New solutions address arc flash energy reduction

The NEC's new set of solutions keep electrical engineers safe during inspections.

09/22/2011


Figure 1: The relative clearing times for circuit breaker trip functions are shown. Courtesy: Eaton Corp.Ideally, work on an electrical system can be 100% safe only if that system is totally de-energized while work is being performed. In many instances, such as in the “process” industries where facilities are required to operate continuously, a totally de-energized system may not always be possible for these situations. The electrical system may need to be energized to conduct maintenance or to perform troubleshooting tasks. The steps involved in confirming that an electrical circuit is indeed de-energized may also put workers at risk.

Employees who work in the electrical inspection industry find that they may be exposed to shock and arc flash hazards while conducting the necessary inspections of electrical systems.

To help address this issue, the 2011 Edition of the National Electric Code has a new set of solutions for addressing arc flash energy reduction in circuit breaker applications. According to Article 240.87:

    240.87 Non-instantaneous Trip. Where a circuit breaker is used without an instantaneous trip, documentation shall be available to those authorized to design, install, operate or inspect the installation as to the location of the circuit breaker(s).

    Where a circuit breaker is utilized without an instantaneous trip, one of the following or approved equivalent means shall be provided:
    (1) Zone-selective interlocking

    (2) Differential relaying

    (3) Energy-reducing maintenance switching with local status indicator

    Informational Note: An energy-reducing maintenance switch allows a worker to set a circuit breaker trip unit to ‘no intentional delay’ to reduce the clearing time while the worker is working within an arc-flash boundary as defined in NFPA 70E, 2009, Standard for Electrical Safety in the Workplace, and then to set the trip unit back to a normal setting after the potentially hazardous work is complete.

In order to maintain selective coordination in some applications, an upstream main circuit breaker may be chosen that does not have an instantaneous trip function. In these cases, if the main circuit breaker were to experience a short-circuit condition, it would remain closed for its preset delay time. And the longer it takes to trip, the higher the arc flash risk.

Therefore, the idea behind the IBEW proposal and the subsequent new NEC requirement is that in cases where the circuit breaker is used without an instantaneous trip, an alternate means shall be provided to reduce the fault clearing time while a worker may be within an arc flash boundary of that circuit breaker. Many circuit breakers are designed so that the instantaneous function may be turned “off.” In cases where the instantaneous trip is turned “off,” the circuit breaker will not trip via its instantaneous function; and per the new NEC requirement, an alternate means for reducing the clearing time shall be provided.

Energy-reducing maintenance switch

The three devices and technologies listed in the new NEC requirement have been available in the electrical industry for several years. While a zone selective interlocking (ZSI ) scheme will provide a faster trip time than the preset short delay (or ground-fault) trip time, its response is typically slower than both the “no intentional delay” of the instantaneous trip times and the trip times of energy-reducing maintenance switching devices. Depending on the circuit breaker manufacturer, ZSI trip times are in the range of 80-130ms, instantaneous trip times are typically 20-80ms, and energy-reducing maintenance switching devices are as fast as less than 50ms.

Figure 1 shows a simple relative comparison of the tripping times for the various electronic circuit breaker trip functions: short time delay, ZSI, instantaneous, and energy maintenance switching. The overlapping of the time bands indicates the typical variation to be expected between different types of circuit breaker designs (molded case versus power circuit breakers) and between different circuit breaker manufacturers.

In situations where it may be impractical to de-energize the electrical equipment, in addition to employing all the other appropriate “traditional” solutions for arc flash reduction, it may be desirable to use an energy-reducing maintenance switch to further reduce the tripping time and subsequent arc flash hazard to personnel working on downstream energized equipment.

An energy-reducing maintenance switch is a device that has been designed specifically to be used by personnel only while they are required to perform work on energized electrical equipment, as permitted by NFPA 70E, Standard for Electrical Safety Requirements for Employee Workplaces. This device is not intended to be continuously active. The inclusion of this device as part of a system’s overall arc flash reduction strategy provides enhanced personnel protection.

Once activated, the energy-reducing maintenance switch option provides a lockable switch feature that should be included in lock-out/tag-out safety procedures. Once the work has been completed, the energy-reducing maintenance switch is deactivated and the system returned to its optimal protection state.

The basic energy-reducing maintenance switching design is one that incorporates an additional electronic control circuit that may be separate from the normal instantaneous or short time protection circuits in the trip unit. The purpose of the separate control circuit is to allow the electrical worker, on demand, to “switch-in” a system that will trip the overcurrent protective device in a time that will provide the minimum possible arcing time should an arc flash incident occur while energized work is being performed on that device.

The tripping times that can be achieved by the energy-reducing maintenance switching schemes may be faster or slower than the “instantaneous” trip times of the trip unit, depending on the circuit breaker manufacturer.

Once the energy-reducing maintenance switching scheme is turned to an active state, a local indicator is turned on and the local protection settings are overridden by the energy-reducing maintenance switching settings.

Manufacturers of energy-reducing maintenance switching schemes provide appropriate written instructions for the safe installation and testing of the schemes. Once qualified personnel complete the energized electrical work, the energy-reducing maintenance switching scheme is switched off and the system returned to its normal operating state.

Figure 1: The relative clearing times for circuit breaker trip functions are shown. Courtesy: Eaton Corp.

Different types of switching

A few different basic types of energy-reducing maintenance switching schemes are available today.

The selection of the settings for the energy-reducing maintenance switching may be integral to the trip unit on the circuit breaker, or done by optional remote settings via a remote switch contact operation or via various communicating devices. One such design scheme provides flexibility in the selection of the pickup level for tripping.

The pickup may simply be set one time at only the lowest, most sensitive level, or it may be set in relation to the calculated arcing fault current, the normal load current, and any possible transient currents from application-specific transformer inrush or a motor starting. An energy-reducing maintenance switching pickup setting is then selected that is above the total load plus transient currents, but below the calculated minimum arcing current. This setting allows the device to trip at the expected arcing current but avoids nuisance tripping from transient load currents.

Table 1 shows the arc flash incident energy is reduced from 10.7 cal/ cm2, by approximately 80%, to 2.2 cal/ cm2 by use of this energy-reducing maintenance switching scheme. In this example, the system data is as follows: 480V System voltage; switchgear, 24-in. approach boundary, solidly grounded.

The total clearing time for this separate energy-reducing maintenance switching scheme may be faster than the instantaneous trip time of the electronic trip unit, as shown in Figure 2. As a result of its very fast tripping time, this energy-reducing maintenance switching design will yield lower arc flash energy even if the circuit breaker tripped via the normal instantaneous trip response.
The benefit of this very fast tripping response is why 2011 NEC requires this type of technology to further enhance worker safety in hazardous arc flash circumstances.

An alternate energy-reducing maintenance switching design scheme is also available in a simpler configuration. With this type of circuit breaker configuration, the switching scheme is switched on or off as needed, with the local indicator present as required by the 2011 NEC. This design does not provide a means separate from the protection settings for selecting a pickup level for the energy-reducing maintenance switching. Instead, it uses the existing short-time delay pickup setting on the trip unit.

For energy-reducing maintenance switching, the short-time current pickup setting must be set below 85% of the calculated minimum arcing current at the system location where it is expected to provide “fast” interruption. At the completion of the energized electrical work, in addition to switching off the energy-reducing maintenance switching scheme, this short-time pickup setting must be also returned to its previous protection setting. When activated, this type of energy-reducing maintenance switching design also provides a faster tripping time (approximately 80mS) than normal short delay tripping, but will not be faster than the normal instantaneous trip time of the circuit breaker.

Figure 2: These time-current curves show faster trip time with energy-reducing maintenance switching. Courtesy: Eaton Corp.

Are all devices equal?

Since the primary purpose of an energy-reducing maintenance switching device is to provide very fast tripping times during an arc flash incident, a key parameter for selecting among manufacturers of these devices is the device’s total clearing time in the event of an arcing fault. While several manufacturers offer energy-reducing maintenance switching designs that meet the 2011 NEC requirements, performance of their designs can vary as the total clearing times of the various design solutions are different.

For some manufacturers, the total clearing time provided by their energy-reducing maintenance switching device is even faster than the normal “no intentional delay” instantaneous trip time. Other manufacturers’ their total clearing time may be slower than their instantaneous trip times.

To ensure that the device with the fastest total clearing time is chosen for an arc flash reduction application, the electrical system designer should consult the manufacturer’s published data to identify and verify the total clearing time performance of each energy-reducing maintenance switching device option.



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