The A9D87420 is a 4-pole Acti9 iC60 RCBO — a combined miniature circuit breaker and residual-current device in one 72 mm wide DIN-rail module. It protects a 4-wire outgoing circuit against overload, short-circuit (curve B, 3 to 5 times In magnetic trip), and earth leakage at 30 mA sensitivity. The 6 kA breaking capacity at 380–415 V AC meets EN/IEC 60947-2 and EN/IEC 61009-2-1, so it's rated for distribution boards in commercial and light industrial panels where the prospective fault current stays under that level.
Rated 20 A with a thermal-magnetic trip unit — the thermal element handles sustained overloads, the magnetic element clears short-circuits above 60 A (3–5 × In). The 30 mA residual-current sensitivity is the standard for personnel protection in most IEC 60364 installations; it trips on leakage to earth, not on line-to-neutral faults. That's adequate for most sub-distribution boards fed by a transformer or a main breaker with a lower let-through. Limitation class 1 per EN/IEC 61009-2-1 means it lets through the full prospective current during a short-circuit — it does not current-limit like a higher-class MCB. That matters for downstream equipment with limited short-circuit withstand; you'd coordinate it with an upstream current-limiting device if the load is sensitive.
Clip-on mount to standard 35 mm DIN rail. The 72 mm width (8 × 9 mm pitches) means it occupies four 18 mm modular spaces — plan your rail fill accordingly. The 9 mm pole-to-pole pitch matches the Acti9 busbar system, so you can feed it with a pre-wired distribution bar without individual ring-tongue terminations. IP40 in the distribution enclosure keeps dust out; IP20 on the terminals is standard for touch-safe finger protection.
The Acti9 iC60 series is a broad, widely stocked platform, so this 4P 20 A 30 mA variant is a standard catalogue item.