The Capacitive Discharge Test:

A primer; tolerance facts and some examples

San Diego, CA 2016-12-13

Jeff Lind - President Compliance West USA

Introduction to the Capacitive Discharge Test

Evaluates the safety of cord- connected products when they are unplugged

Conducted at worst case (standby or other mode where input power is lowest)

The test is specified in IEC 1010, IEC 60065, IEC 60335, IEC 60601, IEC 60950, and IEC 62368.

These Standards specify a maximum voltage after a period of time, or a time at which the residual voltage must be below a certain value.

All tests are conducted to a voltage or to a time, but the requirements differ from Standard to Standard.

In some Standards, no testing is required if the discharging capacitor is less than a given value.

Testing is conducted between L-G, L-N and N-G.

A look at the Standards Requirements

Following are excerpts from Standards, describing the individual requirements. (Standards shown may not be the latest version.)

These excerpts are not comprehensive. Many standards have exceptions if the capacitance value is below a given threshold; and some have additional requirements. This list covers testing parameters for empirical results; a complete discussion of the individual requirements in their entirety is beyond the scope of this discussion.

Tl;dr - Review the Standard requirements completely before testing.

IEC 60065 A/V ApparatusPara. 9.6 and Para 9.1.1.1

IEC 61010 Laboratory Equipment(Plug Caps) Para. 6.10.3 and Para

6.3.1

IEC 61010 Laboratory Equipment(External Circuits and Exceptions) Para. 6.1.2, 6.6.2 and Para 6.3.1

IEC 60601-1 Medical EquipmentPara. 8.4.3

IEC 60335 Household EquipmentPara. 22.5

IEC 60950 ITE EquipmentPara. 2.1.1.7

IEC 62368 Para. 5.5.2.3

Capacitive Discharge Test:Governing Documents

As we have just seen, end-use Standards for the Capacitive Discharge Test areIEC 60065, IEC60601-1, IEC 61010, IEC 60335, IEC 60950 and IEC 62368.

Absent tolerances in the end-use Standards, the Capacitance Discharge Tests are subject to the voltage and time accuracy tolerance requirements of OD-5014.

Input Impedance Specifications:

IEC 60601-1 and IEC 60950 specify an input impedance of > 100 MΩ , < 20±5pF. All other Standards are also required to meet this requirement in accordance with CTL Decision Sheet DSH 716.

Input Impedance requirement per CTL DSH 716

OD-5014 Time and Voltage Tolerances

Oscilloscope Accuracy

Oscilloscope Bandwidth specificationOther amplitude accuracy

specification

Keysight DSO6054A Bandwidth (-3 dB)DC-500 MHz

DC vertical gain accuracy±2.0% of full scale

Tektronix DPO4054 Analog Bandwidth (-3 dB)500 MHz

DC Gain Accuracy±1.5% with offset set to 0 V

LeCroyWaveRunner 6050A

Bandwidth500 MHz

DC Accuracy±1.5% of full scale, ≥ 10 mV/div

Yokogawa DL9140 Bandwidth500 MHz

DC Accuracy±(1.5% of 8 div + offset voltage accuracy)

Fluke 199C Bandwidth200 MHz

Accuracy±(1.5% of reading + 0.04 * range/div)

Vertical gain accuracy for some oscilloscopes is shown,and these results are typical.

As shown here, the DC vertical gain accuracy closely aligns with CTL DSH 251e requirements.

Only two of these scopes are in strict accordance with CTL DSH 251e.

Voltage Measurement Error Comparison

This chart shows the OD-5014 Guard Band in red. Blue shows what a ±2% error guard band would look like.

The test is conducted at a large range (800Vp-p) to capture the mains sinusoid, so the allowed errors can be large because of the small test voltage resulting during this test (34-126V).

Because oscilloscope accuracy mimics OD-5014 tolerances, oscilloscopes can deliver inaccurate results, but still in accordance with OD-5014 tolerances.

Voltage Error comparison

Because of the large voltage swings of the mains sinusoid, the smaller pass/fail points of the Capacitive Discharge Test cause larger errors when the OD-5014tolerances of ±1.5% of range are used.

This graph shows the result of ±1.5% of a ±400V scale on the smaller Capacitive Discharge Test voltages. These results are within the accuracy requirement for the test under OD-5014 tolerance requirements.

Because of these tolerance issues, use a scope with caution.

Real world Capacitance Discharge Testing

Jeff Lind - President Compliance West USA 650 Gateway Center Way, San Diego, CA 92102 (619) 878-9696 Switchboard (619) 878-9687 Direct