We had that problem with DD and the fan...i was terrified she was going to loose a finger and its taken till now....18 months old....for her to understand that its dangerous! And now she tells me ''mummy, fans on, be carful'' just had to say it over and over and over and over.
One of the first words dd knew was 'hot' cause the oven is in a possition where if she is in the kitchen shes always in reach of the oven. Now she puts here hand near it and says 'hot' and pulls her hand away! i just always tried to explain why she couldnt touch it, even though at 8 months she probably didnt have a clue what i was talking about, now she looks and listens and understands....its great!!!
Every draw and cupbourd door in our house was locked or latched because even though she knew she wasnt ment to pull the aluminium foil out all over the kitchen, its so shinny and crackly and fun!! Only now does my 'low tone soft telling off voice' actually work. Like you i was yelled at and screamed at till i was to terrified to not do something....always said id try SO hard to never do that to DD
good luck

ETA: just read back over the previous posts...like MT said you can wear out the word NO....DH said NO to everything and it annoyed me so much, he still does it some times and will just say NO NO NO NO NO NO NO in one long sentence and it just means nothing to her now. So my 'emergency' word for when she REALLY has to not do something is STOP. and she freezes!!