Electromagnetic Shielding

Author: N.H. Crowhurst

The second type of shielding uses a different principle, but is also effective against magnetic fields. Electromagnetic shielding does not use a magnetic material, but rather one that conducts electric current well, such as copper or aluminum. When the magnetic field causing the induction fluctuates, it causes current to flow in the shield. This current in the shield sets up its own magnetic field that opposes the original inducing magnetism, and the two fields tend to cancel inside the shield.

Electromagnetic shielding

Outside the shield, the two fields are in the same direction and the combined magnetic field is increased. Therefore, this kind of shield has the effect of "pushing" the inducing field outward, instead of permitting it to reach the shielded circuits. Because it depends on fluctuating fields, this kind of shield is completely ineffective against steady magnetism. It is also comparatively ineffective against low-frequency fluctuation, and only becomes really effective at higher frequencies.

Triple shielding of input transformer

With input transformers, which are particularly susceptible to magnetic fields because they have a magnetic core, the shielding often consists of an arrangement combining magnetic and electromagnetic shielding into one composite assembly. As well as providing increased protection, the combination makes the arrangement more effective over the entire frequency range. The magnetic shielding takes care of frequencies down to zero or d-c and begins to become ineffective at frequencies between 60 to 300 cycles. The electromagnetic shield^on the other hand, begins to become effective between 60 and 300 cycles, and the combined protection is effective for all frequencies.