BS EN ISO 2746:2015 pdf free download Vitreous and porcelain enamels — High voltage test
1 Scope
This International Standard describes two test methods of high voltage testing:- Test A is used to detect and locate defects in vitreous and porcelain enamels;
一Test B is used to detect and locate defects and weak spots in vitreous and porcelain enamels.The tests are performed using DC or pulsed DC high voltage.
The tests are applicable to dry surfaces of enamel coatings.In the case of moist surfaces, care should betaken to ensure that the locating of any defects is correctly performed.
Since test voltages depend on the coating thickness, the test method, especially with test A, may not besuitable for test specimens for which the coating thickness varies to a large extent.
2Normative references
The following documents, in whole or in part, are normatively referenced in this document and areindispensable for its application. For dated references,only the edition cited applies. For undatedreferences, the latest edition of the referenced document (including any amendments) applies.
ISO 2178,Non-magnetic coatings on magnetic substrates — Measurement of coating thickness 一Magnetic method
IS02360,Non-conductive coatings on non-magnetic electrically conductive basis materials—Measurementof coating thickness —Amplitude-sensitive eddy-current method
IEC/TS 60479-1,Effects of current on human beings and livestock — Part 1:General aspectsIEC/TS 60479-2,Effects of current on human beings and livestock — Part 2: Special aspects
3Terms and definitions
For the purposes of this document, the following terms and definitions apply.3.1
defect
area of an enamel layer where an open path connects the enamel surface with the metal basisNote 1 to entry: Examples of defects are cracks or pores.
3.2
weak spot
area of an enamel layer where the dielectric strength falls below the required value, as determined bythe application of high voltage because of blisters, foreign body inclusions, spalling or cracks
4Principle
A high voltage electrode is passed over the enamel surface. Defects (Test A) or defects and weak spots(Test B) are indicated by a spark and a simultaneous optical and/or acoustic signal.The applied voltagecan be DC or pulsed DC. The test voltage might be different in each case.
At distances smaller than 2 cm of the edges/borders of the enamelled parts, an electrical arc can occurbetween the test electrode and edges/borders with no or only a limited enamel coating thickness.Thehigh voltage technique may,therefore, not be applicable to such border areas.
5Apparatus
5.1 High voltage generator.
5.1.1 Direct-current (DC)-voltage generator, conforming to the requirements of lEC/TS 60479-1,able to deliver a DC-voltage corresponding to the testing voltage, adjustable and measurable at the testprobe with limiting deviation +5 %/-10 %. The total internal resistance shall be high enough to give theshort circuit current of the generator an arithmetical mean from 2 mA to 3 mA maximum.The peak valueof the current during a spark discharge shall be between 10 mA and 50 mA and the amount of charge perimpulse shall be 25 uC maximum.
The negative pole of the generator shall be earthed (USA: grounded) and the positive pole shall beconnected to the test electrode by a screened high voltage cable of suitable length.
5.1.2 Pulsed DC-voltage generator,conforming to the requirements of IEC/TS 60479-2 able todeliver a DC-voltage corresponding to the testing voltage.The voltage shall be adjustable and measurableat the test probe with limiting deviations +20 %/-10 % for test voltage greater than 10 kV and limitingdeviations +40 %/-10 % for test voltages less than 10 kV.
5.2Testelectrode, made out of metal wire or conductive rubber that is unaffected by a spark discharge.NOTE Alternative test electrodes can be used providing they are unaffected by a spark discharge.
5.2.1 Insulated hand-piece, provided externally with an earthed cover for DC-voltage and pulsedDC-voltage devices. Pulsed DC-voltage generators can produce an electric shock to users because ofcapacitive coupling between cable and hand-piece during operation.
5.2.2 Brush holder, of metal wire (used for the test brush), constructed such that it is completelyunaffected by the spark discharge and covers as large an area as possible when sweeping the enamelsurface.
5.3 Indicator device, able to give a clear optical and/or acoustic signal at each spark discharge.
5.4Coating thickness measurement device, such as that described in IS0 2178 or ISO 2360.