DC arc chamber: the magnetic blow-out chamber that drives a direct-current arc into its splitter plates, and how it handles polarity
DC switching · Permanent-magnet and coil blow-out arc chambers, polarity, low-current breaking and series poles
Interrupting direct current is harder than alternating current because there is no natural current zero, and at low currents the arc's own magnetic force may not be enough to move it off the contacts. A DC arc chamber solves this with a field source beside the contacts, a permanent magnet or a blow-out coil with ferromagnetic plates that channel the field, so the arc is driven into the plate stack and stretched until its voltage exceeds the circuit voltage. The decisions for a DC switching device are how the chamber copes with current direction, how small a current it must still break, and which DC rating standard the finished device is tested to.
Arc chambers of DC circuit-breakers, DC switch-disconnectors and DC contactors that use permanent magnets or blow-out coils to move the arc, including polarised and non-polarised arrangements and the magnetic yoke or carcass around the breaking region. Arc chutes of AC circuit-breakers as a whole are covered on the Arc chute page and replacement chutes for air circuit-breakers on the Replacement arc chute for low-voltage air circuit breakers page; the individual steel splitter plates are covered on the Arc splitter plates (deion grids) page, and vacuum interrupters on the Vacuum interrupter pole assembly page.
CNFX editorial research · Sources checked October 17, 2026 · 6 sources
No current zero: why a DC chamber needs a field source beside the contacts
LSIS's patent US11177098 explains that a DC breaker has no natural zero point, that the arc is moved toward the extinguisher by the Lorentz force (Fleming's left-hand rule) and internal pressure, and that this force is proportional to current and magnetic flux. With short-circuit currents the arc moves readily; at small currents the internal pressure is also small, the arc can stay between the contacts, and the likelihood of a failed break rises. [3]
Socomec's patent US11664175 states that breaking DC requires the breaking unit to generate more voltage than the network voltage, which is why magnetic blow-out suits DC: the field moves and stretches the arc as soon as it forms, cooling the plasma raises its impedance and so the arc voltage. [2]
Secheron's patent US8513558, written for DC traction networks of 750 to 3,000 V nominal, adds that the electromagnetic force from the main current is a function of the current value, so at very low current it is not sufficient to drive the arc into the arc chute; a permanent magnet giving a field sufficient to move the arc at low currents is the known remedy. [4]
Eaton's patent US9006601 notes that AC moulded-case breakers applied to DC are connected with multiple poles in series to reach the interruption performance needed for the system DC voltage and current, and that driving the arc into the chamber at relatively low DC current is one of the main difficulties. [1]
State the system DC voltage, the maximum and minimum currents the device must break, whether current can flow in both directions (battery storage, regenerative loads), and the number of poles in series the chamber is designed for.
02
Polarised magnets, non-polarised chambers and blow-out coils
A fixed permanent magnet gives a force in one direction only. Secheron describes how, if the current reverses, the force on the arc also reverses and pushes it away from the arc chute, making the breaker polarity sensitive; a blow-out field generated by the main current reverses with it, so the force stays in the same sense. [4]
Eaton's patent describes a practical detail: if the lower edges of the permanent magnets sit at or near the fixed-contact surface, the field near the contact is reduced or reversed (the fringing effect) and can drive the arc away from the chamber. In its embodiment each magnetic member's lower surface is spaced about 34/128 to 43/128 in. below the fixed-contact upper surface, about 39/128 in. in the example, and an arc runner wider than the contacts carries the arc to the arc plates. [1]
Editorial calculation: 39/128 in. × 25.4 mm/in. ≈ 7.7 mm, and the 34/128 to 43/128 in. range is about 6.7 to 8.5 mm. These are dimensions of one patented embodiment, not a design rule for other chambers. [1]
Socomec also notes that a polarised magnet coupled to two ferromagnetic plates either side of the breaking region (EP 3 242 306 cited) requires the magnet to be polarised according to the current direction, and that channelling the field with ferromagnetic plates behind or around the magnet lets the magnet be made smaller. [2]
Arrangement
How the reviewed patents describe it
Ask the supplier
Single permanent magnet
Field fixed; suitable for one current direction only. [3][4]
Polarity marking and wiring diagram.
Two chambers or dual-sided chamber
Bi-directional operation by two arc chambers, or magnets on both sides of one plate stack with a ferromagnetic centre barrier driving the arc to one side or the other. [1]
Which side breaks which direction.
Non-polarised permanent-magnet chamber
Field oriented in the plane of contact movement; symmetry about that plane keeps the principle non-polarised (FR 3 003 101 cited). [2]
Test evidence in both directions.
Blow-out coil with core plates
Core plates on both sides of the contacts, core rods and a coil, for bi-directional DC. [3]
Coil rating; low-current performance.
Current-driven coil with electrodes
Magnetising coil fed through electrode means for low-current breaking in either direction. [4]
Minimum breaking current.
03
DC ratings the finished device is tested to
The chamber is tested as part of a complete device, so its suitability is set by the device standard. IEC 60898-2:2016 covers single- and two-pole household circuit-breakers for DC operation with rated DC voltage up to 220 V single-pole and 440 V two-pole, rated current up to 125 A and rated DC short-circuit capacity up to 10,000 A. [5]
IEC 60947-3:2020 applies to switches, disconnectors, switch-disconnectors and fuse-combination units up to 1,000 V AC or 1,500 V DC, and its fourth edition added critical load current tests for DC switches, the low-current region where the patents above place the difficulty. [6]
Ask which standard and which DC test report covers the device built with the chamber, the polarity marking required, and whether the rating depends on poles connected in series.
04
Describe the DC circuit and the device the chamber goes into
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
System: [DC voltage; earthing; PV, battery or traction]
Currents: [rated; short-circuit; lowest current to break]
Direction: [one-way or bi-directional]
Device: [breaker, switch-disconnector or contactor; poles in series]
Chamber: [permanent magnet or coil; plate count; housing material]
Evidence: [standard; DC test report; polarity marking]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 17 October 2026
Buyer questions
Why is a DC arc harder to break than an AC arc?
DC has no natural current zero, and at low current the force moving the arc into the extinguisher is small, so the arc can stay on the contacts. [3][4]
Are permanent-magnet DC arc chambers polarity sensitive?
A single fixed magnet pushes the arc the wrong way when current reverses; dual-sided, symmetric non-polarised or coil blow-out designs are described for both directions. [4][1][2][3]
Which standards cover DC switching devices?
IEC 60898-2 for household DC circuit-breakers up to 440 V two-pole, and IEC 60947-3 for switches and disconnectors up to 1,500 V DC. [5][6]
Sources and what still needs a direct check
Sources checked 2026-10-17. Mechanisms and polarity handling are taken from patents held by Eaton, Socomec, LSIS and Secheron. The DC limits of IEC 60898-2 and the 1,500 V DC ceiling of IEC 60947-3 are quoted from their EVS catalogue entries.
