There are two ways to safeguard these services. First there must be at least two generators, the rating of which must be such that essential services can be maintained if one set is out of commission. Secondly, a protection must be provided that if sea load is too much for one generator a system of preferential selection will operate.
In some cases the non essential load is relatively too small to warrant additional switchgear. It is generally in larger installations where loads not under direct control of the engineer that they must be fitted. If the heating, lighting and galley were all switched on without prior warning, then the generators could become overloaded. Without preferential trips this may so overload the generators as to cause a complete shutdown. Therefore non essential services are fed through one or more circuit breakers fitted with shunt retaining coils or shunt tripping coils. Over current relays with time lags are provided for each generator. When overloaded, appropriate relays operate and trip out the non essential services. Some being more important than others, degrees of preference may be given.
Setting
Usual setting is 150% (50% overload) with a time delay of 15 seconds for generator overload protection and the following times come into operation when the generator reaches 110%.| First tripping circuit | 5 seconds |
| Second tripping circuit | 10 seconds |
| Third tripping circuit | 15 seconds |
