In a circuit or system, there are many discrete signals directed to specific destinations only. Often these signals can interfere with other unintended signal destinations or devices. Shielding can prevent or diminish this interference effect. Like a groundplane, a shield can be constructed of one or more conductive homogenous planars. However, the shield differs from the groundplane by its need to be free of conducted or induced electrical currents.
Shielding also has reciprocal purposes.
It protects the circuit it is shielding from outside noise or unwanted signals. Conversely, it contains its own signals and thus protects the outside world from interference of its own making. Shielding is especially useful where open frame transformers or unshielded coils are used, in cases where these would otherwise exchange signals by mutual inductance. This fact is referred to as near field and far field radiation. Ideal shields have no current flowing through them.