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A Review of the IEEE Guide for Grounding System Characterization

and Application in Touch and Step Potential

Estimations

Ahmad Shahsiah, Ph.D., P.E.

March 18, 2015

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Purpose of Grounding

• Grounding is one of the means of safeguarding employees and the public from injury – Other means include, but are not limited to, guarding,

adequate clearance above ground, proper burial depth, etc.

• Grounding also allows protective devices to operate during a fault condition

• The basic theory behind grounding is to keep the voltage of grounded parts as close as possible to the potential of earth, so that a voltage difference does not exist between a person and a grounded metal object (NESC 090)

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Grounding and Bonding

• Normally non-current carrying conductive materials enclosing electrical conductors and equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.

• Normally non-current carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment shall be connected together and to the electrical supply source in a manner than establishes an effective ground-fault current path.

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Examples of the Secondary Grounding Grounding

Y-connected, 3-phase, 4-wire e.g. 120/208 V and 277/480 V

Delta-connected 3-phase, 4-wire e.g. 120/240V, 3φ, 4-W, center-tap

Single-phase, 3-wire system e.g. 120/240V, 1φ, 3-W

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Why Measure Earth Resistivity?

• Earth resistivity is used to estimate:

– Ground impedance of a grounding system

– Earth potential gradients to estimate step and touch voltages

• Earth resistance data provides a quick estimate of the potential rise of the ground electrode at the time of a fault

• Ground grids are designed to limit the surface voltage gradient

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How to Measure Earth Resistivity? • The fall-of-potential method is commonly used to measure the

ground resistance with respect to a grounding electrode • The method involves passing current between a “Ground Electrode”

(E) and a “Current Probe” (G) and measuring voltage between the electrode (E) and a “Potential Probe” (P)

• Distance of the P to the electrode G is 62% of the distance of G to the electrode E – This method assumes soil with uniform resistivity

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Clamp-on Method • This method measures resistance of a grounding electrode by clamping

onto the down-lead-wire • It induces voltage at higher frequencies (~1kHZ). Induced voltage

circulates back and is measured by a second meter probe • Is widely used but has limitations:

– The grounding electrode must have relatively low impedance – Large error can be introduced if the reactance of the path is large compared to the

resistance because of the higher frequency measurements

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Touch and Step Potentials

• Dangerous levels of voltage can develop on grounded equipment and on the ground surface due to a high-current short-circuit fault

• OSHA regulation 1910.333(a)(1): …Live parts that operate at less than 50 volts to ground need not be de-energized if there will be no increased exposure to electrical burns or to explosion due to electric arcs.

• Lower voltage levels of concern may be defined by the user based on the perception level as determined by IEC60479-1-2005

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Measuring Step and Touch Voltages • Current injection method:

– Involves injecting lower level currents into the ground and measuring touch and step potentials

– Scaling the measured potentials to values that would be encountered during a fault based on the ratio of the calculated fault current to the test current

• Examples of locations at which touch voltages can be created: – Steel structures – Grounded equipment housings – Fences – Gates

• Measurement methods: – Inject current between a remote point and simulated fault location – Measure the touch potentials using twisted wire pairs – Measurement equipment should have sufficient accuracy to distinguish

created potentials as a result of the test current from noise – Estimated step and touch potentials can be compared with tolerable

voltages defined by OSHA or minimum perception levels defined by the IEC

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Human Body Resistance

IEC60479-1-2005 (Figure 5)

Dependence of the Total Impedance ZT of one living

person on the surface area of contact in dry condition

and at touch voltage (50Hz)

Touch Voltage

5th Percentile

50th Percentile

95th Percentile

25 960 Ω 1300 Ω 1755 Ω

50 940 Ω 1275 Ω 1720 Ω

75 920 Ω 1250 Ω 1685 Ω

100 880 Ω 1225 Ω 1655 Ω

Values for hand-to-hand paths in saltwater-wet conditions are listed from IEC60479-1-2005 (Table 3)

• The heart-current factor allows

calculation of current through other

body pathways that represent the

same danger of ventricular fibrillation

as the left hand to feet pathway.

• The heart current factor for left-hand to

right hand is 0.4 (IEC Table 12).

• This means the estimated body current

for the pathway from left-hand to right-

hand must be multiplied by a factor of

0.4 to get the equivalent heart effect

between the left hand and both feet.

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Electric Shock Hazard vs Current Through the Body and Exposure Time

• IEC time/current zones for the left hand to feet pathway (IEC60479-1-2005 Table 11): – AC-1: Perception possible but

usually no ‘startled’ reaction. – AC-2: Perception and involuntary

muscle contractions likely but usually no harmful electrical physiological effects.

– AC-3: Strong involuntary muscle contractions. Difficulty in breathing. Reversible disturbances of heart function. Immobilization may occur. Effects increasing with current magnitude. Usually no organic damage to be expected.

– AC-4: Patho-physiological effects may occur such as cardiac arrest, and burns or other cellular damage. Probability of ventricular fibrillation increasing with current magnitude and time.

Source: IEC60479-1 – 2005 (Figure 20)

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Stray Voltage Measurements • Stray voltages and

currents are unavoidable side effects of the grounding system

• Stray voltages and currents should be low enough to avoid affecting livestock

• There has been considerable research performed in this area. Some of the results of this research were used to create laws in states such as Wisconsin and Idaho

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Effect of the Load Resistor • Voltage measurements without a load resistor will result in overstated

measurements of stray voltage • Parasitic capacitances exist due to the presence of leakage and space

charges, and can transfer small amounts of energy into the meter input • The digital multi-meter leads and surfaces being measured can act as

antennas and result in invalid readings

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Primary

Neutral

Wire

Secondary

Neutral

Wire

Neutral

Isolator

Down-

ground

Wires

Distribution

Transformer

Effect of the Load Current

• Some of the load current imbalance returns through the ground creating stray voltage

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Typical Grounding and Bonding Techniques May Not Limit the Ground Voltage for Fast-Changing Currents

IEEE C62.41.1

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Questions