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Generator margin: how much more load before the preferential trips

Why it matters Start a big motor without enough margin and the trips cut other loads; or the whole ship loses power.

Generator margin: from the load now up to where the preferential trips operateThe generator on load is rated 900 kW and its preferential trips operate at 0.92 of that, 828.0 kW. It carries 612 kW now, so the margin is 828.0 − 612 = 216.0 kW. The 780 kW standby is not paralleled — its breaker is open — and is not counted.MAIN SWITCHBOARDon loadrating 900 kWstandby 780 kWbreaker open — not counted100 % = 900 kWmargin 216.0 kWload now 612 kW (68 %)preferential trips at 828.0 kW (× 0.92)72.0 kW above the trips — never usable900 × 0.92 − 612 = 216.0 kWBar to scale over the rating of the machine on load

Words on this page

Generator
the machine, turned by a diesel engine, that makes the ship's electricity.
Rating
the power written on the generator's plate, which it can give continuously.
Preferential trips
switches that cut the less important loads automatically before the generator is overloaded.
Ceiling
the most load the machine can carry before the trips act.
Margin
the load you can still add before you reach the ceiling.
Bars
the thick copper conductors in the main switchboard that the generators feed.
Paralleled
two generators connected to the same bars, so that they share the load.
Synchronised
an incoming generator matched to the bars in voltage, frequency and phase before it is connected.
Standby
a generator ready to run but not connected to the bars.
Black out
the ship loses all her electrical power at once.

An everyday picture

A lift's plate says 1000 kg. But it is set to beep and refuse to move at 900 kg. There are 600 kg inside already.

So you can add 300 kg, not 400. The last 100 kg on the plate is never there to use.

A second lift standing switched off in the basement adds nothing. Someone has to start it and bring it up first.

The rule

  • The preferential trips shed the non-essential loads BEFORE the machine reaches its rating. So the usable ceiling is the rating × the trip fraction, not the nameplate.
  • Margin = that ceiling − the load the machine is carrying now.
  • Count only the machines on the bars. A standby that is not paralleled has to be started and synchronised first. So it adds nothing at the moment a motor is switched on.
  • Starting a big motor pulls many times its running current for a few seconds. If the margin is tight, put the second machine on the board first.

Formula

  • Margin = rating × trip fraction − load on the machine
  • Only machines connected to the switchboard count

Worked example

Rating of the generator on load900 kW
Fraction of rating at which the preferential trips operate0.92
Load being carried now612 kW
Rating of the generator on standby, not paralleled780 kW
  1. Ceiling set by the preferential trips = 900 × 0.92 = 828.0 kWThe trips act before the full rating, so the usable top is only part of it.
  2. Margin = 828.0 − 612 = 216.0 kW (the 780 kW standby is not on the bars)What is left up to that top is the load you can still add; the standby does not count yet.

Answer216 kW

Common mistake

Taking the full 900 kW rating as the ceiling and ignoring where the preferential trips operate.

The trips act at 828.0 kW, so the last 72.0 kW is never there to use. The margin would read 288.0 kW. Starting the fans and the pump on it sheds load, or blacks the ship out.

Note for mariners The kW margin is not the only limit; a big motor's starting current can trip the breaker or dip the voltage, even with kW to spare.