We say “the fan is on” to express its current state.
and “the fan suddenly went on” to express an action.
Normally we use the simple past tense for an action because it happened.
if I say “the fan is going on”, it sounds strange because it just takes 1 second for the fan to go on.
I am sure we can say “the alarm clock is ringing” and “the alarm clock went off”
But I am not sure if we can say “the alarm clock is going off”
Can we say "The fire alarm is going off" to express the state of making sound?
And native English speakers say "The fire alarm is going off" see this Video