b737ng elec question
hello gentlemen
concerning the b737ng elec sytem, by the way much improved over the classics' elec system, one principle is : gpu & apu can never share the electrical load distribution, regardless of the position of the bus transfer switch. it is the winner takes all. all power sources must be manually switched on & the last one takes the load & disconnect the previous one. also, no parallelling possible of power sources, for ac system.
now what's the difference, if the bus transfer switch is open?
first, the dc bus tie R9 relay will be open. this relay opens anyway at glide slope intercept if flight director &/or autopilot is engaged. [ is a classic fail scenario in the sim for the devil(sim instructor, no offence taken) in the back]. the tr3 xfr relay will connect to whatever tfr bus is powered, independently from the tfr bus switch(normally from tfr bus nr 1)
the gpu will connect normally, even with the bus transfer switch open.
for the apu, you must connect both apu switches in order for powering the whole system. you do this all the time anyway, even with the bustfr switch in auto. the difference being that in a normal situation( bus tfr auto), one apu switch connects the whole system & the second apu switch just extinguish the associated source light. in flight apu generator can power the whole system, except for some load shedding, galley busses & main busses.
hope this helps,
bm
Last edited by blackmail; 7th Jun 2007 at 21:03.