HIGH VOLTAGE SPARK GAPS
Clearances recommended by the International Electrotechnical Commission (IEC) Publication 52, are standard.
International Electrotechnical Commission (IEC) heavy wall hollow is standard in aluminum over 4" diameter, Solid non-melting carbon alloy is used for diameters of 4" or less, although larger diameters in solid carbon are available. A copy of the latest international high voltage standards, including charts for calibration and sphere-gap spacing vs. spark-over voltage, is available at extra cost.
Accuracy is approximately ±2% when barometric pressure and temperature conditions are applied (see ''Adjustments'') and the gap is conditioned by spark-overs until consistent breakdown is reached. Considerable variation may occur over long periods of inactivity if gaps are not conditioned or free ions are lacking or excessive.
To prevent damaging the precision voltage measurement type sphere-gap surfaces, current should be limited. Less than 5,000 amps discharge PK current and less than the equivalent of approximately 15,000 joules total system energy at 100 microseconds time constant should be used. Correspondingly, less energy should be used at longer time periods. Long time DC and 60 cycle current of 50 milliamps or even more, depending on the total time, are allowable. Usually values of 100,000 ohms or more are inserted in series with precision gaps. Carbon spheres can handle higher currents and longer time Power arcs where metal gaps would be damaged, Current must be limited as to maximum current level and time for required life (see Interrupt Time Requirements for coulomb capacity).
Where sphere-gaps are used for transient suppression: some resistance in series can be used in order to provide energy dissipation and to reduce oscillatory action which tends to allow the arc to extinguish too early by creating current zeros. To limit peak current flow, 1 to 2 ohms resistive per kV, possibly more, should be inserted in series with any transient limiting device such as spark-gaps and suppression capacitors, Care must be taken, however, not to have excessive resistance which will limit the clamping effect. 5 to 10 ohms per kV appears to be the upper limit in most cases, usually considerably less than the critical damping values would be.
Interrupt Time Requirements
Where power follow current is available, total sensing and interrupt time should be less than 33-40 milliseconds (2 cycles), An interrupt time of less than 8 to 16 milliseconds (½ to 1 cycle) is preferred. This will help to minimize destructive effects, particularly if the current or time can be limited to allow less than 10 to 20 coulombs (amp-seconds), depending on the electrode material and arc spinning arrangement. Carbon and Tungsten alloys have the higher coulomb capacity.
Ratings for these spark-gap units are 760mmHg, 25°C. Derate, approximately 1.5% per 1,000 feet elevation. Pressure and temperature adjustments can be made according to the following formula:
Vact = Vntp [(0.386P)/(273+T)]
where P is barometric pressure in Torr (mmHg, 0°C) and T is ambient temperature in degrees centrigrade.
See IEC Publication 52 for more accurate correction factors if correction is more than ±5%.
Standard spheres and sphere-gap units contain no added radioactive materials and are not hermetically sealed. The carbon spheres do contain slight amounts of naturally occurring radioactive carbon isotopes. If greater accuracy is required, particularly with close spacings and lower voltages, sparking surfaces can be illuminated with an ultra-violet bulb of at least 35 watts, 1 amp. Accuracy at low voltages can also be improved by inserting radioactive material in the order of 0.2 to 0.6 millicurie in the surface of the spheres. Safety rules must be followed in the handling of radioactive materials. Some users also try inserting a needle electrode thru the side of the housing and energize it to make a triggered spark gap.
Some optional features include: calibrated sphere-gap systems with adjustable positioning holder; solenoids for automatic closing; complete current sensing systems to crobar (closing gaps in case of spark-over); motor and air operated units.
Peak Test to Ground
Standard peak test to ground is approximately 120% of maximum sphere spark-over rating or more.
Note: Sphere-gap spark-over will be non-linear at wider spacings for various conditions of AC, DC, polarity, grounding, and proximity of other objects. Consult standards for accurate settings for these conditions.
Specifications and Dimensions are for reference only and are subject to change.
Contact Ross Engineering Corp. for current information.