Introduction

In today’s ever changing environment of electrical power distribution equipment and systems, safety and reliability are becoming the focal point of utility and industrial switchgear users. Many of these locations have upgraded their systems by converting older air circuit breakers to vacuum or by ordering new systems that are “Arc Resistant”. However, many are not aware of the opportunity to upgrade existing switchgear cubicles to arc resistant retrofits achieving optimum safety for operating personnel and reliability of installed systems. Arc Proof retrofits of existing metal clad and metal enclosed switchgear is available for lineups of most manufacturers and vintages.

Members of governing bodies (such as the NFPA 70E) are becoming more concerned about an increasing number of accidents and injuries from electric arcing faults. Extreme temperatures and pressures as well as electromagnetic radiation are all effects that need to be dealt with to provide optimum safety for plant operations personnel. Incident energy and arc flash boundary are becoming increasingly important in the everyday operation of power distribution systems.

New switchgear installations require a more stringent standard than older vintages. Associations such as EEMAC (Electrical Equipment Manufacturers Association of Canada), IEEE (Industrial Electrical & Electronics Engineering Society), IEC (International Electrotechnical Commission) are specifying the requirements for new safety features in switchgear. Through the door racking of circuit breakers, separate instrument sections and various other features are now standard on new equipment to provide additional safety for operators. New switchgear can also be ordered as arc resistant, providing the highest level of safety to operating personnel by reflecting internal arc byproducts in a safe direction.

The following article discusses the options available for upgrading previously installed medium voltage switchgear to an arc resistant design. The designs discussed in this article have been tested to EEMAC G14 -1 and meet IEEE standard C37-20-7. This process can apply to commonly found utility and industrial installations throughout North America.

Electrical Faults & Arcs

There are a variety of circumstances that result in internal arcs in electrical switchgear. Often times this failure occurs when the breaker fails during routine switching or when clearing a through fault. A dangerous situation also exists when a breaker fails to properly open prior to removal or insertion. Other causes of internal failure are due to partial discharge activity that weakens the insulation over time. Normally any line surges on equipment with weakened insulation are subject to internal failure. Operational mishaps can occur to cause internal faults such as tools, jumpers or other equipment left in a cubicle during routine maintenance checks.

Internal Faults in Switchgear
The extent of protection against occurrence of internal faults varies depending upon the type of switchgear installed. The lowest level is plain Metal-Enclosed switchgear. Mid level protection is found in the various forms of “Hybrid” Metal-Enclosed and the highest-level protection is Metal-Clad. Should however an internal fault occur, none of the above is designed to withstand its effects. There is much confusion with the through fault interrupting capability of the switchgear as also applying to an internal fault with stand capability. This is not the case.

Internal Consequences
When an internal arc occurs in a switchgear cubicle, there are a variety of phenomena that occur. Depending on the amount of available fault current and its duration, a certain quantity of hot gases, hot glowing particles and super heated air are produced. There are also potentially toxic components (from insulation and other materials) and vaporized metal particles (plasma). Another major occurrence in this situation is a sudden large internal pressure rise.

External Consequences
There are a number of external consequences that occur during a switchgear arc and failure. Pressure will force particles and gases out of holes, cooling vents and any gaps. With no designed controlled escape point the weakest part of a gear will fail. Pressure at the front door can be between 50,000 and 100,000 pounds static. Front or rear doors are most likely points of failure leading to personnel injury. All of the above occurs in less than 10 cycles. This normally is not enough time for a protection relay and upstream breaker to react.

There up to 4 stages of events during an internal fault.

Stage 1 is the compression stage and starts at “Arc Event” time zero and continues to a point of maximum internal pressure. (less than 10 cycles)

Stage 2 is the expansion stage and starts when the pressure relief vent begins to open, ending gas flow. This stage is characterized by wave motion and possible under-pressure with a duration of 5 to10 milliseconds.

Stage 3, the emission stage, starts when the pressure relief vent has opened and ends when the gas in the cubicle reaches arc temperature. Duration of stage 3 is typically 50 to 100 milliseconds.

The thermal stage in this process is stage 4 and lasts until the arc is extinguished and all combustible material has been consumed. The greatest damage to the equipment occurs during this last stage.

Arc Resistant Switchgear Types

Based on the EEMAC standards, there are 3 types of arc resistant switchgear. Type A requires protection from the effects of an internal arc in the front of cubicle to a height of 2 meters. Type B provides protection in the front, rear & exposed side of cubicle to a height of 2 meters. Non exposed sides are excluded in type B. The last type is type C which is the same as type B with the additional provision of inter-compartmental protection with the exception of the main bus compartment. EEMAC also requires that the building housing the switchgear be considered in the overall design by the end user. IEEE is similar but contains some variations.

Retrofitting Existing Switchgear to Arc Proof

Switchgear retrofits to arc resistant design are available for most manufacturers and also for most vintages of switchgear. Tested designs meet all EEMAC and IEEE standards for protection.

Switchgear cubicles with newly installed vacuum breakers are prime candidates for cubicle upgrades to arc resistant. Vacuum retrofits have already extended the life of the switchgear by adding a new breaker. The circuit breaker is the device that normally shortens the overall life due to characteristically having numerous moving parts. The switchgear cubicles themselves will last for a very long time due to the fact that they have limited moving components.

Modern switchgear arc resistant retrofits allow for system switchgear to be upgraded to include the cubicle front door and steel framework and associated protection or metering if so desired. The doors are a heavy duty design with multiple hinges and locking devices to guarantee that arc blast does not escape through the front door of the switchgear. The extra strength locking devices on the handles work in conjunction with the cubicle modification to ensure the door cannot open under the extreme force. In order to mount such a door a heavy duty steel frame is also installed.











 

One of the major exposures that operating personnel experience when operating metal clad switchgear is during the racking in and racking out of circuit breakers. Anyone that has performed this operation recognizes a distinct sound that occurs when the breaker is just making or breaking the primary connection of the power stabs from the finger clusters to the main bus stabs. That distinct sound is air ionizing and this ionized air can compromise the insulation value of the phase to phase dielectric inside the cubicle.

Through the door racking is a requirement of all switchgear arc resistance retrofits. This upgrade means that the operator will rack the circuit breaker in and out with the door closed. The possibility of exposure during failure is extremely limited during this process.

One of the major factors in releasing energy contained inside switchgear cubicles is to providing an intricate venting system. It is common when a switchgear cubicle fails that the damage cascades to other cubicles or to other equipment located in the same room. A cubicle failure can destroy overhead cable tray or adjacent control system mimic panels or control sections resulting in weeks or months of downtime for equipment and systems outside the immediate cubicle area.

The venting system employed in arc resistant retrofits is a key to releasing the energy in a controlled manner. Optimum safety of the personnel as well as decreased damage to equipment in the surrounding area is the key to each individual design. All this means a higher degree of safety for site personnel and limiting the degree of downtime suffered.

Out door houses are particularly susceptible to major damage when a breaker cubicle or cable entry section fails. It is common that smoke, heat and flash damage can virtually immobilize entire outdoor switchgear and control houses. Venting to outdoor is a key to limiting the damage.

TESTING DETAILS

Retrofit designs are tested at a certified laboratory on typical switchgear cubicles to meet the standards as discussed in this article. An internal short circuit is established to test the retrofit’s capability to withstand the sudden temperature and pressure rise. Indicators are located at numerous points within 10 centimeters of the switchgear. The internal fault is set up to establish the maximum stress on the design and establish the capability of the arc resistant retrofit. Typically, test currents exceeded 30,000 Amps RMS with a peak of 75,000 Amps for a full 60 cycles.

Summary

Operations personnel are exposed to potential hazards during normal operation of power distribution switchgear due to the extremely high levels of energy that are involved when switchgear fails. Arc resistant retrofits are a great option for life extension, improved productivity and most importantly operating personnel safety. These retrofits are available for most manufacturers and vintages of switchgear and should be considered for any application where the safety of personnel or the reliability of equipment is a major concern.

Notes:  

1.0  Arc Flash clothing is an MEC/CIP design/distribtuon.

2.0  Arc Resistance Testing was performed at Power Tech labs in Dec. 1997.   Copies of this test report are available upon request.

4.0  Main Switchgear in Magna brochure # 4.1.9 is at Erco plant and was designed and installaed by MEC.  Arc Flash Clothing is CIP design.

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