Short answer: an isolator and a main breaker are not the same device and are not tested to the same standard. The NH4-125 is a modular switch disconnector to IEC/EN 60947-3: it makes and breaks load current, carries the rated current continuously, withstands a stated short-circuit current for one second, and gives a contact position you can lock. A main breaker — an MCB to IEC/EN 60898-1 or an MCCB to IEC 60947-2 — interrupts overload and short-circuit current. A board often needs both functions, and sometimes one device can legally carry both — but only when its marking and certificate say so.
This is a buying decision that goes wrong quietly. A panel built with a breaker as the incoming device, where the specification asked for an isolator, will pass inspection in some countries and fail in others, because “switch off and lock” is a defined capability, not a habit. We write as an independent trading company: we are not a manufacturer and not an appointed distributor of any brand.
Two jobs that get confused on the drawing
Isolation is a safety function: a break in every live conductor that a person can verify, lock in position and rely on while working downstream. The device that does it has to be rated for the load it switches, survive the fault current while the upstream protection clears, and hold its position. IEC/EN 60947-3 covers that combination: switch, disconnector, and switch-disconnector.
Protection is a different function: detecting an overload or a short circuit and opening within a stated time, at a stated current, without being destroyed. For final circuits that is IEC/EN 60898-1 with an Icn of 6 kA or 10 kA; for industrial feeders it is IEC 60947-2 with an Icu that runs from tens of kA to over 50 kA depending on frame and voltage.
What the NH4-125 is rated to do
The NH4-125 is a modular switch disconnector in the same 9 mm-per-pole format as the NB1-63 and NXB-63 breakers: same DIN rail, same comb busbar pitch. Each published rating answers a different question from the buyer.
| Attribute | NH4-125 published rating | The question it answers |
|---|---|---|
| Standard | IEC/EN 60947-3 | Is this a switch-disconnector, or a breaker being used as one? |
| Rated operational voltage Ue | 240 V AC single-pole, 415 V AC multi-pole | Does it match the system voltage, pole by pole? |
| Rated operational current Ie | 32, 40, 63, 80, 100, 125 A | What continuous load can it carry in the enclosure? |
| Rated short-time withstand Icw | 12 × Ie for 1 s | Will it survive the fault while the upstream breaker clears? |
| Making and breaking capacity | 3 × Ie at 1.05 Ue, cos φ 0.65 | Can it open a loaded circuit, not just an idle one? |
| Electrical life | 5,000 operations | How often can it be used as the daily switch? |
| Mechanical life | 8,500 operations | How long does the mechanism last with no current flowing? |
| Poles | 1P, 2P, 3P, 4P | Three-phase, single-phase, or switched neutral? |
| Published certificate | VDE | Which file can a consultant actually open? |
Two of those numbers carry more weight than the rest. The first is Icw. The second is the making and breaking figure of 3 × Ie at cos φ 0.65 — the 0.65 power factor is what makes it a load-breaking test rather than a no-load test, and it is the number that separates a real switch-disconnector from a terminal block with a handle.
Three devices, three different ratings
Put the three candidates side by side and the difference stops being theoretical. They overlap in current, they overlap in rail width, and they do not overlap in what they are allowed to interrupt.
| NH4-125 switch disconnector | NB1-63 / NXB-63 MCB | NM1 / NXM MCCB | |
|---|---|---|---|
| Standard | IEC/EN 60947-3 | IEC/EN 60898-1 (and 60947-2 manuals for some variants) | IEC/EN 60947-2 |
| Current range | 32–125 A | 1–63 A | 10–1250 A (NM1), 16–1600 A (NXM) |
| Overload and short-circuit protection | None — it is not a protective device | Thermal-magnetic, B / C / D curves | Thermal-magnetic or electronic trip unit |
| Short-circuit rating | Withstands Icw; does not interrupt fault current | Icn 6 kA, 10 kA on the H variant | Icu per frame, voltage and breaking class |
| Isolation and locking | Its primary purpose; padlockable handle position | Switches load, but the isolating claim depends on the marking | Withdrawable or fixed, lockable in some frames |
| Typical position | Incoming isolator, machine local isolation, split point | Final circuit, small board incoming | Sub-main, feeder, transformer outgoing |
Icw is the number that decides coordination
An isolator does not clear a fault. It rides through it. Icw of 12 × Ie for one second means a 125 A NH4 held closed will carry 1,500 A for a full second without its contacts welding. That second is not arbitrary: it is roughly the time the upstream breaker needs to trip on a large fault, and it is the window in which the switch is the weakest element in the circuit. Two consequences follow. The prospective fault current at the point of installation has to be calculated or measured — not guessed from transformer size — and compared with both the switch’s withstand and the breaker’s breaking capacity. And the upstream device has to clear inside the switch’s withstand window at every fault level, not only at the maximum.
Isolation is a rated claim, not a habit
Every electrician has met a board where the “main switch” is a breaker with a padlock hasp added on site. Sometimes that is acceptable; often it is not, and the difference is documented. Three things have to line up before a device can be presented as the point of isolation.
The first is the standard and the marking. A device built and marked as a switch-disconnector to IEC/EN 60947-3 carries an isolating function as part of its definition. A breaker to 60898-1 is a protective device; using it to isolate may still be permitted by the installation rules of a given country, but that is a decision made against the local code, not something the device’s certificate supplies.
The second is the pole configuration. If the neutral is part of the isolation, it has to be switched — a 4-pole or 1P+N device, not a 3-pole device with the neutral taken through a terminal block. Retrofitting a neutral link to a 3-pole isolator defeats the purpose, because the link can be removed and the neutral reconnected while the phase remains open.
The third is the physical interface: a handle that accepts a padlock open, a position indicator readable with the enclosure door closed, and room for the door interlock to work. These are mechanical details decided at the ordering stage and cannot be added by the installer.
Where each device belongs
The NH4-125 sits at the incoming of a board up to 125 A, at a machine’s local isolation point, or at a split where one lockout point is wanted. It is not the incoming protective device, and it is not a changeover switch: an ATS has its own standard and contact arrangement. The MCB stays on final circuits; the MCCB takes the sub-main and feeders, where fault level and current exceed what a modular device carries.
Voltage is not always 230/400 V
The NH4-125 is marked 240 V per pole and 415 V across poles: a direct fit on 220–240 V single-phase and 380–415 V three-phase systems. On a 110–127 V system the marking is a maximum, so the device is not overstressed — what changes is the listing, since a 120 V destination wants its own approval and an IEC 60947-3 type test is not that approval. The same applies at 60 Hz where the certificate file, not the device, is the constraint.
What to put on the purchase order
Series and current rating; poles and whether the neutral is switched; the utilization the switch will see; whether a padlockable handle and door interlock are needed; enclosure type and IP rating; destination country, system voltage and frequency; and the certificate file that destination requires. Add the fault level if it is known — that is what turns an isolator choice into a coordination check, and it is the question worth asking twice rather than discovering on site.
FAQ
Can I use the NH4-125 as the main breaker on a small board?
Only for the isolation function. It has no overload or short-circuit trip, so the board still needs a protective device upstream or downstream. On a small board the usual arrangement is an MCB or MCCB as the incoming protective device and the isolator either ahead of it or at the split point.
Does an isolator need a short-circuit rating if it never clears a fault?
Yes — that is what Icw is for. The switch has to carry the fault current for the time the upstream device takes to clear it. A switch with a lower withstand than the fault duty can weld closed, and a welded switch that reads “off” is the worst outcome on the board.
What does the 3 × Ie breaking figure actually prove?
That the device can open a loaded circuit, not just an idle one. The test is done at 1.05 times rated voltage and a power factor of 0.65, which is an inductive load rather than a resistive one. A switch tested only for no-load operation is a different product.
Which certificate file comes with it?
A published VDE file for the series. Other destinations need their own file, and a certificate covers only the ratings and pole configurations it names.
Related: Isolators, ATS and SPD range · MCB range overview · MCCB range overview · PC class vs CB class changeover · How to read an MCCB datasheet · Selectivity between MCB and MCCB · Cross-reference an existing device · Request a quotation