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Guest Ricardo Sevarant

Question RE: Single System Breaker logic (SSB) on 747

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Guest Ricardo Sevarant

Hi,The PMDG systems manual describes the behavior of the 747's SSB as normally being open if one source of external power is selected whilst the remaining EXT power AVAIL switch/light is not illuminated or available. Thus, only one side of the aircraft's electrical system would be powered with the SSB remaining open until another source of power were available for the other, unpowered side. I have found that with the PMDG 747, the SSB will only remain open during a single EXT power connection (with the remaining EXT power connect switch not being in the AVAIL condition), if both the battery and standby BUS switches are in the OFF position during selecton of EXT power. If the battery switch is ON prior to selection of single-source EXT power, the SSB will close and one source of external power will power all busses with the exception of the pilot's instrument transfer bus. If both the battery and standby power bus switches are ON prior to the selection of single-source EXT power, the SSB will close and one source of EXT power will now power all busses thoroughout the aircraft.My understanding, through reading the PMDG 747 system's manual, is that the SSB should remain open until either another source of external power is AVAIL or the opposite side APU generator(s)are engaged. The PMDG 747 correctly opens the SSB when both APU GENS are online or when one APU GEN and an opposite side IDG GEN is connected to their respective sides. The PMDG 747 systems manual describes programmers implementing a special kind of logic for external power connections to simulate conditions at various world-wide airports. I'm wondering if this logic is being implemented with the SSB behavior or if I am altogher misunderstanding how the SSB logic is supposed to work.Thank you. It's most interesting to gain a deeper understanding of 747 aircraft systems.

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Guest Ricardo Sevarant

SSB = split system breaker, sorry typo on above post.

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Guest Ricardo Sevarant

Ok, found a realworld 747-400 technical brief via Google. It states the following:"If only a single external power receptacle is connected, then selecting that external power source onto an unpowered airplane will power both sides of the sync bus by automatically closing the split system breaker (SSB)."This is the actual behavior of the PMDG 747 SSB as well. The PMDG systems manual however states, "When first beginning to power the aircraft, the behavior of the SSB will depend upon what power sources are AVAILABLE. (AVAILABLE means that the source is available for use, but has not been selected as an ACTIVE power source.) If only EXT1 is showing as AVAIL on the electrical panel, then the SSB will remain open when EXT1 is selected. The only way to close the SSB, thus providing power to the entire airplane, is for a power source to be AVAILABLE for the right side of the airplane, whether it is in use or not. For example, if only EXT1 is available, it will power the left side of the airplane when selected ON. The right side will remain unpowered unless:EXT2 becomes AVAILABLE.APU2 becomes AVAILABLE.It is not necessary to select either EXT2 or APU2 on, it is only necessary that they be available in order to provide power to triggerthe SSB to close."So, it seems the PMDG simulation works like it's realworld counterpart, but the PMDG systems manual possibly has things a little off in this regard.

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"So, it seems the PMDG simulation works like it's realworld counterpart, but the PMDG systems manual possibly has things a little off in this regard."That would be my guess, too, RicoThe #2 Bus Control Unit (one of two elec power control computers) has to be powered to close the SSB. What powers the #2 BCU depends on options, modifications, etc.Here are the optional variations:Battery Busses (APU and Main (Hot) required) and/orGHB (Ground Handling Bus) #1 and/orGHB #2GHB's are powered when the respective EXT power source or APU generators are available (doesn't have to be selected)I can't recall if the PMDG 744 looks at GHB #2Cheers.Q>

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