Capacitor Testing Procedures

All capacitors are tested for dielectric strength on a 100% basis prior to shipment. Upon customer request, certified test reports may be made available.

Dielectric strength is tested by using a low current, high potential source of 60Hz voltage.

Capacitors for applications involving applied DC voltage should be tested on a DC dielectric strength meter for voltage and emission current. Jennings will test capacitors to this measure if specified by the customer.

Most users will find the 60 Hz dielectric strength test adequate and relatively inexpensive. Jennings does not recommend DC testing being performed by the user because of safety considerations. If DC testing is performed, care should be taken not to exceed 60% of the peak test voltage rating of the capacitor.

Testing Procedure-Apply 60 Hz current-limited voltage across the capacitor. One side may be grounded if desired.

Increase voltage gradually. The rate of increase should be from zero to maximum voltage in one minute. Normal test procedure requires that the capacitor be able to withstand the full rated voltage without barnacles occurring after the first minute at the test voltage. A barnacle is a self-healing, non-sustained, momentary breakdown. Weak barnacles that instantly heal are disregarded during the first minute. Under no condition should the test voltage be exceeded.

A slight white or blue fluorescence in glass units is normal in room light. This type of fluorescence is not harmful, nor unusual, as various materials used in the capacitors may tend to fluoresce under high electric fields (such as uranium glass). Capacitors that have not been used for a long time may initially fluoresce.

Arc Detectors-In extremely critical circuit applications, some form of arc detection is necessary. An oscilloscope may be connected across the capacitor to show weak electron discharges, waveform distortion, evidence of strong DC emission currents, or corona. The Jennings type Model J-1005 portable RF voltmeter is ideal for arc detection and voltage measurement of this nature, since it has a voltage range of 0 to 50 kV and oscilloscope connections with a step- down ratio; of approximately 600 to 1. The voltmeter has a frequency response of 60 Hz to 30 MHz at full rated voltage with an accuracy of 3% of full scale.

Another form of arc detection may be improvised by using a small neon lamp with one terminal connected to the hot side of the capacitor under test and the other terminal of the lamp floating. The lamp will flash when the capacitor arcs. A contact microphone and audio amplifier connected to the base of the stand-off insulator supporting the capacitor under test makes a very sensitive arc detector.

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