What does the main fuse box illustration teach about the old Adam?
The main fuse box illustration teaches that the old Adam has been crucified with Christ and has lost his power, but not been annihilated. Like pulling a main lever turns off the lights while the power remains, the old Adam is knocked out of commission; believers should no longer serve him. The unbeliever still lives to satisfy old Adam's desires.
Les explains that the old Adam is dealt with by identification with Christ. Because 'the soul that sinneth shall surely die,' the requirement is fulfilled when Christ died your death on the cross; God saw you in the person of Christ, in the tomb, and in resurrected life. This is identification by faith, not feeling or experience. Knowing this only because the Word says so, you believe that your old Adam is crucified with Him, so the body of old Adam—the body of sin—might be destroyed. In the Greek, 'destroyed' does not mean removed and annihilated; it means knocked out of commission, lost its power. The main fuse box illustrates this: pulling the lever turns off all the lights, but the power is still there, and flipping it back up turns the lights back on. That is what happened to the old Adam: he has lost his power. Henceforth we should not serve old Adam. The unbeliever, by contrast, is living to satisfy the desires of old Adam.