Annex B Safety Knowledge
59
The arc is actually a gas discharge phenomenon, the spark generated as the current flowing
through some insulating medium (e.g., air) with wave as shown in B-4-1. The part circled in red in
Figure B-4-1 (c) is several arc current waves. Factors affecting the arc include voltage, distance,
current, material contamination level and so on.
(a) Wave without arc (b) Wave with arc (c) Arc expanding wave
Figure B-4-1 Arc wave
ACW/DCW
ACW/DCW is a measure to detect and measure the anti-electrical properties of the
electrical equipment, insulation materials and insulation structures, assessment of the
electrical strength. The electrical strength refers to insulation resistance (withstand capacity)
of LV electrical equipment, insulation materials and insulation structures. During ACW/DCW,
apply the specified frequency test voltage onto the insulator to assess its anti-electrical
properties; the purpose of the test is:
Measure the insulation or repair quality of electrical equipment;
Detect damage to the insulation due to raw materials, processing or transportation, reducing
early failure rate of the product;
Measure the insulation electrical clearances and creepage distances;
Measure the dielectric strength of insulation materials.
The standard for high dielectric strength is that there is no abnormal sound during the
ACW/DCW, nor flashover or breakdown. Currently, the valid current (50Hz or 60Hz)
through the insulator measured during ACW/DCW is adopted to determine whether the
insulation properties meet the requirements; This method can effectively detect the change to
insulator current or direct breakdown defects, but difficult to detect arcing arising from
inadequate clearances and creepage distances, discharge or spark discharge and other defects,
producing 3k ~ 250k Hz pulse current, which cannot be recognized during current RMS
measurement. Therefore arc detection is necessary to detect these insulation defects.
B.5 Definition of Discharge, Arc, Flashover and Breakdown
B.5.1 Gas Discharge
Electric conduction in ionized gases. (GB/T 2900.1-2008)
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