BS EN ISO 2746:2015 pdf download – Vitreous and porcelain enamels — High voltage test
4 Principle 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 voltage can 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 occur between the test electrode and edges/borders with no or only a limited enamel coating thickness. The high voltage technique may, therefore, not be applicable to such border areas.
5.1 High voltage generator.
5.1.1 Direct-current (DC)-voltage generator, conforming to the requirements of IEC/TS 60479-1, able to deliver a DC-voltage corresponding to the testing voltage, adjustable and measurable at the test probe with limiting deviation +5 %/-10 %. The total internal resistance shall be high enough to give the short circuit current of the generator an arithmetical mean from 2 mA to 3 mA maximum. The peak value of the current during a spark discharge shall be between 10 mA and 50 mA and the amount of charge per impulse shall be 25 μC maximum. The negative pole of the generator shall be earthed (USA: grounded) and the positive pole shall be connected 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 to deliver a DC-voltage corresponding to the testing voltage. The voltage shall be adjustable and measurable at the test probe with limiting deviations +20 %/-10 % for test voltage greater than 10 kV and limiting deviations +40 %/-10 % for test voltages less than 10 kV.
5.2 Test electrode, 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 pulsed DC-voltage devices. Pulsed DC-voltage generators can produce an electric shock to users because of capacitive 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 completely unaffected by the spark discharge and covers as large an area as possible when sweeping the enamel surface. 5.3 Indicator device, able to give a clear optical and/or acoustic signal at each spark discharge.
5.4 Coating thickness measurement device, such as that described in ISO 2178 or ISO 2360. 6 Specimens The specimens can be commercial items, parts thereof or test pieces which have been subjected to the same processing, and which for testing purposes are representative of the commercial item.
No special preparation of specimens is required.
7 Test voltage
7.1 Test A: Detection of defects (3.1) which extend down to the metal basis (e.g. open pores)
For a correct determination of these defects, the appropriate test voltage shall be used. This voltage depends on the length of the defect gap, which corresponds to the thickness of the enamel layer. Too low a voltage will not result in the determination of all defects. Too high a voltage will result in a breakdown of thin residual enamel layers (destructive testing). In order to look for defects which extend down to the metal base (e.g. open pores), the test voltage shall be as follows:
— DC-voltages shall be set to the values given in Table 1.
Other voltages may be chosen by mutual agreement between interested parties.
— voltage of pulsed DC-voltage generators shall be agreed between the interested parties.
Table 1 shows the minimum voltage needed to arc through a defect which is open to both the surface and the metal substrate. Note that the dielectric breakdown of air is based on measurement at 23 °C and 60 % maximum relative humidity.BS EN ISO 2746 pdf download.