The number that matters is the voltage that appears across a single set of contacts at current zero, not the voltage printed on the panel schedule. A 400 V three-phase wye system with a solidly grounded neutral puts 400/√3 = 231 V from any phase to ground and 400 V phase to phase. A dual-rated device is built and tested for the first number across one pole and the second number across poles in series. A single-voltage device is tested for one number across one pole, whatever the circuit configuration.
Two Rating Schemes on the Same Nameplate
A single-voltage rating — 240 VAC is the common North American value — is a per-pole rating that applies without qualification. One pole may be connected line to neutral or line to line, up to 240 V, and the device is expected to interrupt at that voltage on its own. Ganging poles buys nothing in voltage capability; it only adds simultaneous disconnection of additional conductors.
A dual rating written as 230/400 V is two constraints, not two options. The lower number is the maximum voltage permitted across any one pole, which on a grounded system means the maximum line-to-ground voltage. The higher number is the maximum line-to-line voltage of the system, and it is only valid when two or three poles are ganged so the line-to-line voltage divides across contacts in series. A single-pole device carrying a dual rating is, in service, a 230 V device. It cannot be applied any differently than a single-voltage device of the same per-pole rating.
Trip curve class is an independent axis. A B, C, or D characteristic describes the magnetic instantaneous multiple of rated current and has nothing to do with what the contacts can clear. Choosing the curve for inrush and choosing the voltage rating for system topology are separate decisions made from separate data.
Arc Voltage Across One Gap Versus Two
This is dielectric recovery, not logic. When contacts part, the current continues through an arc until the natural current zero. Interruption succeeds only if the gap regains dielectric strength faster than the transient recovery voltage rises across it. Arc chute geometry, splitter plate count, contact gap, and arc runner length are all sized for a target recovery voltage. Push more voltage across the same gap and the arc restrikes, the device fails to clear, and the fault energy continues into the enclosure.
Ganging two poles on a 400 V line-to-line fault places two arc chutes in series in the fault loop. Each gap sees roughly half the recovery voltage — on the order of 200 V per break — which is inside what the 230 V per-pole construction was verified to handle. That series division is the entire physical basis of the dual rating, and it only exists if both current-carrying poles are actually in the fault path.
The failure case follows directly. On a solidly grounded wye, a phase-to-ground fault returns through the neutral/ground path and only one pole is in series with it, seeing 231 V. That is inside the 230 V rating by design. Change the grounding — ungrounded, impedance-grounded, or corner-grounded delta — and a phase-to-ground fault can put the full 400 V across one pole. The series division disappears and the device is applied above its verified per-pole capability.
Rating Comparison and Where to Read It
| Parameter | Single-voltage rated | Dual-voltage rated | Where to read it |
|---|---|---|---|
| Max line-to-neutral / line-to-ground | 240 VAC | 230 VAC | Nameplate voltage block |
| Max line-to-line | 240 VAC | 400 VAC | Nameplate voltage block, second number |
| Poles required for the higher rating | n/a | 2 or 3 ganged | Catalog application table |
| Single-pole use | Up to 240 V, any configuration | 230 V line-to-ground only | Catalog application table |
| Permitted system grounding for the 400 V figure | n/a | Solidly grounded wye, 230/400 V | System one-line, transformer nameplate |
| Interrupting rating | Declared per voltage | Often different at 230 V and 400 V | Datasheet interrupting table |
| Trip curve (B/C/D) | Independent of voltage rating | Independent of voltage rating | Catalog number suffix |
Dual voltage ratings appear predominantly on supplementary protectors rather than on branch-circuit breakers. In North American practice, supplementary protectors are listed under a different standard than branch-circuit overcurrent devices and are permitted only where branch-circuit protection is already provided upstream — inside control panels, downstream of a listed branch device, protecting individual loads. Read the listing mark on the device, not the shape of the case: a supplementary protector and a branch-circuit breaker look nearly identical on a DIN rail.
System Grounding as the Deciding Variable
Two devices, two answers, and the system decides which one is legitimate.
- Solidly grounded 230/400 V wye, three-phase load: a 3-pole dual-rated device is valid, and it is the only one of the two that is valid, since a 240 V single-voltage device would be applied at 400 V across each of its poles.
- Solidly grounded 230/400 V wye, single-phase 230 V line-to-neutral load: either device works. The single-voltage 240 V device has margin; the dual-rated device is at its per-pole limit.
- Ungrounded, impedance-grounded, or corner-grounded 400 V system: neither of these ratings applies. A first ground fault raises the healthy phases to full line-to-line voltage against ground and no series division protects the pole. Specify a device with a per-pole rating equal to the full line-to-line voltage.
- 240 V single-phase or 240 V delta: the single-voltage device is the correct choice.
Recommendation for the common case of a 400 V control panel fed from a solidly grounded wye: specify the dual-rated multi-pole device with common trip, and verify the interrupting rating quoted at 400 V rather than the headline figure quoted at 230 V. Those two numbers are frequently different by a factor of two or more.
Application Procedure and Verification
- Read the source transformer nameplate and the one-line. Confirm the secondary is wye and the neutral is solidly bonded. Confirm the phase-to-phase voltage.
- Measure with the panel energized: phase to phase on all three pairs, and each phase to the ground bar. On a valid 230/400 V system the phase-to-ground readings land near 230 V and are balanced within a few volts. A phase-to-ground reading approaching phase-to-phase voltage means the system is not solidly grounded wye — stop and re-specify.
- Compare the measured line-to-ground value against the lower nameplate number and the measured line-to-line value against the higher nameplate number. Both must pass.
- Confirm the pole count in the fault path. Two poles for a line-to-line load, three poles for a three-phase load. A single-pole device on a 400 V circuit is a misapplication regardless of what the nameplate reads.
- Confirm common trip, not merely a common handle. A handle tie moves the poles together during manual operation; it does not guarantee that an overload on one pole opens the others. The series arc division depends on all poles in the fault loop opening.
- Verify the interrupting rating at the applied voltage exceeds the available fault current at the device terminals.
- Confirm the listing class matches the application. If the device is a supplementary protector, confirm a listed branch-circuit device is upstream and that it coordinates.
- Label the panel with the system voltage and grounding arrangement so the next person replacing a tripped device does not substitute a 240 V single-voltage part.
Recurring Misapplications
The most frequent one is stocking a single-pole dual-rated device and treating the 400 V number as a per-pole capability. It is not; it describes a system, and it requires the poles ganged. The second is replacing a 3-pole dual-rated device with three individual single-pole devices of the same rating and current. Mechanically it fits, electrically the series division still exists during a phase-to-phase fault, but nothing forces all three poles open on a single-pole overload, so one pole can be left carrying a fault it was never rated to clear alone.
Third: assuming the interrupting rating is constant across both voltages. Arc energy scales with voltage, and manufacturers routinely declare a lower interrupting capacity at the higher figure. Read the interrupting table, not the front label.
Fourth: applying an AC-rated device to a DC circuit. DC has no current zero to help extinguish the arc, and AC voltage ratings do not transfer. A device without a printed DC rating and pole-series instruction has none.
Where the transformer configuration cannot be confirmed, where phase-to-ground measurements do not match a solidly grounded wye, or where the datasheet does not state the pole configuration required for the higher voltage, stop and query the manufacturer's technical support with the catalog number and the measured system voltages. Application beyond a published rating is not something to reason out from first principles in the field — ask for the written application table or a formal statement from the vendor, and keep it with the panel documentation.
Frequently Asked Questions
Can I use one pole of a 230/400 V MCB on a 400 V line-to-line circuit?
No. The 400 V figure requires two or three poles ganged so the recovery voltage divides across contacts in series. A single pole is limited to the lower number, 230 V, which on a grounded wye means line to ground only.
Does a 240 V single-voltage MCB work on a 400/230 V wye system?
Only for line-to-neutral loads at 230 V, where each pole sees 231 V. It cannot protect a 400 V line-to-line or three-phase load, because ganging poles does not extend a single-voltage per-pole rating.
Can I use a dual-rated supplementary protector as branch-circuit protection?
No. Supplementary protectors are listed for use downstream of branch-circuit protection and have limited permitted applications. Check the listing mark on the device and confirm a listed branch-circuit overcurrent device is upstream.