Capacitor Discharge Testing, aka Cap Discharge Testing, is conducted on products that haveaccessible 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:

  • Fixed Voltage Limit Based Standards: If your standard specifies a fixed voltage limit
    (ex. 60V) regardless of the mains voltage, it will be important to insure that the test is
    started at the maximum mains voltage when the power is disconnected = that occurs
    when the mains voltage is at its peak (peak of the AC sinewave). This means that you
    will disconnect the mains and then check the stored image on the oscilloscope to see
    if you got lucky and managed to disconnect the power exactly at the peak of the AC
    sinewave. If you missed the peak, do it again. Repeat until you see that the
    disconnection occurred at the peak of the AC mains.

  • Voltage Decay Rate Standards: Standards that provide a limit based on the decay rate
    (time constant) can make it easier to perform the test. You only need to capture the
    complete decaying waveform without worrying about disconnecting power at the peak
    of the sinewave. Any starting voltage near the peak is adequate in order to calculate
    whether the % of decay is within the specified 1 or 10 seconds (ex. 37% decay rate).

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:

  • Production Testing: We are aware of several product recalls for capacitor discharge problems in ITE equipment = replacing the power supply for all users is not an easy or
    inexpensive process, even if it is an external power supply. It behooves product
    manufacturers to consider performing a production line capacitor discharge test, which is a
    lot less expensive than a recall and it helps reduce the potential for product liability.

  • Impact on Leakage Current: Note that the bleeder resistors included in the EMC filtering
    circuitry that are provided to insure fast discharge of capacitors come with a concern =
    current will flow through the bleeder resistors at all times when the product is operating. This
    creates mains frequency leakage current = mains frequency leakage current (50 or 60Hz)
    presents the highest potential for shock to the human body and therefore has the highest
    potential for causing problems meeting the leakage current limits.

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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