Ensuring the Validity of Regulatory Testing
Verification of electrical safety testing equipment is a procedure that is often overlooked by manufacturers. Running test verification is crucial to ensuring that a safety device will properly detect product failures. While the task of setting up a test verification can seem daunting, it’s a simple matter of having the necessary equipment and procedures in place so that the process is easy to follow.
Why Verify?
International safety testing agencies such as UL (Underwriters Laboratories), the Canadian Standards Association (CSA), the Association of German Electrical Engineers (VDE and TUV) and the International Electrotechnical Commission (IEC) set forth various standards to ensure that electrical devices meet set requirements for electrical safety. Performing electrical safety tests is done to ensure that an electronic product does not pose a shock hazard to the end user. An electrical safety test, however, is only as good as the tester being utilized on the product.
Due to the nature of manufacturing environments, electrical safety testers can be damaged internally without showing physical signs of a problem. As a result, these damaged units can give incorrect readings with regards to insulation resistance, leakage current and withstand potential. Performing regular verifications on electrical safety testing equipment ensures proper operation and testing to U.S. NRTL standards.
According to the UL Mark Integrity Program document titled “Equipment Used for UL/C-UL/ULC Mark Follow-Up Services,” all measurement and test equipment must go through regular inspection:
“IMTE (inspection, measuring and test equipment) used to verify compliance with UL requirements shall be checked daily by the customer to ensure it is functioning properly. If this equipment is not used daily, then this function verification should be performed prior to use.” [1]
The above excerpt outlines the importance of maintaining and verifying measurement equipment which includes electrical safety testers. This program is the driving force behind the requirement to run regular verifications on electrical safety testing equipment. This article will outline the most common electrical safety tests, verification testing for each type of test and efficient means of verification for production line testing.
The Usual Suspects: A Brief Review of Common Electrical Safety Tests
In order to ensure that an electrical product is safe for use, the product is passed through a rigorous gauntlet of testing. Among these tests are electrical safety test which are designed to test the electrical integrity of the product itself. These tests include the ground bond (or continuity) test, dielectric withstand or high potential (hipot) test, insulation resistance test and leakage current test. Each of these tests has unique parameters designed to pinpoint various potential problems with a device. For example, Table 1 outlines common hipot test settings from various NRTL standards.
Ground Bond Test
The ground bond or ground continuity test is used to analyze the integrity of the safety ground on an electrical device. The safety ground needs to be able to handle any fault current that could be imposed upon it due to a product or insulation failure. A low impedance path to ground will allow circuit protection devices such as fuses or circuit breakers to open when fault current flows through them. In order for this system of protection to effectively operate, there must be continuity between conductive components and the product’s ground pin or ground terminal.
Figure 1 shows a standard ground bond test circuit. The ground bond tester injects current onto the ground pin of a product and looks for a return path on the chassis or exposed dead metal. Simultaneously, the instrument must measure the voltage drop across the safety ground circuit to calculate the impedance of the circuit. Common ground bond test parameters are for a current of 10-30A with a maximum impedance of 100-200mΩ and a voltage drop not to exceed 6-12V.
Dielectric Withstand Test
The dielectric withstand test, commonly referred to as the high potential or “hipot” test is an electrical safety test designed to stress the insulation of a device beyond what it would encounter during normal use. The logic behind running such a test is that if the device can handle the force of high potential for a short duration, it should be able to operate at rated voltage without posing a shock hazard to the user.
The hipot test is a versatile electrical safety test. Not only is the test designed to find weak points in the insulation, it can also be used to measure excessively high leakage current, workmanship defects such as pinholes and scrapes, improper spacing with respect to a grounding point and degradation due to environmental conditions. Due to this versatility and the fact that this test can detect a number of insulation faults, this test is generally specified by NRTLs as 100% production line safety test. The method of running a hipot test involves applying the high voltage across the current carrying conductors with a return point on a conductive chassis. The hipot unit will measure the resulting leakage current flowing through the insulation. The potential used in a hipot test varies from standard to standard but a common hipot voltage formula is to take two times the rated voltage (Vr) of the product plus 1000V:
2*Vr + 1000V = dielectric withstand testing voltage
A hipot test circuit can generally be modeled as the device capacitance (C), insulation resistance (RL) and small amounts of contact resistance (RA). This model is shown in Figure 2.
Insulation Resistance Test
While the insulation resistance (often referred to as “IR”) test is the least commonly specified electrical safety test, it can provide the user with some valuable quantitative data. Whereas a hipot test provides a leakage current value, the insulation resistance test gives an actual resistance measurement of the insulation itself. The insulation resistance test potential is generally specified by safety agencies at 500VDC or 1000VDC. Since the test potential is DC in nature, once the capacitive portion of the insulation has charged, the only leakage current flowing through the insulation is resistive and thus allows the user to measure an insulation resistance value.
An insulation resistance test is run in much the same way as a hipot test. The high potential is applied to the current carrying conductors of a device and the return point of the circuit to the chassis. For example, an insulation resistance test run on a solar panel involves shorting the + and – terminal to high voltage and applying the return point to the metal frame. In this way, the insulation is stressed and the IR tester measures the leakage current on the exposed metal chassis. IR tests are usually specified as a test for repaired equipment or just after the hipot test to ensure the hipot test potential did not cause damage to the insulation.
Leakage Current Test
The leakage current test, like the hipot test, measures current flowing through or on the surface of the insulation on a device. However, the leakage current test differs in that this measurement is performed while the product is running at rated voltage (or a high line condition of 110% rated voltage). The other major difference is how the leakage current is measured. For a hipot test, the leakage current is measured through a current sensing resistor on the return side of the circuit (Figure 3).
During a leakage current test, the leakage current is measured through what is known as a measuring device, or “MD.” An example MD is shown in Figure 4. The MD is designed to simulate the impedance of the human body.
Another aspect of the leakage current test that sets it apart from other electrical safety tests is the fact that it incorporates fault conditions. These fault conditions are designed to simulate worst case scenarios that could happen during the instrument’s operation. The three most common fault conditions are the opening of the neutral circuit, the reversal of line polarity and the opening of the ground circuit. A leakage current network is shown in Figure 5.
Switch S1 represents simulation of the neutral fault condition, switch S2 represents the simulation of a polarity reversal, and switch S3 represents the simulation of an open ground condition. The idea behind running tests under these various configurations is to measure exactly how much leakage current a human could be exposed to while the product is running and subjected to a series of fault scenarios. If the leakage current value is sufficiently low enough under all such fault conditions, the product should be able to operate normally throughout its lifecycle without posing a shock hazard.
Leakage current parameters vary greatly from standard to standard. However, some of the most commonly run leakage current tests are for compliance to the medical device standard, IEC 60601-1 3rd edition. For this standard, the leakage current test must be run at 110% line voltage, using the 60601-1 MD (Figure 4) and running a product under the above mentioned fault conditions. Acceptable leakage current values range from 10uA all the way to 10mA.
The Importance of Test Verification
The hipot, ground bond, insulation resistance and leakage current tests are either incorporated into multiple pieces of equipment or even an all-in-one tester. Due to the various functions of the unit(s), it is important to determine if the tester will fail properly when a regulated test value has been exceeded. In a high volume production line setting, it can be easy to miss a unit that should have failed if a test unit is not working properly. Performing verification on all unit functions ensures that a test unit is operating within specified parameters. According to UL procedures, if the test unit is taking measurements to make a determination on electrical safety, that test must be verified.
An example case that outlines this importance is a hipot unit with a damaged measurement circuit. Most hipot testers are not designed to handle an external voltage being input on to the return line of the hipot instrument. There have been instances where an operator on a production line accidently applies line voltage onto the return of the unit. Transient voltage suppressors on the return path of the instrument are installed to protect the other components in the hipot instrument (Figure 6).
If external power is applied, the suppressor will conduct and dissipate this power. However, once the suppressor has been damaged it becomes a direct short and thus the measurement circuit is completely bypassed. When running the hipot test, the hipot instrument will register 0.0mA of leakage current and will pass the test. As far as the hipot unit is concerned, zero leakage current implies infinite insulation value and the electrical device has passed the test. Running a simple verification on an instrument will immediately catch such an issue and warn the operator that there is a problem with the testing system.
The example given above outlines only one possible scenario. Other hazards include harsh environmental conditions such as heat and high humidity. Over time, these conditions can affect an instruments readings and accuracy. If the accuracy of a unit strays too far from the specified accuracy readings, it is possible to get false passes or failures on the device being tested.
Running weekly or daily verifications on the full range of product tests ensures that potential issues with the measurement circuitry are immediately detected. Taking small steps to run verifications can avoid big problems and even product recalls down the road. Verification processes can create an initial time investment up front, but the damage that can be done by the alternative scenario greatly outweighs this allocation of time and resources.
Efficient Verification Processes on Electrical Safety Testing Work Stations
Routine product safety tests are designed to identify faulty insulation, improper grounding, loose connections, defective parts, ground faults in the equipment, unguarded live parts and excessive leakage currents that could represent a potential shock hazard. Simply shorting or creating an open condition between test leads can prove to be an efficient method for ensuring the base failure detectors on an instrument are operating normally. However, such methods cannot catch all potential issues with a unit.
Having simple resistive networks and relays can provide an easy means for an operator to check the functionality of the unit by dropping potential across these resistive networks and setting limits such that the verification tests fail. Additionally, building such a network with easily accessible ports and programmable control allow a verification to be run automatically with each verification process. Providing operators and technicians with a simple means of verification increases the chances that any problem with equipment measurement will be caught prior to product testing.
Figure 7 shows an example of a test box used specifically for verification.
Using a test verification box of this kind, a simple yet effective verification procedure can be setup to be run on a daily basis. This box consists of a series of resistors designed to draw a specific amount of leakage current or have a set resistance value. The following examples outline testing for verification on each type of electrical safety test.
In the example test verification box, there are two posts for each type of safety test, one post for a PASS and the other for a FAIL. This is achieved by connecting separate resistance values from each post back to the RETURN post on the box. (Refer to Table 2 for specific box values in the example test verification box per test type.
Dielectric Withstand Verification
The test voltage for a particular hipot test is 1240VAC. Referring to Table 1, the fail circuit includes a 120kΩ resistor and the pass circuit utilizes a 2MΩ resistor. The hipot instrument would then be set for two separate tests. The first test is a PASS test. The hipot instrument is set at 1240VAC, 10mA high limit, 2sec ramp up, 1sec dwell time. The high voltage lead from the hipot instrument connects to the PASS terminal on the ACW/DCW portion of the box and the return lead is connected to the box’s RETURN terminal. Using a simple Ohm’s Law calculation, it can be determined that the leakage current should be around 620uA:
Leakage Current (Ic) = 1240V/2,000,000Ω = 0.00062A
This should result in a test pass. The next test is a fail test with the hipot instrument set to the same parameters as the pass test. The high voltage lead is then moved to the FAIL terminal on the ACW/DCW portion of the test box. When the test is run, the leakage current should be around 10.3mA.
Leakage Current (Ic) = 1240V/120,000Ω = 0.01033A
To keep in specification with agency regulations, the test should fail within 0.5sec of the dwell cycle. If the test does not fail, the unit is not properly reading leakage current and should be repaired or calibrated.
Ground Bond Verification
The ground bond current for a test is 25A AC. Referring to Table 1 once again, the fail circuit includes a 200mΩ resistor and the pass circuit utilizes a 50mΩ resistor. The ground bond instrument would then be set for two separate tests.
The first test is a PASS test. The ground bond instrument is set at 25A AC, 100mΩ high limit, 6V potential drop, 1sec ramp up, 1sec dwell time. The high current lead from the ground bond unit is connected to the PASS terminal on the GB portion of the test box and the return lead is connected to the box’s RETURN terminal. When the test is run, the resistance should be around 50mΩ which results in a test pass.
The next test is a fail test with the ground bond unit set to the same parameters as the pass test. The high current lead is then moved to the FAIL terminal on the GB portion of the test box. When the test is run, the resistance should be around 200mΩ which should result in an immediate test failure. To keep in specification with agency regulations, the test should fail within 0.5sec of the dwell cycle. If the test does not fail, the unit is not properly reading the resistance value and should be analyzed.
Insulation Resistance Verification
The insulation resistance test voltage for this test example is 500VDC. Once again, referring to Table 1, the fail circuit includes a 4MΩ resistor and the pass circuit utilizes a 1MΩ resistor. The insulation resistance instrument would then be set for two separate tests.
The first test is a PASS test. The IR instrument is set at 500VDC, 2MΩ low limit, 2sec ramp up, 1sec dwell time. The high voltage lead from the IR unit connects to the PASS terminal on the IR portion of the test box and the return lead is connected to the box’s RETURN terminal. When the test is run, the resistance value should be around 1MΩ which results in a test pass.
The next test is a fail test with the IR unit set to the same parameters as the pass test. The high voltage lead is then moved to the FAIL terminal on the IR portion of the box. When the test is run, the resistance value should be around 4MΩ and register an immediate fail. To keep in specification with agency regulations, the test should fail within 0.5sec of the dwell cycle. If the test does not fail, the unit is not properly reading leakage current and should be analyzed.
Leakage Current Verification
Leakage current verification is not as well defined as verification for other test types. The example test box does not contain terminals to test for leakage current values. However, the same concept can be carried over to ensure that a leakage current instrument is operating within specified values. Since a product is operating at rated voltage while running a leakage current test, most leakage current instruments include a universal power receptacle (Figure 8).
As a result, a simple fixture can be made to mate with the receptacle box. A standard plug terminating in two separate resistor values is a simple means of creating a leakage current verification test fixture. Since the leakage meter reads between ground and neutral for normal polarity and ground and line under reverse polarity conditions, separate resistors can be wired between line to ground and neutral to ground.
For example, for a leakage current verification, a plug is wired with a 2MΩ resistor between line and ground and a 200kΩ resistor between neutral and ground. The first leakage current test is set to run at 120VAC, 60Hz, 50uA high limit, reverse polarity, 5sec dwell. With a 2MΩ resistor from line to ground, the leakage current value should read about 60uA:
Leakage Current (Ic) = 120V/2,000,000 = 0.00006A
This leakage current should produce a failure on the test. The second test would check for a failure under normal polarity conditions. The second leakage current test is set to run at 120VAC, 60Hz, 550uA high limit, normal polarity, 5sec dwell. With a 200kΩ resistor wired from neutral to ground, the leakage current value should read about 600uA, thus producing a failure on the test.
While a leakage current instrument also contains various other relays, the main idea is to show that the leakage detectors on the instrument fail in the presence of excessive leakage. The two tests outlined above prove if a leakage current meter is properly reading leakage values.
Automated Testing Verification
Using a test verification box similar to that illustrated in Figure 7 can provide an effective solution for running standard verification tests. However, in a production line setting, it is often advantageous to further streamline the process via automation. Software specifically designed to work in conjunction with a test verification box can allow pre-built verification test files to be automatically downloaded as part of a test sequence. The availability of these verification files means that users can create custom verification test routines in order to verify the functionality of the electrical safety tester prior to executing tests. Most available software offers a number of pre-configured verification routines, but some also provide for the creation of customized verification test files for specific testing requirements.
Conclusion
With the advent of microprocessor technology on electrical safety testing units, setting up instruments for test verification has become increasingly complex. Merging technology on units requires more in depth test verification than on old analog devices. Additionally, electrical safety testing instruments contain built in suppressors that are designed to protect unit circuitry. At the same time, damage to such suppressors can cause false readings on an instrument.
Taking simple steps to run daily verifications can spare a great deal of trouble and effort down the road and can help avoid costly redesigns or product recalls. Using a resistor in series with the output of an electrical safety testing instrument will assist in validating measurements and ensuring that the failure detectors are functioning properly. Setting up a fixture with a series of resistors with calibrated resistor values takes the verification process one step further.
Additionally, microprocessor controlled safety testing devices allow for programming and saving verification procedures. Such procedures can also be set up to include working instructions and data acquisition. Verification methodology gives the operator a simple means of storing the verification test procedure as well as running validations on a daily basis. Performing regular verifications ensures that all equipment is working within NRTL specifications.
References
- Underwriters Laboratories, “UL Calibration Requirements: Equipment Used for UL/C-UL/ULC Mark Follow-Up Services,” Mark Integrity Program, oo-UM-C0025 Issue 4.0, 2012
