Arc Flash hazard

To be affected from shock the operator shall be in touch with a live part. But Arc flash effects can strike even far from the short circuit point, and its energy decreases with distance: this is why the most effective protection against this hazard is based on the use of adequate PPEs rated for Arc Flash protection but mostly from working procedures aimed to prevent short circuit and/or to put the operator as far as possible from live parts.

Arc flash hazard was first brought into the lights in 1981 from a paper presented at the U.S. Industrial Applications Society Annual Meeting from Ralph Lee. Before this, electrical safety was almost entirely focused on shock protection. Lee wrote:

There is another hazard [that] few appreciate – the case where contact is not necessary to incur injury […] This is the radiation burn from the fierce fire of electrical arcs, due to a short circuit that develops from poor electrical contact or failure of insulation.”

Despite the heavy consequences of the arc flash were already clearly highlighted in this first article, it took more than a decade before that 1995 NFPA 70E edition started to address this electrical hazard, and other seven years before that IEEE 1584 guide for performing Arc Flash calculation was published. Nowadays almost all electrical safety international standards consider this risk, although NFPA 70E remains a milestone.

NFPA 70E defines the Arc Flash hazard as a source of possible injury or damage to health associated with the release of energy caused by an electric arc .

The amount of energy can be really huge: a 50kA arc fault is estimated to produce? heat at a temperature of 20.000 C (3-4 times the sun temperature), a pressure wave of 2 atmospheres, project molten metal at a speed of more than 4000 m/sec and generates a sound pressure approaching 165 dB, bigger than the one of an airplane taking off.

The standard defines requirement for personnel skills and PPEs according to the distance from the equipment It can be easily understood that the energy decreases as? distance? increase? from?? the?? fault?? point, and?? that?? at?? a?? certain?? separation - depending on the initial energy value? - corresponds an energy that can no longer permanently arm an operator. The correspondent energy value is 1.2 cal / cm2 and the distance at which this limit is accomplished is called Arc Flash Boundary.

An Arc Flash Risk assessment shall be performed to:

???????? Identify Arc Flash Hazards and related distances

??????? Estimate the Arc Flash likelihood and the potential consequences – this may have reflections on the electrical protective devices type and setting and their maintenance/verification over time

??????? Determine PPEs and Work Practices.

Electrical equipment such as Switchgears, Motor Control centers, Panels etc., likely to require examination, adjustment, servicing or maintenance while energized, shall be labelled with a Warning sign reporting Arc Flash Risk assessment related information.

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Mandatory information, according to NPFA 70E, is:

??????? Nominal System Voltage

??????? Arc Flash Boundary

??????? At least one of the following:

??????? Available incident energy and the corresponding working distance, or the Arc Flash PPE category (but not both);

??????? Minimum arc rating of clothing

??????? Site-specific level of PPE.

Practically labels contain reference to both Arc Flash and Shock Hazards, as both are likely to be present when performing an electric work, and frequently the PPE category isn’t given explicitly as it is determined from the energy.

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For example, the label below tells that:

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??????? The incident energy at the working distance, 18 inches (typical value for LV MCCs and panels), is 32.8 cal/cm2

??????? The fault current is 29.62 kA

??????? The Arc Flash boundary is 168 inches

??????? The voltage is 480 Vac, information necessary to assess the right insulation degree of PPEs and tools, the instruments etc.

??????? The limited approach boundary, that an unqualified operator cannot cross without being escorted or the equipment made safe

??????? The restricted approach boundary, that can be crossed only from qualified personnel using properly insulated PPEs and tools

Finally, the label gives a list of recommended PPEs.

Note: the label format may vary.

Apart from the use of PPEs, as said the risk against arc flash shall mitigated using working procedures that put the operator as far as possible from the electrical equipment.

These may include:

??????? Remote only opening and closing of protection devices;

??????? Using remote controlled racking/unracking devices;

??????? Using tools that allows operation execution from a greater distance

Conclusions:

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??????? Unlike shock hazard protection, there is no automatic protection or technical measure that can mitigate the arc flash hazard: due to its nature, the protection can be achieved only with the use of appropriate PPEs and working procedures.

??????? When reducing the distance from the arc, the energy increases and therefore the effectiveness of the Arc rated PPEs decreases. Maintain distance from potential Arc Flash sources as much as possible.

??????? Arc rated clothing shall allow for movement and visibility, and shall cover all non-arc rated clothing.