Intrinsic safety barriers are devices that limit the current, voltage, and total energy delivered to a sensor in a hazardous area or flammable environment in order to prevent an explosion. There are several types of intrinsic safety (IS) devices. Galvanic isolators provide intrinsically safe connections and are used to isolate circuits, either optically or with a transformer, in both hazardous and non-hazardous areas. Most galvanic isolators require an external power source and are used as signal conditioners. Zener barriers are passive devices that contain Zener diodes, resistors and fuses to limit excess voltage and current. They divert potentially dangerous energy to ground and are the basic building blocks for all other types of intrinsically safe barriers. Because of resistance, there is always a voltage drop across Zener barriers. Some intrinsic safety barriers contain light emitting diode (LED) indicators or replaceable fuses for overvoltage and reverse polarity protection. Other devices provide protection against short circuits.