EPIRBs are small radio transmitters fitted to seagoing vessels. If the vessel were suddenly hit by any disaster, such as sinking, the EPIRB is designed to float free and begin transmission. This is vitally important whenthe crew has not been able to send a distress message from the vessel's main transmitter. This message contains code identifying the vessel as provided by the Administration and retained in a register for this purpose.

The transmission of an EPIRB signal is considered a distress alert. The essential purpose of an EPIRB signal is to help determine the location of survivors during an emergency. A Rescue Coordinating Centre (RCC) receiving an EPIRB transmission would consider that the vessel in distress is unable to send or receive a distress message so a Distress Alert Relay would normally be sent to ships in the area by any suitable means.

All EPIRBs are capable of manual activation or "float-free" release and self-activation. Remote activation from the navigatiin bridge, while the EPIRB is still fixed to the "float-free" cradle mounting, is also possible. The equipment, mounting and hydrostatic release mechanism must be reliable and able to operate under the most extreme conditions likely to be met at sea. Manual distress alert initiation requires at least two independent actions. All types of EPIRBs should be equipped with a low duty cycle flashing light of 0.75 Candela, which is automatically activated in the dark once the EPIRB has been activated.