Short answer: a load-break switch is selected by four things in this order — the utilization category the load demands, the operational voltage of the system it isolates, the continuous current after enclosure derating, and the short-circuit current it must withstand while the upstream device clears. The NF2 series is a load-break switch line rated to 690 V AC with CE and TÜV certificates published per ordering code. Get the order right and the device is unremarkable for twenty years. Get it wrong and the failure appears as burnt contacts on the one day someone opened the switch under load.
Most quotations for a “main switch” start from the current rating, because that is the number printed largest on the old device. That is the last number to choose, not the first. This page sets out the sequence a buyer should follow and the four checks that get skipped. We are an independent trading company — not a manufacturer, not an appointed distributor of any brand — and every rating belongs to a specific ordering code.
What a load-break switch is actually rated to do
A load-break switch is a switch-disconnector under IEC/EN 60947-3. In plain terms it carries rated current continuously, makes and breaks current on a loaded circuit up to its stated category, and provides a contact separation that can be verified and locked for maintenance. It does not interrupt short-circuit current; that belongs to the fuse or breaker upstream, and the switch’s role in a fault is to survive long enough for that device to act.
Three different devices get called a main switch on site: a breaker used as an isolator, a switch-disconnector with no protection, and a combined unit. They are built to different standards and priced differently.
| Device family | Standard | Interrupts fault current? | Isolating function | Typical position |
|---|---|---|---|---|
| Load-break switch (NF2 family) | IEC/EN 60947-3 | No — it withstands fault current, the upstream device clears it | Yes, by definition of the standard, with a lockable position | Machine isolation, sub-board incoming, feeder split |
| Modular switch disconnector (NH4-125) | IEC/EN 60947-3 | No — Icw 12 × Ie for 1 s | Yes, in 9 mm modular format | Small board incoming, split point |
| MCB (NB1-63 / NXB-63) | IEC/EN 60898-1 | Yes — Icn 6 kA, 10 kA on the H variant | Depends on the marking and the national code | Final circuit, small board incoming |
| MCCB (NM1 / NXM) | IEC 60947-2 | Yes — Icu per frame, voltage and breaking class | Fixed or withdrawable versions, lockable in some frames | Sub-main, feeder, transformer outgoing |
Check one: the utilization category, not the ampere rating
IEC 60947-3 assigns utilization categories that describe what the switch may break: resistive, mixed resistive-inductive, motor, and their overloaded variants. The category is printed on the device and in the datasheet, and it decides whether a switch that carries 100 A can also break 100 A of motor current. Breaking a motor circuit is harder than carrying it, because the inductance holds the arc.
Buyers skip this check because the old label rarely states a category, and because the load looks obvious. It is not obvious in three cases: a feeder supplying a mix of lighting and small motors, a capacitor bank, and a DC circuit where the arc does not self-extinguish. Tell us what the switch feeds and whether it opens under load — that one sentence changes the shortlist more than any other input.
Check two: the operational voltage, pole by pole
The NF2 family is rated to 690 V AC, which covers 400/415 V three-phase systems and the 440 V and 480 V systems found in the Gulf, parts of Asia and older industrial installations with margin. What the family rating does not tell you is the per-pole voltage in a single-phase or split-phase arrangement, and that is where 110–127 V markets trip people up.
On a 120/240 V split-phase system, a two-pole device sees 240 V across poles and 120 V to earth. That is usually satisfied by a 690 V family, but the installation still has to be checked against the destination’s own rules, because what a market accepts is its listing — UL, CSA, NOM and their equivalents — not an IEC type test. At 60 Hz the switch is generally indifferent; the certificate file may not be.
Check three: current after derating, not on the bench
A switch rated at a given current in free air will not carry it inside a sealed enclosure at 50 °C with six devices side by side. The datasheet carries the derating table above the reference ambient. Two numbers matter: the ambient inside the enclosure in the hottest month, and the grouping factor. This is the usual cause of a switch that “runs hot but works” for two years and then discolours its terminals — the enclosure was the missing term.
Check four: coordination with the upstream protective device
The switch does not clear a fault, so the fault current passes through it. Two ratings describe how it behaves while that happens: the short-time withstand current, and the short-circuit making capacity. Both have to be compared with the prospective fault current at the point of installation. There is a subtler condition: the upstream device must clear inside the switch’s withstand window at every fault level, not just the maximum. A board where the isolator survives 20 kA but not 6 kA is still a board that fails.
The four checks buyers skip
| Check | What to establish | What goes wrong when it is skipped |
|---|---|---|
| Utilization category | Load type and whether the switch opens under load | Contacts erode early; the switch welds on a motor circuit |
| Voltage, pole by pole | System voltage, voltage to earth, and the destination’s listing | A device that is electrically fine but not certifiable for that market |
| Derating | Ambient inside the enclosure and grouping factor | Terminals discolour after two summers; nuisance failures blamed on the brand |
| Coordination | Prospective fault current and the upstream device’s clearing time | A switch that survives the biggest fault but not a mid-range one |
Mechanical fit decides the last 20 %
Four-pole devices are not two two-pole devices in one housing: width, terminal pitch and handle position all change. Busbar and cable lugs are the second trap — a switch sized for busbar entry may not accept the lug the installer already has. Then the interface: a padlockable handle, a position indicator readable with the door closed, and a door interlock. These are ordering options, not installation details.
Certificates, and what “certified” means here
The NF2 family carries CE and TÜV certificates, published against the series. CE is a declaration of conformity by the manufacturer; TÜV is third-party testing. Both are useful, and neither is a market authorisation. A destination that requires G-Mark, SABER registration, EAC or a national approval needs that file for the exact code being imported, and a CE declaration does not substitute for it. The certificate follows the code, not the family: a file covering one frame and pole configuration does not cover a larger frame.
What to send for a quotation
Load description and whether the switch opens under load; system voltage, frequency and phases; continuous current and the derated design current; prospective fault current if known; poles and whether the neutral is switched; enclosure type, IP requirement and ambient; whether a padlockable handle, door interlock or auxiliary contact is needed; destination and the certificate that market requires. With those, the answer is a series, a frame size and a code. Without them, it is a guess with a lead time attached. We quote the code that fits the grid and the certificate file that exists for that code, and we say plainly when a file does not exist yet — we do not manufacture the device and we are not an appointed distributor of any brand, so the accuracy of the code is the only thing worth selling.
FAQ
Can a load-break switch replace a main breaker?
No. It isolates; it does not protect. A board needs a protective device — an MCB, an MCCB or a fuse — for overload and short circuit, and the switch sits ahead of it or at the split point. Using the switch alone leaves the installation without fault protection.
Is a 690 V rating useful on a 400 V system?
Yes. The rating is a maximum, so a 690 V device on a 400/415 V system has margin rather than excess. The useful part is that the same family covers 440 V and 480 V systems without changing the order line, which simplifies spares across a mixed estate.
How often can the switch be operated?
That is the electrical and mechanical life figure for the frame, and it is in the datasheet for the code. The practical limit is usually not the mechanism but the duty: a switch used as the daily on/off control of a machine accumulates operations far faster than one used once a month for maintenance.
Do I need an auxiliary contact?
If anything downstream needs to know the switch position — a PLC input, a status lamp, an interlock with a second incoming supply — then yes, and it should be ordered with the switch rather than fitted later. The contact reports position; it does not make the switch safer on its own.
What about 110–127 V and 60 Hz destinations?
The device is not the constraint; the listing is. Tell us the grid and the destination and we will quote the code whose certificate actually covers that market, and where no file exists for that combination we will say so rather than propose a device that cannot be registered on arrival.
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