You'll hear me talk a lot in this Blog about the dangers I work with while building: numbers describing how batteries can kill and how electricity will bite you without you knowing its even there. All of these dangers do exist, but my dad and I have taken numberous precautions (though not all - we are still new to this) to protect ourselves and anyone riding in or looking at the car.
Fuses: The most basic safety device in any electrical system is a fuse. A fuse's job is to melt in case too much current flows through it. This can happen in many different forms of a short circuit, where the charge form the battery finds a path around the controller and motor to connect back to the battery. This causes sparks, heat, and massive amounts of current to flow through the pack and "blow" fuses. A short circuit is never a good thing, but some circumstances are better than others. A metal tool dropped into a battery pack is an unfortunate disaster for the pack. Often this will ruin or damage components by welding itself to live metal terminals or throwing sparks over PCB's. A fuse will prevent this kind of damage from exceeding a few seconds. However, with enough voltage, even human flesh can create a short. Though the current will be less as humans are less conductive than metal, this can still seriously injure the limb involved with heat and electrical burns. A short circuit across the chest is capable of stopping the heart. Fuses help mitigate the risk of a burn if a short is created with the pack but they usually don't act fast enough to disconnect without bodily harm.
Contactors: These devices are similar to solenoids or relays and are used primarily to connect parts of the pack or system together. In my case, there are two contactors, one seperating each final terminal of the battery pack (the most positive and negative points of the car's system) from the controller. While they are used primarily for control, they have the added benefit of being rated for a certain seperation at a certain power rating. First, seperation means the two metal contacts inside the contactors actually seperate and break the circuit so electricity can no longer flow. Contactors can try to do this at any time but if there's current flowing through one or even if it sees a voltage, it has the potential to spark and stay stuck closed. The separation power rating says what power the conactor is capable of over coming. Except for emergency disconnects, the contactors actually shouldn't see any voltage as the controller artificially matches the voltage on both sides of the contactor when changing states. The reason these are useful in a safety situation is, one, they keep the motor and controller devoid of voltage potential while you're working on the car and, two, they can turn off controller power in the case of a malfunction just by turning off they key, like a normal car.
The Breaker: The last safety system in place in the vehicle is a massive circuit breaker. Though we hope we never have to use this in a real world emergency, it does have several nice features. First, it can also be actuated inside of the cabin of the vehicle with a shifting cable which runs from the back of the vehicle to the center console right between the occupants' heads. This means that in a true emergency, if the contactors have failed to cut power and welded shut the system, this breaker has a much higher power rating and will almost certainly shut off pack power. It's rated to break automatically at 160V and 600A. This however, takes well over a minute at this power level. Higher power levels will cause this to break even faster. Though our car can easily push 1000A to the motor, it does it in such short bursts that the breaker doesn't trip. However, a short in the pack would easily short the breaker. Also, unlike a fuse, the breaker can be tripped and reset rather than needing to be replaced after each use. This makes it a great tool to kill the pack voltage (its located at the electrical center of the pack) in case someone else needs to work on it. I've used it for this purpose when going in for suspension modifications and alignments.
Work Habits: My dad and I don't have professional insulated tools which is a shame, but they're way too expensive. In stead, we wrap any tools being used in or near the battery pack in either heat shrink or electrical tape. While this isn't rated for a professional environment, it works well enough for us. The other trick to working in high voltage systems without proper professional gear is knowing where you can put your hands. For instance, the best rule of thumb is to always work with one hand behind your back. This way there's no possibility for electricity to make its way across your heart, from one hand to the other. Also be careful not to have two people workng next to each other on the high voltage pack as current will happily short through two people.