these mods were carried out in service under service bulletins, SB A320-33-1057 and A320-24-1120.
the reasons given, "Some operators have reported occurrences of loss of the AC BUS 1 with subsequent loss of the AC ESS BUS and DC ESS BUS resulting in the loss
of 5 upper Display Units and the loss of integral lighting. In this situation flight
crew have reported concerns in reading the standby instruments when the
DOME lights were selected to OFF.
This situation, if not corrected, could increase the workload of the flight crew
which constitutes an unsafe condition.
This Airworthiness Directive (AD) mandates the modification of the electrical
supply logic by adding a back-up supply on the battery hot bus for the under
glare shield flood lighting."
and
"Several occurrences of loss of the AC BUS 1 have been reported which
led in some instances to the loss of the AC ESS BUS and DC ESS BUS
and connected systems. The affected systems include multiple flight deck
Display Units (Primary Flight Display, Navigation Display and Upper
Electronic Centralised Aircraft Monitoring display).
The reasons for these events have been investigated but have not been
fully established for all cases.
Due to the range of system losses some crews reported difficulty in
establishing the failure cause during the events and, consequently, the
appropriate actions to be taken may not be completed in a timely manner.
The loss of multiple display units, if not corrected expediently during a high
workload period, potentially affects the capability of the flight crew and
could contribute to a loss of situational awareness and consequent control of the aeroplane, which would constitute an unsafe condition.
This AD therefore mandates the modification of the electrical network
configuration management logic consisting in adding an automatic
switching of the AC and DC ESS BUS power supply such that upon the
loss of the AC BUS 1, the AC BUS 2 will automatically take over the
power supply. On pre-MOD aeroplanes, this power supply switching can
only be accomplished manually from the cockpit and is covered by an
Electronic Centralized Aircraft Monitoring (ECAM) procedure."