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Load Limit Curve

Load limit curves for direct voltage with resistive load
The load limit curves were determined using low-inductance resistors. Several points on the load limit curves were verified by using 1000 switching operations.

Arc
Plasma current flow between opening relay contacts. An arc is enabled by the electric power of the load circuit (turn off spark) ionizing the gas between the contacts. The stability of hte arc depends on various parameters such as contact material, air pressure, contact gap, system voltage etc. An arc locally produces high temperature causing contact erosion. In cases of strong erosion, spark suppression becomes necessary.

Load limit curve I
(arc quenched during the transit time)

The powers lying below load limit curve I, the values applying to the corresponding relay at the maximum permissible switching voltage, Usmax, and the maximum permissible switching current, Ismax, can be reliably switched on and off in such a way that the arc is extinguished within the transit time of the contacts moved before they reach the opposite contacts. This is important when the break contact and make contact of a changeover contact are at differential potentials.

Load limit curve II
(cut-off, no arc resulting, maximum arc duration £ 10 ms)

The powers lying below load limit curve II, the values applying to the corresponding relay for the maximum permissible switching voltage, Usmax, and the maximum permissible switching current, Ismax, can be switched on and off reliably, i.e. the arc is extinguished. Longer arcs are allowed here than in load limit curve I. Thus, load limit curve II lies at higher switching powers than load limit curve I.

Both of the load limit curves are affected in both position and shape by the contact materials and relay construction (contact distance, break speed of the contacts, etc.). No statement on the electrical endurance is connected with these curves. Characteristics of the endurance are issued separately.

Load limit curve for arcfree switching
Load voltage/current combinations below this load limit curve in general cause no arc at all.

Spark suppression
Reduction of the arc energy between moveable and stationary contact during switching.