chassis "ground" connection, but positive ground may be used in some wheeled or marine vehicles. In a battery electric vehicle, there are usually two separate DC systems. The "low voltage" DC system typically operates at 12V, and serves the same purpose as in an internal combustion engine vehicle. The "high voltage" system operates at 300-400V (depending on the vehicle), and provides the power for the traction motors. Increasing the voltage for the traction motors reduces the current flowing through them, increasing efficiency. Telecommunication Telephone exchange communication equipment uses standard −48 V DC power supply. The negative polarity is achieved by grounding the positive terminal of power supply system and the battery bank. This is done to prevent electrolysis depositions. Telephone installations have a battery system to ensure power is maintained for subscriber lines during power interruptions. Other devices may be powered from the telecommunications DC system using a DC-DC converter to provide any convenient voltage. Many telephones connect to a twisted pair of wires, and use a bias tee to internally separate the AC component of the voltage between the two wires (the audio signal) from the DC component of the voltage between the two wires (used to power the phone). High-voltage power transmission High-voltage direct current (HVDC) electric power transmission systems use DC for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. Other Applications using fuel cells (mixing hydrogen and oxygen together with a catalyst to produce electricity and water as byproducts) also produce only DC. Light aircraft electrical systems are typically 12 V or 24 V DC similar to automobiles. See also CCS DC bias Electric current High-voltage direct current power transmission. Neutral direct-current telegraph system Polarity symbols Solar panel References External links AC/DC: What's the Difference? – PBS Learning Media DC And AC Supplies – ITACA Electrical engineering Electric current Electric power