Transient protection for a single line — both polarities
The 1.5SMC20CA-M3/57T: A single device clamps a positive or negative transient on one line, which simplifies board layout for AC signal lines or bipolar DC rails where the polarity of the surge is unknown. Breakdown starts at 19 V minimum, and the clamping voltage is held to 27.7 V at the rated peak pulse current of 54.2 A. This gives a protection window that keeps the downstream circuitry below its absolute-maximum rating for a 24 V nominal bus.
1500 W pulse — what it means for a 24 V rail
The 1500 W rating is measured with a 10/1000 µs exponential waveform — the standard for telecom and industrial surge testing per IEC 61000-4-5. For a shorter pulse like 8/20 µs (common in ESD or lightning surge), the peak power capability is higher, but the 1500 W figure is the conservative design-in number for the 1 ms pulse width.
The base product number 1.5SMC20 covers both unidirectional and bidirectional variants — the 'CA' suffix marks this as the bidirectional version. For a unidirectional requirement, the 1.5SMC20A (without the 'C') is the sibling. The package is the same DO-214AB, so the PCB layout is identical.