Choosing the Right Test Equipment

The first step in selecting test equipment is to define the types of tests to performed on a given product. For a manufacturer, three items must be considered:

• 1. Tests required by the NRTL for approval and certification. (Includes type tests and production line tests.)
• 2. Optional tests needed by the manufacturer to ensure quality of the product and the manufacturing process.
• 3. Cost/benefit analysis of each optional test.

Since the NRTL specifies the tests they require for certification, the first item is easily defined. For a list of tests required by various NRTLs for different classes of products, click here.

The second item, however, is determined by manufacturing management for the purpose of process control and is largely dependent on the third item, cost/benefit analysis.

NRTLs usually require "type tests" for verifying basic safety of the product design and also "production tests" for ensuring that the approved product continues to meet the safety standards as long as it is sold to the general public. Since the type tests are intended to verify safety of the "design" of a product, they are usually much more stringent than routine production tests performed on every unit as it emerges from the assembly line.

An AC dielectric withstand test is normally a required "type test" for any product. Most manufacturers also choose to perform all the other basic tests as a matter of good design practice, even if they are not specifically required. Production line tests typically required by NRTLs are:

Dielectric withstand - almost always required for all products.

Ground bond - usually required for information technology products, medical equipment, audio/video products, laboratory, control, test & measurement products, household cooking products, and portable electric tools. Always recommended for products sold in Europe with the CE mark.

Ground continuity - usually required for electric air heaters, household cooking and food serving appliances, vacuum and blower cleaners. Recommended by UL for products with 3-prong power cords.

Insulation resistance - usually required for electric air heaters and electric motors.

Line voltage leakage current - usually required for most medical products.

Polarization - usually required for any product supplied with a 2-prong or 3-prong line cord.

 

Recommended Tester Features

In selecting a tester for "type testing", the major considerations are:

1. What tests are required? (AC hipot, DC hipot, IR, Ground Continuity, Ground Bond, Current Leakage)

2. What is the range of test voltage? What leakage current limits are required for the hipot test?

3. How should you connect the tester to the DUT?

4. Is there a requirement for connecting the tester to a PLC, computer, or other external device?

5. Are additional features desirable? Such as:

• • Programmable test setup
• • Store and Recall of previous test setups
• • Minimum current detection (plus maximum current detection)
• • Ease of use in a production environment
• • Operator convenience
• • Displays and alarms
• • Data communication Interface
• • Front panel lockout
• • Arc detection
• • Sequence testing
• • Automatic shutdown
• • Wide range of Accessories
• • Calibration service
• • Competent service and support of the product

 

General Tester Features

Wide Range of Functions

A tester should be capable of performing all of the tests required for a particular product. This is usually much more economical than purchasing several single-function testers. If only a single test is required, of course, a simple, dedicated tester is all you need. Usually, however, a sequence of several tests is desired.

Wide Range of Test Voltage and Current

The tester should be capable of providing all voltages and currents needed for the range of tests to be performed.

• For dielectric testing: Voltage and Current
• For ground bond: Current and Resistance
• For insulation resistance: Voltage and Resistance

Wide Range of Insulation Levels

The tester should handle insulation resistance measurements in the ranges of 1MΩ to 50GΩ at commonly used voltages (from 50V to 1000V DC). For special situations a range as high as 2TΩ may be needed.

Flexibility

The tester should be easily adaptable to different test requirements. Most modern testers provide this flexibility through programmability plus an ability to recall previously stored test setups on demand.

Line and Load Regulation

The tester should be designed so that normal variations in line voltage and connected load do not cause the output voltage and current measured at the DUT to rise above or fall below the levels required for the test. This improves test repeatability and greatly reduces inconsistencies in measurements.

Accuracy

To comply with agency requirements, tester accuracy should meet the following typical specifications:

• AC High Voltage: ±1% of reading +5V
• DC High Voltage: ±1% of reading +5V
• Regulation: <1% +5V Insulation Resistance: ±10-15% of rdg +10 cts
• Leakage Current: ±1.5% + 5 cts
• Breakdown: ±1% of limit + 50mA Ground Continuity
• Output Current: ±1% of setting +0.3A Ground Continuity
• Resistance: ±1% of reading +3 cts

Easy to Use

The tester should have a well-designed front panel with easily read digital displays of measurements, settings, and pass/fail indicators. Audible alarms with acknowledgment are typically required by an NRTL. An ability to hold an alarm condition after operator acknowledgment can be useful for later analysis of the fault. All panel items should be clearly marked so that the function of each is readily apparent to anyone looking at the device for the first time.

The START TEST button should be large, well marked, and protected in a way that prevents accidental activation of a test. The STOP TEST button should also be easily identified (preferably bright red) and placed so as to be found quickly in an emergency.

Pushbuttons for setting, storing, or recalling test values, alarm limits, and test sequences should be clearly marked and easily operated by typical test personnel.

Operator Safety

The tester should be designed so that the test operator cannot be accidentally subjected to hazardous voltages such as those used for a hipot test, a line voltage leakage test, or an insulation resistance test.

Use of safety interlocks can provide this protection by automatically shutting down the high voltage output whenever a safety switch on the DUT is opened. In addition, the tester should be designed so that output current cannot exceed a value that would be harmful to a human (GFI circuit). Cables used for high voltage output and ground clips should be flexible, well insulated, and designed to be repeatedly plugged into and removed from the front panel over a long period of time without becoming frayed, worn, or ineffective.

 

Production Tester Features

Programmability

A modern production tester should be fully programmable, permitting a user to enter a complete sequence of test steps, test parameters, and pass/fail limits. This permits the tester to automatically execute a complete test or series of tests on a DUT.

Store/Recall of Test Programs

The ability to enter a test program and store it for later retrieval is a very useful feature. A user can then recall any of several stored test programs at any time, saving valuable setup and test time in both laboratory and production environments. This feature ensures that tests are always performed the same way.

Minimum Current Detection

The ability to detect minimum current in a hipot test guards against a false positive indication when the ground circuit is open. Without this feature, a fault in the ground circuit may be missed and cause an unsafe product to be released for shipment to customers.

Displays and Alarms

Most modern testers today use digital displays for indications of measurement values, test limits, and pass/fail results. An operator can more easily and accurately read a digital meter than an analog meter.

Data Communication Interface

Any production tester today must be equipped with some type of standard data communication interface for connection to remote data processing, computer, or control equipment. The typical interfaces are an IEEE-488 general-purpose interface bus and an RS232 serial communication line. The tester must also be capable of being remotely controlled by such equipment.

The advantage of an IEEE-488 interface is that it can transfer data at a high rate (typically 1 MB/sec). However, the length of the connection cannot exceed 20 meters. An RS232 serial line can be any length, but is limited to a low data transfer rate.

Front Panel Lockout

To prevent unauthorized operation or setup of the tester, a lockout feature (with password protection) is a desirable option. This feature ensures that changes to test limits or any tester functions are made only by a competent, authorized person.

Ramping

To avoid damage to components in a DUT, the high voltage output of the tester should be increased smoothly over the test range rather as an abrupt step change. A quality tester should provide this feature without introducing spikes or distortion in the AC waveform. Modern testers also provide easily programmable ramp and hold times for each test step.

Automatic Range Sensing

The ability to automatically sense and select the correct range for an insulation resistance test is a very useful feature in an insulation tester. It saves time and ensures maximum accuracy of every measurement.

Automatic Shutdown on Alarm

When a DUT fails a test, the tester should automatically save the test result and interrupt the test immediately to avoid potential damage to the DUT.

Automatic Offset

In production environments, the ability to subtract leakage current due to test leads and test fixtures automatically from the instrument reading can be a great convenience. 4-Terminal Kelvin connections ensure maximum accuracy by preventing errors caused by measurement lead resistance. This feature is typically used to ensure accuracy of a ground bond test.

Arc Detection Before Breakdown

Arc detection is an anticipatory tool that can be used to detect an impending fault that may occur later in the product’s life cycle. Modern testers provide this feature by detecting the presence of high frequency transients in the current waveform. If such variations exceed a specified level and persist for more than 10usec, the tester should instantly alarm and interrupt the test.

Sequence Testing

Because a hipot test is a stress test of the insulation, it may in some cases cause degradation in the insulation resistance of a DUT. By measuring insulation resistance both before and after a hipot test, you can determine whether or not the hipot test damaged the insulation. Therefore, the ability to program the tester to perform these tests automatically in sequence is of great benefit to a manufacturer.

Continuing Calibration Service

The supplier of safety test equipment should be able to offer calibration services on a continuing contract basis to make sure that the testers meet NRTL standards at all times. This is normally accomplished through a periodic on-site or off-site inspection and calibration of all covered equipment. The reference standards used by the supplier should be certified and traceable to NIST standards.

Wide Range of Accessories

A full complement of accessory items such as test probes or guns, high voltage lead sets in various lengths, corded product adapters, and foot switches are a real convenience. Having them readily available saves time and avoids the need for a test engineer to design or build such items.

Accessory Equipment

Some of the accessories typically needed for a lab or production test facility are:

• • High Voltage Probes
• • High Voltage Probe Guns (trigger operated)
• • High Voltage Lead Sets with various lengths of cable
• • Corded Product Adapter Fixture (to accept 2-prong or 3-prong line cords)
• • Foot Switch to start/stop tests
• • Ground Continuity Lead Set
• • Power Entry Adapter Cable for Ground Continuity Tests
• • Multiple Port Scanners for production test applications
• • Rack Mount Kit

 

Tester Calibration

NIST Standards

The voltage, current, and impedance reference standards used by the equipment supplier for calibrating user test equipment should be certified traceable to NIST (National Institute of Standards and Technology) standards. This ensures sustained integrity of calibration accuracy.

120kΩ Leakage Impedance

UL specifies a 120kΩ resistance to check operation of a hipot tester. This requirement is based on a maximum current flow of 10mA when a voltage of 1250V is applied between the circuit of a DUT and ground. Using Ohm's Law, a voltage of 1250V divided by a current of 10mA gives a resistance of approximately 125kΩ.

To verify that a given hipot tester meets this standard for leakage impedance, the user sets the output voltage to the desired value and then connects a 120kΩ resistor across the output terminals. To be accepted, the tester must indicate a fault within 0.5 seconds. If it does not, the tester is not acceptable. The 120kΩ value is the minimum value at which the tester must indicate a fault. It is common for most equipment manufacturers to test their products with higher values of resistance, providing an extra margin of safety rather than testing right at the specified limit.

Refer to our Application Note library for additional information on resistive load boxes.

Keep reading: Applications