Arc Flash in MV Switchgear: Containment, Sensing, and What Can Actually Be Retrofitted

Arc flash in MV switchgear is controlled in two ways: containment, where a type-tested enclosure holds the arc and vents its gases away from people, and sensing, where light and current detectors trip the breaker before the arc does lasting damage. Containment is a property of the switchgear design, proven by the internal arc test in IEC 62271-200, so it cannot be bolted onto an existing panel. Sensing can. ABB’s REA product guide puts the arc relay’s trip command at under 2.5 ms, and states that an arc lasting 500 ms may cause severe damage while one cleared in under 100 ms usually causes limited damage. For an older lineup, New India Electricals Ltd (NIEL) treats retrofit as a question of cutting arc duration, because the enclosure you already have is the enclosure you keep.

TL;DR

• Arc flash protection in MV switchgear is either passive (containment by an internal arc classified enclosure) or active (detection and fast clearing). ABB defines passive protection as containing the arc and directing gases and debris to a safe area, and active protection as limiting incident energy.

• Incident energy is directly proportional to arc duration and arc current, according to ABB’s arc flash mitigation guide. Cutting clearing time is the one lever that works on every panel, old or new.

• Internal arc classification (IAC) under IEC 62271-200 is earned by type testing a specific design at a stated current and duration. A type test certificate covers the design and rating tested, not a modified panel.

• Arc sensing is retrofittable. ABB states that lens-type sensors are especially suitable for retrofit, and the REA system sends trip commands in under 2.5 ms, but the total clearing time still includes about 60 ms of breaker operation.

• Arc quenching goes further. ABB’s Ultra-Fast Earthing Switch (UFES) extinguishes the arc in under 4 ms after detection and is offered as a retrofit through ABB Service, for ratings up to 40.5 kV and 40 kA.

• IEEE 1584-2018, the IEEE incident energy calculation guide, covers 208 V to 15 kV only. A 33 kV board sits outside its model, so containment and clearing time matter more there, not less.

What happens inside an MV panel during an internal arc?

An internal arc is a fault in which current flows through ionised air inside the switchgear enclosure instead of through the conductors. ABB’s guide to IEC arc flash mitigation solutions (1SFC170008N0201, revision C) states that arc temperatures can exceed 20,000 deg C, more than three times the 6,000 deg C surface temperature of the sun, and that the blast pressure wave can reach beyond 3 metres.

The same ABB guide lists the usual causes: dust, corrosion, condensation and animal intrusion; tools dropped or left behind; failed insulating materials; improper installation; loose busbar or cable connections; and missing or poor maintenance. NIEL’s panel engineers make the same point about workmanship. In a factory discussion recorded at NIEL, they note that if a site operator does not fit a cable termination properly, sparking starts, and that sparking can turn into an internal arc.

That list shapes the retrofit question: most of those causes sit at joints and terminations, which a retrofit can inspect and monitor but an enclosure upgrade cannot reach.

What does internal arc classification (IAC) actually certify?

Internal arc classification (IAC) is a rating under IEC 62271-200 showing that a switchgear design, tested with an arc at a stated current and duration, protected people standing on the classified sides. The current edition, IEC 62271-200:2021 (edition 3.0, published 27 May 2021), covers metal-enclosed switchgear above 1 kV and up to 52 kV. It moved the examples not covered by the IAC test into clause 9.103 and added a 1 s rule to Criterion 4 on ignition by hot gases versus glowing particles.

An IAC rating reads as a code. The ABB UniGear ZS1 technical catalogue (1VCP000138) rates that range IAC AFLR, where A means access is restricted to authorised personnel and F, L and R mean the front, lateral and rear sides passed the test. The same catalogue lists internal arc withstand from 31.5 kA to 50 kA for 1 s, depending on voltage.

Two conditions sit outside the certificate. The UniGear catalogue tells specifiers to consider escape routes for hot and toxic gases and the room dimensions, with special attention to height, and describes pressure relief flaps venting into a gas exhaust duct that runs the length of the lineup. An IAC panel installed under a low ceiling, or without the duct it was tested with, is not the panel that passed.

Why can’t containment be retrofitted into an existing panel?

Containment cannot be retrofitted because IAC belongs to the tested design, not to a set of added parts. NIEL’s position on type test certificates is direct: a certificate is tied to the design and rating tested, and a certificate issued for a different current or voltage rating does not cover a different panel. Stiffer doors, extra bolts or a new vent flap change the design the test report describes.

IEC does publish a route for extending type test validity. IEC TR 62271-307:2024 (edition 2.0, published 5 December 2024) gives guidance on extending the validity of type tests for metal-enclosed and solid-insulation enclosed switchgear from 1 kV to 52 kV. It is a technical report for design variants of a tested family, applied by the manufacturer with the test data in hand.

What a retrofit can change is where the arc gases go. NIEL’s catalogue lists an arc duct among its panel products, and the independent exhaust vents on every HT compartment of the Power+ V indoor VCB panel give flue gases a defined exit. Improving venting on an old lineup reduces risk, but the result should be described as mitigation, never as an IAC rating.

How fast does arc sensing clear a fault, and where does the time go?

Arc sensing detects the light of an internal arc, usually confirmed by a simultaneous rise in current, and trips every breaker feeding the fault. Detection is fast; the breaker is the slow part.

The UniGear ZS1 catalogue sets out total tripping times for ABB’s detection options, each including 60 ms of circuit breaker operation:

Every option except UFES lands between 62 ms and 75 ms, because the vacuum breaker needs its own time to open. The REA arc fault protection product guide (ABB, revision B, June 2019) states that an arc lasting 500 ms may cause severe damage, that under 100 ms the damage is often restricted, and that under 4 ms it is insignificant.

The light-plus-current check is what makes sensing safe to install. The REA product guide explains that its overcurrent condition prevents unwanted tripping from light that is not an arc, such as a camera flash. The REF615 feeder relay product guide (ABB, revision M) lists three light detection channels and an operate time of 9 ms minimum, 12 ms typical and 15 ms maximum in “light plus current” mode.

What can actually be retrofitted to an existing MV lineup?

Most arc flash measures that shorten arc duration or reduce the chance of an arc can be retrofitted, while measures that depend on the enclosure’s tested strength cannot. The matrix below sorts the common measures by whether an existing board can take them.

NIEL has delivered most of this list on a single live site. For an aluminium and alumina plant in Karnataka that runs continuously for export markets, NIEL supplied and commissioned more than 65 MV panels across five substations in four project phases, at 33 kV outdoor and 11 kV indoor. The panels combine REF615 relays with arc flash sensors, IEC 61850 communication, busbar chamber insulation coating, a fire quenching system, a draw-out PT mechanism and a LOTO lock mechanism on the breaker doors, all engineered around space restrictions and zero tolerance for downtime.

Arc quenching is the one retrofit that moves the clearing time into single milliseconds. The ABB UFES product page gives a primary switching element operating time under 1.5 ms, arc extinction under 4 ms after detection, and ratings of 36 kV at 40 kA and 40.5 kV at 40 kA. ABB’s mitigation guide lists UFES as available loose, inside ABB switchgear, or as a retrofit solution by ABB Service.

How does an arc flash study fit an Indian MV plant?

An arc flash study calculates the incident energy a worker would receive at a working distance, and it is the input that sets PPE and approach limits. The IEEE 1584-2018 guide for performing arc-flash hazard calculations sets out the calculation method, and its stated scope is three-phase AC equipment from 208 V to 15 kV.

That range matters in Indian industry. An 11 kV or 6.6 kV board sits inside the IEEE 1584 model, but a 33 kV incomer does not, so the study for that equipment rests on engineering judgement and the manufacturer’s test data. At 33 kV, the internal arc type test and the clearing time of the protection become the evidence a plant can actually rely on.

The legal duty in India is broader than the calculation. Regulation 21 of the Central Electricity Authority (Measures relating to Safety and Electric Supply) Regulations, 2023 requires that every person working on an electric supply line or apparatus be provided with personal protective equipment, including devices protecting against mechanical and electrical injury due to arc flash, conforming to the relevant standards. A study tells the plant which PPE rating that means for each panel.

Should a plant retrofit sensing or replace the switchgear?

A plant should retrofit sensing when the enclosure, busbars and breakers are sound and the fault level has not grown past the lineup’s rating, and replace the switchgear when it needs a containment rating the old design never had. Arc sensing shortens the event. It does not make a weak enclosure strong.

Replacement becomes the right call when the fault level has risen above the switchgear’s short-circuit rating after a transformer upgrade, when the panels have no type test record, when the plant wants an IAC rating on the front where staff operate, or when insulation, shutters and interlocks are so degraded that the panel itself is the likely cause of the next arc.

For new boards, NIEL builds vacuum-based panels from 11 kV to 36 kV, 630 A to 3,150 A and up to 40 kA, and holds CPRI and ERDA type test certificates that include internal arc fault tests across its 11 kV, 33 kV and 36 kV ratings. The full medium voltage panel range and our guide on how to specify an 11 kV, 22 kV or 33 kV MV switchgear panel cover the rest of the specification.

What should an arc flash specification or retrofit scope include?

An arc flash specification should state the containment rating, the detection and clearing method, and the installation conditions together, because each one fails without the others. Use this checklist for a new lineup or a retrofit scope:

1. IAC code in full. State accessibility, classified sides, arc current and duration, for example AFLR at the board’s fault level for 1 s, and ask for the test report of that exact rating.

2. Room and venting. Give the room height and the gas exhaust route, and confirm they match the conditions of the internal arc test.

3. Detection method. Specify light plus current detection, the number of sensors per compartment, and whether sensors are lens type or fibre loop.

4. Total clearing time. Ask for detection time plus breaker opening time, not the relay time alone.

5. Quenching. State whether an arc quenching device such as UFES is required, and its voltage and kA rating.

6. Protection coordination. Require a revised protection settings study after any relay or sensor change.

7. Arc flash study. Require a study for boards within the IEEE 1584 range, and state the approach for equipment above 15 kV.

8. Operator exposure. Specify door-closed racking, automatic metal shutters and LOTO provision on breaker doors.

9. Cable compartment workmanship. Require termination inspection and torque records at commissioning.

10. Documentation. Ask for type test certificates, sensor layout drawings and a commissioning test record of the arc protection trip.

Where the upstream breaker, transformer and board are engineered as one system, as in the balance-of-plant packages NIEL supplies, clearing time is easier to control. Our analysis of the real cost of multi-vendor procurement explains why protection coordination often breaks at the interface between suppliers.

Retrofit arc sensing or replace the switchgear?

NIEL builds and commissions vacuum-based MV panels from 11 kV to 36 kV, with CPRI and ERDA type test certificates that include internal arc fault tests. Send us your single-line diagram, the board’s fault level and any type test records you hold, and our engineers will tell you whether your lineup can take arc sensing or needs new panels.

Find the right panel for your board:

• Medium voltage panels: the full indoor and outdoor range, 3.3 kV to 33 kV.

• Power⁺ V indoor VCB, 3.3 kV to 11 kV: independent exhaust vents on every HT compartment.

• Power⁺ V3 indoor VCB, 33 kV: for boards above the IEEE 1584 range, where the internal arc test carries the evidence.

• Power⁺ VO outdoor VCB, 3.3 kV to 11 kV: for outdoor substations.

• Power⁺ R control and relay panels: where protection relays and settings are upgraded.

• Substation systems: transformer, switchgear and protection engineered as one system.

Call +91 80 42434343, email support@newindiaelectricals.com, or contact our team with your panel details.

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