D

DC - Abbreviation for direct current

Delta supply - A three phase power supply with no neutral connection

Differential mode - The voltage that appears between two conductors, neither of which is at earth potential

Digital signal - A signal which operates with a smaller number, usually only two, voltage levels. Because only two easily distinguishable voltage levels are used, digital systems are free from many of the forms of distortion which affect analogue systems. Both can be affected by transient overvoltages.

Dip - A momentary reduction in the voltage for a duration from a half-cycle to a few seconds. Also referred to as a sag.

Direct current (DC) - A steady current in one direction, such as that provided by a battery, a dynamo, or the smoothed output of a rectifier circuit.

Dirty - Cable or supply carrying some form of interference

Discharge current - Refers to the transient current flowing through a protection device or component during operation.

Distribution board - A panel, enclosure, or 'board" containing isolators, fuses, air circuit breakers, or protective devices; feeding one or more outgoing circuits and supplied by one or more incoming circuits, together with terminals of the neutral and earth combination. The term main, or main LV, distribution board refers to the building's principle distribution board, usually supplied directly form an electricity substation or HV to LV transformer. A sub distribution board describes a second tier distribution board supplied from the main distribution board and itself supplying either equipment or third tier distribution boards. The term local distribution board describes the unit that provides the power supply to a particular piece of equipment.

Diverter - General term for a protective device that diverts current away from a circuit

Draw wire - A high tension wire incorporated in many fiber optic and some other data cables, to enable the installer to pull the cable through inaccessible conduits or ducts.

Duct - A pipe or conduit for mechanically protecting cables