Arc Flash and Control Measures

What is an Arc Flash?

Arc flash is an event where current flows through the air between phase conductors. It sounds similar to an electric hazard, doesn’t it? So what is the difference between an electric hazard and arc flash? We can differentiate electric hazards and arc flash mainly by their risk of injury. An electric hazard is typically caused by current passing through the human body. It mainly happens because of poor earthing, faulty insulation, etc.

Whereas, in arc flash, injury is caused mainly by thermal energy and explosion.

Arc flash is the rapid release of energy when an electric current flows through the air between phase conductors. It releases tremendous energy in the form of thermal heat, sparks, and toxic fumes. The temperature exceeds 19000 °C. It can start melting metal in the surroundings. The people around can get burned if they have not used proper personal protective equipment (PPE). It can cause vision and hearing damage as well.

A worker wearing Category 4 Arc Flash PPE, including a specialized hooded face shield (arc hood), flame-resistant heavy-duty coveralls, and insulated gloves.

Causes of arc flash

There can be several causes for arc flash, including human error.

Poor maintenance (dust accumulation may help in insulation (air) breakdown)
Poor housekeeping (after maintenance work, debris is not removed from the panel, which may accidentally cause arc flash when power is restored)
Incorrect wiring (when power is restored, it may result in arc flash)
Improper use of tools
Insulation breakdown
Equipment/material failure
It is more likely to happen in DC equipment than in AC, because it is difficult to quench a DC arc.

Control Measures

We cannot completely avoid arc flash while working near live parts, especially when we are working near batteries (direct current).

But, to minimise the harmful effects of arc flash, we can standardise a procedure to follow.

Create a risk assessment before working.
Safety signs
Find the incident energy of arc flash
The arc flash protection boundary can be calculated.
Find the correct Personal Protective Equipment (PPE) for the work

Safety Signs

Safety signs should be posted near locations where arc flash hazards may occur. Some of the examples of safety signs –

A yellow triangular warning sign with a lightning bolt symbol, labeled "Arc Flash Hazard," stating that appropriate PPE and tools are required when working on the equipment.

A red and white "Danger Restricted Access" safety label for electrical equipment, including fields for Equipment Name, Maximum d.c. Voltage, and Short Circuit Current.

How to find the correct PPE?

Personal Protection Equipment (PPE) is rated according to prospective/incident energy from the arc flash, in calories/cm² (cal/cm²). Incident energy is the basis for determining the level of PPE and the relevant arc flash protection boundary.

AS/NZS 5139:2019, table 6.1, tells what the consequences are for different levels of incident energy from arc flash.

PPE is rated in calories (symbol: cal), so the unit joules is not used in this application.

Incident Energy of Arc Flash

Table 6.1 Consequences levels based on arc flash energy (AS/NZS 5139:2019)
Consequence level	Arc flash incident energy level (cal/cm²)
Insignificant	≥ 0.0, < 1.2
Minor	≥ 1.2, < 4.0
Moderate	≥ 4.0, < 8.0
Major	≥ 8.0, < 40
Catastrophic	≥ 40

DC arc flash current is estimated using the following formula.

IE_M = 0.01 V_sysI_arc(TarcD2) MF

Where,

IE_M = estimated d.c. Arc flash incident energy at the maximum power point in cal/cm²
V_sys= system voltage in volts
I_arc = arcing current in amperes
T_arc = arcing time in seconds
D = working distance in centimetres
MF = multiplying factor, AS/NZS 5139:2019, 3.2.4.3

While, I_arc = 0.5 × I_bf

Where, I_bf = battery system prospective fault (short circuit) current in amps

Multiplying factor

The arc flash energy is calculated according to the above equation and is dissipated radially. A battery system (or systems) is located in an enclosure or in a room, where the area over which the energy is to be dissipated is restricted by walls, enclosure roof or even the battery system. This physical limitation means that the amount of energy dissipated in the direction of a person working on the battery could be increased. The factor by which it is increased is known as the multiplying factor.

Battery placement	Multiplying factor (MF)
Open air	1
Battery system rooms	1.5
Battery system enclosures and BESS enclosures	3

Arc flash protection boundary

A technical illustration of electrical safety boundaries: including Arc Flash Boundary, Limited Approach Boundary, and Restricted Approach Boundary

It is a boundary marked by the distance from electrical equipment at which the arc flash incident energy is equal to 1.2 cal/cm2

Arc Flash Formula

AFB = √ (0.0083 × Vsys × Iarc × Tarc × MF)

AFB = arc flash protection boundary in centimetres

A nominal Distance of 45cm is assumed for the calculation of Arc Flash Energy.

Personal Protective Equipment (PPE)

Depending upon the incident energy of the arc, the type of PPE is decided. Standard 5139 Table 3.3 describes different PPE (AS/NZS 5139:2019, table 3.3 is provided at the end of the blog).