NBPT Inhibition
The NBPT function is inhibited if the system detects that an electrical transfer with nonsynchronized
power sources may occur. This behaviour is aimed at protecting the electrical network
and can originate from an incorrect status of the contactor string or by one of the following actions.
1. For aircraft fitted with the classic EPGS
Action on the "BUS TIE" cockpit pushbutton switch.
APU auto-shutdown or manual shutdown by means of one of the manual emergency shutdown switches (APU FIRE PUSH on overhead panel, APU SHUT OFF at the nose landing gear, APU EMERG SHUT DOWN on refuel/defuel panel at the belly fairing).
If one of the above actions is performed, the GPCU generates the BITE message "GPCU CTRSTGS/PB BUSTIE (13XE)/APU AUTOSHUTDN" and inhibits subsequent NBPT operations. In the event these actions have not been performed and this BITE message is generated, TSM 24-00-00 must be applied.
Reset of the GPCU if the NBPT is inhibited To restore NBPT operation after it has been inhibited, a GPCU cold start is necessary in order to reset the software protection lockout associated with NBPT operation. Two reset
procedures, which consist in removing the power supply from the GPCU, are given in TSM 24-00-00 and are summarized here below:
Remove all AC and DC powers from the aircraft (Batteries, EPA, EPB, APU GEN and
IDGs).
Or:
Aircraft electrical network being supplied with only the APU GEN or with one or more
IDG:
a) Disconnect EPA and EPB.
b) Reset for at least 5 seconds GPCU circuit breaker 13XG.
2. For aircraft fitted with the EEPGS
The NBPT function is not available when the BUS TIE push button switch is released,
however it is recovered when the switch is set back to the normal position. No BITE
message generated and no GAPCU cold start necessary to recover the NBPT function.
The NBPT is not available during the electrical transfer which follows an APU autoshutdown
or a manual shutdown by means of one of the manual emergency shutdown
switches (APU FIRE PUSH on overhead panel, APU SHUT OFF at the nose landing gear,
APU EMERG SHUT DOWN on refuel/defuel panel at the belly fairing). The BITE
Message "49-61-00 APU AUTO S/D /APU MAST SW (14KD)/ GAPCU (40XG)" will be
recorded in the Ground Report. However the function will be available during subsequent
electrical transfers without requiring a GAPCU cold start.
C. The procedures and actions where the NBPT is inoperative:
1. Some actions might be used during aircraft operation and maintenance whereby, per design, the NBPT function is not operative:
Simultaneous connection or disconnection of two electrical sources (Ex: simultaneous starts or stops of two engines).
Disconnection of EPA or EPB by switching OFF the ground power unit, whereas the associated cockpit push button switch is let in "ON" position.
Although remote, the following conditions may render difficult the synchronization within the required time window:
Transfers with a ground power unit providing fluctuating electrical parameters (Ref SIL 24-057).
Transfers while there is a change in the electrical load demand leading to voltage fluctuations.
(e.g Flight controls actuation during the transfer, cargo door opening …)
An IDG with highly worn pistons & block bores might have some difficulties to synchronize with an other power source while still fully able to provide correct power supply to the
aircraft.
D. Energization sequences using transfers where the NBPT is not
operative
In service experience has shown that the following energization sequences can be used during aircraft operation whereas they are not covered by FCOM Standard Operating Procedures.
APU not available
As a general rule, at departure, when the APU is not available the NBPT is operative provided all IDG's are connected on the network before external power disconnection. If airport regulations do not allow starting all engines at the gate then the following sequences apply where some transfers are performed without NBPT.
One IDG not available:
Whatever the sequence, a Break power transfer occurs at APU disconnection only. This does not happen on A330 due to MMEL condition that requires to keep the APU running with one IDG inoperative.
As TURIN says, write it up so that it can be looked at.