Ground Resistance Testers for Electrical Safety: Key Tools for Ensuring Compliance

Electrical safety testing is an essential part of quality control for almost all electronic products. Good grounding is needed to prevent costly damage to an electronic product as well as to avoid service interruptions and potentially dangerous operating conditions to a user. Effective grounding systems support the use of safety components such as protection relays and help protect sensitive electronic components from over-voltage conditions that can occur from manufacturing errors such as poor solder joints or even performance degradation that can occur from faulty insulation. Establishing a safe electrical environment involves ensuring that the resistance of the electrical ground path is suitably low to satisfy the requirements of applicable electrical safety standards, such as those set forth by the Occupational Safety and Health Administration (OSHA). It also involves measurements capable of determining an electronic product's behavior under adverse conditions, such as voltage surges, and how the electronic components and materials within a product will respond when subjected to excessively high voltage, current, and power levels.

The effectiveness of any grounding system requires an understanding of the physical grounding environment, including the resistivity of the soil upon which electrical devices operate. Resistivity is typically represented in units of Ω-cm. By locating the area of lowest soil resistivity, for example, it is possible to construct the safest and most economical grounding installation possible. The resistivity of the soil in an area determines the resistance of a grounding electrode system built upon it. That resistivity varies widely around the world, based upon the amount of electrolytes in the soil and changes seasonally, depending on such factors as moisture, dissolved salts, and minerals in the soil. Dry soil exhibits higher resistivity than soil after a rain fall or in an area of high humidity.

Analyzing the ground resistance of soil and other materials requires specialized test instruments. The depth to which test electrodes are placed in the soil to be measured impacts the accuracy of the resistivity measurements, especially for soil and areas with low ground resistance. Just as numerous electronic materials in printed-circuit boards (PCBs) provide an assortment of electronic responses such as conducting and insulating behaviors, numerous soil components contribute to a range of resistivity values. Gravel and sand exhibit high resistivity compared to ashes and brine, with good insulation usually considered to be resistivity exceeding 109 Ω-cm.

Sizing Up Safety

Determining the ground resistance of an area of interest or the safety of electronic materials, especially in high-voltage, high-power electronic products using starts with test equipment capable of generating and analyzing high voltages. As an example, the Associated Research OMNIA® II 8207electrical safety compliance analyzer is a multifunction test instrument capable of making measurements at the high-potential or hipot voltages that can evaluate the dielectric strength (and safety) of most electronic materials. Such hipot levels are designed to check the insulation safety of electronic designs and confirm that current will not unexpectedly flow or leak from one point to another within an electronic product when operated under stressful conditions or as a result of material deterioration with time.

Over-voltage testing can check whether the insulation in place is adequate for a product's maximum voltage rating. Hipot testing can also be useful for unveiling overly tight component placement, spacing, and tolerance issues that may arise during manufacturing, creating stressful conditions to aggravate problems. Hipot testing can be performed with AC or DC test voltage. For DC hipot measurements, capacitors in a device under test (DUT) should be discharged prior to testing. Leakage currents are more of a concern when performing hipot testing with DC test voltages than with AC test voltages.

The OMNIA® II 8207 provides AC and DC hipot testing and can test for arcing, ground continuity, and ground fault interruptions. The versatile rack-mount instrument offers several test modes, including a run test mode, continuity test mode, and dielectric withstand test mode to check on the upper voltage limits of a dielectric material. It includes a leakage current test mode in which continuous current can be set as high as 16 A for voltages as high as 277 VAC, with the voltage of a DUT displayed as high as 277 VAC full scale with 0.1-V resolution.

The OMNIA® II 8207 simplifies leakage current measurements with the aid on a ground on-off switch and a reverse-polarity switch. It commands high AC and DC "survival" voltages for a DUT, with an AC withstanding test voltage range of 0 to 5000 VAC with 1-V resolution and a DC withstanding voltage range of 0 to 6000 VDC. The ramp up times for both voltage types can be set from 0.1 to 999.9 s.

The Vitrek 954i Industrial Strength Hipot Testercommands a wide AC hipot voltage test range of 20 VAC to 60 VAC with 50 mA maximum standard current and a DC hipot voltage test range from 50 VDC to 11 kVDC with 30 mA maximum current. The instrument incorporates a variety of control interfaces, including Ethernet, and RS-232, with an option for GPIB. It boasts enough internal memory to store as many as 100 automated tests, each with 100 steps per test. Test times range from 20 ms to 1000 s and the 954i includes several measurements not usually associated with hipot or ground resistance testers, such as phase angle measurements and picoamp leakage current measurements.

The Chroma 19032-P Electrical Safety Analyzeris a multifunction tester that includes hipot, insulation resistance, ground bond, and leakage current measurements among its many capabilities. It can perform hipot testing of high-power devices, reaching 500 VA by means of 100 mA for AC power and 25 mA for DC power. It can perform withstanding voltage testing to about 5 kVAC and 6 kVDC and can check ground bonds at current as high as 40 A. The analyzer features a patented open/short check (OSC) function to check for open or short contacts during testing and provide more valid measurement results, such as needed during high-volume production-line testing.

The Associated Research 7850 HypotULTRA® automated dielectric analyzeris well equipped as an electrical safety analyzer, with AC and DC hipot, ground continuity, and insulation resistance measurement capabilities under the control of an easy-to-use touch screen. It can provide AC voltages with 1-V resolution from 0 to 5000 VAC in AC withstanding test mode and 0 to 6,000 VDC with less than 4% ripple in DC withstanding test mode. It commands a voltage range of 10 to 1000 VDC with 1-V resolution for insulation resistance measurements. The analyzer performs continuity testing with current as high as 1 A packing its many capabilities into a rack-mountable housing measuring 16.92 × 3.50 × 15.75 in. (43 × 88.1 × 400 mm).

Additional information on these and other ground resistance testers can be found on the Transcat | Axiom Rentals website (www.transcat.com) or by contacting an advisor at 800-264-4059.