IEEE 21-1976 pdf free.IEEE General Requirements and Test Procedure for Outdoor Apparatus Bushings.
1.1 Scope. This standard applies to outdoorpower class apparatus bushings which havebasic impulse insulation levels of 110 kv andabove for use as components of oil-filled trans-formers，oilfilled reactors，and oil circuitbreakers.
This standard does not apply to the follow-ing:
(1) High-voltage-cable terminations (pot-heads)
( 2) Bushings for instrument transformers(3) Bushings for test transformers
(4) Bushings in which the major insulation isprovided by a gas
(5) Bushings applied with gaseous insulation(other than air at atmospheric pressure) external to tne bushing
(6) Bushingsfor distribution class circuitbreakers and transformers
( 7) Bushings for automatic circuit reclosersand line sectionalizers
(8) Bushingsfor oilless and oil-poor apparatus
1.2 Purpose.This standard defines the specialterms used，service conditions，rating，generalrequirements，and test procedure. See IEEEStd 24-1976，Electrical，Dimensional,and Related Requirements for Outdoor ApparatusBushings (ANSI C76.2-1976)，for specific listings of values of electrical and mechanical characteristics，dimensions，and other related testand quality control requirements which are described in this standard.
For terms not specifically defined in this standard see IEEE Std 10o-1972，Standard Dictionary of Electrical and Electronics Terms(ANSi c42.100-1972).
ambient temperature.The temperature of thesurrounding air that comes in contact with thedevice or equipment in which the bushing ismounted.
basic impulse insulation level (BIL).A referenceinsulation level expressed as the impulse crestvoltage of the 1.2 × 50 us wave which thebushing will withstand when tested in accord-ance with specified conditions.
bushing. An insulating structure，including athrough conductor or providing a central passage for such a conductor，with provision for mounting on a barrier，conducting or otherwise,for the purpose of insulating the conduc-tor from the barrier and conducting currentfrom one side of the barrier to the other.bushing potential tap.An insulated connectionto one of the conducting layers of a bushingproviding a capacitance voltage divider to indicate the voltage applied to the bushing.
bushing test tap. An insulated connection toone of the conducting layers of a bushing forthe purpose of making insulation power factortests.
capacitance (of bushing). ( 1)The main capacitance，C, of a condenser bushing is the valuein picofarads between the highvoltage conductor and the potential tap or the test tap. (2)The tap capacitance,C2 , of a condenser bush-ing is the value in picrofarads between the potential tap and mounting flange (ground). ( 3)The capacitance，C，of a bushing without apotential or test tap is the value in picofaradsbetween the high-voltage conductor and themounting flange (ground).IEEE 21 pdf download.