IEC TR 61641 Internal Arc Testing in LT Panels Guide

IEC TR 61641 is the IEC Technical Report that gives guidance for testing an enclosed LV switchgear assembly under an internal arcing fault. It is not a mandatory part of IEC 61439, and an IEC 61439 compliant panel is not automatically internal-arc tested — the two verifications answer different questions.

The rule that follows from this: a phrase like "arc proof panel" in an RFQ buys you nothing. An internal-arc requirement is only meaningful when it states a current, a duration, an accessibility arrangement, the arc initiation zones and which performance criteria must be met.

We see this in tenders regularly. "Panel shall be internal arc tested" appears in the specification, no test current or duration is given anywhere in the document, and at technical clarification stage nobody on the consultant's side can say what duty was intended.

An IEC 61439 Panel Is Not Automatically Arc Tested

An LT panel can satisfy:

IEC 61439
Correct short-circuit rating
Correct busbar sizing
Correct breaker rating

and yet an internal arc can still represent a severe hazard.

Why?

Because:

Bolted short circuit

and:

internal arcing fault

are not the same physical event.

An arc creates:

Extreme localized heat
Ionized gas
Pressure
Light
Molten metal
Hot particles

IEC TR 61641:2014 provides guidance for testing enclosed LV switchgear/controlgear assemblies under internal arcing conditions. IEC describes the purpose as assessing the assembly's ability to limit risks to people, limit damage and evaluate suitability for further service following an internal arc.

The first important clarification is:

IEC TR 61641 is a Technical Report—not the basic IEC 61439 compliance standard for every LT panel.

IEC 61439 vs IEC TR 61641

IEC 61439

Addresses normal and defined abnormal performance of LV assemblies, including areas such as:

Temperature rise
Short-circuit withstand
Dielectric performance
Protective circuits
Construction

IEC TR 61641

Specifically provides guidance for evaluating an assembly when an:

internal arc fault is intentionally initiated during testing.

IEC TR 61641 is published as a Technical Report, which means it provides guidance for testing rather than mandatory requirements. Internal-arc testing is therefore not a compulsory verification for every IEC 61439 assembly — it is specified when the project needs it.

Therefore:

IEC 61439 compliant ≠ automatically IEC TR 61641 internal-arc tested.

What Can Cause an Internal Arc?

Possible initiating conditions include:

Foreign metallic objects
Loose connections
Insulation failure
Incorrect maintenance
Component failure
Overheating
Contamination
Incorrect operating conditions

In practice the dominant root causes cluster into four groups: manufacturing and workmanship error, installation error, maintenance error, and applying an assembly outside its intended service conditions.

The correct objective is therefore first:

prevent the arc from occurring.

Then:

limit its consequences if it does occur.

Why Ordinary Short-Circuit Protection May Not Be Fast Enough

An arc has electrical resistance.

Therefore the arcing current can be lower than the theoretical bolted-fault current.

This can mean:

Instantaneous threshold is not reached
Short-time delay remains active
Arc continues longer

Because arc impedance limits the fault current, an arcing fault can sit below the instantaneous pickup of an upstream breaker and be cleared only by the short-time delayed element — which is exactly the wrong outcome for an arc.

That is why arc mitigation often focuses heavily on:

reducing arc duration.

Arc Energy and Time

The longer an arc persists:

More thermal energy is released
More pressure develops
More equipment can be damaged

Therefore even a modest reduction in clearing time can materially change the severity of the event.

This is why arc mitigation systems may use:

Optical arc detection
Current supervision
Fast breaker trip
Arc quenching technologies

depending on the switchgear system.

IEC TR 61641 Test Objectives

The IEC TR 61641 test framework evaluates several outcomes following an internal arc.

The criteria commonly referenced from the report cover, among other things:

Doors/covers remaining in place
Dangerous ejection being limited
Avoidance of dangerous enclosure openings
External indicators not igniting
Protective circuit remaining effective
Arc confinement to the defined area
Possibility of limited continued/emergency operation of unaffected assembly sections

The first five criteria relate to personnel protection; additional criteria extend the assessment toward assembly protection and limited continued operation.

What Each Criterion Group Is Actually Protecting

Criteria group	Protection objective	What a pass demonstrates	Typical use case
Criteria 1–5	Personnel protection	Doors and covers stay in place, no dangerous ejection of parts, no dangerous openings in the enclosure, external indicators do not ignite, protective circuit remains effective	Any occupied electrical room; the normal starting point for a specification
Criteria 6	Assembly protection	Arc damage is confined to the compartment where it was initiated	Plants where loss of the full switchboard is unacceptable
Criteria 7	Limited continued operation	Unaffected sections of the assembly can be returned to service, possibly with restrictions	Continuous-process plants, data centres, critical utilities

Our view: most industrial LT projects in India that ask for "internal arc testing" actually need criteria 1–5, because the driver is operator safety in the electrical room. Criteria 6 and 7 cost real money in enclosure design and compartmentation, and they should be specified deliberately, against a business case for process continuity — not copied into the specification because they appear in a reference document.

Bolted Fault vs Arcing Fault vs Arc Flash Study

Three terms get used interchangeably in tender documents and they are not the same thing.

	Bolted short circuit	Internal arcing fault	Arc-flash study
What it is	Zero-impedance metallic fault	Fault current flowing through an ionised air gap	A calculation of worker incident energy for defined tasks
Governing document for the panel	IEC 61439-1/-2 short-circuit withstand (Icw)	IEC TR 61641 guidance for internal-arc testing	Site safety methodology, e.g. IEEE 1584 based
Current magnitude	Highest	Lower than bolted, because arc impedance limits it	Derived from system study
Main hazard	Thermal and electromagnetic force on conductors	Pressure, hot gas, molten metal, light, ejected parts	Energy incident on a person at a working distance
What proves it	Design verification test evidence	Internal-arc test evidence for the specific configuration	Study report plus PPE and work practice
Common error	—	Treating Icw as an arc rating	Assuming an arc-tested panel removes the need for the study

Personnel Protection vs Equipment Protection

This distinction matters.

An assembly may be designed primarily to:

protect persons standing near it

during an internal arc.

A higher performance objective may additionally seek to:

confine damage to the faulted compartment

and retain other sections for emergency operation.

Therefore, when a consultant specifies:

“Internal arc tested panel required”

the RFQ should define the required:

Test current
Test duration
Accessibility
Arc initiation zones
Required criteria/classification
Assembly configuration

rather than relying on the phrase alone.

Internal Arc Rating Is Configuration-Specific

A test applies to a particular:

Enclosure
Busbar system
Compartments
Doors
Breakers
Venting arrangement
Arc initiation location

Therefore:

An internal-arc test on one panel design should not automatically be presented as proof for every modified panel configuration.

This is similar to the principle behind IEC 61439 design verification.

Fault Current and Duration Must Be Stated

A claim such as:

“Internal Arc Tested”

is incomplete without understanding test conditions.

The performance can depend heavily on:

Arc current
Duration
System voltage
Test location

A panel demonstrated at one duty should not automatically be claimed at a higher duty.

Internal Arc vs Arc-Flash Study

These are different.

IEC TR 61641 Internal-Arc Test

Evaluates the physical behavior of an LV assembly during an internally initiated arc.

Arc-Flash Study

Evaluates worker exposure / incident energy for electrical-system operating conditions and tasks according to the selected safety methodology.

An internal-arc-tested switchboard can be part of an arc-risk mitigation strategy, but it does not eliminate the need for safe work practices or automatically replace an arc-flash study where one is required.

Form 4 Does Not Mean Internal-Arc Tested

This is a major misconception.

Form 4 provides a high level of:

internal separation.

But:

Form of separation ≠ IEC TR 61641 arc performance.

A Form 4 panel may or may not have internal-arc verification.

Conversely, an internal-arc-tested assembly's performance depends on its tested configuration.

Short-Circuit Rating Does Not Mean Arc Rating

A switchboard rated:

65 kA / 1 s

for short-circuit withstand is not automatically proven for:

65 kA internal arc for 1 s.

The tests and physical phenomena differ.

Do not use Icw as a substitute for IEC TR 61641 evidence.

Passive Arc Protection

Passive protection can involve switchgear construction such as:

Strong enclosure
Compartment barriers
Arc-resistant doors
Controlled exhaust paths
Arc-resistant partitions

The objective is to manage the consequences of the event without relying solely on active detection.

Active Arc Protection

Active systems attempt to detect the arc and clear it rapidly.

An optical arc-detection system can detect the intense light generated by the arc and send a fast trip signal.

Optical arc-detection systems are typically offered with optional current supervision, so that a light signal alone does not issue a trip. This matters in a working plant, where welding, camera flash and torchlight are all present.

Light + Current Detection

Using light alone can create concerns from:

Camera flashes
Welding light
External illumination

Some arc-protection schemes therefore combine:

Optical detection + overcurrent supervision

before issuing the high-speed trip.

The actual logic depends on the selected product.

Arc Detection Is Not the Same as Arc Containment

An arc relay attempts to:

shorten the duration.

An arc-resistant enclosure attempts to:

manage the physical effects.

The strongest systems may use both.

Where Should Arc Sensors Be Located?

Potential zones include:

Main busbar
Incomer
Vertical bus
Cable compartment
High-risk functional units

But sensor quantity and position should follow:

the selected manufacturer's engineered scheme.

Do not scatter sensors randomly around the panel and call it arc protection.

Breaker Opening Time Matters

Even if an arc relay detects the event extremely quickly:

the breaker still needs time to physically open and interrupt current.

Therefore total clearing time involves:

Detection + Logic + Trip output + Breaker opening/interruption

A slow upstream device can limit the benefit of a fast detector.

Upstream Breaker Tripping

Sometimes the arc occurs:

upstream of an internal feeder breaker

or even:

between the incomer breaker and main busbar.

In such cases, clearing may require:

Upstream LV breaker
MV breaker
Transformer primary protection

depending on architecture.

Arc-protection design must identify:

which device can actually de-energize each arc zone.

Arc Fault on Busbar

A main busbar arc can threaten a large portion of the assembly.

Possible design measures include:

Busbar compartmentalization
Insulation
Arc-resistant construction
High-speed detection
Fast upstream tripping

The exact solution depends on project criticality.

Maintenance and Internal Arcs

Many arc events are associated with periods when:

Doors are open
Equipment is being maintained
Racking operations occur
Testing is underway

Therefore design measures can include:

Remote racking
Remote operation
Shutters
Interlocks
Maintenance mode
Arc-energy reduction functions

depending on equipment.

Internal Arc Requirement in RFQ

Instead of:

“Panel shall be arc proof.”

define:

IEC TR 61641 reference
Required arc current
Required duration
Accessibility arrangement
Test zones
Required performance criteria
Evidence required

“Arc proof” is too absolute and technically vague.

Common Internal Arc Mistakes

Mistake 1 — IEC 61439 panel assumed to be internal-arc tested

Different verification.

Mistake 2 — Form 4 assumed to equal arc resistance

Internal separation and arc testing are different.

Mistake 3 — Short-circuit rating treated as arc rating

Different physical/test requirements.

Mistake 4 — Test certificate not matched to offered configuration

Configuration matters.

Mistake 5 — Arc relay added without identifying the correct tripping device

Detection alone cannot remove energy.

Mistake 6 — Arc current stated without duration

Incomplete specification.

Mistake 7 — Marketing term “arc proof”

Use precise verified performance instead.

Relevant Standards

The current IEC publication remains:

IEC TR 61641:2014 — Enclosed low-voltage switchgear and controlgear assemblies — Guide for testing under conditions of arcing due to internal fault.

The normal LV assembly framework remains:

IEC 61439-1:2020

and:

IEC 61439-2:2020.

How Wisdom Techno Solutions Handles Internal Arc Requirements

Wisdom Techno Solutions builds LT switchgear at Vadodara, Gujarat — PCC and PMCC panels, MCC assemblies, APFC and detuned APFC panels, VFD and bypass panels, ATS, AMF and DG synchronization panels, and PLC control panels — for pharma, chemical, water treatment, solar, BESS and process industries.

Wisdom Techno Solutions is a licensed partner for three design-verified switchgear platforms: Rittal Ri4Power (Rittal Solutions Partner since 2021 — the Ri4Power system carries Intertek ASTA design verification to IEC 61439-1 and -2 Edition 3, with copper and aluminium busbar systems up to 6300 A), C&S CX (Licensed Partner since 2023, up to Form 4B Type II) and Siemens SIEPAN Elite/8PU (IEC 61439 Solutions Partner since 2024). We build to each system's verified configuration and routine-verify every assembly in-house at our Vadodara, Gujarat facility, for industrial, EPC and infrastructure projects across India. That places the arc-fault containment construction inside a verified design envelope.

When a specification calls for internal-arc performance, we start with the specification itself rather than the panel:

Establish the required duty — arc current, duration, system voltage, accessibility arrangement and which performance criteria apply
Match the enclosure and switchgear system to a configuration for which appropriate verification evidence exists
Check that the evidence matches what is being offered — enclosure, busbar system, compartmentation, doors, venting and arc initiation zones
Identify which device can actually de-energise each arc zone, including the case where the arc is between the incomer breaker and the main busbar
Add up the real clearing time — detection, logic, trip output and breaker interruption — instead of quoting the relay's detection time alone

We are deliberately careful about one thing. We do not tell customers that every WTS panel is "IEC TR 61641 arc proof." That claim would be untrue for any manufacturer, because internal-arc performance belongs to a tested configuration, not to a company. What we will do is engineer the panel around your internal-arc philosophy with appropriately verified assembly systems, and show you exactly which evidence covers which part of the offer.

Send us your single line diagram, fault level, protection settings and the internal-arc clause from your specification. We will come back with what the clause actually requires, what it will cost in enclosure and protection terms, and where we think it is over- or under-specified for your plant.

Conclusion

Internal-arc performance is a separate question from IEC 61439 compliance, from Form of separation and from short-circuit rating. It belongs to a tested configuration at a stated current and duration, and the criteria you select decide whether you are protecting people, the switchboard, or process continuity.

The most useful thing an engineer can do here is refuse the word "arc proof." Replace it with a number, a time and a criteria list, and the whole conversation becomes verifiable.

Related Guides

Frequently Asked Questions

What is IEC TR 61641?

IEC TR 61641:2014 is an IEC Technical Report giving guidance for testing enclosed low-voltage switchgear and controlgear assemblies under internal arcing conditions. Because it is a Technical Report rather than a standard, it provides a test methodology rather than mandatory requirements. It is invoked when a project specifically requires internal-arc performance, and it must then be specified with an arc current, a duration and the acceptance criteria that apply.

Is IEC TR 61641 mandatory for every IEC 61439 panel?

No. It is published as a Technical Report giving guidance for testing, not as a mandatory requirement of IEC 61439. Internal-arc verification is carried out when the project specifies it, and it must then be specified with a current, a duration, an accessibility arrangement and the required performance criteria.

Is Form 4 the same as internal arc protection?

No. Form of separation describes how busbars, functional units and terminals are segregated from one another inside the assembly, which is a construction and maintenance-continuity property. Internal-arc performance is demonstrated by testing a specific configuration under an initiated arc. A Form 4b panel may or may not have internal-arc verification, and the two must be specified separately.

Does a 65 kA/1s short-circuit rating mean a 65 kA internal-arc rating?

No. Icw proves that the assembly withstands a bolted short-circuit current for a stated time without unacceptable damage. An internal arc is a different physical event, producing pressure, hot gas and molten metal rather than mainly electromagnetic force. Short-circuit withstand evidence cannot be presented as internal-arc evidence.

What do criteria 1–5 represent?

They are the personnel-protection criteria: doors and covers remain in place, parts are not dangerously ejected, no dangerous openings appear in the accessible enclosure surfaces, external cotton indicators do not ignite, and the protective circuit remains effective. Criteria beyond these extend the assessment to confining damage to the faulted compartment and to limited continued operation of unaffected sections.

Can an arc-detection relay eliminate arc-flash risk?

No, but it can reduce it substantially when correctly engineered. A relay shortens the time the arc burns, and arc energy is roughly proportional to duration, so the reduction can be significant. Total risk still depends on the breaker's own opening and interruption time, the enclosure construction, and the work practices in place — a fast detector upstream of a slow breaker delivers much less than the datasheet suggests.

Why can an arcing fault not trip the breaker instantaneously?

Because the arc has impedance, and that impedance limits the current. The arcing current can therefore land below the instantaneous pickup setting and be seen only by the short-time delayed element, which by definition waits. This is the central reason arc mitigation focuses on detection and duration rather than on conventional overcurrent settings.

How should an internal arc requirement be written in an RFQ?

State the IEC TR 61641 reference, the required arc current, the required duration, the system voltage, the accessibility arrangement, the arc initiation zones and which performance criteria must be met. Then state what evidence you require and that it must correspond to the offered configuration. Avoid the phrase "arc proof" — it is absolute, unverifiable and will produce non-comparable bids.

Does an internal-arc test certificate cover a modified panel?

Not automatically. The evidence applies to the tested enclosure, busbar system, compartmentation, door construction, venting arrangement and arc initiation location. Change the busbar system, the compartment arrangement or the venting path and the evidence no longer describes what is being supplied. This is the same principle that governs IEC 61439 design verification.

Where should optical arc sensors be placed?

Typical high-risk zones are the main horizontal busbar, the incomer compartment, vertical droppers and the cable compartment. Quantity and position should follow the arc-protection manufacturer's engineered scheme for the specific switchgear, including their rules on sensor field of view and reflected light. Distributing sensors around a panel without that scheme does not constitute arc protection.

Does an internal-arc tested panel remove the need for an arc-flash study?

No. The panel test evaluates the behaviour of the assembly; an arc-flash study evaluates the energy a worker could be exposed to during defined tasks on the system. An arc-tested switchboard is a strong input to reducing that exposure, but where a study is required by the site's safety methodology, the study is still required.

Which standards should be referenced?

IEC TR 61641:2014 — Enclosed low-voltage switchgear and controlgear assemblies — Guide for testing under conditions of arcing due to internal fault — is the current IEC publication for internal-arc testing guidance. The assembly framework itself remains IEC 61439-1:2020 and IEC 61439-2:2020.

Internal Arc IEC TR 61641

Internal Arc Fault in LT Panels: IEC TR 61641 Testing, Protection & Arc Mitigation Explained