Installation Precautions

Testing-Vacuum capacitors are well packaged and shipped in a manner to assure safe delivery. However, during shipping they may be subjected to extreme shock which could damage the capacitor elements without damaging the shipping container. Therefore, capacitors should be tested upon receipt and before installation. (See "Capacitor Testing Procedure" TN# 4).

Warning-During installation, avoid twisting or bending strains that could cause failure of the glass- or ceramic-to-metal seal. Mounting Position-Units may be mounted in any position. When large capacitors are mounted horizontally, both ends should be supported by standoff insulators to eliminate excessive stresses and possible damage. (Vertical mounting preferred.)

Cooling-Current ratings are determined on the basis of convection cooling only. It is good design practice to provide an added safety factor by having the variable end mounted on the chassis for heat sinking and by using flexible copper straps for the fixed electrode end. Graphs of recommended minimum strap sizes for various currents and frequencies are available. When a large capacitor is being operated near its maximum current rating at high frequencies, there must be adequate space around the unit for convection cooling.

Water Cooling-Consult factory for specific recommendations. Thermal Expansion-To allow for thermal expansion, at least one end of the vacuum capacitor should have a flexible connection. In variable capacitors, the flexible connection is normally made to the fixed electrode end. Avoid heavy rigid straps or connections that produce a mechanical strain. The connections should be of substantial area to keep losses low and provide cooling by conduction from both ends. Only a small degree of flexibility is required.

Electrical Connections-Both ends of the capacitor may be "hot" or one end may be grounded. Because of the high voltages present, there must be sufficient clearance between the vacuum capacitor and other components to prevent high voltage breakdown. Although the capacitors are designed to ensure optimum electrical field distribution, mounting should be such that adjacent components do not upset this normal electrical field and thus result in excessive heating.

Where the possibility exists of external arcs from transients or parasitic voltages, the capacitor should be protected by adequate corona, or arc shields. Ball gaps are recommended when protection from simple voltage arc cover is required.

Lead Lengths-At high frequencies, avoid long lead length as the reactance of the strap will subtract from the reactance of the capacitor and effectively increase the total capacitance. In some cases, the total reactance may be reduced to the point where the minimum capacity of the capacitor will appear too high.

Other Precautions-Capacitors should not be used as "stand-off" insulators to support heavy assemblies.

Only the part of the capacitor specified as the mounting area should be used for that purpose, and any clamps or straps must be attached carefully to avoid stress on the unit, possibly causing seal failure.

Solder connections should not be made directly to the body of the capacitor, nor should there be any contact with the glass or ceramic insulator during operation.

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