Production Safety Testing Ensures Compliance with Global Safety Standards

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Because virtually all electronic devices and electrical apparatus require safety certification, manufacturers must submit samples of their products to compliance agencies and regulatory authorities to ensure they meet global standards. 

This article provides insights into the safety standards necessary for certification and explains how cutting-edge hipot testers have been developed to expedite and ease the compliance process. It also covers important pre-testing setups and safety procedures that are vital for user safety. Lastly, it elaborates on the four fundamental hipot tests—dielectric withstand, insulation resistance, ground continuity, and ground bond testing—that are typically performed during final production, along with the key test results to monitor.

Understanding Global Safety Standards

During the product development phase, items intended for sale in the United States are typically evaluated by Nationally Recognized Testing Laboratories (NRTLs) for compliance testing. NRTLs are responsible for certifying adherence to appropriate safety standards and regularly audit testing equipment and facilities.

The compliance evaluation performed by an NRTL usually focuses on two significant areas of a product:

1. Construction—Mechanical construction, spacing, clearances, etc.; and

2. Safety—To assure safe operation, even under high-stress conditions.

The specifics of what defines an NRTL-certified product depend on the relevant standard(s) that apply to that item. For products meant for international markets, the applicable standards may vary, complicating the process of achieving global accessibility.

To tackle this complexity, efforts are continuously underway to harmonize standards internationally, such as the IEC -5-1, a standard created by the International Electrotechnical Commission (IEC), addressing safety aspects in adjustable speed electrical power drive systems. In the U.S., the IEC -5-1 requirements have effectively replaced those of UL 508C, which has been withdrawn.

The Evolution of Hipot Testing

Hipot testing has historically been a standard process for a variety of equipment. Hipot testers derive their name from the term "high potential," referring to the high voltages produced for dielectric withstand and insulation resistance tests. Modern hipot testers not only provide precise low-resistance measurements but also offer low-resistance/high-current outputs for testing ground resistance and ground bond integrity.

Earlier commercial hipot testers were primarily simple step-up transformers that increased applied voltage in increments over specific time intervals to test for leakage or component breakdown. However, this traditional method could lead to inaccurate outcomes as voltage output from a high-impedance transformer could drop due to leakage current.

In contrast, contemporary advanced hipot testers leverage electronic source technology to ensure compliance with IEC standards, mandating that "the voltage test equipment shall be able to maintain the required voltage for the specified period of time."

Hipot Testing Setup and Safety Procedures

Given the nature of electrical safety testing, which employs high voltages, strict adherence to safety protocols is essential for test operators. It is imperative for operators to recognize the inherent dangers of high voltages and exercise caution to avoid contact with energized circuits. Training personnel is the critical first step in fostering a secure testing environment.

Station Setup

The placement of the test station is crucial. It should be isolated from the factory assembly area and positioned away from regular foot traffic to ensure the safety of bystanders. Additionally, distractions for the operators must be minimized, and the area should be clearly marked with international warning signs, such as "DANGER - HIGH VOLTAGE." 

During testing procedures, the hipot tester must feature indicator lights to signal the presence of high voltage. Ample and reliable power should be supplied to the test station, and the power wiring must conform to electrical code requirements for grounding and polarization. Always use an outlet with a correctly connected protective ground, ensuring it has been tested for a low impedance path to the panel ground and earth bonded ground.

Figure 2a/2b demonstrates two different setups for a benchtop hipot test. In Figure 2a, operators wearing safety glasses position the device under test (DUT) on the test bench, which is outfitted with palm switches and a footswitch to prevent operator contact with the DUT during testing. Although palm switches are intended for brief repetitive tests, operators may bypass them for extended tests, defeating their safety function.

Figure 2b showcases the DUT beneath a protective cover equipped with an interlock, isolating the operator throughout the test. Employing an enclosure guarantees enhanced safety, especially for tests requiring extended time frames. More sophisticated test stations may incorporate a hipot tester interlock as well.

One efficient safety method utilizing an interlock is a light curtain, where an infrared beam is interrupted if someone crosses it, triggering the interlock to deactivate high voltage. The light curtain should be installed between the operator and the hipot tester or DUT. For an operator to make contact with the high voltage, they must interrupt the light curtain, immediately shutting down the high voltage. 

If the hipot tester is set behind a light curtain, a designated method must be employed to initiate the test, with a footswitch being a reasonable solution. However, it is essential to design the test space to prevent unauthorized access to high voltage around the light curtain.

Inadequate grounding connection of the hipot tester may lead to operator injuries. The work environment and surfaces should not include metallic materials, avoiding metalwork surfaces, and ensuring that any metallic objects are grounded or removed from the testing area. ESD mats are not advisable for test stations, as they can lead to inaccurate leakage readings and are unnecessary in this context.

The test equipment should ensure the prompt and safe removal of output voltage through internal discharge circuitry, either concluding the test or responding to any interruptions. Never disconnect power from the hipot tester, as doing so may necessitate extreme caution when interacting with the DUT. The safest method is to maintain the DUT connected to the hipot tester until power restoration allows for a controlled discharge function.

The test station should offer enough space for both the tester and the DUT while preventing the operator from reaching over the DUT to access the tester. It is ideal for the tester to be positioned at least three inches from the wall for adequate airflow. Furthermore, efforts should be made to isolate the DUT from both the operator and the tester. In cases where larger DUTs require wheeling to the test station, ensure the cart is non-conductive and features locking wheels.

Maintaining cleanliness and order is essential, as is arranging equipment for easy and safe operator access. Numerous safety features can be integrated into test stations to guard against high voltage exposure, such as guards or enclosures. When encasing a DUT, ensure these guards or enclosures are non-conductive and equipped with safety interlocks that disrupt all high voltages upon opening. The layout should guarantee that operators are never put at risk of high voltage exposure.

It is straightforward to implement circuit palm switches that help prevent operator exposure to high voltage during testing. The fundamental operation of palm switches mandates both hands to initiate a test while enabling a footswitch to activate the process. If operators remove one or both hands from the switches during testing, the procedure halts immediately. Positioned shoulder-width apart, these switches are designed to deter operators from pressing both simultaneously using one hand or object.

High voltage cannot be applied to the output terminals or DUT until both switches are engaged. The palm switches ensure that operators cannot touch the DUT or test leads when their hands are properly positioned. These switches connect to the digital I/O on the hipot tester, enabling the start function only when they are firmly pressed down. If either switch is released, the safety interlock engages, terminating the output voltage during the hipot test. This safety mechanism is not only efficient but also crucial.

Routine checks of the tester, typically at the beginning of each shift, should involve connecting the tester to both PASS and FAIL samples. These samples are designed to confirm the correct operation of the tester based on the tests being performed (hipot, insulation resistance, ground resistance, or ground bond). After ensuring all connections are accurate and the prescribed test procedure is selected, operators should verify that all test parameters are visible on the tester screen. Operations can then commence, keeping previously outlined safety considerations at the forefront.

Hipot Testing During Production

Hipot testing during the manufacturing process serves several key purposes:

• Ensure compliance with safety agency labeling requirements;

• Identify defective components or assembly flaws; and

• Minimize latent field failures and the related warranty expenses.

Once products are in production, they must be subjected to comprehensive testing to guarantee compliance with pertinent agency certifications and safety standards. Production tests may be less rigorous than initial certification tests but will usually encompass fundamental dielectric withstand and shock hazard (leakage) assessments.

Devices that connect via plugs are also subjected to ground resistance and ground bond tests, if mandated by relevant standards. Electrical motors, transformers, and similar devices will typically undergo insulation resistance tests.

Regular inspection and calibration of test equipment are standardized requirements for maintaining NRTL certification for produced products. This includes checking the calibration certification of the hipot instrument, usually requiring annual renewal. Commonly, NRTLs mandate a daily functional test of the hipot equipment as well.

Test 1: Dielectric Withstand

The fundamental hipot test involves applying high voltage from conductors to the DUT's chassis. This test, frequently termed as a dielectric test or voltage withstand test, aims to confirm that the insulation and isolation of non-conductive surfaces from the operating voltage are sufficient to prevent shock hazards. The typical specification for this test is V + 2x normal operating voltage.

Both AC and DC hipot tests are feasible, and generally, the test should align with the type of voltage used during standard operation. However, if a DC hipot test is performed on an AC circuit, the hipot voltage should be twice the peak, following the formula (2 x 1.4 x RMS) + V.

Depending on the applicable standard, units will pass this test if:

• The leakage current measured is below the maximum allowable current; or

• No breakdown occurs, indicating no sudden and uncontrolled current flow.

For double-insulated products, higher voltage specifications are often stipulated in the test standard. Furthermore, this category of devices typically necessitates specialized fixturing to connect the non-conductive outer layer with a conductive element.

Common issues identified through hipot tests include contamination (e.g., dirt, debris, etc.) and improper spacing (creepage and clearance) of components. Creepage measures across surfaces, while clearance refers to air gaps between components. Contamination can lead to an unacceptable level of leakage current, while clearance issues may result in breakdown.

Desirable features for hipot testers facilitating dielectric withstand testing include:

• Adjustable maximum output voltage:

  • 5KV suffices for many applications

  • Higher voltages (up to 30KV) may be necessary

  • AC and DC outputs

  • Excellent regulation—both line and load

  • Controllable ramp rates, dwell times, and discharge features

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  • Phase angle measurement of leakage current—capacitive coupling detection

  • Certain standards allow separate measurement of in-phase and quadrature currents, where leakage current due to capacitive coupling may not raise safety concerns.

• Min/max pass/fail current limits:

  • Separate limits during ramp phases.

• Programmable multichannel testing

Test 2: Insulation Resistance

Testing for insulation resistance is frequently required in motor winding, transformer winding, and wiring-related applications. This involves validating that the resistance surpasses a specified high value.

In many cases, insulation resistance measurements should be conducted between several conductors, for example, in cable/connector assemblies, multiconductor cables, and relays. To perform this measurement, all conductors except one are shorted together, and the test voltage is applied to the remaining conductor across the bundled wires. This process is repeated for each wire.

Features desired in hipot testers for insulation resistance testing include:

• A wide range of selectable testing voltages

• Accurate and repeatable high-resistance measurements

• Programmable high voltage switching accessory

• Multichannel programmable testing options

• Pass on steady as well as increasing voltage

Test 3: Ground Continuity

Ground continuity testing ensures that the device’s conductive chassis is securely connected to the earth ground pin on the power plug, thereby providing protection against shock hazards in the event of an internal short to the chassis. The current would redirect through the ground wire, potentially tripping the breaker or blowing the fuse.

To conduct ground continuity testing, a low current (such as 50 mA) is applied, measuring the resistance from the ground pin of the power plug to specified locations on the DUT's exposed surfaces.

Key features desirable in hipot testers for ground continuity testing include:

• Precise and repeatable low resistance measurement

• Plug adaptor accessory for efficient testing

Test 4: Ground Bond

While ground continuity assesses the resistance of the safety ground connection, the ground bond test certifies the integrity of that connection. In this test setup, high current flows through the circuit, confirming that a sound ground bond allows current to pass without alterations in resistance.

Essential features in hipot testers for ground bond testing include:

• Accurate high-current source

• Programmable test currents and durations

• Plug adaptor accessory to enhance testing speed

• 4-wire milliohm meter, providing Kelvin connection for precise low resistance measurements

Conclusion

Hipot testing is a crucial concluding procedure in the production of electrical and electronic equipment. Today's sophisticated hipot testers, with their programmable features and advanced functionalities, facilitate electrical safety testing. However, prior to initiating testing, manufacturers must stay informed on the updated safety certification standards and their prerequisites. Additionally, operators should prioritize the establishment of a safe testing environment and a comprehensive understanding of relevant testing protocols.