"Types II to IV" are variants which attempt to overcome the main limitation of Type I, which is the reference to earth at each end of the circuit. Types II-IV use the Signal Battery (SB) and Signal Ground (SG) lead in conjunction with the E&M wires. This improves noise immunity as the Signal Ground does not carry the same heavy currents as normal ground connections and provides a low resistance return path for signaling. However, if the main ground connection fails all earth current may flow via the interface, causing signaling failure, hum and in extreme cases destruction of equipment.