Capacitor Discharge Testing, aka Cap Discharge Testing, is conducted on products that have accessible electrical terminals that are directly connected to circuits that contain capacitors. The test involves monitoring the voltage over a very short period of time at the accessible electrical terminals beginning with the moment that the mains power to the product is disconnected. This is done to insure that there are no accessible shock hazards after the equipment is powered off = either to the user holding the mains plug or, to someone accessing an internal circuit (i.e. service personnel).

Discharging of Capacitors: Products with circuits that operate at high frequencies such as switch- mode power supplies and microprocessor circuits can have problems with emitting radiated and conducted electrical emissions (invisible electrical noise on the power line or through the air). Radiated and conducted emissions are limited by government agencies such as the FCC and CE Directives as they can interfere with the proper operation of other electrical devices.

In order to limit these emissions, product designers add EMI filter components to the product design. These filter components frequently include capacitors. For several reasons, capacitors make good high frequency noise filters. However, because a capacitor can store energy, they can also present a hidden danger.

A capacitor can store energy at a voltage that exceeds shock hazard levels long after a product has been disconnected from mains power. Product designers combat this by incorporating bleeder resistors that “bleed” the stored energy from the capacitor when the product is turned off. And it’s important that the discharge happens quickly (i.e. 1 second) to prevent someone from being shocked if they touch the terminals shortly after mains disconnect (i.e. when pulling the plug for a cord connected product from a mains power receptacle).

Test Objective: The objective is to identify if any operator accessible terminals have a shock hazard present immediately after the product has been turned off.

Purpose of the Test: The capacitor discharge test insures that adequately sized bleeder resistors or equivalent are included in the product design to quickly bleed any operator accessible voltages at terminals to a safe level.

Test Limits: The pass/fail criteria is based on the accessible voltage at the specified test time. The voltage limit and test time vary by standard. Be sure to check the safety standards that apply to your product. Criteria in the standards includes:

  1. Voltage: Some standards indicate a 60V limit at the test time, others specify that the voltage must decay to 37% of the operating voltage within the test time.
  2. Test Time: The test time starts when the mains voltage to the product under test is disconnected. Keep in mind that the voltage is decaying (going down) over time. The accessible voltage must be below the voltage limit at the test time. The product design controls the decay rate. The standard specifies at what test time the voltage limit applies. Standards that apply to products that are typically found in a home have a 1 second test time. Commercial products generally have test times of 5 or 10 seconds, and some industrial standards have even longer test times (30 seconds or more). 

Test Locations: All operator accessible circuits and terminals should be considered. Some standards allow for waiving the test if the capacitance is below 0.1 uF and a means of discharge is provided that meets the test time specification. However, the circuit is seldom simple enough to allow for an easy calculation. Meaning that the test is performed on most products.

  1. Mains Plug: Most cord connected products with EMI filtering components will need to be tested (ITE-60950; Lab-61010; Medical-60601; A/V-60065; Smart Appliances-60035; etc.). The concern is that immediately upon disconnecting the power by pulling the plug from the receptacle, the user can touch the terminals of the plug. We need to make sure that these terminals are not at a shock hazard level. Measurements are made between all possible pin combinations including Line to Line, Line to Neutral, Line to Ground, & Neutral to Ground.
  2. Replaceable Internal Parts: If internal parts are subject to replacement (i.e. behind an access panel or door), they must also not present a shock hazard from a stored charge in a capacitor. Internal accessible parts typically have a 10 second test time.
  3. Output Terminals: Similar requirements exist for conductive parts of output terminals, usually with a 10 second test time. 

Test Method: There are 2 ways to perform this test. One option is a calibrated cap discharge tester (a fairly new type of product). If you use a cap discharge tester, it will perform the entire test automatically. The other option is an open bench method using more commonly available laboratory equipment - you will need to perform the test manually using a calibrated storage oscilloscope and a multi-pole switch using this method:

1) Test Time: Remember that the test time begins when the mains power is disconnected.

2) Disconnect Point:

3) Multi-Pole Switch: With the short test time, it is not feasible to physically disconnect the power when starting the test = pulling the plug on a cord does not allow you to disconnect all poles simultaneously (the earth pin is longer). Instead, you will need to connect a switch into the mains power connection, a switch that allows all poles to be disconnected simultaneously to start the test (including ground = therefore the product on/off switch will not suffice). A minimum 3 pole switch rated for the input current of the product is needed.

4) Oscilloscope: A storage oscilloscope is required to capture the decaying waveform for examination and measuring the voltage and test time. Begin by monitoring the operating voltage on the oscilloscope. Then start capturing the waveform just before disconnecting the mains. This will insure you capture the entire decaying waveform and be able to identify the exact test time from the point when the mains was disconnected.

5) Measuring the Voltage: View the captured waveform on the storage scope and identify the residual voltage at the test time specified in your standard. Note that you will use the DC voltage setting on your storage scope for measuring the decaying voltage (the decaying voltage is not sinusoidal).

Something to Consider:

Conclusion: As you can see, we don’t simply perform the tests because they are in the standard. Each test in the standard has a set of objectives that relate to the 6 Hazards of Product Safety. The Capacitor Discharge Test is performed as part of the accessibility review for Shock hazards. Protecting the user from stored energy at accessible terminals is crucial to insuring the product continues to provide protection from a Risk of Shock, a potentially serious hazard that could lead to death by electrocution. It is therefore an extremely important test – another test that directly saves lives.

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