Learning the Limitations of Metal-Clad Enclosures

There's a common misconception among many professionals in the electrical industry that can be prove to be deadly under certain circumstances — some technicians, as well as engineers, believe that flame-resistant (FR) clothing is not necessary when working on equipment enclosed inside a metal cabinet (e.g., “metal-clad”).

The “Tabular Approach” for selecting FR clothing in NFPA 70E-2009 exacerbates this problem by classifying most equipment as “Hazard Class Zero” (lowest hazard) when the equipment is contained inside a locked metal enclosure. Because Hazard Class Zero equipment does not require FR clothing, many believe metal-clad enclosures can always be trusted to protect workers from electrical arc blasts when, in fact, they cannot.

Scientific studies have demonstrated that metal enclosures can only contain electrical arc blasts of limited intensity and duration. There are also a number of common work practices and equipment failures that can precipitate arc blasts that exceed the structural limits of metal-clad enclosures and still cause injury to nearby workers. Additional hazards are present anytime the metal-clad enclosures have any intentionally installed openings, such as cooling vents.

In-house research conducted by Praxis Corp., Granbury, Texas, has revealed there are no regulatory or manufacturing requirements mandating that the doors of metal-clad enclosures be of equal strength to the sides of the cabinets. This means that relying on closed and latched doors on enclosures to protect workers from arc blast hazards is sometimes inadequate; therefore, wearing FR clothing (even when working on locked enclosures) is often a reasonable work practice.

Equipment enclosures

There is a surprising lack of available information on the ability of equipment enclosures to withstand electrical arc blasts. One of the guiding documents is the IEEE Std. C37.20.2-1999: “IEEE Standard for Metal-Clad Switchgear.” Although this standard provides excellent guidance regarding proper design and installation of metal-clad equipment, it does have a few noteworthy limitations within the context of this discussion. For example, this standard:

• Assumes fault events will be within the rated interrupting/withstand ratings of the overcurrent protective device (OCPD). Therefore, there is no requirement that the enclosures can contain an arc blast.
• Requires the thickness of the walls of enclosures to be at least 3 mm (steel) while doors may be only 1.9 mm.
• Limits arc duration to 2 sec or less. Normally, this is very reasonable, as arcing faults lasting 2 sec are quite rare in normal circumstances. However, poor maintenance of the OCPD, “dialing up” the setting on the OCPD, or improper replacement of the OCPD could precipitate arcing faults that last much longer than 2 sec.

Other scientific studies have attempted to identify energy levels that will damage electrical equipment, but have achieved general acceptance within the engineering community. One study that contains useful information, “Predicting Damage from 277V Single Phase to Ground Arcing Faults,” by H.I. Stanback, predicts the amount of metal burned away by various combinations of short circuit current (SCC) and time duration of arcing faults.

Although Stanback's models included only single-phase 277V arcs (single-phase to ground or neutral of a 480V wye-connected electrical system), it does provide at least a general idea of the capability of electrical arcs to damage equipment. The following sample calculation using Stanback's models helps illustrate this point.

A single-phase (277V) fault occurs on a 1,000kVA transformer rated at 277/480V with 5% impedance. This transformer is a commonly sized unit for many industrial plants. The arcing fault current on this transformer is approximately 11,480A per phase. The information in the Table (click here to see Table) indicates the amount (in cu in.) of steel, copper, and aluminum that would be burned away for arcs lasting the listed duration (in sec).

Although this table does not precisely predict whether arcs of this magnitude would breach electrical panels, it does clearly indicate the capacity of electrical arcs to burn away metal. At minimum, these arcs would damage equipment, necessitating repair or replacement. It's also quite possible that arcs of this magnitude could breach panel enclosures and still injure personnel in the vicinity of the fault.

NFPA 70E-2009

This standard provides important guidance regarding protection from electrical arcs but only ancillary guidance from electrical blasts. The footnote to the FR clothing table [Table 130.7(C)(11)] on page 34 references the variable “E_bt,” which stands for “break-through energy.” This relates to the force of the arc blast that would blow open or break through clothing, thereby exposing bare skin underneath to incident energy and injury from the blast components.

One potential deficit in 70E relates to the personal protective equipment (PPE) required when working on “exposed” versus “dead-front” equipment. The words “dead-front” refer to equipment with a solid barrier (usually metal panel covers), such that it has no exposed parts energized to 50V or more on the operating side of the enclosure. The circuit breaker panel included in most newer homes is a good example of a dead-front installation. Opening the access door on the circuit breaker panel reveals only dielectric (non-conductive) circuit breakers, a solid metal door that completely covers the circuit breakers, and the rest of the panel opening.

This dead-front is primarily designed to protect the homeowner from direct exposure to the energized elements inside the enclosure. There is considerable evidence that arcing faults can burn through the enclosure cover or sides (Photo) and injure workers/cause fires.

As the name suggests, “exposed” parts have no such barrier; therefore, anyone could touch energized parts without having to remove a barrier to access them. A common example of exposed parts would be a solid blade disconnect installed on A/C units in many homes. Opening the door of this device reveals bare solid-blade disconnect contacts and bare cable terminations.

A review of the FR clothing selection tables on pages 29-31 of NFPA 70E-2009 reveals that FR clothing is not required for work on dead-front installations (listed as having the “doors closed”), unless the equipment contains parts energized greater than 1,000V. The problem with this is threefold:

1. Electrical arcs will exit the equipment through any opening in the enclosure. Many enclosures contain cooling vents that are necessary to keep the internal components within temperature limits for normal operation. Anyone standing in front of the equipment when an arc occurs within could be burned by the arc exiting the enclosure through these openings. Note: It is important never to seal cooling vents in enclosures, as they do serve an important function in the operability of the equipment. Results of arcing also include possible fires.
2. Arcs that last longer than a few seconds can easily generate heat levels that will breach the enclosure and still present a hazard to workers in close proximity to the gear. The arc duration is determined by the length of time it takes for the arc to either operate OCPD or to consume (electrically) the metal in the faulted circuit.
3. The blast generated by the arc can cause the doors of the enclosure to burst open, again exposing anyone standing in front of the equipment to physical trauma. An inspection of typical metal-clad electrical enclosures reveals that the back side of the gear is often secured by four to 10 hardened steel bolts while the access door is often held in the closed position by only two or three “convenience latches,” where a tongue-like metal latch merely slides underneath the lip of the door frame to secure the door. These latches are good when a lock is inserted to prevent the casual access to the interior. However, they're often not able to contain the possible arc blast. They are by far the most likely component of the enclosure to fail when subjected to an arc blast.

The main point of this discussion is that there are many times when a worker would be wise to wear FR clothing even when the 70E Tabular Method for selecting FR clothing indicates that no such clothing is required. Examples of situations where wearing FR clothing as a precautionary measure are warranted, even when working with dead-front equipment, include:

• Resetting circuit breaker (CB) on any fault of unknown origin. Section 130.6(K) of NFPA 70E prohibits resetting devices after a fault trip, unless testing has been performed by a “qualified person” to ascertain the cause of the trip. If, after all such troubleshooting efforts fail to identify the nature of the fault, the CB may be reset. However, this represents a very hazardous situation, warranting additional protective measures.
• Resetting or racking a CB when the enclosure has cooling vents or other openings in the door or walls.
• Operating any device when there is evidence that the device has been subjected to water, heat, or mechanical stress. Examples of this would be carbon on any part of the enclosure, flashover of insulators, hairline cracks on insulators/melting of conductor insulation, or excessive condensation, leaking, rising water, etc.
• Installing personal protective grounds on a circuit of any voltage.

Protection from arc blasts

Protecting workers from the effects of arc blasts begins with proper equipment design and installation to prevent them from occurring in the first place. This includes following the design and installation practices outlined in the National Electrical Code (NFPA 70) and the National Electrical Safety Code (ANSI C2). For the last several years, some manufacturers have included venting of arc blast to a safe area in their designs. However, not all vendors offer this option. Many enclosures rated for high available fault currents do not provide enclosures that will withstand the blasts possible with an arc blast under the maximum conditions.

Although metal-clad enclosures often provide excellent protection from the devastating potential of electrical arc blasts, it's important for electrical workers and engineers to understand the limitations of such enclosures to protect workers in the field. There are a number of circumstances that can compromise the integrity of metal-clad enclosures; however, most are predictable, increasing the ability to effectively control arc blast hazards.

The most effective means to control the hazards posed by arc blasts is to eliminate or mitigate incident energy levels through engineering interventions, such as arc flash hazard analysis plus proper equipment design, installation, and proper periodic preventive maintenance. PPE, such as FR clothing, should always be viewed as a last resort and used only to control residual hazards that remain after all reasonable efforts at mitigating incident energy have been exhausted. NEC Art. 110.16 requires proper arc flash hazard analysis (AFHA) labeling for all equipment that presents an arc blast hazard. Furthermore, workers must be trained to properly interpret AFHA labels and know appropriate safe work procedures.

There are several instances where effectively protecting workers will necessitate them to exceed minimum standards, as set forth by OSHA and the NFPA 70E. It's critical to understand that safe work practices must outline minimum standards, and proper training/good judgment are always necessary when working safely with electrical energy.

Additional research in equipment enclosure development and methods of reducing incident energy exposures is needed to more effectively protect electrical workers. The integrity of electrical enclosures must be tested with specific emphasis on deriving methods for predicting when arc blast events will exceed the design limitations of electrical enclosures.

Kolak is president of Praxis Corp., a firm specializing in electrical engineering and electrical safety training based in Granbury, Texas. He can be reached at jkolak@praxis-corporation.com.