Keep it safe
Hipot testers help you find trouble spots and let you verify your product's electrical safety.
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 4/1/2005 2:00:00 AM
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Manufacturers of hipot testers
Standards bodies |
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Read other articles from this issue:Table of contents, April 2005FEATURES Combining talents Cellular or WiFi? X-ray inspection—Not all black or white Keep it safe Biological pulses |
Nobody wants to build an unsafe product. To meet safety requirements such as those published by UL for the US, by CSA for Canada, and by IEC for Europe and other areas, you must perform electrical safety tests on components, cables, and systems during engineering development and often in production. Components such as transformers and motors that handle AC mains-level voltages often need testing for every production unit.
You should perform a variety of safety tests on your products (Ref. 1, 2), but the most important electrical safety test is the hipot test, also called a dielectric-withstand test. In a hipot test, you apply a high voltage to a product and measure the leakage current passing through, verifying that the product's insulation can withstand high voltages without letting unsafe current pass through to the user.
A hipot test subjects a product to an AC or DC voltage. In an AC-voltage test, you subject a product to voltages that are at least 1000 VAC greater than double the normal operating voltage. Class-I products, those with three-wire AC mains cords that operate at 120 VAC, require hipot tests at a minimum of 1240 V, but the test voltage is typically between 1250 V and 1500 V; in fact, some national standards require even higher voltages. Class-II products, those with two-wire AC cords, need testing at higher voltages, often at 4000 V. Standards specify the length of time that you must apply the test voltage to either class, which is typically 60 s.
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| You can use a hipot tester on engineering prototypes. Courtesy of Clare Instruments. |
You can also perform hipot tests with DC voltages. To find the required DC voltage setting for testing a product, you simply multiply the AC voltage by 1.414. Most standards also require a stepped test voltage, which allows time for a product's internal capacitance to charge. Only after the internal capacitance charges can you measure the leakage current. If you apply the test voltage too soon, the capacitance will cause the product's current draw to exceed the test limit, causing a false-negative result. The measured current represents the normally drawn current plus any leakage currents in the product.
Multiple features
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| Figure 1. Hipot tests often perform both AC and DC insulation-resistance tests. Courtesy of QuadTech. |
Several companies manufacture hipot testers, but the features these instruments offer can vary. Some hipot testers perform dielectric-withstand tests only. Some perform AC tests only, while others test products with both AC and DC test voltages. One line of testers from QuadTech performs AC and DC hipot tests plus insulation-resistance tests (Figure 1). Other testers on the market may perform up to six tests. These multifunction units, such as the Omnia 8100 series from Associated Research (Figure 2), are also called electrical safety analyzers.
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| Figure 2. Multifunction testers, often called safety analyzers, can perform up to six electrical safety tests. Courtesy of Associated Research. |
In most applications, you'll want to automate a test sequence. Generally, you can program a tester's sequence from its front panel. Most testers also offer an IEEE 488 or RS-232 port so you can use a computer to control the tests.
Testers from QuadTech and Associated Research offer LabView drivers. DIT-MCO provides software that lets you automate tests through text-based scripts. The company also offers a test-programming environment. Some units, such as the H111 from AEMC, include a connector that lets you control the unit from a programmable controller through I/O pins.
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| Figure 3. Cable tests often include hipot tests. Courtesy of Cabletest. |
In addition to checking whether an instrument complies with safety regulations, you also need to ensure that instrument cables do not transmit unsafe currents to users. To test how well an interconnect cable's insulation can withstand high voltages, you can use either a bench hipot tester or a dedicated cable tester.
Companies such as Cabletest, Cirris, and DIT-MCO manufacture cable testers that include hipot tests for cables. The Horizon 1500
from Cabletest, for example, can run hipot tests on cable assemblies at voltages up to 1500 VDC and 1067 VAC (Figure 3).
A safe testing environment
Of course, there's more to hipot testing than just buying a tester and running tests. Because hipot testers generate high voltages, you must carefully design the test area. For example, hipot testers should reside in clearly marked, separate areas so unauthorized staff members don't get too close.
The work surface and chair should be made of nonconducting materials, and the tester's safety ground should connect to a rod driven into the floor. You can force test operators to keep both hands away from the product being tested by connecting special palm switches to the hipot tester's interlock terminals—the operator will have to use both hands to activate the tester, so it can't be started accidentally.
A more elaborate way to protect the test operator is by installing a light curtain, or an infrared beam, around the test setup. Whenever a person breaks the beam—even by extending his or her hands through it—the beam will open a switch on the hipot tester that makes the tester unable to turn on its high voltage. (To learn more about this and other safety steps, see "Design a Safe Hipot Test Bench," Ref. 3).
With the right hipot tester and a proper test setup, you are well on your way to ensuring the safety of both your customers, their customers, and your technical staff. Hipot tests do more than ensure compliance to regulations. They ensure that your product is safe to use.
| References |
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Design a Safe Hipot Test Bench
04/01/1999Associated Research Adds Hipot Tester Models
03/24/2005Automate Safety Tests
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