This article is about primary electrical distribution and its implementation into electrical systems, it also contains a description of the control systems applicable to that distribution. Although this article is intended to be purely descriptive, it has by necessity to include some of the formulae associated with electrical distribution.
The primary distribution is that part of the distribution system from the electricity supplier to the first level of LV distribution boards or switchboards. From the load assessment the location of the main distribution boards or panels may be determined. MV loads other than the transformers supplying the first level LV switchboards especially if they require generator back-up will require additional consideration in the design. If standby generators or UPSs are included in the system then the regulations of G59 must be complied with. Some will say that if the generators or UPSs cannot parallel with the mains then this is not a requirement, this is incorrect, it is most definitely a requirement.
The objective of an electrical distribution system is to safely provide a constant voltage to all the electrical loads on a site. This however is impossible, any load that is switched on will affect the voltage all the way back through the distribution system to the electricity supplier; referred to as the PES thoughout the articles on this website.
Any load placed on an electrical system will affect the supply; with most loads the rated current of the load will be relatively constant from the instant that the load is switched on to the instant it is switched off. When designing an electrical supply system the maximum load that can be applied to the system can be calculated to prevent excessive voltage fluctuations in the electrical system during load application and removal, excessive fluctuations can effect other loads connected to the system; either on the same consumers system or other consumers systems.
This is not the case with motors, when a motor is switched on the load presented to the electrical system is much greater than the load when the motor is running. To maximise the size of motor that can be supplied by the electrical system the ‘switch on load’ of the motor can be controlled by varying the method employed to start the motor.
The article on motors examines the various starting methods and the determination of the most appropriate starting method for the electricity system to which it is to be connected.