These devices are not rated in joules because they operate differently from the earlier suppressors, and they do not depend on materials that inherently wear out during repeated surges. SM suppressors are primarily used to control transient voltage surges on electrical power feeds to protected devices. They are essentially heavy-duty low-pass filters connected so that they allow 50 or 60 Hz line voltages through to the load, while blocking and diverting higher frequencies. This type of suppressor differs from others by using banks of inductors, capacitors and resistors that suppress voltage surges and inrush current to the neutral wire, whereas other designs shunt to the ground wire. Surges are not diverted but actually suppressed. The inductors slow down the energy. Since the inductor in series with the circuit path slows the current spike, the peak surge energy is spread out in the time domain and harmlessly absorbed and slowly released from a capacitor bank.