These sites contain specialized equipment to reduce or step-down transmission line voltage. Typically, the voltage is stepped down from 230,000 to 12,000 volts and is connected to a system of wires called the distribution system described below. Substations also contain large circuit breakers to stop the flow of electricity to transmission and distribution lines should the system develop a short-circuit (an electrical leak). SDG&E currently has more than 200 substations.