Glossary terms beginning with D

Terms used within Transpower
Click one of the letters above to go to the page of all terms beginning with that letter.
D.C./D.C. converter

A switch-mode power supply that provides an isolated d.c. output from a d.c. input.


The absolute power level expressed as the reciprocal of the shortest time element in a binary data stream.

DC distribution cabinet (communications)

A cabinet containing distribution fuses or circuit-breakers, visual display for local status indication and facility for remote status monitoring.

DC distribution panel

A cabinet containing distribution fuses or circuit-breakers, visual display for local status indication and facility for remote status monitoring.

DC power cabinet

A cabinet that may contain all or some equipment associated with the power supply such as: batteries, rectifiers, D.C./D.C. converters, secondary distribution, chargers, local alarm indications, etc.

DC power cabinet (control/protection/communications)

A cabinet that may contain all/some equipment associated with the power supply such as batteries, chargers, distribution, local alarms, etc.

DC6 communications system

The communications system between Haywards and Benmore comprising: The OPGW from HAY-OTB, The submarine fibre optic cables OTB-FTB, The OPGW from FTB to ISL, together with optical repeater sites, The NEC DMR from ISL to BEN, The Telecom-leased backup circuit HAY-BEN.


Double Circuit Pole.


Double Circuit Single Tower.


The conducting condition of the main and/or bypass valves of the HVDC system.

de-energised (electrical)

Not connected to any source of electrical supply but not necessarily at zero voltage, e.g. may have an induced voltage.

de-energised line charging

A feature of some auto-reclose schemes, where auto-reclose will occur only if the transmission circuit is de-energised.

dead band

A characteristic of an automatic control system, e.g. governor, voltage regulator, within which a change of value of an input signal may take place without causing a perceptible change in output signal.  Sometimes referred to as dead zone.

dead time (circuit-breaker)

dead time (circuit-breaker).


The status of equipment which is permanently disconnected from the power system, made permanently inoperable, and free of any operational identification.

DEF (relay)

Directional earth fault (relay).

defective equipment (or component)

A line component or a set of components defective to the extent of not fulfilling properly their design purpose or design life, and where such defect is not the result of normally expected rates of wear, corrosion or general deterioration.


To pass from one operator or controller to another the authority and the responsibility for carrying out defined operating actions.


A measure of the rate of consumption of electrical energy.

demand management warning

A warning issued by the grid operator in accordance with section 3.2 of the Grid Emergencies Code.

demand response

Load management in response to high prices or other incentives (See DSP).

dependable strength

MFL x strength reduction (SRF) (SRF allows for variation in material properties, strength reductions in manufacturing processes, wear and other forms of degradation and variables in strength.).

dependant manual operating mechanisms

Where the speed of operation of the switchgear is directly related to to the speed and actions of the operator. The actual fault capability of the switchgear depends upon the actions of the operator, any errors can lead to catastrophic failure.

design spill

The maximum quantity of a possible oil spill.


The person or organisation carrying out the design work. Note: The organisation is usually a Consultant acting on behalf of Transpower but may be Transpower.

determined intent

Motivation of a person (or persons) who has no right to be accessing the grid operating asset.  They have gained access to the grid operating asset with intent on stealing or causing havoc.  They are not naïve in gaining unauthorised access.  They are determined, their entry is not considered a ‘casual’ entry.


Generic description including the sponsor, writer(s), working group, reviewers, illustrators and anyone else contributing to a standard up to the approval stage.


Dry film thickness.


Dissolved gas analysis.


Differential Global Positioning System, a high accuracy satellite navigation system.

diagnostic test

A non-invasive comparative measurement carried out to monitor the condition of the equipment.

differential protection

A protection scheme which measures the sum of the currents at the boundaries of the protected zone.  The sum is zero if the protected zone is healthy:  a fault inside the protected zone will result in the sum not being zero and will cause a protection operation.

digital microwave radio (DMR)

A radio bearer using digital modulation techniques capable of carrying a number of information channels.

direct causes

Acts, conditions, arrangements and circumstances on site which give rise to an accident or near miss.

direct consumer

An end user who is also a connecting party.

direct voltage control (HVDC)

The direct voltage control (DVC) can operate in three different modes:  angle control, voltage control, and on-load control.

direct-on-line starting

The process of starting a motor by connecting it directly to the supply at rated voltage.


Prevent the functioning e.g. of a protection and/or control system.

discharge current (arresters)

The surge current which flows through the arrester.

discharge resistor

The non-linear discharge resistor connected to a spark (series) gap.

discharged (electrical)

Short circuited and/or connected to earth in such a manner as to ensure an effective dissipation of stored electrical energy.


A switch that, when in the open position, provides an isolating distance in accordance with specified requirements.

discrepancy switch

A control switch with an acknowledgment facility, which shows any discrepancy between the actual and the indicated state of the controlled equipment.


Co-ordination of the operating characteristics of protective devices to enable the fault to be identified and the minimum equipment removed from service.


The process of :pre-dispatch scheduling to allocate active and reactive power generation including additional ancillary services and reserve to match expected demand, within the limitations of the grid and equipment; rescheduling to meet forecast demand; and issuing instructions based on the schedule and the real-time conditions to manage resources to meet the actual demand.

dissimilar unit

A unit within a power transformer bank which is of compatible voltage ratio to the remaining 2 units but has a dissimilar MVA rating and/or impedance.

dissolved gas analysis (DGA)

A procedure for testing insulating oil for gases dissolved in the oil.

distance protection

A protection scheme which measures the impedance to a fault on a transmission circuit.  If the fault is within the measured zone (e.g. H1, H2) the relays will operate and remove the circuit from service.

distressed circuit-breaker

A defective circuit-breaker  that has deteriorated to a condition where it is a hazard to personnel and/or  equipment.

distribution (of electricity)

The transfer of electricity between the grid and end users through a local network.

distribution line

An electric line which is part of a local network.

distribution substation

The substation in a local network which provides the voltage level supplied to end users.  It can be an industrial substation, a building substation, kiosk substation or pole-mounted substation.

distribution switchboard

A switchboard that has no link between neutral and earth.

district substation

A major substation in a local network.  It is connected to a point of supply and in turn supplies distribution substations, or other substations which in turn supply distribution substations. Also known as a zone substation.

disturbance recorder

A device that continuously monitors power system electrical analogue and/or digital information.  In the event of a disturbance this information is recorded/stored and is available for subsequent analysis.


Drawing Management and Distribution System.

DMO (lines)

Digital micro ohmmeter for the measurement of joint resistances.


Dispatch Management Tool. Software used to send dispatch instructions to generating companies.

do not operate (DNO) notice

A form of tag consisting of a standardised label for attaching to equipment (or to its control mechanism, valves, switches, fuses etc.) to warn against operating that equipment.

Doble test

A test of ac dielectric loss and power factor, carried out using a Doble type portable insulation test set.


An entity or unit of information compiled of hard copy page/s.

double busbar

A busbar arrangement  in which transmission circuits and other HV equipment are able to be connected to either one of two busbars.

double circuit line

A transmission line carrying two circuits.

double-bit indication (SCADA)

Monitored information represented by two bits characterising two determined states and two indeterminate states of operational equipment.

drainage sump

The holding tank for oil spills and storm water drainage from the bunded areas.

drawing set

A group of drawings with the same basic subject and drawing number.  Drawings within a set are distinguished by a sequence number.

dress mimic (verb)

To attach or remove symbols, earth tags, adjust semaphore indicators, etc so that a mimic diagram (on panel or SCADA display) portrays the status of the equipment with the required warnings.


A governor/ system characteristic, usually adjustable, which matches the change in load to changes in machine speed/system frequency.

drop load test

A test involving the sudden removal of load from a generator to measure speed rise and to test the operation of overspeed and other protective devices.

drop-out fuse

A fuse in which the fuse-carrier automatically drops into a position which provides an isolating distance after the fuse has operated.


A short length of conductor connecting an HV bus (or other item of equipment) and an item of equipment situated below that bus etc.

dry tighten

Precompressing of new unoiled windings during and after dryout.

drying out

A term used in TP.MS 20.03 to include drying out, oil impregnation and oily tighten.


Demand-side participation.  Consumers reducing load or load shifting in response to pricing signals (same as Demand response).

dual battery system

A station d.c. power supply with two batteries and associated battery chargers paralleled to the load through steering diodes.

ductor test

A dc milli-ohm resistance measuring test, normally used to measure circuit-breaker and tap-changer contact resistance.  The test is carried out by injecting a high current and measuring the voltage across the device.  (Ductor is a proprietary name.  Other makes of test set are also used.).

duplicate protection

A protection scheme for a plant item such that any fault on the plant item can be cleared by two independent sets of relays, either of which is able to operate correctly even if the other fails completely.

duty cycle

The specified output of voltage or current to be achieved by the battery for a given period followed by a charge current to restore the batttery charge to an acceptable level.

dynamic node

A communications switching device which interconnects digital channels between radio and optical fibre links, RTUs and PABXs.  The routing of digital channels through the dynamic nodes on the Transpower communications network is controlled by the service centre.