<P> Domestic DC installations usually have different types of sockets, connectors, switches, and fixtures from those suitable for alternating current . This is mostly due to the lower voltages used, resulting in higher currents to produce the same amount of power . </P> <P> It is usually important with a DC appliance to observe polarity, unless the device has a diode bridge to correct for this . </P> <P> Most automotive applications use DC . The alternator is an AC device which uses a rectifier to produce DC . Usually 12 V DC are used, but a few have a 6 V (e.g. classic VW Beetle) or a 42 V electrical system . </P> <P> Through the use of a DC - DC converter, higher DC voltages such as 48 V to 72 V DC can be stepped down to 36 V, 24 V, 18 V, 12 V, or 5 V to supply different loads . In a telecommunications system operating at 48 V DC, it is generally more efficient to step voltage down to 12 V to 24 V DC with a DC - DC converter and power equipment loads directly at their native DC input voltages, versus operating a 48 V DC to 120 V AC inverter to provide power to equipment . </P>

Is a 12 volt system ac or dc