contacts are used on knife switches, laminated (brush) contacts of regulators and other controllers, drum contacts, etc.

Carbon and metallized carbon contacts are unsuitable for carrying high currents for long periods but, as they do not weld together, they are used for arcing contacts on some control gear. Pure silver and silver ally contacts tends to blacken in service but the oxide film has a low resistance. Copper- tungsten (sintered compound), grey I colour, is used in contact facing. This material has a high surface resistance which resists heavy arcing and does not weld. Silver tungsten (sintered) has similar properties to copper tungsten but has a lower contact resistance and is less liable to overheat on continuous load..

Servicing contacts

Copper contacts should be filed up if necessary to restore the profile required to ensure correct wiping action. Copper contacts which have become burnt or pitted or otherwise damaged, may be carefully dressed with a file. Emery cloth should not be used. Some contacts are provided with pressure adjustment, so if the contact pressure is reduced by dressing it should be readjusted. Using a spring balance pulled in a direction normal to the contact surface a reading should be taken when a piece of paper placed between the contacts is released. Inadequate spring pressure may also be due to the pressure springs becoming weak due to fatigue or overheating.

Carbon contacts should receive the same attention as copper contacts except that they should not need lubrication. Silver, Silver alloy and copper tungsten contacts do not require cleaning. As there is no need to remove surface film from pure silver contacts they may be used for light butt contacts.

Where some contacts have the appearance of pitting on both faces this is sometimes referred to as being 'burnt in'. Some manufacturers recommend that the contacts, unless there is loss of material, are not dressed as this may destroy the contact area.


R1-Sets volts value
R2-Trimming resistor (Power factor correction)
Carbon pile-Control resistance for AVR
Operating coil-Along with carbon pile form the controlling elements
CCT and PT-Are the detecting elements, the CCT acts as a feed forward device indicating future voltage changes by detecting variation in current flow
Stabilising element-Is the capacitor across the Exciter (may be replaced by a resistor)

The A.C. voltage is applied to the operating coil through a full wave rectifier. This A.C. voltage supply induced in the potential transformer and the circulating current transformer may vary under varying load conditions such as direct on line starting of relatively large motors. The capacitor connected across the coil smoothes the D.C. output from the rectifier.

If the A.C. applied voltage falls, the field of the solenoid weakens, and the resistance of the carbon pile decreases. With less exciter circuit resistance the current in the exciter field increases thus increasing the output voltage of the A.C. generator.

The automatic voltage regulator voltage output may be adjusted with the hand regulator R1 in the exciter field. Before synchronising the alternator the open circuit voltage is adjusted with the hand regulator R1.

After synchronising, and after the kW loading has been adjusted on the prime mover governor, the field excitation under steady load conditions may be adjusted using the Trimming resistor R2. Using the trimming resistor the power factor of the incoming machine will be equalised with the machines already in use.

If the load power factor now changes then the terminal voltage will regulate badly, e.g. a rise from 0.8 to Unity Power factor will cause a rise in terminal voltage of about 20 %. So a small Voltage Trimmer R3 is provided across each current transformer to adjust terminal voltage when there is a change in overall power factor

Modern A.V.R. (Zener Bridge)

Voltage across the Zener diodes remains almost constant independent of current variations. Smoothed D.C. output is applied to the voltage reference bridge. This bridge is balanced at the correct generator voltage output with no potential difference between 'A' and 'B'.

If the generator voltage fails, current through the bridge arms falls and current flows from 'A' to 'B' through the amplifier.

If the generator voltage falls, current through the bridge arms falls and current flows from 'B' to 'A' through the amplifier.

If the generator voltage rises, Current through the bridge arms rises with current flow from 'A' to 'B' through the amplifier.

The signal from the amplifier will automatically vary the field excitation current, usually through a silicon controlled rectifier ( Thyristor) control element.

The Silicon Controlled Rectifier (Thyristor) is a four layer, three terminal, solid state device with the ability to block the flow of current, even when forward biased, until the gate signal is applied. This gate signal could come from a Zener diode Voltage reference bridge. The gate signal will switch on the forward biased S.C.R. and current flows through the exciter field. When reverse biased the S.C.R. will again block current flow. Due to inductance of the field winding the S.C.R. would continue to pass current for a part of the negative cycle. By fitting a 'free wheeling' diode the current though the Thyristor falls quickly at the end of the positive cycle. In some circuits the excitation current is designed to be excess of requirements, so that the gate signal reduces flow

Insulated neutral system



Note: electrical shock is not reduced by using a non-earthed neutral as large voltages are involved. Both systems are equally dangerous

Earthed neutral system

When an earthed neutral system of generation is used earthing is to be through a resistor. The resistor is to be such that it limits the earth fault current to a value not greater than the full load current of the largest generator on the switchboard section and not less than three times the minimum current required to operate any device against


The armature of the synchroscope carries two windings at right angles to each other and is capable of rotation between field poles F F1

R is a non inductive resistance and XL is a highly inductive resistance both connected to one phase of the bus bars. This produces a field which rotates relative to the armature at the bus bar frequency. When the incoming machine is connected to the coils of the field poles a pulsating field is produced at the same frequency as the incoming machine.

If the two fields are not at the same frequency then the armature will rotate at a speed equal to the difference

In the modern rotary synchroscope there are no slip rings. The rotor has two soft iron pole pieces and with its shaft carrying the pointer it is magnetised by coil R from the bus bars. With this coil is fixed adjacent to the shaft, therefore, there are no moving coils, contacts or control springs.

Single Phase

Single phase synchronising with lamps Lamps Dark

Lamps bright

If using single phase synchronising it is considered better to use the lamp bright method as it is easier to judge the middle of the bright sequence rather than the middle of the dark sequence

Three phase synchronising

Synchroscope with two lamps (lamps dark)

The secondary windings of transformer T1 supplies field coil F of the synchroscope. The secondary windings of T2 supplies the rotating coils R of the synchroscope.
If the incoming machine is in antiphase with the bus bar the voltage difference between the output of the secondary of T1 and T2 is double the normal voltage giving normal volt drop across each lamp. When in phase there is no voltage difference between the outputs of T1 and T2 and therefore lamps are dark when synchronised.

Synchroscope with two lamps (Lamps bright)

Three phase synchronising with lamps (Lamps dark)

No1 Vector is stationary, if the incoming machine is running two slow then the No2 vector moves away from No1 vector in an anti clockwise direction. In the position shown as the No2 vector moves progressively anti clockwise then 'a' will brighten, 'b' will brighten shortly reaching maximum luminosity then darken, 'c' will darken .

When the machines are in phase, then 'R1' and 'R2' will be in align therefore 'a' will be dark, 'Y1' and 'B2' will be 120o apart and therefore 'b' will be approaching maximum luminosity, and the same will be for 'c' with 'Y2' and 'B1' 120o apart.

Reverse power tripping


A non magnetic metal disc can rotate in a magnetic field between two electro magnets. The disc is restrained by a coil spring. The flux produces a torque on the side which rotates the trip lever away from the trip contacts.

In reverse power conditions the flux from the voltage coil and current coil interact to rotate the disc in the reverse direction. The amount of torque/current (and hence power) is set on the current coil tapping.

A permanent magnet is provided on the disc to provide damping. A 3 to 5 second delay is incorporated into the trip circuit to allow for transients when paralleling.


  1. Low voltage coil
  2. Overcurrent trip
  3. Reverse current trip
  4. Reverse current trip

2a and 2b are fitted in case of circulating current via the equalising connection.

Under normal running , fields of '3' and '4' act together to hold the trip contact down. With reverse current fields are in opposition and a spring pushes the plunger against a trip bar to open the reverse current trip relay.

Time delay devices associated with safety circuits

Dash pots-(magnetic time lag)

The usual form of time/delay is an oil dash pot having an inverse time/current characteristic, relies for its operation upon the retarding action of a plunger immersed in a reservoir of oil, together with the magnetic force generated by a flow of current through a solenoid. The plunger is attached to an iron core which is partially enclose in the solenoid. When the solenoid is sufficiently energised the iron core will attract it, but the action is retarded due to the oil, in this way a time lag is introduced

Characteristic of Oil Dash pot with Inverse Time Delay

It is important to note that as the viscosity of the oil varies with temperature, so will the operating times of oil dash pots vary. Makers will supply dash pot oil suitable for the circuit breakers and relays. The recommended oils are selected on the basis of least variation in viscosity over the working range coupled with viscosity's which will give the time delay marked on the calibration plate. These special oils should invariably be used, the time delays are usually calibrated at 15oC, unless otherwise stated and are only correct at this temperature .

Over current devices fitted with oil dash pot time lags do not operate at the current marked on the calibration scale but at a current 25% greater with the appropriate time delay. The current marked on the scale is the value at which they would operate without time delay. Some makers supply an instruction plate indicating the exact current at which the relay will operate with a given setting.

Thermal device

Depends for its action upon the heating effect of an electric current flowing through, either a bi-metallic strip, or a heating coil placed near a strip. The thermal characteristics of the two dissimilar metals is such that when sufficient heat is generated there is a movement of the strip in one direction until the relay contacts are opened.

Induction relay

Similar to the action of the watt-hour meter, consisting of a metal disc pivoted so that it is free to rotate between two poles of two electro-magnets. The disc spindle carries a contact which is arranged to bridge two contacts when the disc has rotated through an adjustable angle. A spring returns the disc to the reset position, and as, during the deflecting period, the torque exerted by the spring increases, this is compensated for by the provision of graduated slots in the discs periphery. The necessary damping of the movement is provided for by incorporating a permanent magnet through which the disc has to rotate. The upper electro-magnet contains two windings, one, the primary normally is connected to a current transformer to a winding on the lower electro-magnet. Because of the graduated slots, the inertia of the moving system prevents the disc form rotating under normal running conditions, but when overcurrent commence to flow through the external circuit, the torque generated by the interaction of the upper and lower electro magnets is sufficient to cause the disc to rotate, until either the current falls to a safe level, or the relay is operated

Thermal Inspection of Switchboards

In service inspection of switchboard, individual starter and distribution panels can be carried out with the use of infra red temperature measurement or Thermal imaging systems. They require no direct contact with the electrical components being measured

This form of inspection is used to locate areas of increased temperature either associated with local overload or with increased resistance.

Infrared Temperature measurements

This takes the form of a relatively inexpensive, readily available temperature reading instrument often with laser guidance. The taking of readings is very simple although care must be taken in the interpretation of results. Due to their low cost these are normally found on most vessels as standard item

Thermal imaging

These tend to be expensive specialist instruments although they may be used by untrained personnel and results are generally simple to interpret. Due to their high cost it is normal to have a single unit available for several vessels delivered as required.

The unit is used rather like a video camera and the results viewed in realtime as an image displayed on the rear of the instrument

A typical unit is that seen in common use with fire brigades who use it in search and rescues

Author Note
When carrying out a thermal imaging inspection on a cargo pump switchboard on a large LNG carrier it was noticed that the areas around the connection of two busbar sections was unusually hot
The power was isolated on the switchboard and the tightness of the joining bolt checked. It was found tight. The decision was made to investigated further and the busbar section was disassembled

It was found that a large portion of the copper material at the contact face had disintegrated and was found absent from both joining bars. The reduced area of contact led to increased resistance and thus the increase in temperature.

The author is not able to offer an explanation as to the mechanism of failure of the busbar. I would suggest that the damage and been caused previously due to the bolt being loose. The bolt had been tightened without further investigation or recording of this action

Discrimation and Fuses


A circuit fed from a distribution board may be fed through three or even four fuses or circuit breakers e.g. a heating circuit may be connected to a 15amp fuse in a fuse box fed from a section box in turn from a 500A circuit breaker on the main board.

Discrimination occurs when the fuses nearest to the fault operates leaving all the other fuses or protective devices intact. Discrimination may be required between fuse and fuse, or between fuse and overcurrent device such as a circuit breaker.


A fuse is a protective device which is there to prevent overloading. If too heavy a fuse or if the fuse is overridden then there is a possibility of overheating, deterioration of insulation and failure.

Materials used are; Tin, Lead, or silver having low melting points. Use of copper or iron is dangerous, though tinned copper may be used.
Unlike some other forms of circuit protection devices (oil switches for instance), which are suitable for a.c. only, solid filled cartridge fuses have an approximately equal breaking capacity for D.C. and A.C. and the action of the fuse does not depend on breaking circuit at the zero point on the current cyclic wave

Requirements of a fuse;

A fuse must not;


I.E.E. and classification society rules now specify high breaking capacity (high category) fuses on main switch boards where the total normal generator capacity exceeds 400kW at 200v, this is for short circuit or low resistance protection of the very high currents that can be generated in these conditions.

In addition;

To control the extent of heavy fault currents on large installations the protective device must have a very high speed performance or High Rupture Capacity (H.R.C.) H.R.C. fuses will operate quickly before the short circuit current exceeds 3 times the full load current.

Cartridge fuses

Are capable of handling large short circuits. Because of standardisation of manufacture they have very consistant time/current fusing characteristics making them accurate, dependable and non-deteriorating in service. Suitable filling powders such as silicon sand are used in cartridge fuses having the property of quenching the arc of the fused element.

Enclosed fuses

The element usually made of silver is much smaller than the tinned copper used in semi-enclosed fuses so that the amount of vaporised metal is less and this contributes to a better performance. The enclosed casing and use of silver ensures no degradation due to oxidation. After the silver element has fused the indicator wire will heat up sufficiently to ignite the indicator powder and the fuse will be shown to be blown.

Except in the lowest ratings there are two or more elements in parallel which increase the contact area in contact with the filler, and this increases the breaking capacity. The ends of the element are reinforced by larger wires to reduce resistance and therefore heat losses.

The indicator type should in the construction below consists of an indicator wire which ignites an explosive powder which chars the indicator paper. On other designs the indicator wire releases a spring and pop up indicator

Semi-enclosed fuses

Tinned copper fuse wire exposed to the atmosphere tends to deteriorate and will vary in performance after long periods in service. Also there is a temptation to increase the gauge of the wire, or the number of wires after a fuse has blown. However, rewireable fuses are cheap, easily replaceable, blown fuses are easily detected and within reason if the circuit is uprated slightly no new fuse holders are required.

Tin-fast heating and failure (expensive)
Copper-Slow heating and failure (cheaper)

On overload the tin will fail rapidly increasing the current through the core speeding up its failure.


Is that current the fuse will carry continuously e.g. for a circuit rated at 30 amp, a 30 amp fuse will be appropriate. Fuses and circuit breakers on switchboards and distribution boards are intended primarily for the protection of the cables and not the apparatus. Overload protection of the apparatus usually provided at the motor starter.

The fusing factor = Minimum fusing current/ Current rating

There are three standards


Class P  Fuses protect against relatively small but sustained overloads with fusing factor of 1.25 (25% overload rating)  

Class Q  Fuses where protection against relatively small overcurrents is not required, with a fuse factor not exceeding 1.5 for cartridge and 1.8 for semi-enclosed fuses. Motor overload protection to back up motor starter protection  

Class R  Fuses require for protection against relatively large overcurrents (e.g. short circuit protection) 3 x Full load current  

Minimum fusing current

Earth fault detection



Preferential tripping

It is essential to prevent interruption of services necessary to maintain propulsion and navigation. These must be safeguarded even if the other services such as domestic supplies are temporarily sacrificed.

There are two ways to safeguard these services. First there must be at least two generators, the rating of which must be such that essential services can be maintained if one set is out of commission. Secondly, a protection must be provided that if sea load is too much for one generator a system of preferential selection will operate.

In some cases the non essential load is relatively too small to warrant additional switchgear. It is generally in larger installations where loads not under direct control of the engineer that they must be fitted. If the heating, lighting and galley were all switched on without prior warning, then the generators could become overloaded. Without preferential trips this may so overload the generators as to cause a complete shutdown. Therefore non essential services are fed through one or more circuit breakers fitted with shunt retaining coils or shunt tripping coils. Over current relays with time lags are provided for each generator. When overloaded, appropriate relays operate and trip out the non essential services. Some being more important than others, degrees of preference may be given.


Usual setting is 150% (50% overload) with a time delay of 15 seconds for generator overload protection and the following times come into operation when the generator reaches 110%.

First tripping circuit  5 seconds  

Second tripping circuit  10 seconds  

Third tripping circuit  15 seconds