In theory, when the going gets tough, a surface layer of high resistivity crushed stone is a grounding designer’s best friend. Touch and step voltages are too high? Well, even if the people we are trying to protect are running around, thoroughly drenched, bare-handed and bare-footed, clutching substation fences and other handy electrodes, that high resistivity crushed stone will save them every time. Right?

Well… We are making some assumptions about the quality of the crushed stone, when installed and then some more assumptions about the quality of the crushed stone after a few years of wear. What is the resistivity of crushed stone with weeds growing through it, anyway?

Clearly, in good conscience, we should test that crushed stone, upon installation and periodically over time, if we really are concerned about achieving the level of safety specified in IEEE Standard 80. Unfortunately, although IEEE Standard 80-2000 provides a methodology to calculate maximum tolerable touch and step voltages of short duration in high voltage installations, as a function of the resistivity and thickness of a crushed stone layer that may be present over the surface of the earth, the standard provides no methodology for measuring the resistivity of the crushed stone. In many instances, rock samples are sent to laboratories for testing; however, these tests are not necessarily representative of the crushed stone, as installed at site, with the associated compaction, degree of drainage, and contamination from dust, fines, underlying soil, or vegetation.

This article provides a methodology for in situ measurements, which not only yield an equivalent resistivity of the crushed stone, but more importantly, also directly measure the foot resistance value, which can be used to compute maximum tolerable touch and step voltages with the IEEE Standard 80 methodology, without any need to know the crushed stone resistivity, crushed stone thickness, and subsurface soil resistivity values. This is certainly the best way to determine if your crushed stone constitutes… the right stuff.

Measurement Method Overview

The test procedure essentially amounts to the following:

1. Wet the crushed rock to be tested. 

2. Place two foot-sized metallic electrodes, spaced about 1 m apart, on the crushed rock. 

3. Measure the (background noise) voltage between the electrodes and the grounding grid (or a nearby structure connected to the grid). 

4. Apply a signal voltage between the electrodes and the grounding grid. Measure this voltage and the current flow. If the signal voltage is on the same order as the background noise voltage, then increase the strength of the signal or change its frequency (this may require the use of more sophisticated equipment, as described later in this document).

5. The foot resistance to be used in the touch voltage limit calculation is the measured voltage divided by the measured current. An equivalent crushed rock resistivity can be calculated by dividing this value by 1.5. 

6. The foot resistance to be used in the step voltage limit calculation can be measured by applying the signal between the two foot electrodes. An equivalent crushed rock resistivity can be calculated by dividing this value by 6.0. Alternatively, an approximate foot resistance for the step voltage calculation can be obtained by multiplying the foot resistance obtained in Step 5 by 4.0.

Construction of the Electrodes

The foot electrodes must satisfy the following criteria:

1. Their dimensions should be 26 cm x 7.7 cm (this provides an equivalent surface area to the circular electrode used for the simple calculation in IEEE Standard 80 and a more realistic, more conservative shape).

2. They must have continuous, compressible metallic surfaces, which will make good contact with the crushed stone, when placed on the crushed stone and weighted down.

3. During the measurements, a weight of about 25 kg or more should be placed on each electrode or made a part of the electrode, such as to press the surface of the electrode on the crushed rock.

4. For the foot resistance test used to calculate the touch voltage limit, the most important test, the electrodes must be connected together with an insulated conductor, rated for the applied voltage and current: a #14 AWG insulated copper conductor with 600 V insulation should do. If a foot resistance test is carried out for the step voltage limit, the electrodes must be disconnected during this test.

5. During both touch and step voltage foot resistance tests, the electrodes must be placed 1 m apart. To enhance the reliability of the testing, a length of nylon string or other such insulating material can be used to tie the electrodes together such that their spacing, from center to center, is 1 m, when the string is fully extended.

The following describes one possible means to construct such an electrode:

1. Cut a 26 cm x 7.7 cm rectangle of wood, whose thickness is great enough to support a 25 kg weight without breaking and which can withstand rough treatment.

2. Staple multiple runs of bare wire to one 26 cm x 7.7 cm face of the wood and bond the wires together. Bond a jumper to the wires to allow the electrode to be connected to other circuitry.

3. Staple wads of metal wool to the face of the wood that is covered with the bare wire. Try to achieve an even, continuous compressible surface, covering the entire surface of the wood, without extending outside the 26 cm x 7.7 cm area, with no significant gaps nor bulges.

4. A layer of rubber could be glued to the opposite side of the electrode, in order to provide an insulating layer between the electrode and whatever weight might be placed on the electrode in the field. Note that wood, when wet, is not a good insulator.

Ideally, the wires, staples and wool would all be made of the same metal, in order to avoid galvanic corrosion. This is not a prerequisite for success of the testing, at least in the short term, but would increase the lifespan of the electrode.

Figure 1 shows a schematic of the equipment connections for the testing. Figure 2 provides the circuit diagram of a connection box that can be used to connect the equipment rapidly in the field.

Foot Resistance for Touch Voltage Limit           Foot Resistance for Step Voltage Limit

Figure 1. Equipment Connections for Foot Resistance Measurement

Figure 2. Circuit Diagram for Sample Connection Box

The following equipment can be used for the testing, if no excessive 60 Hz noise is present (e.g., no more than 10% of the applied signal):

· Foot electrodes described earlier in this document.

· An ungrounded 60 Hz portable generator, supplying 115 V output, as a signal source. Note that a substation’s auxiliary power supply would not be suitable here.

· A voltmeter (e.g., a Fluke 85 multimeter).

· An ammeter (or a precision shunt resistor, with a voltmeter measuring across it).  Here again a Fluke 85 multimeter would be fine as an ammeter.

· The required connectors and wiring to connect this equipment together, as illustrated in Figure 1.

Figure 3 provides a photograph of this equipment.

Figure 3. Cast of Characters: Connection Box, Foot Electrodes (right-side up and upside down),
and Multimeters. Portable generator not shown

In selecting equipment, keep in mind that the voltmeter must be able to measure the full applied voltage from the portable generator, whereas the ammeter function must be able to measure currents on the order of the applied voltage divided by 1.5 – 6.0 times the crushed rock resistivity. Thus, for a 115 V source and a crushed rock resistivity of 5,000 ohm-m, the ammeter must be able to measure 15 mA (115/[1.5*5000]) for the touch voltage foot resistance test or 3.8 mA (115/[6.0*5000]) for the step voltage foot resistance test.

If excessive 60 Hz noise is present (measurement of noise is discussed later in this document), then frequency-selective equipment is required. In this case, the following substitutions can be made:

· For the voltmeter, replace the multimeter by a two-channel dynamic signal analyzer, such as an Agilent Technologies AT-35670A Dynamic Signal Analyzer. This instrument will require its own independent, ungrounded, portable generator (1 kW or more).

· For the ammeter, use the second channel of the dynamic signal analyzer, measuring the current across a precision shunt resistor.

· For the signal source, use a variable frequency power supply, such as a California Instruments Model 1501L-1M Programmable Variable Frequency Power Source. This instrument will require its own independent, ungrounded, portable generator (3 kW or more). Do not use the same portable generator for the signal source as for the dynamic signal analyzer: noise can flow from the variable frequency power supply to the dynamic signal analyzer through the power cord!

Test Procedure

The test procedure is as follows:

1. Select a statistically significant number of test points representing different types of test locations: e.g., 1 m outside the perimeter fence, 1 m away from high voltage equipment, locations where the crushed rock looks particularly thin or contaminated with fines, locations where the crushed rock looks in excellent condition, locations where there is vegetative growth. Flag (for future remedial action, if necessary) and record the locations, along with a short description of each.

2. Wet the crushed rock to be tested with rainwater or bottled spring water. Perform two tests: in the first case, simply wet the crushed rock with a thin spray, so as to avoid washing contaminants off the rock, but ensuring that the rock is wet enough to have dripped down a few inches; in the second case, thoroughly soak the crushed rock and underlying soil. The area to be wetted is a 2-m diameter circle centered midway between the two foot electrodes.

3. Inspect the undersides of the foot electrodes. If they are dirty, rinse them clean.

4. Place the foot electrodes, spaced about 1 m apart, on the crushed rock and weight them down. Connect them up to the other equipment, as shown in Figure 1. For tests involving the grounding grid (i.e., the touch voltage foot resistance test), connect to the grid by means of a pigtail that grounds a piece of equipment.

5. With the signal source off, measure the voltage across the signal source output. If multimeters and a 60 Hz source are being used, this is simply the AC voltage read by the multimeter. If frequency-selective equipment is being used, then this is the voltage corresponding to the test frequency being used (this test frequency is typically on the order of 65 to 70 Hz). This is the background noise and should be recorded for each test.

6. Turn on the signal source and measure the resulting voltage. Also measure the current flow. If the signal voltage is on the same order as the background noise voltage, then tuned-frequency devices are required; if they are already in use, then the frequency must be changed or the applied signal increased in strength.

7. For the equipment connections shown on the left hand side of Figure 1, the foot resistance to be used in the touch voltage limit calculation is the measured voltage divided by the measured current. An equivalent crushed rock resistivity can be calculated by dividing this value by 1.5. This value takes into account the resistivity of the soil beneath the crushed rock: i.e., it includes the surface layer derating factor described in Section 7.4 of IEEE Standard 80-2000.

8. The foot resistance to be used in the step voltage limit calculation can be measured as illustrated on the right hand side of Figure 1. An equivalent crushed rock resistivity can be calculated by dividing this value by 6.0. This value also takes into account the resistivity of the soil beneath the crushed rock. Alternatively, an approximate foot resistance for the step voltage calculation can be obtained by multiplying the foot resistance obtained in Step 7 by 4.0.

When Did You Last Check the Resistivity of the Crushed Stone in a New or Aging Substation?

Robert D. Southey

The Right Stuff

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