Design Considerations.
Methodology.
After a preliminary analysis of the power requirements for the installation, a study of the wiring and electrical protection is carried out, beginning with the origin of the installation, passing through the intermediate circuits and ending with the final circuits. The wiring and its protection in each level must meet several conditions simultaneously, to ensure a safe and reliable installation, that is, they must:
•Support the full load permanent current and normal overcurrents of short duration.
•Do not cause voltage drops that could impair the performance of certain loads, for example: a too long acceleration period when starting a motor, etc. Likewise, the protection devices (automatic switches or fuses) must:
•Protect the wiring and busbars for any level of overcurrent, up to short-circuit currents (inclusive).
•Guarantee the protection of people against the risk of indirect contact, especially in systems with TN and IT earthing, where the length of the circuits can limit the magnitude of the short-circuit currents and, consequently, delay the automatic disconnection (remember that installations with TT grounding are necessarily protected at the source with a DDR, normally set at a sensitivity of 300 mA).
1 Related standards.
Most countries have national regulations and-or standards governing the rules to be strictly observed in the design and realization of electrical installations for residential and similar premises. The relevant international standard is the publication IEC 60364.
2 The power network.
The vast majority of power distribution utilities connect the low voltage neutral point of their MV/LV distribution transformers to earth. The protection of persons against electric shock therefore depends, in such case, on the principle discussed in chapter F. The measures required depend on whether the TT, TN or IT scheme of earthing is adopted. RCDs are essential for TT and IT earthed installations. For TN installations, high speed overcurrent devices or RCDs may provide protection against direct contact of the electrical circuits. To extend the protection to flexible leads beyond the fixed socket outlets and to ensure protection against fires of electrical origin RCDs shall be installed.
3 Distribution boards components.
Distribution boards (generally only one in residential premises) usually include the meter(s) and in some cases (notably where the supply utilities impose a TT earthing system and/or tariff conditions which limit the maximum permitted current consumption) an incoming supply differential circuit-breaker which includes an overcurrent trip. This circuit-breaker is freely accessible to the consumer (see fig.1).
Fig.1 Presentation of realizable functions on a consumer unit
4 Protection of people.
On TT earthed systems, the protection of persons is ensured by the following measures:
· Protection against indirect contact hazards by RCDs (see Fig.2) of medium sensitivity (300 mA) at the origin of the installation (incorporated in the incoming supply circuit-breaker or, on the incoming feed to the distribution board). This measure is associated with a consumer installed earth electrode to which must be connected the protective earth conductor (PE) from the exposed conductive parts of all class insulated appliances and equipment, as well as those from the earthing pins of all socket outlets.
· When the CB at the origin of an installation has no RCD protection, the protection of persons shall be ensured by class II level of insulation on all circuits upstream of the first RCDs. In the case where the distribution board is metallic, care shall be taken that all live parts are double insulated (supplementary clearances or insulation, use of covers, etc.) and wiring reliably fixed.
· Obligatory protection by 30 mA sensitive RCDs of socket outlet circuits, and circuits feeding bathroom, laundry rooms, and so on (for details of this latter obligation, refer to clause 3 of this chapter).
Fig.2 Installation with incoming-supply circuit-breaker having instantaneous differential
Protection.
5 Circuits Subdivision.
National standards commonly recommend the subdivision of circuits according to the number of utilization categories in the installation concerned (see Fig.3):
At least 1 circuit for lighting. Each circuit supplying a maximum of 8 lighting points.
At least 1 circuit for socket-outlets rated 10/16 A, each circuit supplying a maximum of 8 sockets. These sockets may be single or double units (a double unit is made up of two 10/16 A sockets mounted on a common base in an embedded box, identical to that of a single unit
1 circuit for each appliance such as water heater, washing machine, dish-washing machine, cooker, refrigerator, etc. Recommended numbers of 10/16 A (or similar) socket-outlets and fixed lighting points, according to the use for which the various rooms of a dwelling are intended, are indicated in table.1
Fig.3 Circuit division according to utilization.
Table.1 Recommended minimum number of lighting and power points in residential premises
6 Basic and elevated electrification.
The maximum load per dwelling depends on the degree of use you wish to achieve. The following degrees of electrification are established:
• Basic electrification: It is necessary to cover the possible primary needs of use without the need for subsequent adaptation works. The minimum power forecast, regardless of the contracted power that may be lower, will be 5,750 W-7370 W at 230 V. (see table # 2)
• High electrification: It corresponds to homes with a forecast of use of electrical appliances superior to basic electrification or with useful areas of the home exceeding 160 m2.
| Electrification | Power | Automatic general switch gauge |
| Basic | 5.750 W | 25 A |
|
Elevated | 7.360 W 9.200 W 11,500 W | 32 A 40 A 50 A |
Table # 2 caliber of the automatic main switch.
Differential circuit breakers will be installed so as to guarantee protection against indirect contacts of all circuits against residual differential currents of 30 mA maximum. The size of the differential switch will be equal to or greater than the size of the automatic main switch. For basic as well as for high electrification, at least one differential switch will be installed for every five circuits installed.
High electrification The classified homes with a high degree of electrification are those with a forecast of use of electrical appliances superior to the basic one or with useful surfaces of the dwelling superior to 160 m2. The expected power will not be less than 9.200 W (230 V). The degree of electrification will be high when one of the following conditions is met:
• Useful area greater than 160 m2.
• Electric heating forecast.
• Air conditioning forecast.
• Forecast of a dryer.
• Forecasting of automation systems.
• If the number of light points is greater than 30.
Point of light: it is a point of use of the lighting circuit that is commanded by an independent switch and to which one or several luminaires can be connected.
• If the number of points of use of general purpose outlets is greater than 20.
• If the number of points of use of bathroom and kitchen auxiliaries is greater than 6.
• In this particular design case, the high electrification criteria will be used and main protections will be selected for a load greater than 9200W and 40 Amps.
7 Protective conductors.
IEC and most national standards require that each circuit includes a protective conductor. This practice is strongly recommended where class I insulated appliances and equipment are installed, which is the general case.
The protective conductors must connect the earthing-pin contact in each socket outlet, and the earthing terminal in class I equipment, to the main earthing terminal at the origin of the installation. Furthermore, 10/16 A (or similarly sized) socket-outlets must be provided with shuttered contact orifices. Cross-sectional-area (c.s.a.) of conductors The c.s.a. of conductors and the rated current of the associated protective device depend on the current magnitude of the circuit, the ambient temperature, the kind of installation, and the influence of neighbouring circuits (refer to) Moreover, the conductors for the phase wires, the neutral and the protective conductors of a given circuit must all be of equal c.s.a. (assuming the same material for the conductors concerned, i.e. all copper or all aluminium) in the table.3 indicates the c.s.a. required for commonly-used appliances Protective devices 1 phase + N in 2 x 9 mm spaces comply with requirements for isolation, and for marking of circuit current rating and conductor sizes.
Table.3 C. s. a. of conductors and current rating of the protective devices in residential installations (the c. s. a. of aluminum conductors are shown in brackets).
8 Bathrooms and showers
· Bathrooms and showers rooms are areas of high risk, because of the very low resistance of the human body when wet or immersed in water. Precaution to be taken are therefore correspondingly rigorous, and the regulations are more severe than those for most other locations.
· The relevant standard is IEC 60364-7-701.
· Precautions to observe are based on three aspects:
· The definition of zones, numbered 0, 1, 2, 3 in which the placement (or exclusion) of any electrical device is strictly limited or forbidden and, where permitted, the electrical and mechanical protection is prescribed.
· The establishment of an equipotential bond between all exposed and extraneous metal parts in the zones concerned.
· Sub-clause 701.32 of IEC 60364-7-701 defines the zones 0, 1, 2, 3 as shown in the following diagrams (see Fig. 5 below to Fig 7):
Fig.5 Zones 0, 1, 2 and 3 in proximity to a bath-tub.
Fig.6 Zones 0, 1, 2 and 3 in proximity of a shower with basin.
Fig.7 Zones 0, 1, 2 and 3 in proximity of a shower without basin.
The provision of the system ground connection must be properly selected to ensure the safety of people and property. The operation of the different systems must be taken into account with regard to considerations of electromagnetic compatibility. By regulation of gas companies, the main gas pipeline must be placed to the building's grounding system for the safety of the inhabitant. All circuits of electrical outlets, must have a cable referenced to the ground of the building (see Fig.8).
Fig.8 Supplementary equipotential bonding in a bathroom.
9 Cables and busways
Distribution by insulated conductors and cables
9.1 Conductor.
A conductor comprises a single metallic core with or without an insulating envelope.
9.2 Cable.
A cable is made up of a number of conductors, electrically separated, but joined mechanically, generally enclosed in a protective flexible sheath.
9.3 Cableway.
The term cableway refers to conductors and/or cables together with the means of support and protection, etc. for example: cable trays, ladders, ducts, trenches, and so on… are all “cableways”.
9.4 Conductor marking.
Conductor identification must always respect the following three rules:
Rule 1
The double colour green and yellow is strictly reserved for the PE and PEN protection conductors
Rule 2
· When a circuit comprises a neutral conductor, it must be light blue or marked “1” for cables with more than five conductors
· When a circuit does not have a neutral conductor, the light blue conductor may be used as a phase conductor if it is part of a cable with more than one conductor
Rule 3
Phase conductors may be any colour except:
· Green and yellow
· Green
· Yellow
· Light blue.
Conductors in a cable are identified either by their colour or by numbers table.3
Table.4 identification according to the type of circuit.
Note:
If the circuit includes a protection conductor and if the available cable does not have a green and yellow conductor, the protection conductor may be:
A separate green and yellow conductor
The blue conductor if the circuit does not have a neutral conductor
A black conductor if the circuit has a neutral conductor.
In the last two cases, the conductor used must be marked by green and yellow bands or markings at the ends and on all visible lengths of the conductor.
Equipment power cords are marked similar to multi-conductor cables (see Fig.9).
Distribution and installation methods (see Fig. 10)
Distribution takes place via cableways that carry single insulated conductors or cables and include a fixing system and mechanical protection.
Fig.9 Conductor identification on a circuit breaker with a phase and a neutral
Fig.10 Radial distribution using cables and pipes.
For the wiring of the lighting circuits, circuit of outlets, power circuits, and the charge current of each circuit is estimated and a suitable cable gauge is chosen for each connected load. The caliber of the functions depends on the number of conductors that are transmitted in an adequate way.
The following aspects should be taken into account for cable selection:
Power demand:
• KVA to supply.
• Maximum load current IB.
Characteristics of drivers:
• Selection of type of conductor and insulation.
• Selection of installation method.
• Consideration of the correction factors for the different environmental conditions Determination of the sections with tables in which the capacity of conduction of the current is indicated.
Checking the voltage drop:
• Fixed status conditions.
• Engine start conditions.
Calculation of short circuit currents:
• Short circuit power upstream.
• Maximum values.
• Minimum values at the end of the conductor.
Selection of protection devices:
Rated current.
Cutting capacity
Installation of the cascade configuration.
Checking selectivity
Type of wire gauge selected by selected section:
Taking into account the above criteria, the cable cables can be selected for each circuit according to their load.
• Lighting circuits and main outlets are chosen cable # 14 TW.
• The kitchen strength circuit is chosen cable # 10.
• Bath force circuits are chosen cable # 12
• Branch luminaires choose cable # 16 or # 18.
