General Electrical
Understanding IP ratings, and AC-1 vs AC-3 utilisation categories
What the two IP digits are actually tested for, why IPX7 is not a superset of IPX6, and why the same contactor carries two very different current ratings depending on whether it is switching a heater or a motor.
Two of the most quoted specifications in industrial electrical equipment are also two of the most casually misread. An IP rating is not a general measure of ruggedness, and a utilisation category is not a footnote to a current rating — it is what gives that rating meaning. Both answer one question from opposite directions: under what conditions does this number actually hold?
IP: the first digit
The first numeral covers two things at once: protection of people against access to hazardous parts inside the enclosure, and protection of the equipment against solid foreign objects.
| Digit | Protection against solid objects | Protection against access |
|---|---|---|
| 0 | No protection | No protection |
| 1 | Objects ≥ 50 mm | Back of hand |
| 2 | Objects ≥ 12.5 mm | Finger |
| 3 | Objects ≥ 2.5 mm | Tool |
| 4 | Objects ≥ 1 mm | Wire |
| 5 | Dust-protected: ingress not fully prevented, but not enough to interfere with operation or safety | Wire |
| 6 | Dust-tight: no ingress of dust | Wire |
The step from 5 to 6 is worth understanding. IP5X does not mean no dust gets in; it means what gets in does not accumulate enough to matter — for most industrial environments the appropriate and economical choice. IP6X is genuinely dust-tight, and is what you specify where fine conductive or abrasive dust is present.
IP: the second digit
The second numeral covers water only — fresh water, under specified conditions of pressure, volume, duration and direction.
| Digit | Protection against water |
|---|---|
| 0 | None |
| 1 | Vertically falling drips |
| 2 | Drips with the enclosure tilted up to 15° |
| 3 | Spraying water up to 60° off vertical |
| 4 | Splashing water from any direction |
| 5 | Water jets from any direction (6.3 mm nozzle) |
| 6 | Powerful water jets from any direction (12.5 mm nozzle) |
| 7 | Temporary immersion, to a defined depth for a defined time |
| 8 | Continuous immersion, on agreed conditions |
| 9 | High-pressure, high-temperature close-range jets (ISO 20653, as IP9K) |
Where a digit is not specified — not tested, or not relevant — it is replaced by X: IPX4 is water protection to level 4 with solids unstated, IP6X is dust-tight with water unstated.
The IP rating of an assembly is not the IP rating of its parts
This is where specifications most often come apart. A device rated IP20 on its front face can be entirely correct inside an IP65 enclosure. Conversely, an enclosure sold as IP66 achieves that rating only as tested: door closed and latched, gasket intact, cable entries made with correctly fitted glands of at least the same rating.
Every hole changes the answer. A lower-rated gland sets the assembly’s rating; an unused entry left open sets it to nothing. A ventilation louvre or filter fan typically caps the assembly around IP54 — often the right trade, because a sealed enclosure cannot shed the heat its contents produce.
For a low-voltage switchgear assembly the governing standard is IEC 61439, which treats degree of protection as a property of the assembly, verified for the assembly alongside temperature rise, dielectric properties and short-circuit withstand. A board built from compliant components is not automatically a compliant board.
Utilisation categories: what the current rating depends on
Why does one contactor have two different current ratings, and which one applies? The IEC 60947 series defines utilisation categories because the severity of switching a load has almost nothing to do with the steady current and almost everything to do with what happens at the instant of making and of breaking.
AC-1
AC-1 covers non-inductive or slightly inductive loads with a power factor of at least 0.95 — resistance heating is the archetype. The contactor makes and breaks essentially at rated current, with no significant inrush and with voltage and current nearly in phase, so the arc extinguishes readily at the natural current zero. This is the easy duty, and the AC-1 rating is correspondingly the device’s highest.
AC-3
AC-3 covers squirrel-cage motors, and both ends of the duty are hard. At making, the stationary motor behaves as a short-circuited transformer: the contactor closes onto locked-rotor current — several times rated current, the exact multiple a characteristic of the specific motor and stated on its nameplate — and the contacts must survive the electrodynamic repulsion and local heating without bouncing into a weld. At breaking, the current is near rated, but as the contacts part the still-turning motor acts briefly as a generator and the recovery voltage across the gap is severe.
That combination is what a contactor’s arc chutes, contact material and contact force exist for, and why the AC-3 rating sits well below the AC-1 rating.
AC-4, and the rest
AC-4 covers plugging and inching: breaking the motor at or near locked-rotor current, repeatedly. It is the harshest common AC duty, and both current rating and contact life fall sharply relative to AC-3. Beyond these: AC-5a and AC-5b for lamp loads, AC-6a for transformers and AC-6b for capacitor banks, AC-8a and AC-8b for hermetic compressors, and AC-15 and DC-13 for control circuits.
The DC categories deserve separate mention. A DC arc has no natural current zero, so it must be actively lengthened and cooled until its arc voltage exceeds the supply voltage — which is why DC ratings are strongly voltage-dependent, sometimes polarity-dependent, and sometimes achieved only by connecting poles in series. A device with no declared DC rating has not been tested for DC service.
What this means when you are selecting
- Establish the load type first, then read the rating for that category at your actual supply voltage. Applying an AC-1 figure to a motor is the classic error.
- Take full-load current from the motor’s nameplate, not from a generic kW-to-amps table.
- Mixed loads take the most severe category present — a circuit switching a heater and a motor is an AC-3 selection.
- Categories interact with duty. Endurance curves are published per category, and a device switching an AC-3 load many times an hour is a different selection from one switching it twice a day.
- A contactor feeding a variable speed drive is not switching a motor: the drive’s input stage is the load, with its own inrush as the DC link charges. Size it from the drive’s documentation.
How the two subjects meet
They meet in the enclosure. A high IP rating seals it, raising the internal air temperature and derating every device inside — and utilisation-category ratings are declared at a reference ambient, typically the IEC 60947-1 conditions of -5 °C to +40 °C. Specify IP66 without a thermal calculation and contactors can end up operating outside their declared conditions on a warm day, with no sign of it except that they fail early.
How to check this on the product page
Every product on this site carries a specification table under the product image. For these two subjects, look for:
- Degree of protection (IP), and on an enclosure whether the figure is as supplied or as installed with specified glands.
- IK rating where impact matters — a separate field, because it is a separate standard.
- Rated operational current (Ie) listed per utilisation category, each with the voltage it applies at.
- Utilisation categories declared, and ambient temperature range with any derating.
Each value is a link that filters the catalogue to every product sharing it — the fastest way to see everything available at a given AC-3 rating or IP rating. Where a field is absent we do not hold a verified value for it; take it from the manufacturer’s datasheet and ask us if you would like it confirmed. Derating and endurance curves are published per device and we can send you the document for a specific part. Add your shortlist to a quote request and the sales desk will confirm stock, lead time and price.
A closing note on responsibility
An IP code and a utilisation category describe how a product performed in a defined test. Whether that test represents your environment and your duty — the actual contaminants, the washdown regime, the ambient inside your enclosure, the load and switching frequency — is a judgement about your installation, and it belongs to the person designing it. Final selection is the responsibility of the designing engineer, working from actual application data and the manufacturer’s published data for the specific device. RMS can assist with selection, supply datasheets and derating data, and check a proposed arrangement against the manufacturer’s published tables — but that assistance supports competent design, it does not replace it.