IEC 61621:1997 pdf download – Dry,solid insulating materials -Resistance test to high-voltage,low-current arc discharges

IEC 61621:1997 pdf download – Dry,solid insulating materials -Resistance test to high-voltage,low-current arc discharges.
IEC 61621 describes a test method which can provide preliminary differentiation between similar insulating materials, with respect to their resistance to damage when exposed to high-voltage, low-current arc discharges, occurring close to their surfaces.
The discharges cause localized thermal and chemical decomposition and erosion and eventually a conductive path forms across the insulating material. The severity of the test conditions is gradually increased: in the early stages a low-current arc discharge is repeatedly interrupted, whereas in the later stages. the arc current is raised in successive steps.
Because of its convenience and because of the short time required for testing, the test method is applicable for preliminary screening of materials, for detecting the effects of changes in formulation and for quality control testing.
Previous experience with this test, showed acceptable reproducibility with thermoset materials. Using thermoplastics, some testing laboratories report unacceptably large variation in test results which lead to the recommendation not to use the test for thermoplastics.
NOTE — Attempts are being made to reduce the variability of the results of tests on thermoplastics by controlling the electrode pressure and depth of penetration into the material during the test. Without such electrode control. tests on many thermoplastics may not be sufficiently meaningful to be performed.
This test method will not, in general, permit conclusions to be drawn concerning the relative arc resistance rankings of materials which may be subjected to other types of arcs.
The ranking of materials may differ from that found in wet tracking tests (e.g. IEC 60112, IEC 60587 and lEG 61302) and from their performance in service, where the intensity, recurrence frequency and time of exposure to arc discharges are very different.
2 Normative references
The following normative documents contain provisions, which through reference in this text. constitute provisions of this International Standard. At the time of publication, the editions indicated were valid. All normative documents are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. Members of lEG and ISO maintain registers of currently valid International Standards.
IEC 60112: 1979, Method for determining the comparative and the proof tracking indices of solid insulating materials under moist conditions
3 Definitions
For the purpose of this International Standard, the following definitions apply:
31
fa I lure
Failure is considered to have occurred when a conducting path is formed in the material; failure is also considered to have occurred if the arc causes a material to burn and the burning continues when the arc is interrupted.
NOTE 1 — When the arc disappears into the material, the circuit current usually changes and a noticeable change in sound takes place.
NOTE 2 — For some materials, the trend towards failure increases over a fairly long Interval of time before all parts of the arc between the electrodes have disappeared. Failure Only occurs when the entire arc has disappeared.
NOTE 3 — For some materials, a persistent scintillation may be observed close to the electrodes after the arc has disappeared. This scintillation shall not be considered as part of the arc.
NOTE 4 — Burning of the malerial accompanying the arc is only considered as failure if the burning continues between arc interruptions. Otherwise the test is continued until a conducting path is formed.
NOTE 5 — The first disappearance of the entire arc Is considered as failure even if the material recovers for subsequent arcing.
3.2
arc resistance
total time in seconds from the start of test until specimen failure
4 Apparatus
4.1 Test circuit
The principal components of the electrical circuit for the apparatus are shown in figure 1.
NOTE — In the secondary circuit wiring the stray capacitance should be less than 40 pF. Large stray capacitance
may disturb the arc shape and influence the test results.
4.1.1 Transformer, T
A transformer with a rated secondary potential (on open circuit) of 15 kV, and a rated secondary current (on short circuit) of 60 mA, line frequency (48 Hz to 62 Hz).
4.1.2 Variable autotransformer, T0
Rated 1 kVA and suitable for the line voltage.
NOTE — A constant primary voltage supply ±2 % is recommended.