A shock from an electric fence pulser is modified and applied to the tower base. The other end of the pulser is attached to a cable that runs along the ground. The cable acts like a traveling wave antenna, providing a temporary path for the return current. The tower base potential is measured between the tower and a second cable, running 90° to 180° away from the first cable. A computer and digitizers capture and store the fast- changing voltages and currents and process them into a value of impedance.
2. What instrument checks and calibration procedures do I need to do before going out to the field, and how do I do them?
The Zed-Meter® instrument should be operated with a short circuit on the terminals, using the tower clamp, and also with a small calibration resistor inserted in series. 3. How is the Zed-Meter instrument connected to a structure? How do I check that my
connections are right?
The Zed-Meter instrument should be connected to the structure using a welding clamp or antenna ground clamp that provides a low impedance at high frequency. Rust should be removed to make the connection. If the current into the tower does not equal the current into the reaction lead after some initial oscillations, lasting no more than 200 ns, or if it does not stabilize to a constant value for about 700–1000 ns, then there is probably something wrong.
4. What lead lengths should I use and why? How does soil resistivity impact my choice? Leads should be at least 90 m long. Longer leads give a wider sweet spot in the measurement, which is hoped to be constant from 500–700 ns after the initial pulse. Coaxial cable (RG58 or RG59) is sold in 500-ft (152-m) lengths at most large hardware stores, so this length is a good choice for most situations. The cables should always be pulled all the way off their wind-up reels. Longer 125-m or 150-m leads are needed in frozen soil, which has a high resistivity; in rocky areas, which also have high resistivity; in grassy areas, where the cable can run well above the ground surface in tall grass; and on towers with guy wires.
5. What external influences will affect the measurements and how?
The presence of power-frequency, static, and radio broadcast current and voltage on the test leads is common, and the instrument will average many pulses to reduce their
influence. Test results will vary (increase) slightly if the overhead groundwires (OHGWs) are disconnected.
6. How do I orient the measurement leads for various electrode and structure types? What precautions are there?
For towers that do not have buried radial wires or continuous counterpoise as
supplemental electrodes, the two leads should normally be oriented along the right of way, 180° apart, and stood off from any nearby metal structures by at least 1 m.
For lines that do have counterpoise, the best results will be obtained if the test leads run at 90° to the line direction. This is a good configuration for all towers.
Especially if the remote ends of the cables are not terminated in short ground rods, they should be treated as live until proven safe by measuring the cable-to-tower potential with a voltmeter. Test cables with the upper limit for bare-hand work (50 V) might not spark when teased against the tower steel, so this check is inconclusive.
7. How do I orient the leads when obstructions such as fences or buildings are in close proximity?
The Zed-Meter instrument results are relatively insensitive to lead layout, as long as the leads are kept away from obstructions that might be well grounded. One suitable rule is the one-diameter goal—for example, keep the leads about 5 m away from a vehicle or shed or 1 m away from a tower leg foundation or fence. In cases where the choices are limited, perform a dipole test of the two leads to confirm that the impedance of the leads is constant and that the duration of the test signal is sufficiently long.
8. What measurement lead height above vegetation can be tolerated before the results are compromised? How can I tell my results have been compromised?
In theory, the Zed-Meter instrument can work even if the test leads are suspended several meters off the ground. Some problems can occur when this is done. The waves travel faster, so the leads need to be longer. It can be difficult to maintain a constant height. The lead should be close to the ground as they approach the Zed-Meter instrument location at the tower leg. A dipole test should be recorded to confirm that the impedance of the leads is constant and that the duration of the test signal is sufficiently long.
9. Can a structure without a shield wire be measured, and if so, what results can I expect if that structure is in high resistivity soil?
The Zed-Meter instrument is designed from the ground up to report the impedance of the local tower, in parallel with the surge impedance of the shield wire. If the soil resistivity is quite high (over 5000 Ωm) and the tower base has a small surface area and geometric radius, the tower resistance can be much less than the surge impedance of the OHGWs, which is typically 170–250 Ω. This will give a rising Z(t) profile and a result with a high standard deviation.
10. Must the measurement leads be grounded? And will this affect the results?
The leads do not need to be grounded except when there are high induced potentials and currents. Tests with and without grounding give the same results in the 500-1000 ns sweet spot when a sufficient number of test pulses are averaged. If the test leads are grounded with resistance of 2000 Ω or less, it might be possible to obtain a Z(t) profile in the time range from 2000 to 10,000 ns that shows the influence of adjacent towers. 11. How do the impedance (Z) results differ from typical resistance measurements and why?
This depends on the type of grounding system. If there are buried wires, the value of Z should be much higher than the low-frequency value, especially at the critical
measurement time of 500 to 700 ns. The impedance profile will fall with increasing time. If, instead, there are only the foundations of the tower and a few local ground rods, Z will be less than the low-frequency resistance, and Z will increase as time moves from 500 ns to 1000 ns. In the first case, the series inductance of the grounding system gives a result with an L/R time constant that is initially high and falls to the level value. In the second case, the capacitance of the soil behaves in the opposite way, initially low and rising to settle at the level value.
12. What does “zed” stand for?
The normal symbol for impedance is Z. In some countries, such as Canada and South Africa, this letter is pronounced “zed.” The original concept was developed in New Brunswick and refined through projects managed by EPRI staff of South African origin who also pronounce the letter “zed” rather than the American “zee.” Zed-Meter® is a registered trademark of EPRI.
B
SPECIFICATIONS
Tables B-1 and B-2 list the physical and electrical specifications of the Zed-Meter® instrument. Table B-3 describes the software outputs. Table B-4 lists the accessories that are supplied, and Table B-5 lists recommended additional accessories. Figure B-1 is a photograph of the
instrument.
Table B-1
Zed-Meter® instrument physical attributes
Parameter Specification
Power supply 100–240 V, 50–60 Hz
Power consumption 100 W maximum
Mass 30 kg
External dimensions 450 × 300 × 200 mm (width × height × depth) Operating time before charging 1–8 hours, depending on ambient temperature
Operating conditions -20 to 40°C, 5% to 95% relative humidity non-condensing
Table B-2
Zed-Meter® instrument electrical outputs
Parameter Specification
Internal communication USB 2.0 (3 devices)
USB wireless support No
Internal signal system
Electric fence pulse generator meeting Underwriters Laboratories (UL) and Canadian Standards Association (CSA) standards; proprietary square pulse-shaping circuit Square pulse output 500 V open circuit, 1 A short circuit; 250 V at 0.5 A into 500 Ω typical Square pulse rise time <10 ns
Square pulse settling time <50 ns with typical leads
Square pulse duration >20 μs
Digitizer sample rate 100 MS/s, four channels simultaneous
Digitizer resolution 8 bits (0.4%)
Digitizer memory 80,000 points (80 μs), 4 channels
Digitizer bandwidth 200 MHz
Current sensor rise time 8 ns (CM-100-M)
Current sensor droop 50% per ms
Table B-3
Zed-Meter® system software outputs
Parameter Specification
Software platform Labview 8.1 for Windows XP operating system Log file
Text (.txt) file for all tested towers, consisting of the following fields: date, line name, tower number, impedance (Ω), voltage, current (mA), comments
Line report
Text (.txt) file for every tower with the same line name, consisting of the following fields: date, line name, tower number, impedance (Ω), voltage, current (mA), comments
Data file
Text (.txt) file for every tested tower, consisting of the following fields:
File name Operator name Date and time Line name
Structure (tower) type Structure (tower)number Line voltage
Target impedance Comments
Impedance measurement start (ns) Impedance measurement stop (ns) Ground impedance
8000 rows of the following data: time (ns), remote voltage (V), lead current (mA), structure current (mA), lead impedance (Ω), structure impedance (Ω)
Table B-4
Supplied accessories
Item Quantity
50-cm RG58 or RG59 coaxial cable terminated in BNC connector 1 Sheet metal welding clamp or truck mirror-style portable CB antenna mount with BNC connector 1
10 Ω, 100 Ω, and 500 Ω calibration resistors 1
Table B-5
Recommended accessories
Item Quantity
Voltmeter 1
Insulated gloves and covers in accordance with local safe work practices for grounded equipment 2 Opaque jacket to shield screen and instrument from direct sun 1
Ground spikes and hammers 2
Figure B-1