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.
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 
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