Sphere Gap Breakdown Voltage Tables

Content on this page from the web site of Jim Lux

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 these tables will need to be corrected for current atmospheric conditions, particularly humidity.

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)

Gap Spacing	5	5	10	10	15	15	25	25	50	50	100	100	150	150	200	200
	A	B	A	B	A	B	A	B	A	B	A	B	A	B	A	B
0.5	17.4	17.4	16.9	16.8	16.9	16.9
1	32	32	31.7	31.7	31.4	31.4	31.2	31.4
1.5	44.7	45.5	44.7	45.1	44.7	45.1	44.7	44.7
2	57.5	58	58	58	58	58	58	58
2.5			71.5	71.5	71.5	71.5	71.5	71.5	71.5	71.5
3			85	85	85	85	85	85	85	85
3.5			95.5	96	97	97	97	97	97	97
4			106	108	108	110	110	110	110	110
5			123	127	127	132	135	136	136	136
7.5					181	187	195	196	199	199
10							257	268	259	259	262	262	262	262	262	262
12.5							277	294	315	317
15							309	331	367	974	383	384	384	384	384	384
17.5							336	362	413	425
20									452	472	500	500	500	500	500	500
25									520	545	605	610
30									575	610	700	715	730	735	735	740
35									725	755	785	800
40											862	885	940	950	960	965
45											925	965
50											1000	1020	1110	1130	1160	1170
75											1210	1260	1420	1460	1510	1590
100															1870	1900

symmetrical sphere gaps 20deg C, 760 torr, either polarity impulse, AC, DC
spacings <0.5D Accuracy +/- 3% ; spacings >=0.5D accuracy +/- 5%

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