Choice of power-supply sources

The importance of maintaining a continuous supply raises the question of the use of standby-power plant. The choice and characteristics of these alternative sources are part of the architecture selection, as described in chapter D.
For the main source of supply the choice is generally between a connection to the MV or the LV network of the power-supply utility.
In practice, connection to a MV source may be necessary where the load exceeds (or is planned eventually to exceed) a certain level - generally of the order of 250 kVA, or if the quality of service required is greater than that normally available from a LV network.
Moreover, if the installation is likely to cause disturbance to neighbouring consumers, when connected to a LV network, the supply authorities may propose a MV service.
Supplies at MV can have certain advantages: in fact, a MV consumer:

• Is not disturbed by other consumers, which could be the case at LV
• Is free to choose any type of LV earthing system
• Has a wider choice of economic tariffs
• Can accept very large increases in load

It should be noted, however, that:

• The consumer is the owner of the MV/LV substation and, in some countries, he must build and equip it at his own expense. The power utility can, in certain circumstances, participate in the investment, at the level of the MV line for example
• A part of the connection costs can, for instance, often be recovered if a second consumer is connected to the MV line within a certain time following the original consumer’s own connection
• The consumer has access only to the LV part of the installation, access to the MV part being reserved to the utility personnel (meter reading, operations, etc.). However, in certain countries, the MV protective circuit-breaker (or fused load-break switch) can be operated by the consumer
• The type and location of the substation are agreed between the consumer and the utility