Temporary power
Distribution apparatus assumes a supply to distribute. Where there is no supply, or not enough of one, generation provides it: and the distribution equipment downstream has to be arranged for it.
When generation is needed
Three situations come up repeatedly. A site has no permanent connection yet, and work cannot wait for one. A permanent supply exists but is too small for a particular load or a particular period. Or a process cannot tolerate an interruption, and standby generation stands behind the mains supply.
Sizing
A generating set is rated in kVA, and sizing it is not simply a matter of adding up loads. Motor starting draws several times running current for a short period, and a set that comfortably carries the running load may stall or dip unacceptably when a large motor starts. Non-linear loads, drives, rectifiers, uninterruptible supplies, distort the current waveform and derate a set further.
Sizing therefore starts from the load schedule, the largest single starting load, the starting method used, and how much voltage and frequency dip the connected equipment will accept.
| Characteristic | Why it matters on site |
|---|---|
| Mass | Several tonnes. Determines the hardstanding required and the lifting or haulage arrangements. |
| Footprint | Sets of this size are measured in metres. Access and manoeuvring space have to be planned, not assumed. |
| Fuel capacity | Base tank capacity governs run time between refuelling and the bunding required beneath it. |
| Acoustic canopy | Reduces noise at the boundary; often the difference between a consented installation and a complaint. |
| Exhaust and ventilation | Discharge position and cooling airflow must be clear of occupied areas and of air intakes. |
| Output arrangement | Determines the connection method, cable sizing and the protective device at the point of supply. |
Changeover
Where a set stands behind a mains supply, the changeover arrangement is the critical part. Mains and generator must never be paralleled unless the installation is specifically designed and protected for it, because back-feeding a supposedly dead network is lethal to whoever is working on it. Mechanically and electrically interlocked changeover, correctly rated and correctly tested, is the whole of the safety case.
Earthing
A generator on a temporary supply changes the earthing arrangement, and the protective arrangements downstream have to suit it. This is settled at design stage rather than on the day the set arrives.
Distribution downstream
Generation is only useful once the power is distributed to where it is wanted, which brings the work back to the apparatus described elsewhere on this site: a distribution assembly rated for the source, protected appropriately, and arranged so circuits can be isolated safely.
Hired rather than bought
Generating plant for a defined period is normally hired rather than purchased. Hire is arranged for the period concerned, and the set goes back at the end of it. What is agreed at the outset is the rating, the run time expected, who fuels it, who maintains it, and what happens if the period extends.