The primary line of defense is a solid grounding system; both in your individual circuits as well as your main service. This is the third conductor that will take fault current and return it to ground in your panel causing the breaker to open (or trip) in the event that you have a short to ground.
Imagine a wire coming loose on the inside of your toaster and then touches the metal exterior cabinet. If the grounding conductor is missing on your circuit, then the shell of the toaster becomes energized and when you touch it YOU are the path to ground. This will result in a shock that could lead from a jolt to a cardiac arrest. Electricity is unforgiving and you can not predict where it will leave your body to ground. If the current crosses your heart you could end up in a full blown cardiac arrest. If you grab an energized component with your hand the current can cause your muscles to contract, closing your hand on the live wire. Most people that are caught in a circuit burn to death. It is a horrific and painful way to leave this earth. You can be killed by current as small as 12V. It is imperative that your systems are grounded to the Standards of NEC 2014 ART 250 to insure the ability of your system to respond to ground faults.