Naturally there can be no set rules for applying diversity factors and each installation has to be considered in the light of past experiences and present knowledge. A guide is given in as to how typical loads can be assessed for current rating in different types of installation. It will he noticed that figures for factories are not shown. This is because the loading in one factory may be completely different from that in another factory employing the same equipment, and so no generalization can be given. Thus the electrical engineer will have to consider the loading individually on each item in the factory on information provided by the consumer. Size of Switchgear Main switchgear should be rated according to the maximum current which is likely to flow at peak periods, and very often this may be 100% of the installed load. The table in Fig. 11 can be used to find rating of switchgear but if there is any doubt, higher ratings should be chosen. Additions are always likely to an installation, especially in large premises, so due account of increased load must be taken. Thus if a maximum load at peak periods is estimated at 100 amps, then it would be better to install a s itch rated at say 150 amps, rather than risk later overloading and possible breakdown of a 100 amp switch. Size of Cable See Section 2 on Choice of Cables for a description of diversity’ factor applied to main, sub-main, and final sub -circuit cables. Fuse Ratings I.E.E. Regulation 108 states that no cable should have a current rating less than that of the fuse which protects it (or less then one half the operating current of the protective circuit breaker).
This is a very simple regulation, but a fundamental one, and one that is so often ignored. A common example of this is 15 amp fuses inserted to protect cables rated at only 5 amp. A fuse rated at 15 amp is designed to carry 15 amp without overheating and it will not melt until about 30 amps is flowing through it. So a cable rated at 5 amps kind “protected” by a 15 amp fuse can have currents of up to 28 amps flowing through it unchecked. This could very easily make the cable red hot and cause considerable damage. Thus it is essential for the electrical engineer to make sure that no fuse has a higher rating than the cables it is protecting (the only exception being motor circuit, see Section 6).
If a diversity factor ‘is used to install a smaller size of cable in a circuit, then the fuses protecting that circuit must also be reduced in rating to comply with Regulation 108. ie. If the maximum loading of a section of an installation was 80 amps but use of diversity brought this down to an estimated current demand and hence cable rating, of only 65 amps, then the fuse would have to be rated at 65 amp and not 80 amp. If however, the cable size is increased to avoid excessive voltage drop in the circuit, then the size of the fuse must NOT be altered. This is because conditions of operation have not altered, and the fuse rating must therefore remain the same. e.g. Assuming a single phase circuit is designed to be fused at 15 amp, then 7/.029 cables, rated at 17 amps (from Table 1), are the initial choice. If however, after calculation, it is found that the volt drop in this cable is excessive, then the size will have to be increased within the voltage limits, The fuse size must not be correspondingly increased since the same current will flow, so it remains at 15 amp. Complete safety is maintained as the fuse rating remains smaller than the cable rating. Solid Neutral Links I.E.E. Regulation 2 states that no fuse, non-linked witch or circuit breaker, must be inserted in a conductor connected with earth.
The neutral point of the three phase supply system is connected permanently with earth, then the above rule and two wire final sub-circuits. This means that no fuses should be inserted in the neutral or common return wire, and the neutral should consist of a solid link or part of a linked switch which completely disconnects the whole system from the supply.
The practice of fusing is often not well done. Frequently the phase and neutral conductors are connected wrongly at switchfuse and distribution boards, leaving the relItra1 fused, and the phase conductor linked throughout the installation. Another similar aid common fault is to put a fuse in the neutral as well as the phase conductor, thus giving double pole fusing on a single phase supply. Danger could then occur if, for some reason, the neutral fuse only blew on a fault, leaving the phase conductor fuse still intact. Since the supply would be cut off, the consumer would assume that all the terminals were dead and could receive a shock at mains voltage if he investigated a socket outlet, or similar point, unsuspectingly. Colour Coding D.C. SYSTEMS
Two wire d.c. system:
Red for positive or switch wire.
Two wire d.c. from a 3 wire d.c. system.
Red for outer or switch wire.
Black for middle wire.
Three wire d.c. system.
Red for positive or switch wire.
Black for middle wire.
White for negative or switch wire.
AC.SYSTEM
Two wire single phase a.c system:
Red, White or Blue for phase line or switch wire.
Black for neutral.
Green for earth when flexible cords used.
Three wire two phase a.c. system:
Red for one phase.
Black for common return.
White for other phase.
Three or Four wire three phase a.c. system.
Red for first phase.
White for second phase.
Blue for second phase.
Black for neutral.
For two wire final sub-circuits, whether a.c. or d.c supplying lighting or power circuits, the neutral or “middle” wire must always be black, and the phase or outer wire (no matter which phase it is connected to) must always be red. For lighting, the red wire should always feed the switch, and a red wire should always be used from the switch to the light.
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