Product Safety and the use of EMC techniques to evaluate the 30 Amp Ground continuity test

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1 Product Safety and the use of EMC techniques to evaluate the 30 Amp Ground continuity test Ron Duffy Product Safety Engineer Agilent Technologies, Inc Richard Georgerian Product Compliance Engineer Carrier Access Corporation Technical Presentation - October 3,

2 TDR continued. Plot 1 is a good ground connection. Plot 2 is the same, a good ground connection with the ground lead dressed against the rear panel. By examining the two plots one can conclude the TDR is not a good method to prove a good ground connection. Plot 1. Plot 2. Technical Presentation - October 3,

3 Impedance Analyzer Test Setup An impedance analyzer was used to examine the grounding system. The impedance analyzer was first connected to a known good ground. Figure 3. Technical Presentation - October 3,

4 Impedance Analyzer Test Setup continued. Plot 3 shows the result of the good ground as tested with the impedance analyzer. The screw holding the ground lug was then loosened one-quarter turn. The impedance analyzer did not show the loose screw. This is shown by Plot 4. Horz = 78mV/div Vert = 2.60V/div Start Freq = 5Hz; Stop Freq = 13MHz; Step = 500kHz Horz = 78mV/div Vert = 2.60V/div Start Freq = 5Hz; Stop Freq = 13MHz; Step = 500kHz Plot 3. Plot 4. Technical Presentation - October 3,

5 Triggered Oscilloscope Method Apply a direct current high impedance source across the grounding system. 5kohms To oscilloscope 5Vdc Ground System Figure 4. Schematic 1. Technical Presentation - October 3,

6 Triggered Oscilloscope Method continued. Inducing mechanical energy into the product will trigger the oscilloscope if the connection is loose. Shaking, or a two to four inch bench drop, will work. Horz: 1ms/div Vert: 2V/div Plot 5. Technical Presentation - October 3,

7 Thirty Ampere Ground Test The right value to insure the incoming mains power protection system, fuse or circuit breaker, will operate. Some standards may specify other currents and/or times. Previous 30 amp testing did not prove defective ground connections would be found. Visual inspection was the only way. Technical Presentation - October 3,

8 Thirty Ampere Ground Test continued. A measurement method that was accurate and allowed control of time and current was used. Digital Voltmeter Precision resistor Current probe Pulse generator Constant Current generator Oscilloscope Current flow Ground System Differential probe Current flow Figure 5. Schematic 2. Technical Presentation - October 3,

9 Thirty Ampere Ground Test continued. The thirty ampere ground test was divided into five parts. 1. Bench mark: Using the split-half method. 2. Loose screw: One-quarter turn loose. 3. Wire crimped, no solder. 4. Loose crimp 5. Cut strands Technical Presentation - October 3,

10 Thirty Ampere Ground Test continued. Bench mark Thirty amperes was conducted through a known good ground as a bench mark. Refer to Plot 6. Plot 7, Benchmark 47.7 amperes without damage. Horz = 20sec/div Vert = 20mV/div Delta = 9mV Plot 6. Horz = 20sec/div Vert = 40mV/div Delta = 43.8mV Plot 7. Technical Presentation - October 3,

11 Thirty Ampere Ground Test continued. Loose screw Before applying benchmark current, a thirty ampere ground test was performed. Comparing the voltage waveform to the benchmark, the loose screw was detected. Technical Presentation - October 3,

12 Thirty Ampere Ground Test continued. Loose screw continued. Apply the bench mark current of 47.7 amperes: The loose screw increases the resistance. Result: Smoking at the crimp, insulation overheating, and discoloration of the lug. Horz = 20sec/div Vert = 100mV/div Delta = - 206mV Plot 8. Technical Presentation - October 3,

13 Thirty Ampere Ground Test continued. Wire crimped, no solder. Apply the bench mark current of 47.7 amperes: A wire was fastened but no solder was applied. Result: The ground passed the thirty ampere test but not the bench mark test. Horz = 20sec/div Vert = 200mV/div Delta = 56.3mV Plot 9. Technical Presentation - October 3,

14 Thirty Ampere Ground Test continued. Loose Crimp Apply the bench mark current of 47.7 amperes: The wire strands were inserted in the lug but not crimped. Result: Heating occurred at the lug, causing blistering of the insulation. Horz = 20sec/div Vert = 60mV/div Delta = 80.6mV Plot 10. Technical Presentation - October 3,

15 Thirty Ampere Ground Test continued. Loose Crimp continued. Cautionary note: If the strands in the uncrimped lug are making good electrical contact because of sideways pressure the benchmark will not work. The sideways pressure might be caused by the lead being dressed with a cable tie so that the wire in the lug is forced to one side of the crimp. Technical Presentation - October 3,

16 Thirty Ampere Ground Test continued. Cut strands 18AWG stranded wire, 34 strands. Groups of four strands were cut until it was determined that the benchmark test would show a cut strand connection. Result: After a total of 20 strands out of 34 (59%) were cut, the ground system failed the benchmark test. In all cases the thirty ampere ground test found no failures. Technical Presentation - October 3,

17 Thirty Ampere Ground Test continued. Cut strands continued. Chart 1 details the cut strand experiment. If one was very familiar with the benchmark system, one could see the resistance changes with the cut strands Cut 4 strands out of 34 stranded wire. Cut 8 strands out of 34 stranded wire. Cut 12 strands out of 34 stranded wire. Cut 16 strands out of 34 stranded wire. Cut 20 strands out of 34 stranded wire Constant Current (amps) Rdelta(ohms) Chart Passed Passed Passed Passed Passed Passed Passed Passed. Began to smoke at 90 sec. Passed Failed S T U V W X Y Z AA AB Experiment # Technical Presentation - October 3,

18 Redundant grounds One hundred amperes was passed through this ground system. Result: No failure. Technical Presentation - October 3,

19 Recommendations Benchmark during product development. Archive the benchmark's voltage waveforms and current value. Technical Presentation - October 3,

20 Conclusion TDR and sweepers cannot find faulty ground. The triggered oscilloscope method may find loose screws. The benchmark current test can find loose screws, crimped but not soldered joints, and cut strands if more than half the strands are cut. Redundant grounds are robust enough to defy all high current testing. There will always be cases that require visual inspection to determine good ground systems. Technical Presentation - October 3,

21 Abstract Required by most standards. Test performed to ensure that ground has been assembled properly. The test does not ensure quality earthing connections. Explored other techniques to detect poor quality grounding system. Technical Presentation - October 3,

22 Introduction Different techniques were used. A more reliable and robust method to determine good ground continuity was found. Methods were found that would non-visually prove a good ground connection. Technical Presentation - October 3,

23 Description of ground system Earth ground wire Solder lug Crimp lug Screw Standoff Figure 1. Technical Presentation - October 3,

24 TDR Test Setup The TDR could not detect a bad connection. Found to be sensitive to lead dress. Figure 2. Technical Presentation - October 3,

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