Short answer: read an IEC 60947-2 MCCB datasheet in this order — Ue and Ui, then In, then the Icu and Ics pair for the voltage row that matches your system, then the utilisation category with Icw, then the trip settings. The biggest kA number on the cover is rarely the one that applies to your installation: a device sold as “50 kA” can carry a much lower figure at 690 V, so a kA value without a voltage attached is not a specification.
We are an independent trading company — not a manufacturer, and not an appointed distributor of any brand — so the figures below are quoted from the range data for the exact ordering code, and the datasheet that ships with your code governs the order.
Why the cover number misleads
An MCCB is a frame with a trip unit inside it. The frame sets the shell size, the pole spacing and the maximum current the body can carry; the trip unit sets the rating you actually get. That is why one page carries NM1-125 and NM1-250, and why breaking capacity is never one number but a table: one column per voltage, one row per breaking class.
CHINT’s NM1 range is rated to Ue 690 V AC at 50/60 Hz, In from 10 to 1250 A, fixed or plug-in, with thermal-magnetic or electronic trip. Its capacity is coded in letters — C basic, S standard, H higher, R current-limiting — and the catalogue’s cascade table shows the pattern: NM1-125S and NM1-250S around 25 kA, the H versions of the same frames around 50 kA. Re-read those figures per frame and voltage in the official table; the point is that “NM1” alone says almost nothing until the class letter and the voltage column are fixed.
The rows that actually decide the order
| Parameter | What it states | How a buyer should use it |
|---|---|---|
| Ue — rated operational voltage | The voltage at which the published ratings are valid, per pole configuration | Match it to your system voltage. A 690 V device on a 400 V system is fine; a 400 V device on a 690 V system is not, whatever the kA column says |
| Ui — rated insulation voltage | Dielectric reference for clearances and creepage | Always at or above Ue. A design value, not an operating value |
| Uimp — impulse withstand | Peak voltage survived without flashover, in kV | Relevant when coordinating with surge arresters |
| In — rated current | The current the trip unit is calibrated for, at a stated ambient | Choose the trip unit, not the frame. In is a property of what sits inside the shell |
| Frame | Shell size: 125 / 250 / 400 / 630 / 800 / 1600 | Decides busbar size, lug range, accessory family and enclosure space |
| Icu — ultimate breaking capacity | Highest fault current it can break, serviceable only after inspection | Must exceed the prospective fault current at your installation point, at your voltage |
| Ics — service breaking capacity | The fault current it breaks and stays in service afterwards, usually a percentage of Icu | Decides whether a trip means “reset” or “replace”. Industrial users pay for Ics, not Icu |
| Icw — short-time withstand | Current carried without tripping for a stated time, e.g. 1 s, on category B devices | The basis of time-based selectivity with the downstream breaker |
| Category A / B | A = no intentional short-time delay; B = intentional delay with an Icw rating | Category B is what you need if discrimination is part of the design |
| Ir | Long-time (overload) setting | Set to the cable’s ampacity, not to the breaker’s In |
| Isd / Ii | Short-time pickup with delay, or fixed instantaneous pickup | Thermal-magnetic devices usually fix the instantaneous value; electronic trips let you move it |
| IP and pollution degree | Protection of the device itself and its contamination environment | NM1 and NXM are IP30, which assumes life inside a board, not on an open wall |
| Temperature range | Ambient window for the trip calibration | NXM states -25 to +70 °C; the NM8N electronic trip -35 to +70 °C. Above the calibration ambient the device drifts and trips early |
Icu versus Ics: the pair, not the headline
IEC 60947-2 proves the two values with different sequences. Icu is established by an O-t-CO sequence: the device breaks a fault, waits, then breaks and close-breaks again, and afterwards only has to be safe, not necessarily serviceable. Ics adds a second O-t-CO round and requires the device to carry its rated current and trip correctly afterwards. Because Ics is normally a percentage of Icu, one device can advertise 100 percent at one voltage and 50 percent at another.
In practice: on a feeder where a fault means “find the cause and replace”, Icu is enough. On a main incoming breaker in a plant that cannot afford a shutdown to swap hardware, specify on Ics. A datasheet that prints Icu and omits Ics is telling you something.
Worked example: the same frame at two voltages
| What you are reading | What it looks like on paper | What it means for the order |
|---|---|---|
| Range-level claim | NXM series: up to 70 kA at 400 V AC, up to 50 kA at 690 V AC | A ceiling reached by the highest class in the biggest frame, not the rating of the unit you are buying |
| Nameplate-level value | NXM S-250H: Icu / Ics = 50 / 36 kA at 400/415 V | The figure to compare with your calculated fault level. Ics at 72 percent of Icu means one trip at the limit still leaves a serviceable device |
| Class effect | NM1-125S and NM1-250S around 25 kA; the H versions of the same frames around 50 kA | The letter after the frame number is a price and size decision as much as a technical one |
| Voltage effect | The same device carries different values at 240 V, 415 V and 690 V | Always quote the voltage with the kA. “50 kA breaker” on a purchase order is an invitation to a dispute |
The NM8N series shows how far a family stretches: frames from 125 to 1600, In from 16 to 1600 A, Ue to 690 V, AC and DC versions, with the AC 690 V column falling to about 10 kA on the smaller frames. Compare an NM8N with an NXM on the cover figure alone and you will pick the wrong one.
Category A or B, and why Icw matters
Time-based selectivity needs the upstream device to hold a fault current for a defined window without tripping, and only a category B device with a published Icw can do that. Current-limiting selectivity instead relies on the downstream device clearing fast enough that the upstream never sees the full let-through energy — it works, but only on a pair tested together, and the manufacturer’s selectivity tables are the only acceptable evidence. Small frames rarely offer category B: the NM8N range publishes Icw of 1 to 3 kA on its small frames, which constrains how much delay you can dial in. If your single-line diagram assumes 300 ms of upstream delay on a 250 A frame, that drawing is wrong and the datasheet is where you find out.
The trip settings are a separate document
On a thermal-magnetic device the thermal element is often fixed, so you select the rating rather than set it, and the instantaneous pickup comes from the factory: you get a curve, not a dial. An electronic trip unit gives you Ir, Isd with a time delay and often a separate override, and those settings live in the trip-unit manual, not the breaker datasheet. This is where ordering errors happen — the buyer specifies frame and kA, the trip unit arrives with a fixed Ir of 250 A, and the cable was sized for 160 A. Ask for the trip-unit code and its Ir range.
Five questions to send with your enquiry
System voltage and frequency, single or three phase; prospective fault current at the installation point and the device ahead of it; cable size and installation method, which sets Ir; the ambient inside the enclosure, since a board in a Gulf plant room is not a laboratory; and the destination country, because the certificate that matters is the one accepted at that border. Those five pin the code down — send them here. If you are still choosing between device types, start with MCB vs MCCB vs ACB, then the NM1 / NXM / NM8N guide; IEC 60898 vs IEC 60947 covers the standards behind the columns.
FAQ
Q: Is a higher Icu always better?
A: Not by itself. Once Icu exceeds the prospective fault current with a margin, the money is better spent on Ics, on the trip unit, or on discrimination. An oversized frame also raises the minimum setting, which can leave a small feeder unprotected.
Q: Why does the datasheet list different kA values for the same breaker?
A: Because breaking capacity falls as recovery voltage rises. Each voltage column is a separate test, and the value at 690 V is not derived from the 415 V figure — it is measured, and usually lower.
Q: Can I use an AC breaker on a DC circuit?
A: No. DC has no natural current zero, so the arc must be forced out magnetically and ratings are given per pole count with poles in series. Ranges that serve both publish separate DC versions such as NM8N-DC.
Q: The datasheet gives Icu at 40 °C but my enclosure runs hotter?
A: Derate. A trip unit calibrated at a reference ambient trips earlier once the air around it is hotter, so reduce Ir, enlarge the frame or ventilate the enclosure. The temperature row is a calibration condition, not a tolerance.
Q: What does the letter after the frame number mean?
A: It marks the breaking class — C, S, H and R on NM1; E, S, F and H on NXM. Same shell, different arc chutes and contacts, different price and different kA.
Related: MCCB range · MCCB selection: NM1, NXM, NM8N · MCB vs MCCB vs ACB · IEC 60898 vs IEC 60947 · Selectivity between MCB and MCCB · 6 kA vs 10 kA breaking capacity · How to read MCB marking · How to verify genuine product