E and M signaling

The signaling unit and trunk circuit communicate their status over the E and M leads, using a combination of battery and earth (also known as ground) levels.

The maximum distance between the signaling unit and the trunk interface is determined by the resistance of the wire, but is normally less than 100 meters for adequate noise immunity.

The interface is vulnerable to poor earthing at either end, and interference from external electrical noise which act on inferior voice signal quality.

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.

SSDC5 is commonly used in the United Kingdom and unlike Type V the on- and off-hook state, are backwards to allow for fail-safe operation.

E&M defines three methods of "start" signaling: The choice of letters for the E and M leads was fortuitous, unrelated to any names or meanings.