Generator Duties and Ratings

One engine, several ratings

The same generator set can carry different power ratings depending on how it will be used. A set that runs at a steady load for years must be rated more conservatively than one that runs a few hours a year during supply failures. Choosing the rating that matches the real duty is one of the first and most consequential design decisions.

This course is for engineers who design, specify, commission and run generation for industrial and remote sites. It assumes the basics of synchronous machines, governing and synchronising covered in the existing lessons, and moves on to design decisions, edge cases and the judgement needed when things go wrong.

Why the ratings differ

Engine wear and the time between overhauls depend on load and running hours. At an emergency standby rating, the maker accepts high load because the set will rarely run. Used continuously at that rating, the same engine would wear quickly and its warranty would not apply. The difference between ESP and PRP for the same set can be significant — check the maker's data sheet for the exact figures for each model.

Worked example 1 — the camp that grew

A remote construction camp was supplied by a set bought on its emergency standby rating, because that figure looked largest on the data sheet. The camp ran on it twenty-four hours a day. Within two years the engine needed a major overhaul far earlier than planned. The set should have been selected on its prime rating, which would have meant a larger model from the start.

Worked example 2 — a hospital standby set

A hospital standby set runs only during supply failures and monthly tests. It is correctly selected on its emergency standby rating. But a proposal to use it every afternoon to reduce peak demand charges changes the duty to limited-time or prime running. The maker must confirm the rating for the new duty before the change is made.