ABB vs Siemens Contactor: When the load doubles — which one fails first?

The myth: “A contactor rated 9 A AC‑3 handles 18 A for a few seconds — it’s just a momentary overload, the contacts will survive.”
The reality: If you double the load on a contactor, the failure mode changes from electrical wear to thermal runaway in the arc chamber. The datasheet AC‑3 rating assumes a specific make/break duty; pushing it past the rated breaking current even briefly can weld the contacts or cause a phase-loss cascade. This is not a safety factor — it’s a physics limit.

1. Breaking capacity — the number that actually kills the contactor

Numbers. ABB AF09 is rated AC‑3: 9 A / 4 kW at 400 V; Siemens contactor SIRIUS 3RT2016 (size S00) is also 9 A / 4 kW at 400 V. Both meet IEC 60947‑4‑1 utilization category AC‑3. But the breaking capacity under overload (e.g. 18 A) is not a linear multiple: the standard only certifies the contactor to make and break the rated current at the rated voltage. At 2× rated, the arc energy roughly quadruples (arc energy ~ I² × t_arc).

Mechanism — why the number changes the outcome. In a contactor, the arc is drawn when the contacts separate. The arc chute (de‑ion plates) is sized to cool and split the arc for the rated current. At 9 A AC‑3, the arc quenches within half‑cycle. At 18 A, the arc energy is higher, and the chute saturates — the arc can restrike or sustain into the next half‑cycle, causing contact material transfer (pitting) or, in a worst case, a welded contact. The electronic coil in the ABB AF09 does not protect against this; the coil is for control voltage flexibility, not overload protection.

Worked consequence — translate to a decision. If you double the motor current (say a 4 kW motor that draws 9 A FLA, but you accidentally use it on an 8 kW motor drawing 18 A), the contactor will not trip — there is no built‑in overload. The contactor will attempt to break 18 A. After ~50–200 operations (depending on power factor), the contacts may weld shut. That means the motor stays connected even after the stop command — a classic “run‑on” failure that can destroy the motor or cause a safety hazard.

Reversal — when it does NOT matter. If your load is purely resistive (AC‑1, e.g. heating elements), the same contactor can handle 25 A continuous. In AC‑1, the arc is much smaller (zero‑crossing at low voltage). Doubling a resistive load from 12.5 A to 25 A is still within the AC‑1 rating. The failure mode changes only when you are at the AC‑3 boundary.

2. Thermal accumulation in the coil — a hidden failure that mimics contact weld

Numbers. The ABB AF09 uses an electronic wide‑range coil consuming ~2.5 W hold power; the Siemens 3RT2016 uses a conventional DC or AC coil, with hold power roughly 4–6 W (depending on voltage). Both are within IEC 60947‑4‑1 temperature rise limits.

Mechanism. When the contactor stays closed for extended periods (e.g. motor running for hours), the coil dissipates heat into the contactor housing. Under normal load (9 A), the contactor’s internal temperature stabilises. If the load current doubles (18 A), the I²R losses in the main current path (power contacts, bimetal links, terminals) increase by a factor of 4. That extra heat raises the ambient temperature around the coil. For the ABB AF09, the electronic coil has a wider temperature tolerance (rated up to 70 °C ambient), but the Siemens 3RT2016’s conventional coil is more sensitive: the insulation class (Class F, 155 °C) can be approached if the contactor is enclosed in a small panel with poor ventilation.

Worked consequence. The coil may overheat and burn open (open‑coil failure) — the contactor drops out, motor stops. But the symptom is identical to a contact weld: the motor stops unexpectedly, and a technician might think the contacts welded shut. In reality, the coil failed open due to accumulated heat from the overloaded power path. This misdiagnosis leads to replacing the contactor (correct) but not fixing the root cause (the doubled load).

Reversal. If the duty cycle is short (e.g. intermittent operation

3. Overload relay mismatch — the coordination that breaks

Numbers. ABB AF09 pairs with ABB contactor overload relays (e.g. TA25DU); Siemens 3RT2016 pairs with 3RU2 thermal overload relays. Both are sized to the motor FLA. The overload relay is set to 9 A (motor FLA). If the load doubles to 18 A, the overload relay should trip after a few seconds (Class 10 or 20). But here is the trap: the overload relay protects the motor, not the contactor.

Mechanism. Under a sustained overload (e.g. 18 A), the bimetal in the overload relay heats up and trips the contactor after, say, 8 seconds (for a Class 10, at 200 % load, trip time ~4–10 s). But during those 8 seconds, the contactor’s main contacts are carrying 18 A — see dimension 1. The contacts may already be damaged (softening, material transfer) before the overload relay opens. After a few such cycles, the contactor fails despite being “protected” by the overload relay.

Worked consequence. The overload relay extends the apparent survival time — the contactor works for weeks before failing. The failure appears random, but it’s cumulative: each overload event erodes the contacts a bit. Eventually, the contactor welds shut during a normal start (9 A) because the contact surface is already cratered. This is a common field complaint: “the contactor was correctly sized, the overload relay was set correctly, yet the contactor failed after 6 months.” Root cause: the motor was pulling 18 A during a jam, and the overload relay didn’t trip fast enough to protect the contactor.

Reversal. If the overload relay is set to a lower multiple (e.g. 1.1 × FLA) and the overload is repetitive (e.g. every hour), the contactor will still degrade. Only a contactor with a higher AC‑3 rating (e.g. ABB AF16, 16 A / 7.5 kW) would survive the 18 A peaks without cumulative damage.

Decision tree — when to upsize vs. when you can ignore the myth