What does an impulse relay's contact rating actually tell you? It tells you how the relay's contacts are qualified to switch a specific type of load — resistive, discharge lamp, or incandescent — under a defined utilisation category such as AC-1, AC-5a or AC-5b, not simply how many amps the device can carry. The ampere figure printed next to the category is only valid for that category. Apply it to a different load type, especially a modern LED driver, and the number stops meaning what you think it means.
This matters because most lighting circuits built today run LED fittings, and LED drivers do not behave like the resistive or ballasted loads the classic utilisation categories were written around. An engineer who reads "16 A" on a relay nameplate and multiplies by the wattage of an LED fitting is doing arithmetic that the standard never promised would work.
Why utilisation category matters more than the ampere number
Every impulse relay contact is tested and rated against a defined utilisation category from IEC 60947-5-1 (the general low-voltage switchgear standard that the relevant control-circuit device categories borrow from). The category describes the load's electrical behaviour at switch-on and switch-off — resistive, inductive, capacitive — because that behaviour, not the steady-state current, is what actually stresses the contact.
A resistive heater and an LED driver can draw the same nameplate current and still be worlds apart for a relay contact. The heater's current rises smoothly to steady state. The LED driver's input capacitor charges in a few milliseconds, pulling a current spike many times the steady-state figure. The contact sees that spike as an arcing event, not as a current reading, and category ratings exist precisely to capture that difference.
AC-1
Utilisation category for non-inductive or slightly inductive resistive loads — heating elements, incandescent filaments treated as pure resistance. It is the least demanding category and the baseline figure most impulse relay datasheets lead with.
AC-5a
Utilisation category for switching discharge lamp circuits — fluorescent and other gas-discharge lamps with ballast. The ballast's inductance means switch-on inrush and switch-off arcing are more severe than AC-1, so the rated current for AC-5a is typically lower than the same relay's AC-1 figure at the same voltage.
AC-5b
Utilisation category for switching incandescent lamp circuits, where the cold filament's low resistance produces a brief but sharp inrush current at switch-on before the filament heats up and its resistance rises. It is a different stress profile from AC-5a even though both sit under the same "lamp switching" umbrella.
None of the three classic categories was written with LED drivers in mind, because LED drivers barely existed when the categories were defined. That gap is exactly where impulse relay contact failures on modern lighting retrofits originate, and it is the subject of a dedicated discussion in our LED Inrush Current and Impulse Relay Derating guide — worth reading alongside this one if your project is an LED retrofit rather than a new installation. For the mechanism and coil behaviour that sit behind all of this, our Impulse and Latching Relay Engineering Guide is the pillar reference for the whole family.
Key takeaway: A contact rating is a load-type qualification, not a flat ampere ceiling. The same relay can carry a different current depending on whether the load is resistive, ballasted, incandescent or capacitive.
Reading a manufacturer's rating table correctly
A proper impulse relay datasheet lists more than one current figure per relay. You will typically see an AC-1 rated current, a lower AC-5a figure, and sometimes a separate AC-5b figure or a maximum incandescent wattage per pole. Selecting purely off the AC-1 number and ignoring the others is the single most common sizing mistake we see referenced from installers troubleshooting a relay that welded shut within its first year.
For background on the modular family this article sits under, see the Impulse and Latching Relay Engineering Guide, which covers the mechanism and coil behaviour that make impulse relays distinct from contactors in the first place.
I_selection ≥ I_load × k_derate
Where: I_load is the relevant steady-state or manufacturer-published equivalent current of the connected load for the applicable utilisation category, and k_derate is the safety margin the relay manufacturer publishes for that category — read it from the datasheet rather than assuming a fixed number, because it varies by relay family and by load type.
For discharge-lamp and LED circuits the practical approach is to work from the manufacturer's lamp-count or load-current table for the specific relay, rather than back-calculating from wattage and an assumed power factor. The table already embeds the inrush behaviour the manufacturer measured; a hand calculation from nameplate wattage usually does not.
Resistive, ballasted, incandescent, capacitive — how the load types compare
The table below is a qualitative comparison, not a source of numeric ratings — always confirm the actual current figures against the datasheet of the specific relay you are specifying.
| Load type | Utilisation category | Switch-on stress | Typical failure mode if under-rated |
|---|---|---|---|
| Resistive heating | AC-1 | Low — current rises smoothly | Thermal overload of contact |
| Discharge lamp (ballasted) | AC-5a | Moderate — ballast inrush and arcing on break | Contact pitting, reduced switching life |
| Incandescent lamp | AC-5b | Moderate-high — cold-filament inrush | Contact erosion over repeated switching |
| LED driver (capacitive) | Not a classic AC-5 category — treat as capacitive | High — brief, very high inrush peak | Contact welding, the dominant field failure on LED retrofits |
Key takeaway: LED loads are capacitive, not inductive like the lamp categories the standard was written around, so neither AC-5a nor AC-5b tells you the LED story on its own — go to the manufacturer's LED-specific table.
One recurring field pattern is worth flagging: a relay speced correctly for the original fluorescent fixtures on a circuit, then left in place when the building does an LED retrofit years later. The contact rating that was conservative for the old ballasted load can be marginal or wrong for the new capacitive one, even though the relay itself never moved and the wiring never changed. This is one of the more common causes of a relay that worked for a decade and then starts welding within months of a lighting upgrade.
Mechanical wear, switching frequency and contact life
Contact rating and mechanical life are related but not the same figure. A mechanical ratchet-type impulse relay is rated for a mechanical operations life (how many times the latch can physically actuate) that is usually far higher than its electrical operations life at rated current — the electrical figure is what falls when the contact is repeatedly asked to break an inrush current above its category rating. In a stairwell or corridor circuit with dozens of daily operations, that gap compounds quickly; a relay pushed just beyond its LED-load rating can visibly age within a year where an AC-1 resistive load would have shown no wear at all.
Electronic latching relays behave slightly differently: some ranges offer more stable switching behaviour under capacitive inrush because of how the switching element is driven, at the cost of more sensitivity to control-wire noise. Neither construction removes the need to size against the correct utilisation category — it changes the margin, not the principle.
Key takeaway: Contact welding on an LED circuit is almost never an overload problem in the classic sense. It is an inrush problem, and the fix is derating against the LED table, not upsizing the relay's AC-1 amperage.
Selecting between Schneider Acti9 iTL and ABB E290 on contact rating
Both major modular impulse relay ranges available through Stoklink publish current ratings per utilisation category rather than a single blanket figure, and both offer more than one current tier per pole count. The ABB E290-16-10/230 and the Schneider Acti9 A9C30211 iTL both sit in the common 16 A modular tier, while a project with higher lighting load per circuit will look at the 32 A tier — the ABB E290-32-10/230 is the direct step-up in the same range. Selecting between brands on paper current alone is not enough; compare the AC-5a and LED-specific figures in each datasheet for the actual load mix on the circuit, not just the AC-1 headline number.
If the project is genuinely closer to a contactor application — continuous holding rather than momentary pulse control — it is worth reading the Contactor Engineering Guide before committing to an impulse relay purely because it is already the modular default on the board.
Practical sizing sequence for a lighting circuit
Before ordering, work through the load in this order: identify the lamp/driver technology on the circuit, pull the manufacturer's lamp-count or load table for the shortlisted relay (not the bare AC-1 rating), check whether the pole count and control voltage match the panel, and only then compare price and stock across the shortlisted parts. Our companion piece on How Many Lamps Can One Impulse Relay Switch? A Capacitive Load Calculation walks through that table-based sizing step in more depth, and is the natural next read once you understand why the category matters.
The full modular range — both brands, both pole counts and both current tiers — is browsable in the relay collection if you are shortlisting parts for a bill of materials rather than reading for background.
Key takeaway: Size against the load's actual electrical character — resistive, ballasted, incandescent or capacitive LED — using the manufacturer's category-specific table, and treat the AC-1 nameplate figure as a starting point, not the answer.
Conclusion
An impulse relay's contact rating is a set of numbers tied to specific load behaviours, not one number that covers everything wired to the output terminal. AC-1 covers resistive loads cleanly. AC-5a and AC-5b cover the classic lamp technologies the standard was written for. LED drivers sit outside that neat structure and need their own manufacturer-published figures. Get the category right for the actual load on the circuit, and the relay will outlast the lighting retrofit that eventually replaces the fittings around it — get it wrong, and welded contacts are usually the first symptom anyone notices, generally at the worst possible time.
Frequently Asked Questions
What is the difference between AC-1 and AC-5a ratings on an impulse relay?
AC-1 covers resistive or near-resistive loads, where current rises smoothly and there is little switching stress on the contact. AC-5a covers discharge lamp circuits with ballast, where the ballast's inductance creates a harsher inrush at switch-on and more arcing at switch-off. Because of that added stress, the AC-5a current figure on a given relay is normally lower than its AC-1 figure at the same voltage, even though it is the same physical device.
Can I just use the AC-1 amp rating to size a relay for LED lighting?
No. LED drivers present a capacitive inrush that neither AC-1 nor the classic AC-5 lamp categories were written to describe. Using the AC-1 figure as if it applied to LED loads is the most common cause of contact welding on retrofit circuits. Use the manufacturer's LED-specific lamp-count or current table for the relay instead, and derate from there.
What is AC-5b and when does it apply?
AC-5b is the utilisation category for switching incandescent lamp circuits. A cold incandescent filament has low resistance and draws a brief high inrush current before it heats up and its resistance rises, which is a different stress pattern from the ballast inrush covered under AC-5a. It is relevant mainly on legacy circuits that still run incandescent or halogen fittings.
Why did my impulse relay contacts weld after an LED retrofit if the relay was fine for years before?
The relay was likely sized correctly for the original ballasted or incandescent load, which has a different inrush profile than the LED drivers that replaced it. The wiring and the relay did not change, but the load's electrical character did. This is a sizing mismatch introduced by the retrofit, not a sign the relay itself degraded on its own.
Do ABB E290 and Schneider Acti9 iTL relays use the same contact rating categories?
Both ranges are rated using the same family of utilisation categories, since both are qualified against the relevant IEC low-voltage switchgear standards. The specific current figures per category differ by model and current tier within each range, so compare the datasheets for the exact part numbers under consideration rather than assuming parity between brands at the same headline amperage.
Does a higher AC-1 current rating always mean a relay can switch more lamps?
Not reliably. A higher AC-1 figure reflects resistive-load capacity, but lamp and LED circuits are limited by their own category ratings and, for LED, by the manufacturer's capacitive-load table — not by the AC-1 number. Two relays with the same AC-1 rating can have meaningfully different LED lamp-count limits.