Nautel NX100, Preinstallation Manual

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NX100 Pre-installation Manual RF output requirements
Page 6-2 Issue 3.2 2017-03-31
Insulator flashover.
Surface flashovers along an insulator occur when there is an electrical
breakdown in the gas (normally air) at its surface. The mechanism that triggers a flashover depends
on the insulation surface conditions. Gas breakdown flashover occurs when one or more of the
following conditions are met:
• The voltage field at the insulator surface or at an adjacent electrode reaches the critical break-
down level for the gas.
• The electrode or insulator interface is poor or dirty, resulting in a three material junction.
This creates excessive high voltage stress on the air insulator, causing a gas breakdown flash-
over.
• Moisture damage on the insulator surface creates regions of high voltage that can cause a gas
breakdown flashover.
Three-material junctions:
A frequent cause of breakdowns at an insulator surface is the junction
of three materials: a metal conductor, a solid insulator, and a gas insulator. The insulators form a
capacitive RF divider between the metal conductor and the grounded periphery. Since the dielectric
constant of the solid is higher than the gas (air), the high voltage stress is concentrated on the air.
It is very important to minimize the junction's stress gradient and to avoid triple junctions wherever
possible.
Bowl type bushings.
Use bowl type bushing insulators to increase the surface tracking distance
from the central conductor to the grounded periphery or wall.
Insulator Breakdown/Damage.
An insulator can be damaged during gas breakdown flashovers.
When current flows across the surface of an insulator, especially when it is coated with a conductive
contaminant that is slightly damp, carbon tracks can form. Once this occurs, the track provides a
conductive path and reduces the effectiveness of the insulator.
Pitting and erosion of the insulator may also occur. To reduce the possibility of tracking damage,
clean all insulator surfaces periodically, and use stress control techniques at the conductor/insulator
junction.
Other causes.
Other causes of breakdowns may include improperly adjusted spark balls or the
disruption of air gaps by rain, snow, insects, birds, grass, or an accumulation of pollution (soot) on
insulator surfaces.
WARNING: FAILURE TO PROVIDE CORRECT STRESS CONTROL AT HIGH RF
VOLTAGE CONDUCTOR/INSULATOR JUNCTIONS CAN CAUSE INSULATOR
BREAKDOWNS OR EVEN FIRES.