Testing dielectric strength measurement

Thermal Expansion. Early BNL dielectric strength measurements were made with eight to ten layers of tape helically wound on a 1.27 cm diameter stainless steel mandrel. Most nonoriented polyethylene and polypropylene tape candidates fractured during these tests, which were made at 6 to 8 K. A comparison of the 293 to... 

On sheet samples, dielectric strength measurements can be used to spot poor dispersion, particularly of carbon black. The applied potential can be AC or DC, depending on anticipated service. With AC testing, fiequency influences the values obtained. In both cases, the potential may be steadily increased until failure occurs (the rapid rise method). Alternatively, the potential may be inereased in steps per UL and ASTM protocols. Rapid rise tests generally yield higher values of dielee-tric strength. 

The efficiency of transformer oils as dielectrics is measured by electric strength tests. These give an indication of the voltage at which, under the test conditions, the oil will break down. Various national standards exist that all measure the same basic property of the oil. There is an international specification, lEC 296/1982, which may be quoted by equipment manufacturers in their oil recommendations. 

Constant, dissipation factor dielectric strength. A literature search indicated that little work of this nature has been done with expl materials. Further, such electrical measurements can be used as supplementary criteria for evaluating the purity, homogeneity and, with the exception of the dielectric strength determination, as possible nondestructive tests of expls. Hence the data listed in the following table were deed at Pic Arsn (Ref 2) for the common readily avail expls which are believed to be of interest in research... 

The measurement of dielectric strength (Figure 3.52a) is usually carried out either by the short-time method or by the step-by-step method. In the former method the voltage is increased continuously at a uniform rate (500 V/sec) until failure occurs. Typically, a 1/8-in. thick specimen requiring a voltage of about 50,000 V for dielectric failure will thus involve a testing period of 100 sec or so. 

The dielectric strength of PES, as measured in oil by the short term tests ASTM D 149, is 800 KV/em for a 0.23 mm section and 158 KV/cm for 3 2.3 mm. The tracking behaviour of PES when subjected to high voltages is very complex and this phen< nenon is currently under investigation. There are indications that electrical breakdown associated with carbonisation tends to occur across the surface of the material. However, results obtained are similar to those found for other amorphous aromatic polymers like polycarbonate. 

Dielectric strength (electric strength) n. A measure of the voltage required to puncture an insulating material, expressed in volts per mil of thickness (SI V/rrmi). The voltage is the root-mean-square voltage difference between the two electrodes in contact with opposite surfaces of the specimen at which electrical breakdown occurs under prescribed test conditions. Ku CC, Liepins R (1987) Electrical properties of polymers. Hanser Publishers, New York. Weast RC (ed) (1971) Handbook of chemistry and physics, 52nd edn. The Chemical Rubber Co., Boca Raton, FL. 

Dielectric breakdown is a measurement of breakdown voltage which is made parallel rather than perpendicular to the plane of a sample, as illustrated in Fig. 9.7(b). In the standard test (D149) the electrodes are taper pins inserted into the sample on 1 in centres. The voltage is applied at a controlled rate of increase with the sample immersed in oil. Dielectric strength depends on both surface and bulk characteristics of a material and is, of course, reduced by increases in humidity, water absorption and ionic or other conducting contaminants. Typical values for polymeric material are in the range 10-50 kV. 

Similar to the measurements with non-exposed specimens, the unfilled material exhibits higher electrical strength over the entire measurement period, both in the ramp and in the step test. While pure resin exhibits lower dielectric strength and dielectric withstanding values with increasing moisture concentration in the early time range, no moisture influence could be detected in the quartz powder-lilled epoxy resin compound after more than 5 years in tropical climate despite saturation. 

A conductor has a resistivity on the order of p < 10, whereas an insulator has a p > 10. Another electrical property is the dielectric strength, which is a measure of when the structure of an insulator breaks down. In the ASTM test D149, this property is measured by placement of a thin specimen between two steel plates and application of a voltage across the thickness of the specimen. 

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