Chapter 2: Final Circuit & Load Estimation Hasnizam Hanafi Pusat Pengajian Kejuruteraan Sistem Elektrik Chapter 2: Final Circuit & Load Estimation

Definition of a final circuit A circuit connected directly to current using equipment, or to a socket outlet or socket outlets or other points for the connection of such equipment An outlet is defined as the termination of fixed wiring feeding a luminaire, socket, or any current consuming appliance. From this it will be seen that a final circuit might consist of a pair of 1.5mm2 cables feeding a few lights or a very 3 core cable feeding a large motor direct from a circuit breaker or main switchboard.

Definition of a final circuit Socket outlet: A device, provided with female contacts, which is intended to be installed with the fixed wiring, and intended to a receive plug. A luminaire track system is not regarded as a socket outlet system

Definition of a final circuit

IEE Regulations for Final Circuit

Final Circuit Distribution Final circuits can be divided into the following types, all of which will need different treatment when planning the size of the conductors and the rating of the overcurrent devices: The general of final circuits are: Final circuit feeding 13A sockets to BS 1363 Final circuit feeding sockets to BS EN 60309-2 (industrial types 16A, 32A, 63A and 125A) Final circuit feeding fluorescent or types of discharge lighting Final circuit feeding motors Final circuit feeding cookers

Sockets

Final circuit feeding 13A sockets to BS 1363 The main advantages of the 13A socket with fused plug are that any appliance with a loading not exceeding 3.12kW (13A at 240V) may be connected with perfect safety to any 13A socket, and under certain conditions an unlimited number of socket may be connected to any one circuit .

Final circuit feeding 13A sockets to BS 1363 13A socket outlets circuits can be fed by either radial or ring circuits.

Final circuit feeding 13A sockets to BS 1363 (Radial circuit arrangement)

Final circuit feeding 13A sockets to BS 1363 (Ring circuit arrangement) A ring circuits utilises one additional conductor to loop back to the sending end. In other words, the socket outlets in the ring circuit are fed by two parallel conductors.

Final circuit feeding 13A sockets to BS 1363 (Ring circuit arrangement) The sharing of the load between the two parallel conductors will depend on the load distribution within the ring.

Final circuit feeding 13A sockets to BS 1363 Recommendations exist in the IEE Regulations for standard circuit arrangements with 13A sockets. These permit 13A sockets to be wired on the final circuits as follows (subject to any derating factors for ambient temperature, grouping or voltage drop):

Final circuit feeding 13A sockets to BS 1363 An unlimited number of socket outlets connected to a final circuit serving a floor area not exceeding 100m2 wired with 2.5mm2 PVC insulated cables in the form of a ring, and protected by 30A or 32A overcurrent protective device. In actual practice, 10 number of socket outlets connected to a final circuit wired with 2.5mm2 PVC insulated cables in the form of a ring.

Final circuit feeding 13A sockets to BS 1363 An unlimited number of socket outlets connected to a final circuit serving in floor area not exceeding 75m2 with 4mm2 PVC insulated cables on a radial circuit and protected by an overcurrent device of 30A or 32A rating.

Final circuit feeding 13A sockets to BS 1363 An unlimited number of sockets connected to a final circuit serving a floor area not exceeding 50m2 with 2.5mm2 PVC insulated cables on a radial circuit and protected by an overcurrent device not exceeding 20A.

Spurs Non fused spurs: A spur is a branch cable connected to a 13A circuit. The total number of non fused which may be connected to a 13A circuit must not exceed the total number of sockets connected direct the circuit. Not more than one single or one twin socket outlet or one fixed appliance may be connected to any one spur. Non fused spurs may be looped from the terminals of the nearest sockets, or by means of a joint box in the circuit. The size of the cable feeding non fused spurs must be the same size as the circuit cable.

Spurs Fused spurs: The cable forming a fused spur must be connected to the ring circuit by means of fused connection unit or spurbox. The rating of the fuse in this unit shall not exceed the rating of the cable forming the spur and must not exceed 13A

Malaysia Practices for 13A Socket Outlet (BS 1363) Types of 13A Socket Outlets Area Malaysia Practices Size of wires Fuse/Circuit Breaker Rating Single socket outlet 20m2 2.5mm2 PVC cables 16A Double socket outlet 20A Ring (10 Nos 13A socket outlet provided they are all located within an area of not more than 1000 sq feet) 100m2 32A Radial (Max 6 Socket Outlets) 50m2 4mm2 PVC cables

Final circuit for socket outlets to BS EN 60309-2 These socket outlets are of the heavy industrial type, and are suitable for single or three phase with a scraping earth. Fuses are not fitted in the sockets or the plugs. Current ratings range from 16A to 125A (16A, 32A, 63A & 125A).

Final circuit for socket outlets to BS EN 60309-2 The 16A sockets whether single or three phase, may be wired only on radial circuits. The number of sockets connected to a circuit is unlimited, but the protective overcurrent device must not exceed 20A. It is obvious that if these 16A sockets are likely to be fully loaded then only one should be connected to any one circuit. The higher ratings will of course each be wired on a separate circuit.

Final circuit for socket outlets to BS EN 60309-2

Final circuit for motors Final circuit for motors need special consideration, although in many respects they are governed by the regulations which apply to other types of final circuits. The current rating cables in a circuit feeding a motor must be based upon the full load current of the motors, although the effect of the starting current will need to be considered if frequent starting is anticipated (IEE Regulation 552-1-1 pg 141).

Final circuit for motors IEE Regulation 552-1-1 states all equipment, including cable, of every circuit carrying the starting, accelerating and load current of a motor shall be suitable for current at least equal to the full load current of the motor when rated in accordance with appropriate British Standard. Where the motor is intended for intermittent duty and for frequent starting and stopping, account shall be taken of any cumulative effects of the starting or braking current upon the temperature rise of the equipment of the circuit.

Final circuit for motors Every electric motor exceeding 0.37kW shall be provided with control equipment incorporating protection against overload of the motor. Several motors not exceeding 0.37kW each can be supplied by one circuit, providing protection is provided at each motor.

Final circuit for motors All isolators must be ‘suitably placed’ which means they must be near the starter, but if the motor is remote and out of sight of the starter then an additional isolator must be provided near the motor. All isolators, of whatever kind, should be labelled to indicate which motor they control.

Final circuit for motors The cutting off of voltage does not include the neutral ins systems where the neutral is connected to earth. For the purposes of mechanical maintenance, isolators enable the person carrying out maintenance to ensure that all voltage is cut off from the machine and the control gear being worked upon, and to be certain that it is not possible for someone else to switch it on again inadvertently. Where isolators are located remote from the machine, they should have removable or lockable handles to prevent this occurrence.

Final circuit for motors (Motor Starters) It is necessary that each motor be provided with a means of starting and stopping, and so placed as to be easily worked by the person in charged of the motor. The starter controlling every motor must incorporate means of ensuring that in the event of a drop in voltage or failure of supply, the motor does not start automatically on the restoration of supply, where unexpected re starting could cause danger. Starters should be fitted with undervoltage trips, which have to be manually reset after having tripped.

Final circuit for motors (Motor Starters) Every motor having a rating exceeding 0.37kW (1/2 hp) must also be controlled by a starter which incorporates an overcurrent device with a suitable time lag to look after starting current (IEE Regulation 552-1-2). These starters are generally fitted with thermal overloads which have an inherent time lag, or with the magnetic type usually have oil dashpot time lags. These time lags can usually be adjusted, and are normally set to operate at 10% above full load current. Electronic protective relays are also available now and these provide a fine degree of protection.

Final circuit for motors Rating of protective device IEE Regulation 433-2-2 states that the overcurrent protective device may be placed along the run of the conductors (providing no branch circuits are installed), therefore the overcurrent protective device could be the one incorporated in the starter, and need not be duplicated at the commencement of the circuit.

Final circuit for motors Short circuit protection must be provided to protect the circuit, and shall be placed where a reduction occurs in the value of the current carrying capacity of the conductors of the installation (i.e such as in a distribution board). The device may, however, be placed on the load side of a circuit providing the conductors between the point where the value of current carrying capacity is reduced and the position of the protective device do not exceed 3m length and providing the risk of fault current, fire and danger to persons is reduced to a minimum (IEE Regulation 433-2-2).

Final circuit for motors When motors take very heavy and prolonged starting currents it may well be that fuses will not be sufficient to handle the starting current of the motor, and it may be necessary to install an overcurrent device with the necessary time delay characteristic, or to install larger cables.

Final circuit for motors With three phase motors, if fuses protecting the circuit are not large enough to carry the starting current for a sufficient time, it is possible that one may operate, thus causing the motor to run on two phases. This could cause serious damage to the motor, although most motor starters have inherent safeguards against this occurrence.

Final circuit for motors The ideal arrangement is to back up the overcurrent device in the motor starter with HBC fuselinks which have discriminating characteristics which will carry heavy starting currents for longer periods than the overload device. If there is a short circuit, the HBC fuses will operate and clear the short circuit before the short circuit kVA reaches dangerous proportions.

Final circuit for cooker (IEE Regulation 476-03-04) A cooker is regarded as a piece of fixed equipment unless it is a small table-mounted type fed from a plug by a flexible cord. Such equipment must be under the control of a local switch, usually in the form of a cooker control unit. This switch may control two cookers, provided both are within 2m of it.

Final circuit for cooker (IEE Regulation 476-03-04) In many cases this control unit incorporates a socket outlet, although often such a socket is not in the safest position for use to supply portable appliances, whose flexible cords may be burned by the hotplates. It is often considered safer to control the cooker with a switch and to provide a separate socket circuit. The protective device is often the most highly rated in a installation, particularly in a domestic situation, so there is a need to ensure that diversity has been properly calculated (see Table 2).

Final circuits feeding fluorescent and other types of discharge lighting Electric discharge lighting may be divided into two groups: those which operate in the 200V/250V range, and the high voltage type which may use voltage up to 5000V to earth. The first group includes tubular fluorescent lamps which are available in rating 8W to 125W, sodium lamps which are rated from 35W to 400W, also high pressure mercury vapour lamps rated from 80W to 1000W, and other forms of discharge lighting. The second group includes neon signs and similar means of high voltage lighting.

Final circuits feeding fluorescent and other types of discharge lighting Low voltage discharge lighting circuits: Regulations governing the design of final circuits for this group are the same as those which apply to final circuits feeding tungsten lighting points, but there are additional factors to be taken into account. The current rating is based upon the ‘total steady current’ which includes the lamp, and any associated control gear, such as chokes or transformers, and also their harmonic currents. In absence of manufacturers’ data, this can be arrived at by multiplying the rated lamp power in watts by 1.8, and is based on the assumption that the power factor is not less than 0.85 lagging.

Final circuits feeding fluorescent and other types of discharge lighting (summary) For low voltage discharge lighting: Total Loading (W) = Prated, lamp x 1.8 Based on assumption that: The power factor, cos θ not less than 0.85 lagging Total steady current which includes the lamp and any associated control gear such as chokes or transformers and their harmonic current For some fluorescent lamp circuit (especially the 125W switch start type) Total Loading (W) = Prated, lamp x 2

Final circuits feeding fluorescent and other types of discharge lighting Circuit switches: Circuit switches controlling fluorescent circuits should be designed for purpose otherwise they should be rated at twice that of the design current in the circuit; quick-make and slow break switches must be used. Quick break switches must not be sued as they might break the circuit at the peak of its frequency wave, and cause a very high induced voltage might flash over to earth.

IEE ON SITE GUIDE DIVERSITY FACTOR Diversity factor, DF is the ratio of the sum of the maximum power demands of the subdivisions, parts of a system, to the maximum demand of the whole system or part of the system under consideration. IEE ON SITE GUIDE DIVERSITY FACTOR

Maximum Demand Maximum demand (often referred to as MD) is the largest current normally carried by circuits, switches and protective devices; it does not include the levels of current flowing under overload or short circuit conditions

Example 1 A shop has the following single-phase loads, which are balanced as evenly as possible across the 415 V three-phase supply. 2 x 6 kW and 7 x 3kW thermostatically controlled water heaters 2 x 3 kW instantaneous water heaters 2 x 6 kW and 1 x 4 kW cookers 12 kW of discharge lighting (Sum of tube ratings) 8 x 30 A ring circuits feeding 13 A sockets. Calculate the total demand of the system, assuming that diversity can be applied. Calculations will be based on Table 2.

Solution Example 1 The single-phase voltage for a 415V three-phase system is All loads with the exception of the discharge lighting can be assumed to be at unity power factor, so current may be calculated from

Solution Example 1 Water heaters (thermostatic) No diversity is allowable, so the total load will be: (2 x 6) + (7 x 3) kW = 12 + 21kW = 33kW This gives a total single-phase current of

Solution Example 1 Water heaters (instantaneous) 100% of largest plus 100% of next means that in effect there is no allowable diversity. Single-phase current =

Solution Example 1 Cookers 100% of largest 25.0A 80% of second 20.0A 60% of remainder 10.0A Total for cookers 55.0A

Solution Example 1 Discharge lighting 90% of total which must be increased to allow for power factor and control gear losses. Lighting current

Solution Example 1 Ring circuits First circuit 100%, so current is 30 A 75% of remainder Total current demand for ring circuits = 187.5A Total single phase current demand = 486.2A Since a perfect balance is assumed, three phase line current

Example 2 A 240 V domestic cooker has the following connected loads: top oven 1.5kW main oven 2.5kW grill 2.0kW four hotplates 2.0kW each The cooker control unit includes a 13 A socket outlet. Calculate a suitable rating for the protective device. Calculate the total demand of the system, assuming that diversity can be applied.

Solution Example 2 Assume the power factor, cos θ is 1. The total cooker load is 1.5 + 2.5 + 2.0 + (4 x 2.0) kW = 14 kW Total current

Solution Example 2 The demand is made up of: the first 10 A 10A + 30% of remainder 14.5A + allowance for socket outlet 5A Total 29.5A

LOAD ESTIMATION No Types of Load JKR 1. 13A Switch Socket Outlet 250W (Domestic) 300W (Industry) 2. Lighting: a) Fluorescent Fittings i) 1 x 18W 24W ii) 2 x18W 48W iii) 3 x 18W 72W iv) 4 x 18W 96W v) 1 x 36W 42W vi) 2 x 36W 84W vii) 3 x 36W 126W viii) 4 x 36W 168W

No Types of Load JKR b) 100W down light 100W c) Emergency light Table Appendix 2.1 d) Exit light 20W 3. Ceiling fan c/w regulator 80W 4. Exhaust fan 70W 5. Wall fan 6. 15A Switch Socket Outlet 500W 7. Isolator 20A TPN 8. Isolator 30A TPN 9. Isolator 60A TPN 10. Fire Alarm System 250W 11. Water Heater 3kW Cooking Unit 7.5kW