Advanced Protection Technologies
14550 58th St. N.
Clearwater, FL 33760
(727) 535-6339
A Preliminary Investigation of Grounding Equipment Performance at Surge Amplitudes and Frequencies: A Stainless Steel Ground Rod Compared to a Copper‐Clad Steel Ground Rod Report Date: July 22, 2013 Prepared by: Lou Farquhar, PE, CEM, GBE VP – Engineering Services Advanced Protection Technologies Rationale and Executive Summary: Surge suppression generally relies upon Surge Protection Devices (SPDs) coupled with grounding methods. Several surge standards explain how inductive impedances attenuate surges and limit surge propagation. This is governed by the formula V = ‐L di/dt, which quantifies voltage drops across inductive components. (This mathematically explains why SPD leads should be short.) During a recent grounding roundtable, there was discussion about how far apart ground rods should be placed to avoid interaction. Traditional practices suggest farther distances to minimize undesirable interactions. Plugging a typical 6,000V, 3,000A 8x20µs surge into the above formula, an APT engineer assessed what affect that a second ground rod might offer, specifically with regard to surge suppression, if driven various distances from the first ground rod. The inductance of wire, L, is approximately 0.3µH/foot. Change in current, di, is ≈3,000A. Change in time, dt, is ≈8µs. (Note that changing surge ampacity and/or rise time can yield dramatic changes.) V = ‐ L di/dt = ‐ 0.3 x 10‐6 Henrys/foot x (3,000A / 8 x 10‐6s) = ‐112.5 V/foot. A ‘crossover point’ can be calculated whereby inductive impedance attenuates the original 6,000V surge voltage down to 0V. In this example, 6,000V / 112.5V/foot = 53.3 feet. Beyond 53.3 feet, there is no longer any driving voltage to push the surge any farther. As it applies to a regional specification requiring 40 foot ground rod spacings, driven 20 feet, the math suggests that no surge current will reach the bottom of the second ground rod. During discussion, one group lamented over ground rods. Pure copper rods are stolen and copper‐clad steel ground rods have rusted away within two years near Florida salt water environments. Stainless steel rods are not stolen, but are reported to have substantially higher ground resistance. Appreciating the issues above, APT hypothesized that stainless steel ground rods might offer similar impedance to copper‐clad at surge frequencies. APT then evaluated the performance of a ten foot, 5/8” diameter stainless steel rod against an identical length and diameter copper‐clad ground rod:  At 50A, 60Hz, the stainless rod offered an additional 178% impedance. (‘Low’ freq.)  At 6kV, 3kA, 8x20µs surge impulse, the stainless rod offered an additional 8.7% impedance. (‘High’ freq.) The comparative impedance differences at different frequencies are substantial. This suggests that ‘low‐frequency’ data may lead to erroneous conclusions during ‘higher‐frequency’ surge events. For example, these results suggest that stainless ground rods may offer cost effective life expectancy benefits, while not seriously degrading surge frequency grounding ability. We caution that this test evaluated ground rods only; no conductor or attachment mechanisms were evaluated. No equipment from the ground measurement industry was used in this evaluation. Disclosure: APT has no financial interest in the ground rod industry. This testing was driven purely by curiosity and not funded by any outside party. APT supplied the test equipment and qualified test personnel. TransCore Tampa provided new, undamaged ground rods and ancillary hardware. Equipment Under Test (EUT): Copper‐clad steel ground rod: Erico 63500 Sectional Ground Rod Stainless steel ground rod: Erico 685800S Sectional Ground Rod Date of Testing: June 2013 Location of Test: Advanced Protection Technologies Lab 1 14550 58th St. N. Clearwater, FL 33760 (800) 237‐4567 Test Personnel: Dan Proffitt: Electrical Technician Paul Rogulski: Electrical Technician Paul Saa: Engineering Labs Quality Manager Michael Gerlach: Director of Engineering Lab Qualifications: ISO 9001‐2008 Quality Management System ISO 17025 – per UL Client Test Data Program Test Facility: Test facility is APT’s test facility at 14550 58th St. N., Clearwater, FL 33760. This consists of Lab 1 and Lab 2. Lab 1 is essentially low current (10kA and below), where Lab 2 is a high current facility (up to 175kA). These labs and personnel have been qualified by Underwriters Laboratories to submit data for UL’s Client Test Data Program. (This testing is not sanctioned by UL; rather we are addressing lab qualifications.) Atmospheric conditions are controlled via air conditioning to accomplish the following: Ambient Temperature: 21°C – 22°C Relative Humidity: 45‐65% Barometric Pressure: 980 – 1030mBar Test Equipment: Instrument Type Make/Model/Serial Number
Last Cal. Date
Next Cal. Date
Oscilloscope Tektronix/TDS‐380/B012514 10/11/2012 10/11/2013 High Voltage Tektronix/P6015A/B033474 10/18/2012 10/18/2013 Probe High Voltage Tektronix/P6015A/B033475 10/18/2012 10/18/2013 Probe Surge Generator KeyTek/ECAT E501A/9701202 Daily Cal Cal before use DMM Fluke/87/68760779 03/05/2013 03/05/2014 DMM Agilent/34401A/US36042259 09/12/2012 09/12/2013 Variac SHLGJT/TDGC2J‐3/‐ NA – use DMM NA – use DMM Weather Station Sunbeam/‐/001 09/12/2012 09/12/2013 Table 1 Lab 1 Environment: KeyTek Surge Generators
Digital Oscilloscope & Electronic Recording Equipment Figure 1 Methodology: Surge Frequency: We selected a 6kV, 3kA 8x20µs surge impulse for this testing. This is the ANSI/IEEE C62.41.2‐2002 Category B Combination Wave, the waveshape used by UL 1449 for VPR determination, and also used by IEC 61643. This is probably the most widely accepted waveform in the surge industry. This wave has two segments, hence the name ‘Combination Wave’. Figure 2 depicts a 6,000V open‐
circuit voltage with a 1.2 µs rise time and a 50µs half‐amplitude duration. Figure 3 depicts a 3,000A short‐circuit current with an 8µs rise time and a 20µs half‐amplitude duration. Figure 2 Figure 3 Open Circuit Voltage Short Circuit Current Important caveats:  Note that the 3,000A is determined by short‐circuiting the surge generator. There is nothing else in the circuit. After calibration, and when an EUT is inserted into the circuit, the actual current through the EUT decreases below 3,000A.  Note that the fronts of the waveshapes have shorter time durations than the tails. This means that the fronts represent higher frequencies than the tails. Surge frequency testing is fraught with unintentional and sometimes unknown measurement error. Seemingly trivial issues make huge differences. For example, air loops in wiring at surge frequencies can create massive error. Figure 4 shows our test setup. Our KeyTek surge generator is calibrated for 6kV, 3kA, assuming the EUT is a relatively small SPD. However, the generator’s leads would not reach five feet in either direction to span a ten foot ground rod. We chose to not alter the surge generator’s calibration, rather we added wire. Addition of wire could create air loops creating measurement error. Consequently, wires were tie‐wrapped to the rod every six inches to eliminate air loops and reduce inductive affects. The addition of any wire adds impedance, which decreases actual surge current flow. Note the symmetry of wire from the generator to each end of the rod. Also note the tie‐wraps. Both rods were tested exactly the same way with the same number and location of tie‐wraps. The surge current comes out of the generator, up the wire to the connection terminal at the end of the ground rod, through the ground rod, through the second connection terminal, back to the generator. Figure 4 Below are wavecaptures of voltages at the probes, with auto‐measured voltages shown in the upper right of each graphic. Stainless steel voltage = 647.9V Copper‐clad steel voltage = 595.9V Figure 6 Figure 5 Our ‘true knowns’ are: identical test conditions and voltages measured at the probes. Knowing that copper‐clad is 595.9V and stainless is 647.9V, we may conclude that stainless offers an additional 8.7% impedance. Percent Change Calculation: (Stainless ‐ Copper‐clad) / Copper‐clad * 100 = (647.9‐595.9)/595.9 * 100 = 8.7% We caution anyone attempting to obtain simplistic ohmic values from these specific results. In theory, the formula V = IZ is appropriate. (For non‐electricals, we have replaced the purely resistive term ‘R’ with ‘Z’ which denotes all impedances; resistive, capacitive and inductive.) Z = V/I, however, we do not exactly know ‘I’, albeit it is less than 3,000A; qualified estimate: 2,500A. In hindsight, we would have measured current. In addition, we would have also compared results to 10,000A impulses, which are available from an adjacent KeyTek generator. Estimated ohmic impedance at 6kV, 3kA 8x20µs: ZEstimated copper‐clad = V/I = 595.9V/≈2500A = 0.238 Ω ZEstimated stainless = V/I = 647.9V/≈2500A = 0.259 Ω 60Hz Frequency: Using V = IR, R = V/I. We flowed 50A of 60Hz current through each length of rod and measured the voltage drop across that length. Copper‐clad: Rcopper‐clad = V/I = 0.195V/50.07A = 0.00389 Ω Stainless: Rstainless = V/I = 0.548V/50.60A = 0.01083 Ω Percent Change Calculation: (Stainless ‐ Copper‐clad) / Copper‐clad * 100 = (0.01083 ‐0.00389)/ 0.00389 * 100 = 178%