|Recommendation to NTSB on the grounding of DC power system neutrals
||[Mar. 25th, 2013|04:10 pm]
To the NTSB, 787 Battery Investigation:
I have been following this investigation as an electrical engineer interested in power systems.
I believe this incident highlights the drawbacks of the apparently standard practice of bonding one side of aircraft DC power buses to aircraft ground, i.e., of grounding the DC system neutral. The NTSB should officially question this practice. What follows is my rationale.
It is clear that large ground fault currents flowed at some point in the 787 battery failure sequence as evidenced by overcurrent damage to the green ground bonding jumper wire and to the stainless steel shield on the J1 signal cable, and by the arcing between Cell 5 and the battery box.
The ground fault current arced from Cell 5 to the battery box, flowed out in parallel along the ground bonding jumper and the shield of the signal cable, through the aircraft grounding system to the (presumably single point) connection to the aircraft (-) 28V DC bus, back to the battery via its (-) lead, and back into Cell 1 via the battery box's (-) bus bar.
I see no over-current protective devices to interrupt this fault current. Even the battery disconnect contactor would not have helped as its contacts were only in the battery (+) bus bar to the (+) terminal of Cell 8.
You report that the battery cases nominally float with respect to each terminal, so this ground fault must have occurred after an earlier fault caused an internal electrode in Cell 5 to short to its case.
Nevertheless, I believe it should be noted that even with these internal cell and cell-to-box faults, no damaging ground fault currents would have flowed had the (-) side of the 28 VDC aircraft power bus not been intentionally bonded to aircraft ground.
I believe it is time to re-examine the apparently standard practice of bonding the (-) side of aircraft DC power systems to aircraft ground.
This presumably follows the long-standard practice of grounding one side (the neutral) in terrestrial AC power systems to protect against lightning strikes on the power lines, although even here I think we should carefully re-think the practice. But aircraft DC power systems are isolated, so I see only downsides to this practice -- as shown by this incident.
In contrast, the high voltage DC systems in the current generation of electric and hybrid vehicles are NOT grounded to the vehicle body.
Please note that I am only questioning the bond between the negative current-carrying DC power bus conductor and aircraft ground. There is no question that every exposed metal component of an electrical device must be grounded to protect against electrical shock.
If aircraft practice parallels terrestrial AC practice, where every device has separate neutral and ground wires, all DC loads already isolate both sides of the DC supply from aircraft ground to prevent objectionable return currents (ground loops) through the aircraft ground system (and body in a metallic aircraft) under normal conditions. That's why there's only one connection between the (-) DC bus and the aircraft ground. Because of this isolation, this single bond could probably be removed with no impact on or redesign of existing 28 V DC aircraft equipment.
But the DC bus should not just be left to float. To drain any static charges the low impedance bond should be replaced with a high impedance bond (i.e., a resistor) monitored for voltage drops by an alarm. Under normal conditions, no current would flow through this resistor and there'd be no voltage drop. Should a ground fault occur anywhere in the aircraft DC system -- generators, batteries, loads or wiring -- a potential would appear across this resistor and sound an alarm. (The potential difference could also be logged by the aircraft flight data recorder.) Yet the DC bus and equipment would continue to operate normally until the fault is located and repaired by maintenance personnel.
Continued operation in the event of a ground fault is no small advantage considering that much of this equipment is undoubtedly critical to safe aircraft operation. Indeed it's why batteries are present in the first place. Continued normal operation with an alarm is preferable to having the DC system fail due to a circuit breaker or fuse opening, and vastly preferable to uncontrolled ground fault currents flowing from a battery capable of supplying extremely large currents.
Thank you for your time.
San Diego, CA