AC-1 Load Category (IEC 60947)
This is the switching category defined for resistive or lightly inductive alternating current loads with a power factor above 0.95. It covers heaters, resistors and similar loads. The high ampere ratings highlighted in relay and contactor catalogues mostly belong to this category.
In AC-1 the current and voltage are almost in phase. Because the current is interrupted as it passes through zero at the moment the contact opens, the arc is short and contact wear stays limited. For this reason the same contact appears to have a high ampere rating in AC-1.
The error arises from assuming that this rating can be applied to every load. The typical load driven by the relay output in a crane panel is a contactor coil, and this load is not resistive:
- The coil draws roughly 10 times the current when pulling in.
- At the moment of interruption the coil inductance produces a voltage that creates an arc across the contact gap.
- These two effects together erode the contact surface.
The correct reading is this: if only an AC-1 rating is given for a relay, its capacity on an inductive control load (AC-15) is assumed to be roughly 15–20% of the AC-1 rating. The 2×10A output of the LF100R is sufficient for driving small loads directly; with large motors, let a contactor carry the current and leave only the coil to the relay.
The context in which this term is used: Wireless Crane Remote Control Selection Guide