## Sphere Gap Breakdown Voltage Tables

The following two tables give the breakdown voltages for a standard sphere gap of the specified sphere size and spacing. The standard accuracy is ±3% for gaps less than half the sphere diameter and 5% for gaps larger than that. The first table is for a gap with one terminal grounded, typically a vertical gap. There are two columns for each sphere size depending on whether it is a positive or negative impulse. The second table is for a gap with both terminals at HV. The values from both of these tables will need to be corrected for current atmospheric conditions, particularly humidity. More details on sphere gap construction and procedures is available.

### Sphere gap with one terminal grounded

A -> AC, DC, either polarity and full negative standard impulse voltages (one sphere grounded)
B -> positive polarity standard impulse voltages and impulse voltages with long tails (either polarity)

As the field gets more uniform (i.e. the gap is a smaller fraction of the sphere diameter), the difference in voltage for the two polarities becomes less. In general, voltages for spheres larger than the largest for which there is a voltage will be the same. That is, 200 cm spheres with a 0.5 cm gap will breakdown at the same voltage as a 15 cm sphere at the same spacing.

Sphere Diameter (cm) 5 10 15 25 50 100 150 200 A B A B A B A B Gap Spacing cm 0.5 17.4 16.9 16.8 16.9 1.0 32.0 31.7 31.4 31.2 31.4 1.5 44.7 45.5 44.7 45.1 44.7 45.1 44.7 2.0 57.5 58.0 58.0 58.0 58.0 2.5 71.5 71.5 71.5 71.5 3.0 85 85 85 85 3.5 95.5 96 97 97 97 4.0 106 108 108 110 110 110 5.0 123 127 127 132 135 136 136 7.5 181 187 195 196 199 199 10.0 257 268 259 259 262 262 262 12.5 277 294 315 317 15.0 309 331 367 974 383 384 384 384 17.5 336 362 413 425 20.0 452 472 500 500 500 25.0 520 545 605 610 30.0 575 610 700 715 730 735 735 740 35.0 725 755 785 800 40.0 862 885 940 950 960 965 45.0 925 965 50.0 1000 1020 1110 1130 1160 1170 75.0 1210 1260 1420 1460 1510 1590 100.0 1870 1900

### Symmetrical sphere gaps

This data applies to gaps where neither sphere is grounded, or, where the gap is so far from the surroundings that the fields relative to ground have no influence.

All voltages in kiloVolts with air at 20deg C, 760 torr, either polarity impulse, AC, DC
spacings <0.5D Accuracy +/- 3% ; spacings >=0.5D accuracy +/- 5% (shown in italics)

Sphere Diameter (cm) 5 10 15 25 50 100 150 200 gap (cm) 0.5 17.5 16.9 16.5 1 32.2 31.6 31.3 31 1.5 46.1 45.8 45.5 45 2 58.3 59.3 59.2 59 2.5 69.4 72.4 72.9 73 3 79.3 84.9 85.8 86 4 107 111 113 112 5 128 134 138 138 137 137 137 8 177 194 207 214 10 248 263 266 267 267 12 286 309 14 320 353 16 352 394 18 452 20 495 504 511 511 25 558 613 628 632 30 744 741 746 35 812 848 860 40 902 950 972 50 1070 1140 1180 60 1210 1320 1380 70 1490 1560 80 1640 1730 90 1900 100 2050

Copyright 1998, Jim Lux / spherev.htm / 29 Sep 2000 / Back to HV Home / Back to home page / Mail to Jim