Electrical noise can be annoying when it causes audible or visual interference in radios and televisions. If it interferes with a depth sounder, or a back-up camera on a motor coach, then human life may be at risk.
Noise categorization is complex, but falls into two major types, radiated and conducted.
Radiated noise is propagated through the air, while conducted noise is present on power and signal lines. The best rule is to prevent noise from escaping its source, but this isn't always easy to do, and in some cases, proper filtering would cost more than a manufacturer is willing to pay. In other cases, noise exists because no one has considered the problem. In such circumstances, noise might be prevented with easy and inexpensive methods. High frequency noise is easier to filter than low frequency noise.