it1 u14 - wiring systems - complete

CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology

Unit 14 – Wiring Systems

The College At Clacton Unit 14 March 2012

Wiring Systems

Unit Aims

By the end of the unit participants should be able to:

Identify appropriate wiring systems, enclosures and equipment with reference to the a) use of single and three phase power systems b) installation of lighting circuits c) use of radial and ring circuits

Syllabus Reference: 2.3.02

Wiring Systems

There are many standard circuits in use in electrical installation work covering lighting heating and power with supplies either being single phase or three phase. A brief description of these follows.

Lighting Circuits Lighting Circuits - Points to Note

All lighting points, unless specific light sources with known power ratings are being used, are assumed to have a rating of 100 watts minimum.

To ensure there is not a complete black out when a fault occurs, we should install at least two lighting circuits in premises.

Earthing terminal and c.p.c. must be provided at ALL lighting points including switches and ceiling roses.

5/6 amp protective devices are usually used to protect domestic lighting circuits (5 amp if BS 3036 semi-enclosed rewireable fuse; 6 amp if a circuit breaker).

Domestic lighting circuits are generally wired in 1.0mm2 or 1.5mm2 cables.

Two types of wiring system are used for lighting circuits:

Two-Plate System: Used where single core cables are used, e.g. in conduit and trunking systems.

Three-Plate System: Used where multi core cables are used, e.g. domestic installations using twin and earth.




Practical Lighting Circuits – Two-Plate

One-Way Switching

Points to Note

Single pole switches and protective devices must be installed in the line conductor only.

For Edison Screw (ES) lamp holders the line conductor must be connected to the centre contact of the lamp holder.

Two-Way Switching




Two-Way and Intermediate Switching

Point to Note

An intermediate switch can be used as a one-way or two-way switch as well as an intermediate, e.g.




Practical Lighting Circuits – Three-Plate One-Way Lighting Circuit

Two-Way Conversion Lighting Circuit

Point to Note

Although this is called a ‘conversion’ circuit, most electricians use this circuit as a matter of course in domestic installations. For example, the landing light in a house would be connected to the upstairs lighting circuit with the switch drop going to the switch on the landing. The three-core & cpc would be run from the landing switch downstairs to the switch at the bottom of the stairs.




Intermediate Switches in the Two-Way Conversion Lighting Circuit

Intermediate switches can be inserted in to the two-way conversion circuit as shown below:

Two-Way Lighting Circuit using a 6-Terminal Joint Box




Plug and Socket Outlets Many electrical appliances can easily be connected and disconnected from the electrical supply by means of plugs and sockets. A plug top is connected to an appliance by a flexible cord that should normally be no longer than 2 metres. To comply with BS7671 Regulation 553.1.7, socket outlets should be located adjacent to where portable equipment is likely to be used. Pressing the plug top into a socket outlet connects the appliance to the source of supply. Socket outlets therefore provide an easy and convenient method of connecting portable electrical appliances to a source of supply.

Socket outlets can be obtained in various ampere ratings but the 13-ampere flat pin type complying with BS1363 is the most popular for installations in Great Britain. Each 13-ampere plug top contains a cartridge fuse to give maximum potential protection to the flexible cord (but not the appliance which, if required, must be provided with additional protection).

Socket outlets may be wired to a Ring or radial final circuit. In order that every appliance can be fed from an adjacent and convenient socket outlet, the number of sockets is unlimited provided that the floor area covered by the circuit does not exceed that given in Table H2.1 (reproduced below) in Appendix H of the „On-Site Guide‟.

Table H2.1 - Final circuits using BS1363 Socket-Outlets and Connection Units

Minimum conductor cross-sectional area*

Overcurrent protective

device

Copper conductor

thermoplastic or thermosetting

insulated cables

Copper conductor

mineral insulated cables

Maximum Floor area served

Rating A mm2 mm2 m2

1 2 3 4 5 6

A1 Ring 30 or 32 2.5 1.5 100

A2 Radial 30 or 32 4 2.5 75

A3 Radial 20 2.5 1.5 50

*The tabulated values of conductor size may be reduced for fused spurs.

Radial Final Circuits

In a radial final circuit each socket outlet is fed from the previous one. Line is connected to line, neutral to neutral and cpc to cpc at each socket outlet. The fuse and cable sizes are given in Table H2.1 in Appendix H of the „On-Site Guide‟ but circuits may also be expressed with a block diagram, as shown below:




Where two or more circuits are installed in the same premises the socket outlets and permanently connected equipment should be reasonably shared-out amongst the circuits, so that the total load on each is balanced.

When designing ring or radial final circuits special consideration should be given to the loading in kitchens, which may require separate circuits. This is because the maximum demand of current using equipment in kitchens may exceed the rating of the circuit cable and protective devices.

Ring and radial final circuits may be used for domestic or other premises where the maximum demand of the current using equipment is estimated not to exceed the rating of the protective devices for the chosen circuit.

Ring Final Circuits

Ring final circuits are very similar to radial final circuits in that each socket outlet is fed from the previous one, but in ring final circuits the last socket is wired back to the source of supply. Each ring final circuit conductor must be looped into every socket outlet or joint box that forms the ring and must be electrically continuous throughout its length. The circuit details are given in Table H2.1 in Appendix H of the „On-Site Guide‟ but can also be expressed by a block diagram, as shown below:




Spurs From Socket Circuits

A spur is defined in Part 2 of BS7671 as “a branch cable from a ring or radial final circuit”.

Non-Fused Spurs

The total number of non-fused spurs must not exceed the total number of socket outlets and items of stationary equipment connected directly in the circuit. The cable used for non-fused spurs must not be less than that of the final circuit. The requirements concerning spurs are given in Appendix H of the „On-Site Guide‟ but the various circuit arrangements may be expressed by the block diagrams, as shown right:

A non-fused spur may only feed one single or one twin socket outlet or multiple socket outlet or one permanently connected piece of equipment.

Non- fused spurs may be connected into the final circuit at the terminal of socket outlets or at joint boxes or at the origin of the circuit.

Fused Spurs

The total number of fused spurs is unlimited. A fused spur is connected to the circuit through a fused connection unit, the rating of which should be suitable for the conductor forming the spur but should not exceed 13 amperes or by a circuit breaker of rating not exceeding 16A. The requirements for fused spurs are also given in Appendix H of the „On Site Guide‟; the block diagrams below express the various circuit arrangements:




The general arrangement shown below shows 11 socket outlets connected to the ring; 3 non-fused spur connections and 2-fused spur connections.




Cooker circuits Cooker circuits are usually a single outlet supplied from a separate protection device in the distribution board. The outlet is normally a double pole switch which controls the supply to the cooker. The rating of the protection device and cable is determined on the basis that not all of the cooker will be switched on at the same time. Even once everything is switched on the thermal control devices, such as thermostats and simmerstats, would be continually switching sections off. Making allowance for this process is known as applying „diversity‟. So that calculations can be made as to the possible load of a domestic cooker a formula has been devised. This is as follows:

Take the first 10 amperes of the total possible being taken at 100% of load

The remainder, after the 10 amperes are taken off, are taken at 30%

Plus 5 amperes if a socket outlet is incorporated in the control unit.

Example

The assumed demand for a cooker which contains:

2 x 1.5kW hob plates 2 x 3.0kW hob plates 1 x 2.0kW oven/grill 1 x 4.0kW oven

Maximum total power = 15kW

Maximum current = P V

= 15 x 1000 230

= 65.22A

The assumed current demand, allowing for diversity, is:

the first 10 amperes are at 100% = 10A

that leaves 55.22 amperes at 30% = 16.57A

Total = 10 + 16.57

= 26.57A

This means that the cable supplying this cooker would have to have a rating of at least 26.57A. If the control unit contained a socket outlet the rating would have to be at least 31.57A.




Try this The rating for a 230V electric cooker is:

4 x 2.75kW hob plates

1 x 2.5kW top oven

1 x 4.5kW main oven

Calculate the maximum assumed current demand allowing for diversity.

Total power of cooker = (4 x 2.75) + 2.5 + 4.5 kW

= 18 kW

Total current = 18000 230

= 78.26 A

Take the first 10 A = 10 A

Take 30% of the remainder = (78.26 – 10) x 30 100

= 20.48 A

Total current demand after diversity = 10 + 20.48

= 30.48 A

Water heating Unlike cookers, there is no reduction off the total rating on water heaters. They are either ON or OFF, full load or no load, even though they are thermostatically controlled. It is usual to connect immersion heaters (see picture right) to their own circuit, as they are often rated at 3kW.

Instantaneous Water Heaters Instantaneous water heaters with immersed and uninsulated elements must be permanently connected to the electricity supply through a double-pole linked switch. Plugs and socket outlets must not be used.




Space heating

Total electric space healing generally makes use of low-cost electricity at night. These systems are controlled by tele-switches on tariffs such as Economy 7, which charges customers about half price for electricity used over a particular seven hours at night. Such heating devices have to he designed so that heat produced at night can be stored and given off during the day.

Motor circuits For most industrial purposes motor circuits are usually three-phase, whilst for domestic and office use single phase supplies are more common.

BS7671 requires the provision of an isolating device to allow the disconnection of the supply and a method of preventing the automatic resuming of a motor after a stoppage or failure of supply. Additionally, every motor over 0.37kW must be provided with a means of overload protection.

All of these requirements can he included in a motor starter (see right), although a separate starter and isolator are often used.

Some three-phase motors have windings connected in star/delta for easier starting purposes and six connections are necessary between the motor and starter (see right).

For more information see pages 122 to 135 of „Electrical Installations Level 2 2330 Technical Certificate‟ – revised for the 17

th Edition IEE Wiring Regulations (ISBN 978 0 435401 09 2).

it1 u14 - wiring systems - complete

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