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)
Gap Spacing 5 10 15 25 50 100 150 200
cm A B A B A B A B A B A B A B A B
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)
gap (cm) 5 10 15 25 50 100 150 200
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