# Explain to me the mechanism that interrupts the arc

When turning off the switching device, in an AC circuit in which the direction of current flow changes, the arc disappears each time the voltage becomes OV. It is relatively easy to interrupt without the need for additional technical ingenuity.

In a DC circuit, however, as the direction of current flow is constant and the arc is continuous, the arc must be forcibly interrupted. Consequently, with high-capacity relays for DC circuits, the larger the voltage or current, the more difficult it is to interrupt the arc.

##### Arc continues from the point where the contacts separate until the next point where the current goes to zero (zero crossing)

There are three main ways to interrupt a DC arc.

Method of extending and interrupting the arc by a larger contact gap Method of extending and interrupting the arc with a magnet Method of cooling and interrupting the arc with enclosed gas

The larger the voltage and current, the greater the arc (discharge) energy and the more difficult it is to interrupt. Therefore, a combination of arc interrupting methods is used depending on the magnitude of voltage and current.

The simplest way to interrupt an arc is to interrupt it by the contact gap. This method is generally applied in conjunction with other interrupting techniques due to the larger relay size. Some load conditions, however, create areas where the arc is interrupted by the contact gap alone.

##### (1) Interruption by contact gap

The method mainly used in OMRON’s PCB power relays is to use magnets to extend and interrupt the arc. Fleming’s left-hand rule is applied as a means of extending the arc. The power relays are designed to maximize the arc distance in a limited space by adjusting the distance and angle at which the arc is pulled by the position and type of magnet.

##### The arc generated between the contacts is pulled in the direction of the force, which causes the arc to curve and gain the distance necessary for interruption.

The length of the arc that can be extended depends on the Lorentz force, which is determined by the current value and the magnetic field. If the current is low, it is difficult to extend the arc.

Therefore, when the current is low, the arc length cannot be extended long enough for the required interruption, and in some cases, the arc cannot be interrupted.

##### The arc cannot be sufficiently extended to interrupt.

OMRON’s DC power relays mainly use an enclosed gas to cool and interrupt the arc. In gas-sealed relays, a pressurized gas with high thermal conductivity is sealed in the contact area.

Normally, an arc is generated between the contacts when a DC high voltage is interrupted, and the arc is extended to both ends of the contacts by the force of the magnetic field according to Fleming’s left-hand rule. The gas enclosed in the space impedes this arc flow, and the high thermal conductivity of the gas efficiently dissipates arc energy to the outside. The resulting arc extension causes a sudden increase in arc voltage, and the gas effect quickly interrupts the arc inside the air-tight enclosure.

Easy-to-understand explanations of the unknowns when using high-capacity power relays

Section 4 of 9

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