B747-200 Essential Power Switch

[forsyh]

1. Why is it that when you select the ESS Power switch away from normal, you should select Gens No. 3, 1 and 2 in that order?

2. Why is it that when Essential Power is selected to Generator No 1, No 2 or No 3, the No.3 DC Bus isolation relay will open automatically?

[Short Hauler]

It has been a little while since I was full bottle on this stuff but I'll have a go.

As far as I understand it -

1. Normally the essential AC bus is powered by AC BUS 4 (primarily sourced from GEN 4, but backed up by the other generators through the bus tie breakers and the split system breaker).

If the essential bus off light illuminates you select GEN 3 first, I assume because it is the closest selection. This connects GEN 3 directly to the essential AC bus.

The F/O's instruments are sourced from GEN BUS 2 and so you select GEN 1 next. If not, you would be connecting the Captain's instruments (on essential power) to the same source as the F/O. If GEN 2 failed and the GEN 2 BTB tripped you would lose all the instruments up the front.

You then select GEN 2.

We were always taught that with an ESS BUS OFF light illuminated, you were allowed only one selection away from normal. Apparently this is to prevent shorting out all the electrics, one bus at a time, if there is a short in the essential bus.

2. The DC busses are paralleled during normal operation so that if one TR fails the others take up the load.

I believe the automatic tripping of the number 3 DC BUS isolation relay, when selecting the essential power source away from normal, is to prevent the essential TR having to power more than two DC busses in the event the number 3 TR fails.

With the number 3 DC bus isolation relay open, the number 1 and 2 DC busses are separated from the number 3 and essential DC busses.

I am not sure what happens if both TR's fail on one side of the open number 3 DC bus isolation relay. I guess it comes under the umbrella of Boeing not considering double failures??

I hope this helps. If someone can pick out errors please set me straight.

[Flight Detent]

Hi all,
I essentially agree with what is offered, but add one extra point, my understanding regarding the DC seperation, is from the opposite point of view.
I think the DC power supplies are seperated for the very reason that if one side does fail, it doesn't overload the other, with the resultant total power loss.

When 'land' mode is selected, the same DC seperation occurs, for the same reason!

Cheers

[Cornish Jack]

Always looks a little odd when viewed as an isolated statement of fact. Just try thinking WHY such a condition would occur - given the natural redundancy levels of the 47 electrics.
IF such a condition occurred, it would be because of a pretty catastrophic electrical problem and what you then want is separated systems, to prevent one system's problems taking out one or more of the others. Sticking to the 'book' selections will give you a good base from which the FE can start to diagnose and, perhaps, re-instate working systems. It's always worth taking on board the TWO functions of the SSB - connecting up in parallel when all is going well, BUT, when it turns to worms, giving you four separate sytems, any one of which will get you home.