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HIGH VOLTAGE
SPARK GAP
Clearance
Standards
Clearances recommended by the
International Electrotechnical Commission (IEC) Publication 52, are
standard.
Diameter Standards
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
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.
Current Limitations
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).
Transient Suppression
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.
Elevation-Temperature-Pressure
Adjustments
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%.
Increased Accuracy
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.
Options
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.
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