Glossary terms beginning with I

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

lnternational Accreditation New Zealand.


The luminous flux incident on unit area of a surface (lux).

important load

An important load in the context of this policy document would be a switchboard supplying a load which may be industrial, commerical, residential or a combination of all three but where the consequences of a loss of supply has the potential to cause major disruption or incur high costs.

impressed current

The current supplied by rectifier or other direct current source (specifically excluding a sacrificial anode) to a protected structure in order to attain the necessary protection potential.

impulse (voltage or current)

A unidirectional wave which rises rapidly to a maximum and falls a little less rapidly, to zero. Its wave shape is expressed by two numbers (T1/T2). It refers to the virtual front -time and T2 to the virtual time to half-value of the tail; both expressed in microseconds, eg lightning current impulse 8/20 µs.

impulse sparkover voltage (arresters)

The highest value of voltage attained during an impulse of given wave shape and polarity applied between the line and earth terminals of an arrester with series gaps, prior to the flow of discharge current.

in service

The state of equipment which is connected to a source of energy or may be connected to a source of energy by an operating action.

in-service inspector

A person who carries out inspections - refer to AS/NZS 3788. Note: Inspections are performed "out of service".


An unplanned event resulting in, or having the potential for injury, illness, damage, transmission system loss, asset related outage, interruption to supply, or other loss.

incoming machine

A generator or synchronous condenser that is being prepared for paralleling to a running machine or to the power system.

incoming voltage

The voltage at the terminals of an incoming machine.

indirect causes

Hidden or background causes, on or off site, including personal, job and management factors which give rise to the direct causes.

indirect or intermediate earths

Conductive running blocks rigged on structures during conductor stringing or destringing operations.

individual locking

A system of security padlocks and keys where each disconnector and earth switch is fitted with a lock having its own individual (exclusive) key. (See also lockout box.).

indoor switchgear

This may be metalclad withdrawable air insulated, or non-withdrawable for isolation purposes.

induction (electrical)

The phenomenon causing voltage to be present in a conductor (line, bus, etc.) due to the influence of an adjacent energised conductor. Induction can produce very high voltages. This includes both electromagnetic and electro-static effects.

industry procedures

Procedures which are recognised by the industry generally, or by a particular sector of the industry, or by an organisation within the industry, including contractors working for the industry, as being the recommended method of achieving the required outcome, in a way that avoids harm to any employee(s) and to the public. [SR -EI April 1995].

industry sector

A part of the electricity industry comprised of organisations carrying out a similar function within the industry, for example, generation companies, energy retailers, line businesses.

information assets

Includes documents, files and information stored within fileshares, databases, applications systems and services used to create, access, store and transmit this information. Also includes any other representation of this information regardless of medium (e.g. paper, diskette, CD-ROM, and magnetic tape).

infrared radiation

Electromagnetic radiation whose wavelengths lie in the range from 0.8 micrometer (the long wavelength of visible red light) to 1000 micrometers (the shortest microwaves).

infrared spectrum

The range of wavelengths of infrared radiation.

inhibited (insulating oil)

A mineral insulating oil which contains an anti-oxidant.

inrush current

The transient current which flows into a power transformer upon energising.

inspection body

An inspection body accredited under EN45004 and recognised by the Secretary of Labour in the terms of the PECPR regulations.

instantaneous load

The maximum instantaneous current drawn. It consists of continuous, non-continuous and momentary currents.

instantaneous protection

Protection not having an intentional delay.

instantaneous reserve

The reserve which is able to counter a contingent event or extended contingent event, the response being fast enough either to limit the fall in frequency to within the limits set out in section 2 of the Power Quality Code, or assist in the recovery of frequency to within the limits set out in section 2 of the Power Quality Code. Note: There are two types of instantaneous reserve: Fast instantaneous reserve, being fast enough to limit the fall in frequency; and Sustained instantaneous reserve, being able to assist in the recovery of frequency to within the limits set out in section 2 of the Power Quality Code.

instrument transformer

A term used in TP.GS 22.01 to include current transformers (CT's), voltage transformers (VT's), capacity voltage transformers (CVT's) and HVDC resistive voltage dividers (RVD's).

insulating base (surge arrester)

An insulated base that is installed between the base of the surge arrester and the earthed stand for the use of a surge counter.

insulation co-ordination

The process of correlating the insulation strengths of electrical equipment with expected voltages and with the characteristics of surge protective devices.

insulation level

An insulation strength expressed in terms of a withstand voltage.

insulation resistance test

A test for measuring the resistance of insulation under specified conditions. See megger.

insulator fitting

Any component of a suspension or tension insulator set other than an insulator unit or preformed fitting.

insulator set

An assembly of one or more insulator strings connected together by insulator fittings, complete with fixing and protective devices as required in service.

insulator string

One of more insulator units coupled together by their ball and socket connections.

insurance spare transformer

A transformer that is being held as a dedicated spare for one or more transformers.

intermittent fault

A fault which recurs at apparently random intervals, in the same place, and due to the same cause.

interrupter (circuit-breaker)

A device that contains the main contacts and arcing contacts and also the arc quenching devices.

interrupter bottles

Fixed and moving contacts contained within a sealed ceramic evacuated (vacuum), or SF6 filled bottle. The interrupter bottles are permanently sealed and are not designed to be maintained or repaired.

interrupter unit

A discrete device that contains the main contacts and arcing contacts and also the arc quenching devices. The device incorporates its support structure which may be a porcelain rainshed as on outdoor circuit-breakers, an insulating surround as in bulk oil and GIS circuit-breakers or a sealed metal enclosed bottle as in indoor metal-clad switchgear.

interruptible demand

Demand which, by commercial arrangement between the grid operator and a provider, may be disconnected without prior warning for the purposes of grid security.


A protection signalling system whereby a signal initiated at one station trips a circuit-breaker at another station.

inverse-time overcurrent relay

A time-dependent relay in which the time taken to operate is inversely proportional to the overcurrent.


An independent qualified person as defined in Section 44(9) of the Building Act 1991.

iron loss

The rate of energy conversion in to heat in the core of a transformer when subjected to an alternating magnetic field.

islanded operation

The condition which arises when a section of the power system is disconnected from and operating independently of the remainder of the power system. [GOSP uses the IEC term "isolated operation"].

isolate (verb)

To deliberately disconnect primary or secondary equipment. Examples of this are: opening a disconnector, removal of VT secondary fuses.


Deliberately disconnected from external sources of harm, e.g. energy or asphyxiating, toxic or flammable gas, and rendered incapable of being reconnected without predetermined deliberate action, e.g. opening of disconnectors, closure of valves. Note: Despite equipment being isolated, it may still contain hazards, eg induced voltage, stored energy, toxic gas. [SR-EI April 1995].


Construction designed to avoid contact between electrically conducting materials and earthing conductors.

isolation device

A device that prevents the transfer of hazardous voltages between Transpower's equipment (including cabling), and a telecommunication network operated by a carrier.


Information Systems Strategic Plan.

issuer (authorisations)

An employee who issues, modifies, receives back, transfers, or cancels authorisations and who, for the purpose of carrying out the operating for such authorisations at specific stations, or in a specific area, or on specific equipment, is: > deemed competent by their employer in terms of the asset owner’s standards; > authorised or appointed by the asset owner. Note: The "issuer" who for example, issues an authorisation may not be the same employee as the "issuer" who for example, receives it back, e.g. where a control centre is staffed on a rotational shift. [SR-EI April 1995].


A protection signalling system whereby a signal initiated at one station trips a circuit-breaker at another station.