Had one last year with an intermittent starting prob.
Apu batt & various relays changed to nil effect.
During ecb change as a last resort, one of the c.b.'s that should be pulled during ecb removal was found already pulled & with a black tiewrap around it - hence nobody had spotted it.
Think it was called "d.c. cntl" but not sure, on the o'head pnl anyhow.
Thinking back the problem was always with a dead aircraft, ok with a gpu on.