Page 27 - Volume 15 Number 10

In the previous paragraph I used the term “wrong” while referring to the direction of current flow. HEDs only respond to excessive current in the undesired, wrong direction. To explain further: Except for the power needed by the starter to turn the engine, it is undesirable for the battery to discharge quickly at a high rate of current flow. On the other hand, after the battery has been partially depleted during the start, it is desirable to have it receive current at a high rate for recharging itself. Similarly, since the charging current comes from the generators, it is normal to have a very high current flowing from the generators into the battery after starting. To get from the source – the generators – to the draw – the battery – current must flow from each generator through its respective generator bus to the center bus to which the battery connects. So at times it is normal and expected to have high current flow from a generator bus into the center bus.
The overhead panel where the loadmeters and voltmeters tell their stories.
But having high current flow from the center bus into a generator bus is rare and almost always would be due to a problem: That missing wrench finally bridged the metal of that bus to the airframe ... a direct ground short. To conclude, all three HEDs – the two between the respective
generator buses and the center bus and the one between the battery and the center bus – only react when more than 275 amps of current flows in the wrong or bad direction.
The time that it is normal to experience very high current flow out of the battery is, of course,
OCTOBER 2021
KING AIR MAGAZINE • 25