Power network architecture

MV refers to medium voltage, the distribution networks that feed an industrial site from the public grid or a delivery substation. LV refers to low voltage, which reaches machines and sockets after transformation. Public lighting covers street and yard lighting.

One project often spans all three: the transformer substation is MV, the main switchboard and its outgoing ways are LV, and roadway lighting is public lighting.

It depends on what an outage costs. A set is justified as soon as a stoppage compromises safety, continuous production, a cold chain or data.

Sizing is not based on total installed power, but on the circuits to be backed up and on their starting peaks — a motor draws several times its rated current at start-up, and it is that peak, not the steady-state power, that dictates the size of the set.

It is the device that switches the supply from the grid to the generating set, then hands it back when the grid returns, with no human intervention.

It also interlocks the two sources: they must never be closed at the same time, otherwise the set would feed back into a network that crews believe to be dead. It is as much a safety device as a continuity one.

By its environment first. The IP rating states what must not get in — dust, splashing, jet washing — and the IK rating what impacts it must take. A clean plant room and a dockside do not call for the same enclosure.

Next comes the power dissipated by the equipment inside, which decides ventilation: a sealed, poorly ventilated enclosure cooks its own contents. Only then come dimensions, material and access for maintenance.

Armour is mechanical protection: a metal layer that lets the cable be buried or exposed to damage without additional conduit.

Screening is electromagnetic protection: a conductive foil or braid that contains disturbances and protects against interference. A buried power cable needs armour; a measurement or control cable running alongside variable-speed drives needs screening. The two needs are independent and can be combined.