Reversion and full rethink of backup battery system and essential bus concept

Recent posts

https://webuildplanes.com/adam-dickson-sling2/p/7325/move-boost-pump-to-essential-bus-and-consequences-

and

https://webuildplanes.com/adam-dickson-sling2/p/7328/revised-panel-cb-arrangement-and-busbar-structure

significantly extend the role of the essentials bus and consequently the load under some circumstances on the IBBS 6AH backup battery.

Online AI searches have revealed the requirement that the 10A IBBS (ATC) blade fuse should not be uprated under any circumstances, even if a 10CB is placed at the output. Warranty will be voided otherwise, and there are safety issues.

More searches have revealed that the maximum size of a Klixon 2TC2 CB at the output of the IBBS guaranteed to trip before the fuse blows is 5A

Similar searches have revealed that the IBBS is not intended as a power source for things like flaps, trim, and boost pump. Given the maximum size of CB is 5A, the planned for current load on the IBBS is approximately 4A.

This immediately forces some changes:

Dump flaps, trim, and boost pump from the essentials bus

Also dump the (main) EFIS from the essentials bus

In the event of a main bus failure, rely on the 912ULS mechanical pump only, and plan for a flapless landing where the lack of ability to alter trim will not be too problematic.

Searches have also contained the suggestion that the cabin lighting be also moved onto the main bus

So the main bus will contain the following:

Feed from the main busbar (split between top and bottom rows, on the left) the list of CBs is

• BK CHG - IBBS charge input
• REG - regulator
• TAXI LT - taxi lights
• LDG LT - landing lights
• 4 hole 12.5mm wide busbar
• AP - autopilot servos
• NAV LT - navigation lights
• STROBE - strobe lights
• FLAPS - flaps
• TRIM - trim
• BOOST - boost pump
• CAB LT - cabin light
• EFIS - which then feeds the (main) EFIS toggle switch

After this first change all that is left on the essentials bus list is the following

• AVNCS - which then feeds the AVNCS toggle switch

So the essentials bus loses its point, and can be eliminated as a separate entity

The list of CBs on the main bus then becomes

• BK CHG - IBBS charge input
• REG - regulator
• TAXI LT - taxi lights
• LDG LT - landing lights
• AP - autopilot servos
• NAV LT - navigation lights
• STROBE - strobe lights
• FLAPS - flaps
• TRIM - trim
• BOOST - boost pump
• CAB LT - cabin light
• EFIS - which then feeds the (main) EFIS toggle switch
• AVNCS - which feeds the avionics toggle switch

From the previous post, the AVNCS CB can be a 2TC2-5A, the same as that of the CHG OUT at the output of the IBBS

The list of CBs on the avionics bus remains as

• COM - com radio
• IC - intercom
• XPNDR - transponder/ads-b in
• FLARM- flarm
• BK EFIS - backup efis
• HDST -headset

There are 19 CBs listed above, the 20th position is taken by the (now 5A) CHG OUT CB at the output of the IBBS

The START and USB CBs will still be moved out of the main group as follows:

• START CB below the master switch
• USB CB alongside the USB outlet

The role and type of the avionics toggle switch is changed, it will now be a SPDT type that switches between the IBBS output (via a 5A CB) and the main bus (via a 5A CB). That there should be two 5A breakers, one on the main bus and one on the backup bus is necessary to protect the 15A rated MS35058 switch. Otherwise, in the event of a short, the main bus could cause a much larger fault current up to and exceeding 30A to pass through the switch which would likely weld or damage it. This matter was not addressed in the previous configuration and would have required an additional circuit breaker

The need for original main/backup switch is effectively eliminated and is rolled into the avionics switch function. However the main backup switch could select between these two sources and the avionics switch could turn on and off. A combined switch would need to combine these functions, having an off, backup, and main bus position. In line with the aesthetics, this would need to be a progressive type. It would also leave one switch hole unoccupied.

Another option is to revert to the use of the bypass functionality of the IBBS, eliminating the need for a main/backup switch function entirely. Only the on/off avionics switch would be needed. There would still be a spare switch hole position, and there would be the need for an additional circuit breaker

As for the spare hole, the 30A 3TC7 breaker could be moved into the location alongside the master switch, but then the out-of-place hole alongside the fuse grouping would remain.

Work in progress.

In a future panel, the 30A main bus breaker would be alongside the master switch, and there would be two extra locations for the Start and USB CBs