6. Apparatus

6.1 Voltage Source—Obtain the test voltage from a step-up transformer supplied from a variable sinusoidal low-voltage source. The transformer, its voltage source, and the associated controls shall have the following capabilities.

6.1.1 The ratio of crest to root-mean-square （rms） test voltage shall be equal to =2 6 5 % （1.34 to 1.48）, with the test specimen in the circuit, at all voltages greater than 50 % of the breakdown voltage.

 6.1.3 The controls on the variable low-voltage source shall be capable of varying the supply voltage and the resultant test voltage smoothly, uniformly, and without overshoots or transients, in accordance with 12.2. Do not allow the peak voltage to exceed 1.48 times the indicated rms test voltage under any circumstance. Motor-driven controls are preferable for making short-time （see 12.2.1） or slow-rate-of-rise （see 12.2.3） tests.

6.1.4 Equip the voltage source with a circuit-breaking device that will operate within three cycles. The device shall disconnect the voltage-source equipment from the power service and protect it from overload as a result of specimen breakdown causing an overload of the testing apparatus. If prolonged current follows breakdown it will result in unnecessary burning of the test specimens, pitting of the electrodes, and contamination of any liquid surrounding medium.

6.1.5 It is important for the circuit-breaking device to have an adjustable current-sensing element in the step-up transformer secondary, to allow for adjustment consistent with the specimen characteristics and arranged to sense specimen current. Set the sensing element to respond to a current that is indicative of specimen breakdown as defined in 12.3.

6.1.6 The current setting is likely to have a significant effect on the test results. Make the setting high enough that transients, such as partial discharges, will not trip the breaker but not so high that excessive burning of the specimen, with resultant electrode damage, will occur on breakdown. The optimum current setting is not the same for all specimens and depending upon the intended use of the material and the purpose of the test, it is often desirable to make tests on a given sample at more than one current setting. The electrode area is likely to have a significant effect upon the choice of current setting.

6.1.7 It is possible that the specimen current-sensing element will be in the primary of the step-up transformer. Calibrate the current-sensing dial in terms of specimen current.

6.1.8 Exercise care in setting the response of the current control. If the control is set too high, the circuit will not respond when breakdown occurs; if set too low, it is possible that it will respond to leakage currents, capacitive currents, or partial discharge （corona） currents or, when the sensing element is located in the primary, to the step-up transformer magnetizing current.

6.2 Voltage Measurement—A voltmeter must be provided for measuring the rms test voltage. If a peak-reading voltmeter is used, divide the reading by =2 to get rms values. The overall error of the voltage-measuring circuit shall not exceed 5 % of the measured value. In addition, the response time of the voltmeter shall be such that its time lag will not be greater than 1 % of full scale at any rate-of-rise used.

6.2.1 Measure the voltage using a voltmeter or potential transformer connected to the specimen electrodes, or to a separate voltmeter winding, on the test transformer, that is unaffected by the step-up transformer loading.

6.2.2 It is desirable for the reading of the maximum applied test voltage to be retained on the voltmeter after breakdown so that the breakdown voltage can be accuray read and recorded.