A look at arc-resistant switchgear

Switchgear sections with compartments containing power circuit breakers, bus, and cable are the primary means for managing an industrial facility’s electrical distribution. In fact, switchgear is the heart of such facilities.

Switchgear is an engineered-to-order product, meaning it is custom-built by the manufacturer to the exact specifications that meet a given facility’s needs. It’s also built to handle the rigors of day-to-day power needs over multiple decades, and if well-maintained, it will operate optimally to clear any fault condition.

But while maintenance is critical and should be performed on a regular schedule, situations arise that can’t be predicted, such as an arc fault that occurs within a switchgear section. (For recommendations on proper maintenance intervals, see the 2006 edition of the National Fire Protection Assn.’s NFPA 70B consensus standard “Recommended Practice for Electrical Equipment Maintenance.”) The likelihood of that happening is rather remote—if one occurs, it’s typically caused by an external source. Improper maintenance can also cause an arc fault.

No matter the source, the heat and pressure generated by an internal arc fault can have devastating consequences for anyone in close proximity to the switchgear or the equipment itself, and ultimately the facility owner and his business. That’s why many facilities have deployed arc-resistant switchgear, which traditionally has encompassed a heavy, sheet-metal enclosure with venting, designed to direct heat and pressure of an arc fault away from nearby personnel. This configuration also typically results in heavy damage to the switchgear, which is why there is growing interest in alternate methods of limiting fault current damage, such as fast-acting breakers, differential relaying, and interruption and active fault mitigation systems that channel the damaging energy release from an arc fault through a bolted connection. In this fashion, the chances increase that a switchgear section can be saved, and the time and monetary costs to completely replace a destroyed section can be avoided.

Arc-resistant standard

When an arc fault occurs within a confined space, such as a circuit breaker compartment within a switchgear section, the arc energy is converted into heat, resulting in a rapid pressure increase than can cause an explosion that will heavily damage the switchgear and endanger nearby personnel. As defined in ANSI/IEEE C37.20.7-2007, the intention of arc-resistant switchgear is “to provide an additional degree of protection to the personnel performing normal operating duties in close proximity to the equipment while the equipment is operating under normal conditions.” (See ANSI/IEEE C37.20.7, section 1.2.2) According to the standard, normal operating conditions entail:

• Opening or closing switching devices

• Connecting and disconnecting withdrawable parts

• Reading of measuring instruments and monitoring equipment.

This is a performance standard, not a construction standard. It does not specify how switchgear should be built to increase arc resistance, but rather what the results in a test laboratory must be in order for switchgear to be considered arc resistant. It even goes a step further by declaring it does not apply to personnel working in, on, above, or below the equipment, including:

• On top of the switchgear for cleaning and maintenance

• Activities that require a person to be elevated above the base level of the switchgear via a ladder, lift, or on a catwalk

• Switchgear installed on an open grate

• Installations over a cable vault large enough for someone to enter the vault.

Because ANSI/IEEE C37.20.7 does not dictate how arc-resistant construction should be achieved (though a 2007 revision does provide guidelines in this area), facility owners have more latitude beyond the traditional method of using a vented sheet-metal enclosure to protect the switchgear, which is essentially a passive solution. True, it will likely protect personnel walking by or working in close proximity of the switchgear from the effects of an arc fault, which is what it’s designed to do, but when the arc fault has finally been cleared, chances are the equipment will be damaged beyond repair.

That could mean a facility owner’s concerns have just begun. It’s possible the affected switchgear sections could be rebuilt, but depending on the extent of the damage, that could take a couple of days to a couple of weeks. In a worst-case scenario, brand-new switchgear sections will have to be built from scratch, which requires several months of lead time. Meanwhile, manufacturing processes and budgets need to be reconfigured to account for an unexpected capital outlay, and the expectations of customers suddenly need to be managed.

Active fault mitigation

If you remove the source of heat from an arc fault as fast as possible to limit the pressure increase, potential damage to the switchgear and danger to nearby personnel is contained. That is the premise behind an active fault mitigation system, which is typically a bolt-on piece of equipment that is installed within the switchgear at the time of manufacture. (For more detailed information on active fault mitigation systems, see the IEEE article “Arc Terminator an Alternative to Arc-Proofing,” authored by Ruben Garzon, paper No. PCIC-2001-19).

Such systems use a high-speed electromechanical switch to control and direct the current flow of the arc, as opposed to allowing the arc to continue in open air.

The switch is closed by a signal from an electronic control module, which receives virtually simultaneous signals from two types of sensors:

• Current sensor, which detects discontinuity in the current waveform and the exceeding of a threshold current level

• Optical sensor, which visually detects the arc fault.

The switch could be closed via the optical sensor signal alone, but the current sensor helps prevent a false trigger due to activation of the optical sensor by an irrelevant light source. When the switch closes, it provides a low-impedance parallel path to effectively transfer the fault current from the arc to the switchgear’s three-phase main bus assembly. The main bus carries the fault current while it is being sensed and cleared by the switchgear’s current transformers, protective relaying and main breaker. Creating an alternate route for the arc fault by converting heat and pressure in this manner does result in mechanical stress within the capabilities of the switchgear on the main bus assembly, but that’s more desirable than switchgear replacement costs, or worse, accident litigation.

In addition to reducing equipment damage, active fault mitigation systems also allow arc-resistant switchgear performance in rooms with low ceiling heights, since top-venting mechanisms are not required. Traditional passive systems must be vented to allow escape of ionized gases away from personnel in the immediate area. If inadequate ceiling height is available to allow the vented hot gases to cool before reaching personnel, special ducting is required to remove the gases from the equipment room.

The speed with which the electromechanical switch must close in order to prevent arc duration and the resultant heat and pressure buildup is substantial.

Is it necessary?

Though ANSI/IEEE C37.20.7-2007 was expanded to include all types of low- and medium-voltage switchgear, it might be surprising to learn that arc-resistant construction may not be necessary for all applications. Keep in mind that a facility owner’s most important priority is protecting his employees and anyone who works nearby the switchgear. If the equipment is behind a locked door, personnel likely won’t be present if there is an arc fault, thus decreasing the chances they’ll be hurt.

The best rule of thumb for an industrial facility owner considering arc-resistant construction for new switchgear is to review current processes and work flow (or anticipated, in the case of a new facility). For example, arc-resistant construction is highly recommended for the following situations:

• If the switchgear is on the manufacturing floor, near machinery and personnel, or in close proximity to highly traveled or frequented areas, like a lunch room or break room

• If a facility does not have switchgear redundancy

• In an equipment room where routine access is acceptable for monitoring or controlling equipment.

Making the choice

Arc-resistant switchgear is never a replacement for personal protective equipment (PPE) as described in the National Fire Protection Assn.’s NFPA 70E 2004 “Standard for Electrical Safety in the Workplace.” Though ANSI/IEEE C37.20.7-2007 specifically does not cover personnel working in, on, above or below switchgear, it may be tempting for an electrical worker to forgo PPE because he knows the equipment has a vented sheet-metal enclosure or an active fault mitigation system. Put simply, this is not a safe work practice, and a facility owner or manager must discourage this behavior.

But what arc-resistant switchgear can provide is cost management—an arc fault event that is averted due to an active fault mitigation system can prevent a major capital expenditure for new switchgear, or lost business due to processes that have to be halted because power simply isn’t available. That also doesn’t begin to cover the potential healthcare and litigation costs due to worker injury.

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