Sunday, December 9, 2012

CHAPTER 3 LOW VOLTAGE SWITCHGEAR



3.1       GENERAL


Switchgear is required to enable power sources to be connected to and disconnected from the low-voltage distribution system.  This switching is necessary both for normal operational purposes and also for the rapid and automatic disconnection of any circuit that becomes faulty.  The switchgear also allows any circuit to be isolated from the live system and for that circuit to be made safe so that work may be carried out on the equipment connected to it.

This chapter deals with switching devices as applied to low voltage (typically 415V or 440V).  Three types of LV switchgear are considered:

(a)    Circuit-breakers

Circuit-breakers are used to control inputs from transformers, section breakers on switchboard sections, interconnectors between LV switchboards and inputs from auxiliary LV generators.  All LV circuit-breakers are of the air-break type.

(b)    Contactors

Contactors are used to control mainly motor circuits.  They are always of the air-break type and are usually enclosed in the individual cubicles which form that part of a low-voltage switchboard referred to as a ‘Motor Control Centre’
(MCC).

Contactors are designed only to make and carry fault current for a short time, not to break it.  Where the system fault level exceeds the limited breaking capacity of the contactor, fuses are inserted in series with the contactor contacts.  Contactors are designed for remote operation and to undergo repeated and frequent operation without undue wear.

(c)    Moulded Case and Miniature Circuit-breakers

These form a special class of lightweight compact circuit-breakers for mounting onto or behind panels.  They are designed for hand operation only but have built-in protective tripping arrangements.

3.2       CONSTRUCTION


3.2.1    Main Air Break Circuit-breakers (ACB)

The main low-voltage circuit-breakers are always of the air-break type whose construction and operation are similar to those for high-voltage ACBs described in Chapter 2 and shown in Figure 2.2.  Being designed for low-voltage systems their insulation levels are of course lower, but, by the same token, their normal rated currents and their short-circuit current ratings are considerably higher.  This leads to generally heavier copperwork, to large arc chutes and especially to heavy switching contacts and isolating contacts.

Like their HV equivalents, LV circuit-breakers are horizontally isolated, with similar interlocks to ensure the correct sequence of operations when being withdrawn or reinserted.

Being smaller in size they are usually mounted in pairs, one above the other in an LV switchboard, presenting a dead-front panel face.

Most LV systems are 4-wire.  Some main circuit-breakers are 4-pole, but most are 3-pole with an unswitched neutral connecting link.

LV circuit-breakers are rated from 800A to 4 000A normal current.  They come in standard ranges of breaking capacity, which in British Standards is 35kA, 43kA and 70kA rms symmetrical.  These currents at 415V are equivalent to 25MVA, 31MVA and 50MVA respectively, or at 440V are equivalent respectively to 27MVA, 33MVA and 53MVA.  Because of the heavier normal and short-circuit currents found in LV systems, the circuit-breakers usually have much heavier breaking and isolating contacts than those of the HV types.

The circuit-breaker closing mechanism may be operated by solenoid or motor/spring.  Descriptions of both these methods will be found in Chapter 2, para. 2.3.  Tripping is by a separate shunt-trip coil, always powered from an independent battery-supported d.c. supply.

The basic circuit-breaker control circuits are essentially the same as those described in Chapter 2 for high-voltage switchgear.  Some circuit-breakers are equipped with an additional release device in series with the main circuit which trips the circuit-breaker instantaneously if it is closed onto a fault; it does not operate under any other circumstance.  This series tripping release is part of the circuit-breaker, not a protective relay, and requires to be reset by hand after operation.  Where this device is fitted, the anti-pumping circuit is unnecessary and is omitted.

Closing and tripping of low-voltage circuit-breakers is usually carried out by operating switches on a remote electrical control panel.  In some cases this remote control facility is not provided, and switching is done locally at the switchboard.

The procedure for disconnecting a circuit-breaker and removing it from its panel depends on the type of switchgear; the maker’s instructions must be consulted in each case.  To avoid personal injury, the closing spring must always be discharged before removing a spring-closed circuit-breaker from its switchgear panel.

3.2.2    Contactors

All contactors form a part of the individual distribution cubicles which make up an MCC.  Almost all are unlatched.

Each contactor is rated according to the service which it feeds, which may vary from a fractional horsepower motor drawing one ampere to a large 250kW motor drawing over 400A.  Consequently the contactor cubicle may vary in size from ‘one tier’ deep up to ‘seven tiers’ deep (see para. 3.3.2).

Each contactor operating coil is supplied, through its control circuits, from the switchboard busbar either direct at 415V or 440V, or through a small step-down transformer.  This ensures that, if busbar power fails, all connected contactors ‘drop off’ and keep their motors disconnected until each can be individually restarted.

Every contactor is backed up by a set of high rupturing capacity (HRC) fuses housed in the same cubicle.  These are rated according to the fault level at the switchboard (typically 31MVA or 50MVA, equivalent to 43kA or 70kA at 415V).  The correct size of fuse is chosen so that, in the event of a fault in the feeder circuit which exceeds the ability of the contactor to clear it, the fuse will blow first, leaving the contactor to open on a dead circuit.  The choice of back-up fuse is further discussed in the manual ‘Electrical Protection’.


Facilities are provided for testing the contactor while isolated from the busbar.  While so isolated a separate test supply can be applied to the contactor coil to check its operation without actually starting the motor.

3.2.3    Moulded Case Circuit-breakers

A type of low-voltage circuit-breaker widely used in most installations is the Moulded Case Circuit-breaker, or MCCB for short.


Shown in Figure 3.1, it consists of a moulded plastic case containing a switching element which is operated manually by an external handle or ‘dolly’.  Because the original design was American, the dolly position is down for ‘Off’ and up for ‘On’.  MCCBs can be used for switching either a.c. or d.c. circuits.  They are usually mounted, when used on distribution panels, behind the panel, and only the dolly shows.  Other arrangements however, such as surface mounting, are also found.

FIGURE 3.1

MOULDED CASE CIRCUIT-BREAKER


Most of the MCCBs used onshore and offshore are 3-pole, but very occasionally a 4-pole version is fitted.  They are also supplied as 2-pole (for example for d.c. switching), but this is usually a 3-pole type with one pole omitted.

MCCBs are very compact and have a high breaking capacity for their small size.  Where the system fault level at the point where an MCCB is used exceeds its fault-breaking capacity, separate HRC back-up fuses must be used in series, as described above for
contactors.  MCCBs are made by a number of manufacturers, and different makes and sizes are used in installations.  The following description, therefore, can be no more than very general.

A MCCB as used in onshore and offshore installations is normally fitted with two separate overcurrent devices.  One is a thermal element in each pole having an inverse-time characteristic, and the other an instantaneous ‘high-set’ electromagnetic element in two of the three poles; this operates instantaneously but only on the highest fault currents and then overrides the thermal element.  Both trip the circuit-breaker when the current reaches the set operating level in any of the poles.

Typical MCCBs used in installations have normal current ratings of either 125A or 250A, according to the circuits they control.  The breaking capacities of these two sizes are given below.

Normal Current
Breaking Capacity to BS Rules
MVA (3-phase, 440V)
Equiv. kA (a.c.)
(rms symmetrical)
kA (d.c. 250V)

125A

250A

7.5 MVA

15 MVA

10kA

20kA

18kA

25kA



The maximum currents that can be handled by these two sizes of MCCB are therefore 125A and 250A, but they can be arranged to trip at lower currents.  This is achieved by fitting a separate ‘trip unit’ to the breaker.  In the 250A size the trip units are interchangeable, but in the 125A size they must be fitted at the time of ordering and cannot thereafter be changed; if a different trip setting is needed, the whole 125A MCCB must be replaced by one with the new setting.  Sizes larger than 250A are manufactured but are not installed in platforms.

The following trip units are available:

                                    125A size                               250A size

                                     125A                                       250A
                                     100A                                       200A
                                       75A                                       160A
                                       50A                                       125A
                                       25A                                       100A
                                       15A                                         60A

There is normally no adjustment of current setting in any of the thermal units, but the electromagnetic elements have settings adjustable in five steps, numbered 1 to 5, by means of a control knob on the trip unit.  The range of adjustment of the high-set instantaneous overcurrent trip element is typically from 6 to 13 times the full-load rating of the trip unit (note, not of the MCCB itself).

When a closed MCCB self-trips due to operation of either of its overcurrent trip elements, the main contacts open fully, but the dolly goes to a mid, ‘half-cock’ position where it shows a white line in the dolly window (see Figure 3.1).  This indicates which of a number of MCCBs has tripped and enables it to be distinguished from those which were already open.  Before the MCCB can be reclosed by hand, the dolly must first be moved to the ‘Off’ position.

MCCBs are ‘latched’ breakers in that they do not, like contactors, fall out automatically if the service voltage disappears; they must be individually opened by hand.  However the 250A size (only) can be fitted with an undervoltage release, but none is so fitted on offshore installations.

Although MCCBs are self-tripping, they are not normally remote controlled.  However, the 250A size (only) can be provided with a shunt-trip release which enables it to be electrically tripped from a remote point.  It cannot be remotely closed without an additional operating mechanism.  Neither of these features is employed in most installations except for the shunt trip on certain offshore d.c. distribution boards.

Many MCCBs are used in onshore and offshore installations as incomer isolators for sub-distribution boards such as lighting or sundries panels.  When used in this way they are pure isolators and do not have a protective function.  Such MCCBs usually have their trip units removed and can only be opened by hand.

3.2.4    Miniature Circuit-breakers


 

The range of MCCBs already described extends down to a series of smaller breakers of ratings up to 70A maximum. These are known as Miniature Circuit-breakers (MCBs). (The term MCB should not be confused, as it often is, with MCCB for the moulded-case type.)
FIGURE 3.2

MINIATURE CIRCUIT-BREAKER


A typical single-pole MCB is shown in Figure 3.2.  In operation it is generally similar to, though physically smaller than, the moulded-case design.  The MCB has a moulded plastic case and is manually operated by an external dolly.  It is manufactured as a 1-pole, 2-pole, 3-pole or 4-pole unit, but the commonest types in most installations are the 1-pole and


2-pole models which are used in single-phase-and-neutral sub-distribution panels.  Such panels usually mount the MCBs horizontally in blocks of 6 or 12.  Other forms of mounting are also found.

Sometimes where the 3-phase switching is desired, instead of using a 3-pole MCB, three 1-pole units are physically ganged together by a bar joining the dollies.

Each pole of an MCB is protected by two separate overcurrent devices.  One is a thermal element having an inverse-time characteristic, and the other an instantaneous ‘high-set’ electromagnetic element; this operates only on the highest fault currents and then overrides the thermal element.  Both trip the circuit-breaker when the current reaches the set operating level in any of the poles.

The size ranges of MCBs vary from one manufacturer to another, but only a limited number are used in most installations.  Two typical sizes have normal current ratings of 70A and 32A, but they may be fitted with one of several different trip units, as follows:

70A size
32A size




3A
25A
1A
10A
6A
32A
2A
15A
10A
40A
3A
20A
16A
50A
5A
25A
20A
70A

32A






The trip unit fitted in any given MCB is usually denoted by a figure on the dolly.  The unit is fitted during manufacture and cannot be changed. (Figure 3.2 shows a 6A trip unit fitted.)

The fault current breaking capacity of an MCB differs according to the number of poles, the working voltage and, in some cases, the trip unit fitted.  The maximum breaking currents are as follows:

70A size

Trip Unit
Breaking Current (kA)
250V a.c.
440V a.c
3-pole
125V d.c.
2-pole
1-pole
2-pole
3A
6A
10A
16 to 70A
1kA
1kA
5kA
5kA
1kA
1kA
8kA
8kA
1kA rms symm (=0.75MVA)
1kA rms symm (=0.75MVA)
5kA rms symm (=3.8MVA)
3kA rms symm (=2.3MVA)
1kA
1kA
10kA
10kA


32A size

Trip Unit
Breaking Current (kA)
250V a.c.
440V a.c
3-pole
125V d.c.
2-pole
1-pole
2-pole
1 to 5kA
10 to 32A
6kA
6kA
10kA
8kA
3kA rms symm (=2.3MVA)
3kA rms symm (=2.3MVA)
5kA
5kA


The electromagnetic trips in the 70A size operate at 7 to 12 times the current rating of the thermal trip unit fitted (note, not of the MCB itself).  In the case of the 32A size the range is 6 to 9 times.

Both sizes of MCB are also manufactured in an alternative ‘high breaking capacity’ version giving approximately 50% higher breaking current, but these are not used on offshore or onshore installations.

If the system fault level at the point where the MCB is used exceeds its fault-breaking capacity given in the tables opposite, a properly selected back-up fuse of correct characteristic must be placed in series, as described in the manual ‘Electrical Protection’.

MCBs are ‘latching’ devices and do not, like contactors, fall out automatically if the service voltage disappears; they must be individually opened by hand.  They cannot be fitted with an undervoltage release, nor can they be remote-controlled.

3.2.5    Earth Leakage Circuit-breakers

A special type of miniature circuit-breaker, very sensitive to small earth-leakage currents, is described in the manual ‘Electrical Protection’, Chapter 5.

3.3       LOW VOLTAGE SWITCHBOARDS


3.3.1    Centre (Incomer) Section

An LV switchboard is usually supplied from one or two step-down transformers fed from the HV system. LV operating voltages are normally 415V onshore and 440V offshore.


FIGURE 3.3

TYPICAL ONSHORE 415V SYSTEM



Figure 3.3 shows diagrammatically such an onshore LV system with two transformer incomers, a bus-section breaker, a heavy feeder and two grouped distributing sections, left and right, also called Motor Control Centres (MCCs).  (Note: This is a somewhat misleading term, as by no means all the feeder circuits supply power to motors.)

The circuit-breakers usually form the centre section, with power being passed to left and right by the busbars.  Heavy-current feeders and the larger interconnectors feeding power to or from other LV switchboards sometimes require circuit-breaker protection and are then brought into the centre section.


FIGURE 3.4
PART OF TYPICAL OFFSHORE 440VSWITCHBOARD

Part of a typical offshore 440V switchboard is shown in Figure 3.4.  The part shown consists of five panels mounted side by side; the centre three panels contain cubicles for the incoming feeder, bus-section and heavy feeder circuit-breakers; the associated protective relays, control switches and indication equipment are mounted on the fronts of each panel.

On each side of the circuit-breaker panels are MCC panels, one of which is shown in Figure 3.4 on each side of the centre panels.  Further MCC panels are added as required.  Each contains a number of motor control contactor cubicles and fuse-switch cubicles mounted one above the other to control the outgoing circuits.  The fuse-switch cubicles control those circuits not associated with motors such as sub-distribution boards or welding sockets.

Additional MCC panels are mounted on each side of the centre section to house the feeder cubicles necessary to meet the requirements of the system concerned.  A large switchboard may include as many as 30 or more MCC panels.

The arrangement of the busbars and circuit connections is shown diagrammatically in colour on Figure 3.4; the main busbars are shown in red for the phases and blue for neutral and run through busbar chambers at the top and to the rear of the panels; they are connected through the length of each section of switchboard.  Power is supplied to each outgoing feeder cubicle by a set of dropping busbars (also shown in red and blue) housed in a vertical enclosure at the rear of each MCC panel.

On many switchboards each incoming switchgear panel has provision for earthing the neutral busbar through a bolted link, shown in black.  It is not switched with the breaker. When the incoming supply is from a transformer, this link is closed, earthing both the neutral busbar and the star-point of the transformer; this provides an earth for that particular part of the LV system.  On systems where the transformer star-point is earthed direct, this feature is not provided at the switchboard.

On some installations where dry-type encapsulated transformers are used, the transformers themselves form part of the LV switchboard, installed behind a panel and with their LV terminals connected directly onto the copperwork of the incomer panel.

3.3.2    MCC Section







 
FIGURE 3.5
TYPICAL MCC CUBICLES
 

Each outgoing circuit (other than interconnectors) is controlled by a feeder cubicle on one of the MCC ‘wings’ of the switchboard.  These cubicles are of different types, depending on


the manufacturer.  The following description is typical and is widely used in both onshore and offshore installations.

The MCC feeder cubicles occupy the full width of an MCC panel, but their vertical height depends upon the rating and function of the unit.  The smallest unit occupies one module of height - a ‘one-tier’ cubicle - and there is space for ten of them.  The largest is seven tiers high.  In practice a panel usually contains a mixture of cubicles of different heights to suit the particular distribution requirement.

In the smallest (one-tier) feeder cubicles the HRC fuse bases are permanently fixed to the busbar droppers and, although they are shrouded, care is needed when withdrawing or replacing them.  Such feeder cubicles have a simple rotary isolating switch on the hinged panel-front door, as shown in Figure 3.5(a).

Larger sizes of switch panel are provided with a fuse-switch for isolation.  This is operated by an isolating handle on the door of the unit with a mechanical drive to the fixed fuse-switch through a dog-clutch which is engaged only when the door is closed.  The fuses are dead when the fuse-switch is off.  This is shown in Figure 3.5(b).

Both isolating switches and fuse-switches are interlocked with the doors of their associated switch unit so that the door cannot be opened unless the switch is off.

All MCC feeder cubicles used for motor control have a contactor in the circuit following the isolator switch or fuse-switch; the two types are shown in Figures 3.5(c) and 3.5(d).

It is possible to test the contactors without actually starting the motor.  When the cubicle door is opened, the isolator having first been opened, a switch inside can be closed to provide an alternative supply to the contactor coil.  The contactor can then be operated while its main contacts are isolated from the mains.  A small cubicle at the top of the MCC panel provides the test supply through small distribution fuses; it is labelled ‘TEST’ or ‘CONTROL’.

Motors are normally started and stopped by remote control from the control room or the motor site.  Starting pushbuttons or switches at those points cause the contactor at the MCC to close.  It is very rare for provision to be made to start a motor at the MCC cubicle itself (some ventilation fans are exceptions).  However each motor cubicle at the MCC has an emergency stop pushbutton.

3.4       FUSES


Fuses are used with low-voltage switchgear:

(a)    as back-up for distribution contactors, or

(b)     for various control and instrumentation circuits.

In all cases they are of the HRC type.

When used as back-up the fuses are inside the individual distribution cubicles on the MCC section of the switchboard.  In one design they are either direct on the busbars (for one-tier units) or embodied in the isolating switch as a ‘fuse-switch’ in larger units - see Figure 3.5.  In this case protection against accidental contact is afforded by the cubicle enclosure itself.  Access to a fuse-switch is only possible after the fuse-carrying blades of the switch have been put in the isolated (open) position and the door opened.

Great care is needed with the busbar fuses in one-tier cubicles.  Although the door cannot be opened until the isolating switch has been opened, the fuses themselves are still

connected to the live busbar, though not carrying current - see Figure 3.5(a) and (c).  Although the fuse links are well shrouded, caution should be shown when removing or replacing them.




Low-voltage control and instrument fuses are usually panel-mounted in their own carriers. Their physical size is determined by their normal current rating.  Their breaking capacity is determined by the fault level current of the circuit in which they are connected

 
FIGURE 3.6
COMPLETE LV FUSE UNIT (TYPICAL)

A typical low-voltage fuse assembly is shown in Figure 3.6.  The replaceable ceramic cartridge with its metal terminal caps is known as the ‘fuse link’.  It is held in an insulated ‘fuse carrier’ which completely shrouds all live metal.  The carrier is supported on an insulated ‘fuse base’, where it is firmly fixed by various mechanical means, among them tongue contacts, butt contacts held by insulated screw pressure, or wedge contacts pressed in by insulated screws.  A tongue-contact type is shown in Figure 3.6.

A full description of how an HRC fuse operates to interrupt current is given in the manual ‘Electrical Protection’.

3.5       BUSBAR BRACING – SHORT-CIRCUIT FORCES


Figure 3.7 shows two busbars, A and B, installed side by side, supported on post insulators and carrying a single-phase current.


FIGURE 3.7

FORCES ON A BUSBAR


This current, at the instant shown, is assumed to be flowing down (into the paper) in the left-hand bar A, and up (from the paper) in the right-hand bar B, as indicated in the figure.

By Oersted’s rule the current in bar A will give rise to a circular magnetic field around it in a clockwise direction (corkscrew motion), whereas the current in bar B will give rise to a circular magnetic field in the anti-clockwise direction.  These fields are shown in the figure, and each field system cuts the other busbar.

By Fleming’s Left-hand Rule (see the manual ‘Fundamentals of Electricity 1’) the interaction between the current in A and the magnetic field from B which cuts it is to produce on bar A a mechanical force F to the left.  Similarly the interaction between the current in B and the magnetic field from A produces on bar B an equal mechanical force F to the right.

Half a cycle later both the currents and both the magnetic fields are reversed, so that the mechanical force on each, produced according to Fleming’s Left-hand Rule, is unaltered in direction.

Thus both bars are subjected to outward mechanical forces trying to push them apart.  They are resisted only by the post insulators which undergo a shearing and cantilever stress tending to break them.  This force alternates in magnitude as the current cycles, but not in direction.  It reaches a peak value twice each cycle: once when the currents peak as shown in the figure, and again half a cycle later when they peak in the opposite direction.

The magnitude of the force on a bar depends on the strength of the current in the bar and also on the strength of the magnetic field cutting it, which itself depends on the strength of the equal and opposite current in the other bar.  The force is thus proportional to the current squared.  It also depends on the distance between the bars, becoming less as the spacing is increased.

The following calculation gives some idea of the scale of these forces, particularly under conditions of short-circuit when the currents may be very large indeed.  If the currents were, say, 50 000A d.c. in each bar, and if the bars were spaced 3 inches (7.5cm) apart, the outward force on each bar would amount to no less than 670 kgf per metre run of bars, or nearly ¼ ton-force per foot run.

If the current were 50 000A alternating (rms), the peak current would be Ö 2 times this, namely over 70 000A, and the peak forces would be doubled to nearly ½ ton-force per foot run.  It does not end there: on switching on, or at the onset of a fault, the current might be 100% asymmetrical (see Chapter 5 of the manual ‘Fundamentals of Electricity 3’), in which case the current could peak to 2.55 times the rms value, namely to nearly 130 000A, which would give momentary forces of over 1½ tons-force per foot run.

It can easily be seen that such enormous forces would instantly break up the busbar system unless the bars were firmly braced at close intervals against lateral movement.  The forces due to one single asymmetrical peak could cause the damage which would initiate a catastrophic break-up.  Short-circuit currents of the order of 50 000A rms are not uncommon in LV systems.

Care must be taken in switchboards to brace not only the busbars themselves, but also any ‘droppers’ or tee-offs through which a short-circuit current may pass.

In 3-phase switchboards, where three busbars are usually arranged side by side, the currents, being 120° apart in time, are not equal and opposite as described above for single phase.  In different parts of the cycle the currents in adjacent bars will sometimes be in opposite directions and sometimes in the same direction.  Therefore the mechanical force on any bar is at one instant acting to blow it outwards, and at the next to draw it inwards.  If the spacing is not sufficient (bolt-heads can be a source of trouble) or if the bar or bracing is not stiff enough, a bar might even touch its neighbour, so transferring the original external short-circuit into the switchboard itself, with a high risk of fire.  Possible mechanical resonance of the bars or of any droppers to the 50Hz or 60Hz supply must also not be overlooked.

Although busbar forces have been described here in relation to low-voltage switchgear, they occur equally in high-voltage switchgear.  However the current levels there, even short-circuit levels, are generally much lower and the busbar spacing greater; consequently the problem is most acute on low-voltage switchboards.

3.6       HOLLOW BUSBARS - SKIN EFFECT


If a conductor, such as the one shown in Figure 3.8, is carrying a current, that current will normally make use of the conductor’s whole cross-section area.  If the cross-section is regarded as made up of a number of equal, thin elements each carrying some of the total current, then all these elements are in parallel and have the same resistance for a given length of conductor.  The current will divide equally between them - that is to say, it will distribute itself uniformly over the whole cross-section.

This is certainly true of direct currents and also of alternating currents at power frequencies if they are not too large.





FIGURE 3.8

SKIN EFFECT - ELEMENT OF CONDUCTOR


To examine how the conductor behaves when carrying an alternating current, consider a solid conductor of circular section as shown in Figure 3.8.  Take a short length of this conductor, for example between points A and D.  Assume the current (I) to be flowing upwards as indicated, the direction being taken at a certain instant of time during the alternating cycle when the current is rising.

Consider now a rectangular element ABCD within the conductor, the side DA along the axis, AB and DC in a radial direction, and the side BC inside the conductor and parallel with AD.  The thin strip of conducting material around the sides of the rectangle forms a closed conducting loop.

That part of the main current I flowing through the centre part DA of the conductor causes a circular magnetic field around it, anti-clockwise as seen from above and as shown in the figure, and in phase with the current causing it.  This alternating flux passes through the conducting loop ABCD, causing an emf to be generated in it by Faraday’s Law, that emf being anti-clockwise at the instant shown in the figure, but lagging 90° in phase on the flux, and therefore on the conductor’s main current.

The emf induced in the closed rectangular element ABCD causes a current i to flow round it, again anti-clockwise at the instant shown.  Because the loop is mainly inductive this current will lag almost 90° on the emf causing it, which, as shown above, itself lags 90° on the flux and so on the main current. Therefore the loop current lags a total of 180° on the main current - that is to say, it is anti-phase with it.





FIGURE 3.9

SKIN EFFECT - CURRENT IN A CONDUCTOR


Although Figure 3.8 suggests that the current flow i in the loop is anti-clockwise at the instant shown, in fact, because of its 180° phase lag, it is opposite in sign, and the actual directions at that instant are as shown in Figure 3.9.

This figure shows clearly that the loop current opposes the main current at the centre but adds to it towards the outside.  The current density at the centre is therefore reduced, but it is increased as one goes outwards.  The effect is dependent on the magnitude and on the rate of change of the flux.  The former is directly proportional to the conductor’s main current, and the latter to its frequency.  The effect is also greater with large diameter conductors.

The result is an increasing concentration of the main current towards the outer layers and a decrease at the centre.  This phenomenon is known as ‘skin effect’.  It is not present at all with d.c., since the circular magnetic field does not alternate and so produces no emf.  With normal levels of a.c. power currents the effect, though present, is negligible.  But when currents of several thousand amperes are flowing, the internal magnetic field is large and the skin effect becomes pronounced.

By the time currents of the order of 3 000 to 4 000A are reached, the effect is so strong that the bulk of the current is flowing in the outer parts of the conductor and very little in the centre.  The centre, in fact, is hardly being used and is a waste of expensive copper.  With currents above these levels it is not uncommon to install hollow busbars, which are in effect a continuous skin having no centre.  A further advantage of using hollow busbars is that

cooling air or other medium may be passed along their full length.  There is no need for the hollow busbar to be circular; it may be a rectangular extrusion or made up of plates.

Although LV switchboards with busbar ratings up to 4 000A are found on some offshore platforms, hollow busbars have so far not been used.  They may however be found on shore installations of very large current capacity.

The phenomenon of skin effect is well known in the radio world where high frequencies up to many megahertz are used, but it is not generally realised that it also occurs at power frequencies.
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