Insulation Co-Ordination - Part 1: Definitions, Principles and Rules

This part of International Standard IEC 71 applies to three- phase a.c. systems having a highest voltage for equipment above 1 kV. It specifies the procedures for the selection of the standard withstand voltages for the phase-to-earth, phase-to-phase and longitudinal insulation of the equipment and the installations of these systems. It also gives the lists of the standardized values from which the standard withstand voltages shall be selected.

This part recommends that the selected withstand voltages should be associated with the highest voltage for equipment. This association is for insulation co-ordination purposes only. The requirements for human safety are not covered by this Standard.

Although the principles of this part also apply to transmission line insulation, the values of the withstand voltages may be different from the standard withstand voltages.

The apparatus committees are responsible for specifying the withstand voltages and the test procedures suitable for the relevant equipment taking into consideration the recommendations of this Standard.

NOTE - In IEC 71-2 Application Guide, (under revision), all rules for insulation co-ordination given in this Standard are justified in detail, in particular the association of the standard withstand voltages with the highest voltage for equipment. When more than one set of standard withstand voltages is associated with the same highest voltage for equipment, guidance is provided for the selection of the most suitable set.

1A.1 Power transformers and reactors
This Clause applies to all liquid-filled power transformers, auto-tr ansformers, booster transformers, load-tap-changing transformers, series-voltage regulators, grounding transformers, phase-shifting transformers, and shunt reactors. This Clause deals with the rated insulation levels and dielectric test prescriptions for this equipment.

Dielectric tests for equipment covered by this Clause are generally made in accordance with the requirements of CSA Standard CAN/CSA-C88. Impulse tests, unless otherwise noted, shall precede the low-frequency tests. While all impulse tests are standard impulse withstand tests (as defined in Clause 5.3 of this Standard), the number of impulses is not given, nor, except as noted above, is the order in which the tests are to be carried out. For these and other details the reader should refer to CSA Standard CAN/CSA-C88.

1A.2 Distribution transformers
This Clause applies to all liquid-filled distribution transformers, types ONAN and LNAN, rated at 1000 kV·A and below for single-phase and 3000 kV·A and below for three-phase, with 36 kV highest voltage for equipment and below. This Clause deals with rated insulation levels and dielectric test prescriptions for this equipment, and addresses the requirements of CSA Standard CAN/CSA-C2. Dielectric tests for equipment covered by this Clause are generally made in accordance with the requirements of the latter Standard. This Clause also provides the dielectric requirements for self-protected transformers.

1A.3 Dry-type transformers
This Clause applies to single-phase and three-phase dry-type transformers with highest voltage for equipment from 1 to 46 kV. This Clause deals with rated insulation levels and dielectric test prescriptions for this equipment, and is based on CSA Standard C9.

1A.4 Capacitor voltage transformer
This Clause applies to all capacitor voltage transformers intended to be connected between line and earth, composed essentially of a capacitor divider and an electromagnetic device. The capacitor voltage transformer may have provision for power line carrier coupling. This Clause will also be valid for capacitor dividers used as coupling capacitors. It does not apply to field-adjusted capacitance potential devices, including bushing potential devices and out-of-phase potential devices.

This Clause deals with rated insulation levels and dielectric test prescriptions for this equipment and addresses the requirements of CSA Standard CAN3-C13.1.

1A.5 Instrument transformers
This Clause applies to all instrument transformers comprising elec tromagnetic voltage transformers and current transformers. This Clause deals with rated insulation levels and dielectric test prescriptions for this equipment, and addresses the requirements of CSA Standard CAN3-C13.

1A.6 Power circuit breakers
This Clause specifies the rated insulation levels and dielectric tests for single-phase and three-phase power circuit breakers with highest voltage for equipment in Ranges I and II. The test requirements are generally in accordance with those of IEC 60056 and IEC 60694. Where ANSI C37 test requirements differ, these differences are noted.

1A.7 Disconnectors
This Clause specifies the rated insulation levels and dielectric tests for single-phase and three-phase disconnectors with highest voltage for equipment in Ranges I and II. The test requirements are generally in accordance with those of IEC 60129 and IEC 60694. Where ANSI C37.34 test requirements differ, these differences are noted.

1A.8 Power transformer and reactor bushings
This Clause applies to outdoor bushings having lightning-impulse withstand voltages from 110 to 1950 kV installed as components of liquid-filled transformers and reactors installed on systems with Um from 17.5 to 765 kV.

This Clause deals with rated insulation levels and dielectric test prescriptions for this equipment and addresses the requirements of CSA Standard CAN/CSA-C88.1.