115 VAC Bus Question

[Shore Guy]

Recently had a situation where we had sequential failures of numerous seemingly unrelated components on our aircraft (B757). Fuel pumps, air data, equipment cooling, radar, signal generators, etc. Got the aircraft on the ground and maintenance found a 115 VAC bus breaker popped…….explanation it was one of the phase breakers (one of three breakers on a three phase system). Maintenance said this will sometime cause sequential failure of items on that bus as they overheat or become intolerant to operating in this condition.

I’m far from an Electrical Engineer….would someone expand on losing one phase in a three phase system?

Thanks in advance.

[Engineer]

The aircraft generator delivers a 3 phases system to A B and C busbars. Single phase equipments is fed from these respective busbars such that the power drawn through out the aircraft is evenly distributed across them.

Hence if you lose No1 generator A phase bus bar then all equipment on that bus bar is lost.

Throughout the aircraft there is 3 phase equipments fuel pumps, radio rack cooling fans etc. The effect of losing a single phase depends on the total current drawn by that equipment. If a cooling fan is running when a phase is lost it may still operate at a reduce output or trip the CB If it is not running an attempt to start it will result in a tripped CB due to an excessive starting current taken by the other 2 phases.

Hence the reason why if a a 3 phase CB pops you should reset all three of course this is not applicable to fuel pump circuit breakers which only maintenance should reset :ok:

[Flash2001]

Losing 1 phase on a 3 phase bus is not good. Underloaded motors that are still turning will slow down and overheat. Fully loaded motors will stop. No motor will start. However, any unloaded or lightly loaded motor still turning will tend to re-create the 3rd phase assuming that there is not a bus overload in the system. DC devices that rely on rectification of the 3 phase AC line will suffer seriously increased ripple and in turn will return additional and perhaps unexpected harmonics to the bus. These are consequences of general 3 phase theory. I don't know how well it applies to aircraft.

After an excellent landing you can use the airplane again!

[L Peacock]

Aren't three phase CBs ganged such that 1 phase fault takes out the other two? Think they are in the FJ world.
Wouldn't think it's ideal having two phases maintained and one down. Unbalanced loads and all that.

[Bolty McBolt]

Shore Guy

I agree with engineer..I doubt you can lose a phase with out the Gen tripping due to unbalanced loads.
But 757 has 3 buses you may have lost supply power to a bus thru a breaker tripping and therefore you lose the power to eqipment supplied from it. If memory serves me correct 757 has Capt Bus FO bus and essential bus. These buses would definitely supply the equipment you mention Fuel pumps, air data, equipment cooling, radar, signal generators for they are flight critical etc
Hope this helps as its only my 2 cents

[Shore Guy]

All,

Thanks for the responses. Here is the official response from maintenance.

A review of the logbook corrective actions found the following:
"CB 313 located BUS 1 SECL 1 popped. Reset the CB and ran both engines with all systems functioning normally. As a precaution, CB 901 (Left Generator Power) was replaced. Ran engines again with all systems operating normally."

The Left Generator Control circuit breaker accounts for all of your electrical problems, primarily, the aft equipment cooling fan. Airline Safety will continue to monitor this aircraft's performance. If we find a repeat discrepancy, we will involve Tech Service to investigate further and correct.

....Still doesn't answer all of my questions.....the failures were not all at once, they happened in sequence (3-4 minutes between each, approx).

Something else interesting that occurred during the event. I spoke with someone who has been on and teaching the aircraft since introduction and had never heard of this.....during the sequence of events, both ADI's went to black and white display. Odd looking and initially did not know whether info on it was accurate. Eventually, the color came back. A gent from maintenance said that the color "raster guns" will shut down when overheated, and he implied that losing one phase in a three phase system can cause overheating of components.

Just glad we were VFR and not halfway across an ocean.

[mono]

Without knowing exactly what failed it is difficult to give a full explanation.

However CB 313 is a 3 phase CB whose phases are split to power a range of equipment including EICAS displays, various lights, pack power (in auto and manual), zone contoller, pressurisation, vib monitors, EICAS computer (L) among other things. While some of these seem to reflect your failures they don't explain them all. Particularly the air data and symbol generator failures.

It would seem that you lost a cooling fan. This would certainly explain the loss of the colour display (raster) on the CRTs but not the lines and marks (stroke). As the CRT cooled the raster would have come back again only to fail again as the thing overheated again. (raster generates far more heat

Reading between the lines of the maintenance response it would however seem that you actually lost an entire bus (left gen bus) possibly as a result of a phase trip, which certainly affect the equipment cooling system. This could in turn have then caused the sequential failures you mention as various pieces of equipment shut down as they overheated. Vital equipment would have transfered over to the good bus automatically but some loss of systems including cooling would have occured.

It is unlikely that you ONLY lost a phase as differential protection circuits would have automatically switched out the entire gen.

[Shore Guy]

Thanks one and all for the responses……I’m going to do an article for our in-house safety publication on this topic. If anyone has some good online resources for explaining 3-phase AC, I’d appreciate if you could post them.

Thanks in advance.

[Plastique]

Fuel pumps are designed to stop if a phase is lost.
This is due to the way the motor is wound (a star rather than pi motor winding).
The fuel pump will stop rotating and the windings will heat until non resettable thermal switches in the windings cut the power to the windings.
This is a safety feature to stop the pump running at low rpm and eventually overheating.
Many of the high powered motors (e.g. recirc fans) operate in a similar way.