The whole idea of an essential loads bus(ELB) is that no failure modes should separate it from available battery power, so it is crucal that there be no switching or contactors of any kind between it and the battery. The simplest ELB is the battery post itself, but all should be located as close as possible to the battery as they are always ?hot?. Truely essential loads like electronic ignition and electric primary fuel pumps sould always connect directly to ELB?s and never to the main bus. Each essential device must be breaker-protected, switch-connected to the ELB, but unaffected by master switch position. Yes, you have to manually turn off each essential load at shutdown.
My RV has twin elecronic ignitions AND twin primary electric fuel pumps. I run a single B&C alternator so have identical-twin 16AH EarthX batteries. Each battery has a contactor to the main bus and its own ELB. It is completely symmetrical, so the master switch is an on-off-on switch allowing either but not both batteries to act as primary(*-see note). Each essential load is also powered through an on-of-on switch protected by two breakers that can route power from either but not both ELB?s. The batteries, the ELB?s, the essential load switches and breakers, and the contactors are all located in very close proximity in a console separating the pilot and passenger? feet, with the switches in a vertical stack above the fuel selector valve. There is a fifth on-off-on switch there energizing a sub-bus of breakers feeding alternate power inputs on several Garmin devices with dual power inputs, allowing selected access to avionics and communications with the main bus shut down. This system is simple, symmetrical, and fool-proof. There is no ?emergency mode?, just alternate, normal, everyday operation of essential loads. I can fly an entire tank of gas with one fully charged battery.- Otis
(*)- There is one exception- A guard-protected, spring-loaded to ?OFF?, DPST switch will activate both main bus contactors when held ?ON?. This allows switching between the two batterys in flight without interrupting main bus power, and it allows the option of starting the engine with both batterys working. Both batteries, the added busses, breakers, wiring and contactors all together weigh less than one PC680, but have four times the cold cranking Amps.