Low-Voltage Switchgear Maintenance and Care
The low-voltage switchgear discussed in this section involves power circuit breakers and enclosures of indoor or outdoor type. The frequency of inspection and maintenance should be 3–6 months when new equipment is installed and 1–2 years for existing equipment. However, if problems with switchgear are encountered, the frequency can be shortened. Similarly to medium-voltage switchgear, the conditions that call for frequent inspection and maintenance are high humidity and temperature, corrosive atmosphere, excessive dirt or dust, frequent fault interruption, and age of the equipment. The following guide is provided for the general maintenance of low-voltage equipment; where neces- sary it should be supplemented by the manufacturer’s detailed instructions.
General Guidelines for Inspection and Maintenance of Switchgear
The ultimate long-term performance of switchgear depends on the reliability of its insulation system. An important factor in the insulation reliability is its regular switchgear inspection and maintenance program. The frequency of inspection should be based on the number of scheduled shutdowns, frequent emergency shutdowns, long periods of sustained overloading or abnormal operating conditions, numerous switching operations, number of fault occurrences, and extremes in atmospheric conditions. The following guide is offered on how to inspect and what to look for when inspecting switchgear. This guide may also be used for inspecting medium-voltage switchgear.
On energized equipment
• Listen for popping, spitting, or cracking sounds produced by electrical discharges—also humming noises or vibration produced by resonance.
• With lights out, look for blue or purple corona halos. Orange or red sputter arcs are created by intermittent sparking.
• Ozone, produced by corona or overheating of organic materials, can usually be detected by their odors.
On de-energized equipment
• Look for physical damage—cracks, breaks, delaminations, warping, blisters, flaking, or crazing of insulated parts.
• Check for foreign objects and loose hardware, warped or distorted insulated bus, and rusty or bent structural framework.
• Powdery deposits, carbon tracks, moisture stains or rust, flaking paint, or varnish are signs that moisture is, or has been, present; look for probable source of entry.
Specific areas to inspect
Although the inspector should check the whole insulating structure, there
are a number of specific areas where distress is more likely to occur.
• Boundaries between two contiguous insulating members
• Boundaries between an insulating member and the grounded metal structure
• Taped or compounded splices or junctions
• Bridging paths across insulating surfaces; either phase-to-phase or phase-to-ground
• Hidden surfaces, such as the adjacent edges between the upper and lower members of split type bus supports, or the edges of a slot through which a bus bar protrudes
• Edges of insulation surrounding mounting hardware—either grounded to the metal structure or floating within the insulating member
Physical damage
Broken or cracked insulating supports can allow the supported components to
be subjected to mechanical stresses for which they were not designed, resulting
in ultimate failure. Damaged or contaminated insulation materially reduces
voltage striking and creepage distances, ultimately resulting in a flashover.
Physical damage can stem from several causes:
• Mishandling of the switchgear during shipment, installation, over- loading, or maintenance
• Mechanical forces induced by heavy faults
• Thermal cycling of insulating members
• Strains induced by improper mounting of insulating members
• Combinations of the above
Heat
Temperatures (even slightly over design levels) for prolonged periods can significantly shorten the electrical life of organic insulating materials. A pro-longed exposure to higher than rated temperatures will cause physical deterioration of organic materials, resulting in lower mechanical strength.
Localized heating (hot spots) sometimes occur. They are hard to detect because the overall temperature of the surroundings is not raised appreciably.
Loosely bolted connections in a bus bar splice or void spaces (dead air) in a taped assembly are examples of this.
Since power is usually removed prior to inspection, it is unlikely that apparatus temperature can be relied upon as an indicator of damaging heat.
But observed conditions can be used as the basis for determining heat damage. The signs of heating are as follows:
• Discoloration—usually a darkening of materials or finishes
• Crazing, cracking, or flaking of varnish coatings
• Embrittlement of tapes and cable insulation
• Delamination of taped conductors or laminated insulation
• Generalized carbonization of materials or finishes
• Melting, oozing, or exuding of substances from within an insulating assembly
Moisture
The term moisture, usually associated with water, includes vapors which can readily conduct leakage currents. They are often present as air pollutants in industrial atmospheres. The main source of moisture is highly humid air which is subject to climatic type cycling. The drop in temperature between daytime and dark can cause relatively still air to pass through a dew-point, resulting in condensation. Sudden temperature drops can cause condensation, even inside of buildings housing switchgear. Detection of moisture usually depends on signs, rather than the presence of actual moisture. Look for these indications:
• Droplet depressions (or craters) on a heavily dust-laden bus
• Dust patterns, similar to those on an auto subjected to a light rain shower after driving on a dusty road
• Deposits which remain if a film of dirty water evaporates on a flat surface
• Excessive rust anywhere in the metal housing
• Actual condensation on metallic surfaces, even though the insulation is apparently dry
Tracking
Tracking is an electrical discharge phenomenon caused by voltage bridging
insulating members—phase-to-phase or phase-to-ground. Normally considered to be a surface phenomenon, it can occur internally in some materials.
Materials that are known to track internally are never applied in metal-clad switchgear. Tracking may be detected in various ways:
• Active streamers or sputter arcs may occur on insulating surfaces adjacent to high-voltage conductors. These arcs are very tiny, usually intermittent or random in nature, and of variable intensity. One or more irregular carbon lines (trees) eroded into the insulating surface is a sign that tracking has occurred.
• Materials specifically designed for track resistance seldom, if ever, exhibit carbon lines. Instead, these materials usually develop ero- sion craters after extensive bombardment by electrical discharges.
• Tracking can propagate from either the high voltage or ground ter- minal. It will not necessarily progress in a regular pattern or by the shortest possible path.
For tracking to occur, five conditions must exist simultaneously. Remove any one condition and tracking will cease. These conditions are
• Appropriate temperature
• High local field intensity or gradient
• Contamination on the insulating surface
• Moisture on the insulation surface
• Susceptible insulating material, forming a bridging link over which leakage current can flow; phase-to-phase or phase-to-ground
Corona is an electrical discharge phenomenon occurring in gaseous substances. High electrical gradients, exceeding the breakdown level of the gas, lead to corona discharges. Pressure, temperature, humidity, and the type of gas affect breakdown levels. In metal-clad switchgear, corona (if it occurs) is usually localized in the tiny air gaps between the high-voltage bus bar and its insulation or between to contiguous insulating members or at sharp
* It should be noted that corona usually occurs in switchgear rated at 5 kV and higher. Corona is not a problem in 600 V switchgear. The inspection for corona is listed here only for completeness since this inspection guide may also be used for inspecting medium- and high- voltage switchgear.
corners of the uninsulated bus bars. Corona can be detected in various ways without using instruments as follows:
• A visible, pulsating, blue or purple haze (or halo) may surround the overstressed air gap. The halo is generally of low light intensity and invisible, except in the dark.
• Popping, spitting, crackling, or frying noises may accompany the corona discharge.
• Corona ionizes the surrounding air, converting the oxygen to ozone.
It has a distinctive penetrating odor.
• Its presence may be indicated by erosion of the organic materials adjacent to an overstressed air gap. A white powdery deposit is often present along the edges of the eroded area. In some materials, corona deterioration has the appearance of worm-eaten wood.
• Interference with radio reception may be a sign of corona. If the audible noise level increases as a radio is moved closer to switchgear, corona could be the cause.
Corona discharges create problems in a number of different ways:
• Ionization of the air releases ions and electrons. These bombard near- by organic materials affecting their molecular or chemical structure.
• Ozone, formed by corona, is a strong oxidizing agent; it can also react with many materials.
• Nitrogen in the air will also react to ionizing. When ionized under humid conditions, it forms nitric acid which is harmful to insulation.
Insulations are generally selected from materials having acid resistance, but acids can become the conducting fluids causing the tracking phenomenon. Although switchgear is designed for corona-free performance, there have been cases, in specific applications, where corona has developed.