Bus distribution, B737-800

[KristianNorway]

Hello

Would anyone tell me what the practical difference is between the battery bus and the switched hot battery bus on the 737-800?

I know there must be an operationally significant difference since they have after all divided them into two different buses, but I can't see it.
They are both operated by the battery switch in the same manner and in the same situations.


All inputs are very welcome :)

[aveng]

The battery bus receives power from the 28v dc battery
bus bar or TRU 3, via spcu (supply depends upon whether AC power available). The switched hot battery bus receives power from the battery bus bar -via the spcu (standby power control unit) - through a relay driven by the battery switch (essentially direct power from battery bus bar when battery switch closed).:ok:

[KristianNorway]

Thanks aveng.

But the question stands unanswered. We know the buses are separated, but why?
They both energize - or de-energize by manipulating the battery-switch.
(Apart from when the stby pwr switch is set to bat and the battery switch is set to off. But there's no procedure for this in our manuals).

[doncas]

Just dusting off my conehead hat, but I believe that the Hot Battery Bus is always connected to the battery, regardless of which position the battery switch is in. There's only a few items hooked up to the Hot Battery Bus, the airstairs standby relay is one... hope this helps!

[KristianNorway]

Hello doncas

You're thinking of the hot bat bus. I'm discussing the switched hot bat bus.
But thanks for posting anyway!

In fact it surprises me that I even can't find the answer to this in the engineers & technicians pages.
I hope one of you out there may have the answer.

[aveng]

Could be your looking to far into it. They are probably named the way they are so that engineers know that they are different buses. Not really meant for pilots to look too deeply at. Often the terminology denotes their relative priority. Hot battery buses generally supply power to stuff that needs power at all times - Clock internals etc.