Short answer: selectivity — also called discrimination — means the breaker closest to the fault trips and the one upstream stays closed. Between an MCB and an MCCB you can get it in three ways: a sufficient current ratio between the two ratings, a deliberate short time delay on the MCCB trip unit, or an energy comparison where the downstream device limits let-through I²t below the upstream device’s threshold. Which of the three you actually have is proven only by the manufacturer’s tested selectivity table for that specific pair.
Key facts
- Selectivity is a property of a pair of devices, not of one device. Two good breakers can still be non-selective together.
- An MCB to IEC/EN 60898-1 has a fixed, non-adjustable magnetic and thermal element. You cannot tune it, so with an MCB downstream the coordination has to come from the ratio or from the upstream device.
- An MCCB to IEC/EN 60947-2 usually gives you adjustable Ir and Im, and the electronic versions add a short-time delay — that adjustability is what buys time selectivity.
- Series rating and selectivity are different things. A pair tested as back-up protection may both open on a heavy fault. That is acceptable for breaking capacity and unacceptable if you needed continuity of supply.
- The supplier’s table is the only evidence that counts. Extrapolating from ratings is how boards end up tripping at the incomer.
Four mechanisms, in the order you should consider them
| Mechanism | How it works | Typical application | Where it fails |
|---|---|---|---|
| Current | The upstream instantaneous threshold sits above the maximum fault current the downstream device will pass | Small MCB downstream of a much larger MCCB | Close to the transformer, where fault current is high |
| Time | The upstream device deliberately waits before tripping in the short-time region | MCCB with an electronic trip unit feeding a sub-board | Very high faults that reach the instantaneous override |
| Energy | The downstream device clears so fast that its let-through I²t stays under the upstream threshold | Current-limiting MCBs and MCCBs | When the upstream device is also fast and has a low threshold |
| Zone interlocking | The downstream device signals the upstream one to hold its delay | Large installations with communicating trip units | Needs wiring and compatible units; not a retrofit afterthought |
The ratio rule, and why it is only a starting point
Manufacturer guides commonly quote a ratio of about 1.6 to 1 between two MCB ratings for current selectivity, and roughly 2 to 1 or more when an MCCB sits upstream. The idea is straightforward: the upstream instantaneous element must not see the fault current that the downstream device is willing to pass before it clears.
The catch is that the ratio is derived from typical magnetic bands, not from your devices. A C-curve MCB trips magnetically somewhere between five and ten times its rating, so a 32 A device can start responding anywhere from 160 A to 320 A. The upstream device has to be clear of the top of that band, not the middle of it. This is why a ratio that works on paper with a 40 A upstream and a 25 A downstream can fail in the field when the actual curves are at opposite ends of their tolerance.
There is a second catch: an MCB to IEC 60898-1 and an MCCB to IEC 60947-2 are tested to different requirements, so a mixed pair cannot be reasoned from first principles. Ask for the table.
The ratings you are actually coordinating
| Family | Role in the board | Range | Breaking capacity | Built-in adjustability |
|---|---|---|---|---|
| NB1-63 (IEC/EN 60898-1) | Final circuit MCB | 1–63 A, B, C and D curves, 1P to 4P | 6 kA | None; fixed thermal and magnetic elements |
| NB1-63H | Final circuit MCB, higher fault level | Same frame, up to 63 A | 10 kA | None |
| NM1 (IEC/EN 60947-2) | Sub-main or incoming MCCB | 10–1250 A, up to 690 V | Breaking class codes C, S, H and R | Thermal-magnetic or electronic trip, depending on frame |
| NXM (IEC/EN 60947-2) | Sub-main or incoming MCCB, higher ratings | 16–1600 A, up to 690 V | Up to 70 kA at 400 V, up to 50 kA at 690 V | Thermal-magnetic, magnetic-only or electronic; NXMS electronic versions add settings |
Two practical notes on that table. First, the breaking class letter on an NM1 is a capacity grade, not a selectivity statement — an H-class device is not automatically selective with everything below it. Second, the electronic versions are where time selectivity becomes possible: an adjustable short-time delay holds the incomer closed for the few cycles the downstream device needs, across a much wider range of fault currents than a ratio ever will.
Let-through energy: the number that decides the hard cases
Two devices can have the same breaking capacity and completely different let-through behaviour. A current-limiting device opens before the prospective fault current reaches its peak, so the energy it lets through is a fraction of what the installation would otherwise see. That is the mechanism behind energy selectivity, and it is also the reason an upstream device can be rated lower than the prospective fault current at its terminals when the downstream device is current-limiting.
The let-through figure applies to a stated prospective current and voltage: change the transformer or the cable length and the number moves. I²t data is only safe inside a tested combination.
Series rating is not selectivity
These two get conflated constantly in tender documents. A series-rated combination is a pair tested together so that the combination’s breaking capacity exceeds the downstream device’s own rating — the upstream device helps the downstream one survive a fault. Both may trip. That is fine when the requirement is “the board must not explode”, and it is not fine when the requirement is “only the faulty final circuit goes dark”.
On a hospital, a data room or a cold store, those requirements are not interchangeable: if the specification says selectivity, ask for the selectivity table, not the coordination table.
Coordination checklist
- Draw the fault path and note the prospective fault current at each board, not just at the origin.
- List both devices by family, rating, curve or trip setting, and breaking class — not by “32 A breaker”.
- Ask for the selectivity table for that pair at that voltage, and check the table covers your prospective current.
- Confirm which device the installation can afford to lose. If the answer is “neither”, you need time or zone interlocking, which means an electronic trip unit upstream.
- Record the combination in the panel schedule so nobody substitutes a different curve.
What we can and cannot confirm
We can confirm ratings, curves, breaking classes, trip unit types and pole options, and send the certificate for the exact ordering code. We cannot invent a selectivity table for a pair the manufacturer has not published. HAZAM is an independent trading company — not a manufacturer and not an appointed distributor — so our value here is the document trail and the honest answer.
What to send for a quotation
Upstream device and downstream device with full codes or at least family, rating and curve · system voltage and frequency · prospective fault current at the sub-board · required selectivity or just back-up protection · destination country · quantity. You get unit price, packing data, the certificate for the code, and the published coordination data we can actually source. Request a quotation.
FAQ
Q: Can a 63 A MCB be selective with a 100 A MCCB?
A: Sometimes, in the low fault current range, because the MCCB’s instantaneous threshold sits above what the MCB passes. It usually stops being selective as the fault current rises towards the MCCB’s instantaneous setting. That boundary is a number in a table, and you need it before you sign the panel schedule.
Q: Is a higher breaking capacity device more selective?
A: No. Breaking capacity is about surviving a fault; selectivity is about which device opens. Two MCCBs of the same rating but different breaking classes can behave identically against a given MCB.
Q: Why do electronic trip units make selectivity easier?
A: Because you can set a short-time delay and a current threshold, so the upstream device can be told to wait for a defined period. A thermal-magnetic MCCB has a fixed magnetic element, so the only lever is the rating ratio.
Q: Does a 30 mA RCD upstream ruin selectivity?
A: It can. Two 30 mA devices in series are not selective by time or current in any meaningful way; one will trip first, and it may be the upstream one. Where earth fault selectivity matters, use a selective or delayed device upstream and a 30 mA device at the final circuit.
Q: How do I check selectivity for a combination I already installed?
A: Get both exact codes off the devices, get the manufacturer’s selectivity table for that pair, and compare it against the measured prospective fault current at that board. With no table, treat the combination as back-up protection.
Where to go next
For the frame-level comparison behind this decision, read MCB vs MCCB vs ACB and MCCB selection across the NM1, NXM and NM8N frames. The standard behind the two families is explained in IEC 60898 vs IEC 60947, and the capacity question in 6 kA vs 10 kA. For board-level planning see how to size a distribution board, and browse the MCCB range or the MCB range.
Related: MCB vs MCCB vs ACB · MCCB selection guide · IEC 60898 vs IEC 60947 · 6 kA vs 10 kA MCB · MCCB range · How to verify genuine product