I have a neon sign transformer that converts 120 V to 12 kV which is the rated voltage. Using the 1:100 ratio would it be safe to have the input voltage be 140 V converted to 14 kV when the specified input voltage is 120 V? I am working with 60 mA. Is it also safe to connect the NST to a variac?
It possibly might work, but more information is needed to answer that question.
- How close to saturation is the transformer? It's unlikely the manufacturer added more silicon steel than needed, so it is probably close to saturation at 120 VAC. In that case, the extra voltage goes into heating the transformer, not output.
- How much power can the transformer dissipate? Increasing voltage 20% increases heating by 44%, if resistive heating predominates.
- What is the breakdown voltage of the insulation? If the transformer arcs over inside, it's kaput.
Would there be any safety issues if the input voltage on the primary coil is bigger than what the rated voltage is?
Depends. Is the possibility of the transformer catching fire a safety issue? Just asking.
Sigh. Of course there is a safety issue. Whenever you operate a device beyond its ratings you must account for the possible failure modes. In this case, an overvoltage on the primary might cause the insulation to break down, allowing too much current to flow in either the primary or secondary side of the transformer, and the i^2 R losses can become too great.
The breakdown of winding insulation may permit the secondary to be connected to the primary, resulting in a circuit which used to be galvanically isolated from the power line becoming connected to it. In the worse case, this can cause whatever is connected to the secondary to become a death trap. Granted, this would imply a considerable level of incompetence of the part of the designer, but in this case the designer is you.
Is the safety issue overwhelming in your case? Probably not. 140 is not that much greater than 120. But if the whole thing bursts into flames you have no recourse, no one to blame other than yourself. You knew what it was rated for and you went ahead and exceeded the ratings. Your appetite for risk is unknown.
Following the ratings is not a guarantee that failure will not occur. There is always the possibility that flawed materials will be used, or some other unforeseen circumstance will raise its ugly head. But it is the way to go unless you have a great deal of experience with the equipment you are using.
As the saying goes, "Good judgement comes from experience. Experience comes from bad judgement."