The chance of a successful jam is a fractional affair. The strength of the jammer is the numerator, and your ship’s sensor strength is the denominator.
For example, if your attacker has a jam strength of 3 vs gravimetric, and your ship has a gravimetric sensor strength of 4, then the odds of a jam going off successfully are 3/4, or 75%.
Because of this, no amount of sensor strength will ensure you won’t be jammed. The numerator will always be at least above 0 and even a one in a hundred chance will prove it is not zero about 1% of the time.
Jams are not as scary as they used to be, though. Every jammed ship can target every ship jamming them. If a fleet comes after you with two of them getting successful jams, you can target both of the jamming ships freely. The more ships with successful jams on you, the more ships you’ll be able to target. You just won’t be able to target any ships that aren’t jamming you until you deal with the jammers first.