Electrical Installation - Basic vocational knowledge (Institut für Berufliche Entwicklung, 201 p.) 4. Low-voltage switchgear and distribution systems (introduction...) 4.1. Low-voltage distribution systems (introduction...) 4.1.1. Open-type switchgear 4.1.2. Enclosed switchgear (factory-finished switchgear) 4.1.3. Encapsulated switchgear 4.1.4. Installation distributions 4.2. Cable connections to switch and distribution systems 4.3. Preparation and installation of bus bars 4.4. Conduct during working with switchgear 4.5. Summary

Electrical Installation - Basic vocational knowledge (Institut für Berufliche Entwicklung, 201 p.)

4. Low-voltage switchgear and distribution systems

Low-voltage switchgear and distribution systems are an essential part of electrical supply. These are operated by consumer’s voltages up to 1 kV ac voltage or 1.5 kV dc voltage.

Low-voltage distribution systems serve to supply all consumer’s installations the connection of which to the low voltage network is customery. Except for the station service installations of power plants and transformer stations these are individual consumer groups in industrial plants, in craftman’s businesses as well as the public network with its many consumers like dwelling and public buildings as well as offices. Such systems are operated foremost in centres of consumption. This is already required due to the necessity to keep cable lengths as short as possible for limiting the power losses arising from the high currents in the low-voltage network to a reasonable quantity.

The basic purpose of low-voltage distribution systems is therefore to feed the electrical power supplied by a feeder to bus bars and from there via many taps to the individual consumers. According to the requirements, the load necessities, the place of operation, the switching necessities and accessibility there are different types of low-voltage distribution systems.

4.1. Low-voltage distribution systems

In accordance with local and spatial conditions low-voltage distribution systems are classified in

- indoor systems.
- outdoor systems.

Grouping the distribution systems according to their design

- open-type switchgear,
- enclosed switchgear (factory-finished switchgear),
- encapsulated switchgear,
- installation distribution systems are being discerned.

4.1.1. Open-type switchgear

This is preferably applied in case of higher operating currents. Its advantage is to be easily accessible and well arranged making work with it convenient. Its disadvantage is, however, that it can only be installed in enclosed electrical operation rooms to which authorized staff has got access only. Due to that, it will need more space.

These assemblies generally consist of frameworks of rolled steel sections which may be put up free-standing.

Fig. 4.1. Schematic representation of an open-type switchgear

1 frame, 2 feeding bars, 3 current converters, 4 power switch, 5 terminal board, 6 auxiliary relays, 7 fuses 8 bus bars, 9 lower parts of low-voltage high-power fuses, 10 bar for protective conductors

4.1.2. Enclosed switchgear (factory-finished switchgear)

This means standardized switchgear for indoor use. In case of this type of switchgear panels with different insertions may be arranged together without problems.

Due to factory-finished supply there will be but little installation work required. The switch cubicles are being bolted to each other and connected at the bus bars only. These systems are distinguished by good connecting and extension facilities. All cable connections are performed from below in wiring spaces provided for this purpose. Application of the plug-in method enables quick replacement of modules. The connections of the plug-in units are being made by run-in contacts. The bus bars are screened from the devices. Because of the shape of the switch-gear the installation room will be well utilized.

Fig. 4.2. View of a factory-finished heavy-duty station

1 transformer box, 2 supply panel, 3 output panel, 4 capacitor panel, 5/6 contactor panel

Enclosed switchgear (factory-finished switchgear) and open-type switchgear are placed and fixed above openings through which the cables are passed into the switching system. The cubicles are being aligned and bolted to each other.

4.1.3. Encapsulated switchgear

Encapsulated switchgear may be used indoors as well as outdoors. It will always operate there, where high demands are put to protection against humidity, dust, mechanical damage and explosion. The encapsulation may consist of cast iron, weather-resistant steel sheet or plastic.

Encapsulated switchgear has rubber sealing preventing penetration of foreign matter, humidity and explosive matter into the interior of the switchgear.

The switchgear consists of switch boxes assembled like modules containing all required equipment including bus bars. They are mounted to framework or directly to the wall.

The design of the encapsulation depends on the purpose of the switchgear.

When using encapsulated switchgear in explosion-prone rooms the special design of the encapsulation will prevent the switchgear from causing an explosion.

Fig. 4.3. Schematic representation of an encapsulated switchgear

4.1.4. Installation distributions

Installation distributions are foremost employed to supply power to dwelling and public buildings, offices and in a limited scope also to industrial establishments (lighting distributions).

They are the simplest form of low-voltage distribution systems. In this case a difference is made between cabinet distributors, switch distributions for underplaster and surface installations as well as central distributors (distribution of fuses in dwelling buildings).

Generally the distributions consist of a frame or framework and a sheet resp. plastic cover. Decorative coverings by wooden or glass doors in niches may be made.

Fig. 4.4. Distribution cubicle

1 terminal strip, 2 terminal strip of protective conductors, 3 terminal strip for neutral conductors, 4 fuses, 5 screening sheet for electrical devices (protection against contact), 6 connecting terminals of feed line, 7 master switch, 8 mounting framework

4.2. Cable connections to switch and distribution systems

The connection cables are entered into the wiring space of the switchgear and distribution system and connected after removing the insulation. In front of the wiring space the connection cables are to be secured by pull relief against being torn off.

For that purpose a part of the switch and distribution systems has got rails to which relieving clamps may be fitted.

By wiring space that part of the switchgear is meant, in which the cables will be connected to the switching devices of the switchgear.

As relieving clamps double-tongued clips or clamps (quick-fixing cable clamps) are being used.

If cables are to be fixed by double-tongued clips the cables will have to be lapped with cable paper for cushioning purposes.

Fig. 4.5. Relieving clamps for cable

1 cable, 2 double-tongued clip, 3 clamp (quick fixing cable clamp), 4 mounting angle, 5 cable paper

Depending on the type of connection points and according to the conductors’ cross sections the cable will be connected as described in section 3.

Maintaining the required degree of protection in case of encapsulated switchgear is important. Screwable cable entries are to be tightened and sealed by putty. Conductors wich must be installed along or mounted to metal parts, in particular edges, are to be protected at the exposed point, e.g. by plastic tape or hose.

4.3. Preparation and installation of bus bars

In switch and distribution systems bus bars as main current conductors will distribute the electrical power supplied by the feeder. They are therefore an important component of switch and distribution systems.

Solid, non-insulated conductors are called bus bars!

The conductor material of bus bars will be rolled aluminium or copper.

For low-voltage switch and distribution systems mainly flats are being used. Besides them, further sections, e.g. rounds or channels are being taken.

Prior to processing the bus bars will be cleaned from dirt and adhering oxide residues which may have come on to the metallic bright surface during shipment or storage.

If the bus bars are distorted they will have to be straightened on a parallel or straightening machine. When straightening bus bars on a parallel a rubber mallet will be used in order not to damage the bus bar’s surface. Distortions on the wide side are being eliminated by short and strong blows of the rubber mallet. Distorting of the narrow side of the bus bars are more difficult to remove. For straightening the bus bars the inner edges of the distortions are to be worked by the rubber mallet until the bends vanish.

Fig. 4.6. Straightening a bus bar by rubber mallet

1 edges of the bus bar to be worked on

For marking of bus bars pencils or copying pencils are to be used. Scribers are generally not permitted for marking busbars, the reason being that on bending points marked by scriber the material will start to crack and may break.

Bus bars are sawed to length and are not to be sheared. When sawing bus bars clamped in a vice they are to be protected against impressions by the inserts. The sawed off surfaces of bus bars are to be deburred. If it becomes necessary to bend bus bars, bending devices will have to be employed.

In order to avoid cracking of the burs bars, a bending radius of 1 d (bar thickness) must not be undercut. The max. bending angle of bus bars will be 90°. If the bus bar is to be bent beyond this angle, bending will have to be done in two sections.

The bending angle on bending flats around the narrow side will be max. 30°!

Fig. 4.7. Bus bar bent in two sections

Mounting holes are punched or drilled into bus bars, oblong holes are to be punched into bus bars. It is not permitted to file oblong holes into bus bars. The edges of the mounting holes must be well deburred.

The climatic conditions may require to paint the bus bars on their overall length and to tin the connection points of copper bars. Moreover, by painting bus bars the heat dissipation from the bus bars to the environment will be improved.

Care has to be taken to have the connecting faces completely plane for obtaining the largest contact area possible.

The type of bolted mounting depends on the bars’ thickness and their cross sections.

Bus bars will be mounted sliding, not rigid Due to warming-up caused by the current flow the bus bars must be allowed to extend in length. For that purpose it will be mounted to bar holders of insulating material and supporting insulators. Fig. 4.8. represents a bus bar mounting. For avoiding strain due to extension in length each section of the bus bar is fitted by a fixed support, the other supports being sliding supports. Also by fitting so called expansion strips the elongation of bus bars may be compensated. The expansion strips will serve further to absorb possible shocks caused e.g. by operation of power switches. They consist of strips of metal foil put one upon each other being welded on both sides to solid pieces of the bus bar.

Fig. 4.8. Examples for installation of bus bars

1 bus bars. 2 fixed supports, 3 sliding supports, 4 expansion strip

Fig. 4.9. Bar support of insulating material

Fig. 4.10. Expansion strip

1 expanding portion, 2 connecting portion

The bus bars are to be mounted at the cubicles resp. switch panels in such a way as to have the sequence of conductors L1, L2, L3, PEN (PE, N) fixed

- from left to right
- or from above to below
- or from front to rear.

Fig. 4.11. Arrangement of bus bars in a cubicle

1 cubicle, 2 bus bars, 3 operation side

Bus bars for protective conductors will be installed on insulators in the lower portion of the cubicles. Mounting the protective conductors’ bars in this location will be favourable due to the protective conductors of the connection cables being connected at this place of the cubicles.

Connection of bus bars to each other can be made by the following methods:

Bar’s outgoing: connection between bus bars and electrical devices.

Overlap (unilaterally offset): connection of the same bus bars in each case, e.g. from one cubicle to the next one.

Strap connection: for same purpose, however, to be applied if overlap will not be possible for design reasons.

Connection of joining faces via oblong holes in both parts of the bus bars is not permissible!

Fig. 4.12. Representation of bars’ outgoings

For bolting bus bars galvanized or cadmium plated steel bolts are being used. The bolt heads may be painted with a temper colour. In case of a dangerously high temperature increase at the joining points of the bus bars the temper colour will change to another one.

Fig. 4.13. Connections between bus bars

1 unilaterally offset overlap, 2 strap connection

The bolted joints are to be fitted in such a way as to have the bolt heads visible. The nuts are to be tightened by wrench only. Mounting an extension piece on the wrench is prohibited, since the bolt may be overloaded!

4.4. Conduct during working with switchgear

Working on live electrical switchgear is not permitted! By modern working methods this will be actually possible, however, specially insulated tools and a special working apparel, but in any case extensive vocational qualification will be required for that.

For protecting the electricians working at the switchgear from possible accident caused by the electrical current, prior to starting work measures will have to be taken as described in the following.

Disconnection of working site

The switching action - disconnection of the switchgear resp. a portion of same - must be performed only by an expert authorized by the management. The fitters employed in the work have to be instructed regarding possible risks which may arise in the switchgear installation as well as regarding a special code of conduct. One of the electricians, generally an experienced expert with a long-time standing, will be appointed to manage the work as responsible expert.

The user will have to confirm disconnection of the switchgear or part thereof in writing. The form resp. certificate, also called clearance report in writing, is to contain a.o. the following information:

- place and time as well as start and end of the work to be performed,
- type and scope of work,
- scope of disconnection,
- special hints and instructions,
- name and signature of the participating fitters.

All concerned fitters have to sign the clearance report prior to commencement of work and to resign after completion of work or the working day.

Thereby the user - to whom the written clearance report is to be returned - can be sure that no fitter is staying any more in the switchgear installation.

Never must a fitter work alone in a switchgear installation being partly alive. Always a second person must be present in the visual or audible range!

Safeguarding against reclosing

Safeguarding the switchgear against unintentional or self-acting reclosing is an important safety measure.

In case of power drives the driving power (compressed air, spring force, control voltage) will have to be cancelled. Manually operated switchgear can be mechanically locked if such a device has been provided.

Other voltage-carrying parts of the switchgear installation being alive are to be screened for avoiding contact to live parts of the installation.

By switching off circuit breakers, disconnection in the lower voltage range will not yet be finalized. By removing the fuses (screw-type fuses or low-voltage high-power fuses) a visible isolation will be achieved, and work safety will increase.

The removed fuses are to be safely stored. Under no condition they are to remain at the switching point or even left there just slackened!

At the switching elements by which the switchgear was disconnected a caution board must be reliably fixed for the duration of the work, its contents should read as follows:

Do not switch on! work is going on! place...... Board to be removed only by: (name of the fitter)

This is particularly important if the working site is located at a distance from the switching point.

Except for overhead-line systems, earthing and short-circuiting of bus bars at the incoming feeder may be renounced. On earthing and shortcircuiting the sequence mentioned will have to be kept under all circumstances!

Ascertaining dead condition

The dead condition of the disconnected switchgear will have to be ascertained by means of a two-pole voltage tester. Prior to and after application its functionability is to be checked.

Fig. 4.14. Two-pole voltage tester with load testing button (1)

After the above mentioned steps have been taken, the work may be started.

On completion of the assembly work, tools and materials not required any more are to be removed. All dispensable hands may be drawn back. Now the protective measures can be cancelled. Only thereafter the plant may be reconnected to voltage. After these measures have been completed, the overall plant is considered to be alive.

4.5. Summary

Low-voltage switchgear and distribution systems are being operated in centres of power consumption with consumers’ voltages up to 1 kV ac voltage and 1.5 kV dc voltage.

Low-voltage switchgear and distribution systems serve to distribute electrical power. They may be installed outdoors and within buildings. Their design determines the place of installation.

A distinction is made between:

- open-type switchgear systems
- enclosed (factory-finished) switchgear systems
- encapsulated switchgear systems and
- installation distributions.

Cables are connected to the switchgear in the wiring space of the switchgear and distribution systems.

Bus bars serve as main current conductors within switchgear and distribution systems.

Bus bars are solid, non-insulated conductors. They are made in different shapes and with standardized cross sections.

For processing bus bars special attention will have to be paid to:

- Prior to further processing, bus bars are to be cleaned.

- Distortions at bus bars are to be eliminated.

- Bus bars are not to be marked by scriber.

- The bending angle of bus bars is 90°. The minimum bending radius has to be maintained at that.

- Holes are to be drilled or punched into bus bars.

- The contact surfaces of bus bars are to be plane and metallic-bright.

In order to allow bus bars to extend in case of temperature variations they are to be mounted in sliding position.

Installation of bus bars in switching panels or cubicles is to be performed according to a fixed sequence of conductors:

- from left to right
- from above to below
- from front to rear.

Joining bus bars to each other may take place by:

- bar’s outgoing
- unilaterally offset overlap
- strap connection.

Prior to commencing assembly work at live switchgear, the parts to be worked on are to be disconnected. For that purpose the following steps will have to be taken:

- disconnecting the working site,
- safeguarding against reclosing
- ascertaining dead condition.

The sequence of the above individual actions has to be observed. After completion of assembly work

- the working sites are to be inspected
- workmen, not required any more, are to be withdrawn
- the protective measures undertaken are to be cancelled
- the system is to be put under voltage again.