60-Hz resistance of 120 ohms. At peak surge currents above 6 kA, it can be seen that ZSURGE is less than 40 ohms, a 67 percent decrease. For grounding resistances of 10 ohms or less, the surge impedance is not appreciably smaller than the 60-Hz resistance value.
Different kinds of soil and types of ground can also be compared by looking at the surge char- acteristic of grounds shown in Figure 5.3. Here, the ratio of surge impedance to 60-Hz resistance (ZSURGE/R60-Hz) is plotted against peak surge cur- rent. In this figure, curve 2 represents a 10-foot galvanized steel rod one inch in diameter driven into moist clay with a 60-Hz resistance measured at 27.5 ohms. Curve 1 shows four of the same rods as shown in curve 2, spaced in a square 10 feet apart with a measured R60-Hzof 9.7 ohms. As the surge current increases above 12 kA, the ZSURGE/R60-Hzratio of the single rod is less than 0.4, while the four rods in parallel will not have a ratio substantially below 0.7 at higher currents.
To summarize,
• The surge impedance (ZSURGE) of a ground rod or ground rod group is defined as the ratio of peak voltage to peak current.
FIGURE 5.2: Variation of Surge Impedance with Surge Current for Various Values of 60-Cycle Resistance. Source: Westinghouse T&D
Reference Book, 1964, page 593.
FIGURE 5.3: Surge Characteristics of Various Ground Rods. Source: Bellaschi, Armington, and
Snowden, 1942, page 353. 60-Cycle Resistance
Rods In Sand
Rods In Clay
2,000 4,000 6,000
Peak Surge Current (Amperes)
Peak Surge Current (Kiloamperes)
ZSURGE (Ohms) 8,000 10,000 12,000 0 20 40 60 80 100 120
Four 10-ft Rods in Parallel, in Clay
10-ft Rod in Clay 1. 0 2 4 6 8 10 12 14 16 18 0 0.2 0.4 0.6 0.8 1.0 2. 8-ft Rod in Sand
8-ft Rod in Gravel & Stones with Clay Mixture
8-ft Rod in Stones with Clay
Ratio of ZSURGE to R60-H z
• ZSURGEis always less than or equal to the measured 60-Hz resistance of the ground rod(s).
• ZSURGEdecreases with increasing surge current magnitude.
• The proportional reduction of ZSURGEis less for grounds of low resistance than it is for grounds of high resistance.
Arrester Discharge Paths
Surge arresters are applied on distribution lines for two main reasons:
1. To shunt lightning current surges to ground, which reduces the magnitude of surge voltages propagating on overhead and under- ground systems, and 2. To limit overvoltages on
protected equipment.
For the first application to be effective, there must be a low surge impedance to ground. In the second application, ground resistance is not a consideration because the voltage across equipment is limited to the arrester discharge voltage plus the voltage drop produced by the arrester lead(s). However, other elements must be considered when arresters are applied to pro- tect JCN cable.
At the riser pole on wye-connected distribu- tion systems, the arrester down lead is con- nected to the pole ground conductor, the multigrounded system neutral, and the concen- tric neutral of the jacketed cable. Because pri- mary and secondary neutrals are tied together at the pad-mounted transformer, the JCN provides a direct path for discharge currents to flow to the neutrals of premises that the transformer serves. The amount of surge current that flows on the various neutrals is determined mainly by the surge resistance of the pole ground. Surge voltages induced by discharge currents can dam- age the cable jacket and consumer appliances. Various arrester discharge paths that occur at a riser pole have an effect on cable insulation pro- tective margin, cable jacket neutral-to-ground voltage rise, and how current surges on the sec- ondary neutral can damage consumer equipment.
There are also various ways to reduce the magni- tude of discharge currents on the neutral circuit.
Arrester Leads
Lightning is a current generator. Surge arresters are applied at riser poles to protect cables from lightning-induced overvoltages by shunting the surge current to ground. Surge voltages pro- duced by a lightning flash are a function of the current magnitude, its rate of rise, and the dis- charge path impedance. The arrester is con- nected to the overhead conductor and the pole
ground conductor. The dis- charge path that determines the voltage impressed across cable insulation is the arrester and its connecting leads that carry lightning current in paral- lel with the cable termination.
This concept is illustrated in Figure 5.4. Two riser pole installations are shown; the lightning discharge paths are highlighted. Pole 1 represents the desirable connection where no current flows through leads L1and L2. Cable phase insulation will “see” only the ar- rester discharge voltage. Pole 2 is not desirable because the level of protection provided by the arrester is reduced when lead voltages L1and L2 are added to the arrester discharge voltage.
Arrester lead length must be considered in calculating protective margin when evaluating current rate of rise. The protective margin is the difference between the arrester discharge voltages plus the lead L di/dt drop and cable withstand level, where di/dt is the change in current with time expressed as kA/µs (kiloamperes per micro- second). Protection standards suggest using an average rate of rise of 4 kA/µs. Tests have shown that the conductor normally used for leads has an inductance, L, of about 0.4 µH/ft. The lead lengths connecting the arrester to the termina- tion will contribute approximately 1.6 kV/ft to the total voltage across the insulation if they car- ry lightning surge current. The 1.6 kV/ft figure is based on an average probable rise time. Field investigations have shown that this figure will be exceeded 30 percent of the time. Some applica- tion engineers believe 6 kV/ft or higher should be used. To minimize the effect of current rate