# What happens to a relay when a large current (short-circuit current) is energized?

This section describes the behavior of a relay when a large current, such as a short-circuit current, flows through the relay.

Depending on the magnitude of the current, three major phenomena occur.

Current value Small Medium Large
Phenomenon Electromagnetic repulsion is small and the contacts do not open.

(Time setting is necessary because contacts may melt)

Electromagnetic repulsion causes the contacts to open, but welding occurs. Electromagnetic repulsion causes the contacts to open, and the arc continues.

(Continued use is possible in case of fuse coordination)

Relay status Continuous use is possible if the contact area is energized for a period of time during which melting and welding does not occur. Electromagnetic repulsion causes the contacts to open, and welding occurs when the contacts re-contact with the arc generated. Continued use is not possible. Contacts open by electromagnetic repulsion, but the repulsion force is so large that it is difficult for the contacts to re-contact. In the case of fuse coordination, the contact that interrupts the current by the fuse before it is re-contacted is worn, but can continue to be used.

(However, if the arc continues for a long time, it will explode and become unusable)

Electromagnetic repulsion due to Lorentz force

When a current flows through a relay, the Lorentz force, which is proportional to the

magnitude of the current, causes a repulsive force (electromagnetic repulsive force Fa) between the contacts. Since the contacts come to contact at a point, current flows through the contact surface as shown in the figure below.

Electromagnetic repulsive force generated on the contact surface

The contacts come to contact at a point, the current flows in the opposite direction between the contacts, and an electromagnetic repulsive force acts.

The spring force exerts a force (contact pressure Fb) on the relay contacts that pushes them against each other. If the contact pressure (Fb) is sufficiently large, the electromagnetic repulsive force (Fa) has no effect and contact welding does not occur. Contact can be opened without a problem.

In the region where the electromagnetic repulsive force (Fa) does not exceed the force (Fb) pressing the contacts to each other, the contacts do not open. But the force pressing the contact points against each other is reduced due to the generation of Fa. This increases the contact resistance of the contact area, which in turn increases the Joule heat generated there, melting the contacts and possibly causing welding.

Furthermore, if the current increases further and the electromagnetic repulsive force (Fa) exceeds the contact pressure (Fb), the contacts open. In the process, an arc is generated between the contacts, and the arc heat can melt the contacts and the area around the contacts, causing burnout (smoke and fire) and, in some cases, an explosion due to rapid heating.

Thus, when a large current is energized, the relay may be destroyed depending on the magnitude and duration of that current.

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

Section 8 of 9

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