In the event of a loss of power from the surface (via the main bell umbilical), there is a semi-automatic power failover system inside the bell which routes the 24V DC power supply from the Emergency Battery Pod to a switching system, allowing the divers to energise the emergency light and scrubbers. This switching system is contained inside the Emergency Power Junction Box.
The Emergency Power junction box is fitted with a self-luminous legend to allow it to be identified in darkness. The controls and indicators on the unit are clearly marked, and have the following functions:
A digital voltmeter shows the DC voltage supply from the surface. This is typically between 23.5V and 24.5V DC. There is a green indicator lamp that is illuminated when the surface supply is healthy.
A second digital voltmeter shows the DC voltage supply from the battery. This meter only displays the battery voltage when the red pushbutton next to the meter is pressed.
Light Power Controls:
There is an illuminated green ‘ON’ button used to turn ON the emergency light. A non-illuminated red button turns the light OFF. Note – these buttons do nothing when the surface supply is healthy.
Scrubber Power Controls:
There are two illuminated green ‘ON’ buttons used to turn ON the two CO2 scrubbers. Two non-illuminated red buttons turn the scrubbers OFF. Note – these buttons do nothing when the surface supply is healthy.
Under normal surface supplied operations, the only illuminated indicator on the box is the ‘Surface Supply’ lamp and voltmeter. If the surface supply fails, the SDC will be plunged into darkness, and the ‘Surface Supply’ lamp will be extinguished. At the same time, the internal switching system automatically illuminates the red ‘Battery Supply’ lamp (inside the ‘Display Battery Voltage’ pushbutton). In addition to this red indicator, the three green ‘ON’ pushbuttons for the emergency light and scrubbers are illuminated at half-intensity. These indicators allow the diver to quickly orientate themselves – and to find the controls to energise the light and scrubbers.
The reason for using a semi-automatic power changeover system is that it avoids accidental discharge of the Emergency Battery in the event that the surface supply is accidentally disconnected.