Protection against arc flash (assembly internal arc faults)

While shock protection during electrical work is generally well-managed, arc flash hazards remain a critical risk.

Arc flash (also called assembly internal arc fault), a short circuit through ionized air, can lead to devastating explosions, producing extreme heat, light, and molten metal.

Fig. F94 – Consequences of an arc flash event, including intense heat, pressure waves, shrapnel, toxic gases ... and psychological trauma.

Origin of arc flash (assembly internal arc fault)

An arc flash can occur in areas where bare conductors are present. This hazard is therefore always present in metal-enclosed switchgear or electrical assemblies with busbars and power connections.

Assembly internal arc fault may occur in normal operation without any worker activities, and may be caused by:

• Insulation aging
• Intrusion of foreign bodies (rodents, water, etc.) in the enclosures
• A short circuit on a feeder that does not adhere to the safety perimeter causes gas exhausted by the circuit breaker of the faulty feeder to ignite an arc upstream on the busbar

Electrical worker activities can also lead to an internal arc fault:

• Directly: The electrical worker, while performing his duties, is introducing a conductive object near live components, such as a tool or wire
• Indirectly: The electrical worker, when operating the equipment, uncovers a pre-existing issue:
  • Closing a feeder on a short circuit without maintaining the safety perimeter can cause gas released by the circuit breaker of the faulty feeder to ignite an arc upstream of the busbar
  • The shock or moving part of switching devices during racking in, racking out, opening or closing switchgear causing the fall of an unexpected foreign part (screw, washer, piece of wire, filings remaining from cutting, etc.)

A pre-existing issue can be difficult to predict because the equipment can pass a dielectric test without indicating any problems.

MV installation standard IEC/EN 61936‑1 has addressed this risk for several years, but this was not true for LV installations before 2024.

The 2024 edition of IEC 60364‑4‑42 now requires consideration of protection against assembly internal arc faults.

Note: most incidents occur in low-voltage switchgear, affecting qualified electrical workers, who account for 66% of fatalities in the US.

Consequences of arc flash

Arc flash incidents result in significant human and financial costs, affecting both workers and businesses with serious consequences:

• Severe burns and life-altering injuries
• Operational downtime, impacting business continuity
• Regulatory fines and legal liabilities for non-compliance
• Damage to company reputation and employee morale

Solutions to mitigate the risk of arc flash

While the most effective risk reduction method is to work on de-energized installations, electrical work often requires operating near live components, which could still pose a risk of triggering assembly internal arc faults due to factors like insulation aging or foreign object intrusion.

To mitigate arc flash risks, businesses must adopt a multi-layered approach:

• Design for safety – Incorporate internal arc fault mitigation systems
• Use monitoring and automation – Ensure workers understand proper procedures and reduce direct worker interaction with live equipment
• Follow standards evolution – For example, EN50110 edition 2023, IEC 60364-4-42 update 2024

Building a strong safety culture with advanced mitigation technology protects workers and businesses from the devastating consequences of arc flash incidents.

Focus on IEC 60364-4-42 update 2024

Since its 2024 update, IEC 60364‑4‑42 includes internal arc fault protection (arc flash) requirements in Clause 427, covering:

• 427.1 – Protection of persons
• 427.2 – Protection of the assembly

Protection of persons

For assemblies rated ≥ 800 A and installed where the presumed short-circuit current is ≥ 25 kA (AC), Clause 427.1 requires measures to protect persons from internal arc faults.

Accepted solutions include:

• Locating assemblies in an area with access restricted to authorized and trained people only
• Using assemblies tested for internal arc withstand
• Implementing an Internal Arc Mitigation System (IAMS)
• Applying fast-acting instantaneous protection (permanent or temporary), such as ERMS

Where work on energized assembly is frequent, IEC recommends using IAMS or fast instantaneous protection (ERMS) to reduce arc fault energy.

Protection of the assembly

Clause 427.2 focuses on protecting the equipment itself in critical continuity of service scenarios. Internal arc faults not only endanger personnel, they can also cause significant damage to equipment and surrounding structures.

Where service continuity is essential, IEC recommends installing an Internal Arc Mitigation System (IAMS) that conforms to IEC TS 63107.

Support for operational continuity

Operational continuity is the ability of an electrical system to remain functional under fault conditions. This depends on:

• Redundant design
• Preventive maintenance
• Protection strategies like IAMS or ERMS

IEC 60364‑4‑42:2024 supports these goals with clear provisions that balance safety and resilience — especially in environments where downtime carries financial, operational, or safety consequences.

General guidelines to help select arc flash mitigation measures

These guidelines should be seen as a basis for your own consideration. The choices you will make for the design of your own project remains under your responsibility. Note that IEC 60364-4-42 requirements are applicable for AC systems with a rated current >= 800A and with a maximum prospective short-circuit current >= 25kA.

Fig. F95 – Selection guidelines - Solutions to mitigate arc flash risks

Installation characteristics			Worker Operations		Recommend equipment type / options
Ik3max (400V)	Main incomer	Panel is NOT accessible to ordinary person	Operation in front of live panel is forbidden	Maintenance or evolution of live panel is forbidden	IEC 61439-2 type tested switchboard	Arcing class C TR 61641	Optical Internal Arc Detection TS 63107	ERMS on the main incomer	Arc Quenching Device
< 25 kA	<= 630A	-	-	-	☑
	> 630A	-	Yes	Yes	☑
			No OR don't know	Yes	☑	☑
			No OR don't know	No OR don't know	☑	☑		☑
>= 25kA < 50kA	<= 630A	-	-	-	☑
	> 630A	Yes	Yes	Yes	☑	☑
				No OR don't know	☑	☑		☑
			No OR don't know	Yes	☑	☑       OR        ☑
				No OR don't know	☑	☑	☑	☑
		No	-	Yes	☑	☑       OR        ☑
				No	☑	☑	☑	☑
>= 50kA	<= 630A	-	-	-	☑
	> 630A	Yes	Yes	Yes	☑	☑		☑
				No OR don't know	☑	☑	☑		☑
			No OR don't know	Yes	☑	☑	☑	☑
				No OR don't know	☑	☑	☑		☑
		No	-	-	-	-	-	-	-