Short answer: Uc is the highest continuous voltage an SPD may see across its terminals, and it is chosen from the voltage between the live conductor and earth – not from the line-to-line voltage and not from whatever step is cheapest. On a 230 V system you move up to the 275 V step; on a legacy 127 V system the lowest step in the range already covers the to-earth voltage. Earthing arrangement matters next: a TT system is usually specified at a higher Uc than a TN system on the same grid, because the live-to-earth voltage under a fault is not clamped by the supply neutral.
HAZAM is an independent trading company – not a manufacturer, and not an appointed distributor of any brand. What follows is how an SPD’s Uc is chosen for the grids we quote, and why the same device can be right for one site and wrong for the next.
What Uc is, and why it is the first number
An SPD (surge protective device) sits in parallel with the installation and waits. Most of the time it must simply not conduct, and it must survive whatever continuous voltage the installation presents it with. That maximum continuous voltage is Uc. Get Uc too low and the device works continuously, heats up and degrades – or fails safe, tripping its own disconnector and leaving the panel unprotected. Get it too high and the protection level is looser than it needs to be.
So Uc is a floor, not a target: it must exceed the highest sustained voltage between the conductors it protects, including normal supply tolerance. It is declared per IEC/EN 61643-11.
Choosing Uc by system voltage
The number to work from is the voltage to earth. A rough working rule is to take about 110% of that value and then move up to the next available step in the range. The steps available in the NU6 family are 275, 320, 385 and 440 V, with a separate 255 V figure for the neutral-to-earth module.
| System | Voltage to earth (approx.) | Working step | Why |
|---|---|---|---|
| 230 V, single phase | 230 V | 275 V | About 110% of 230 V lands at the 275 V step; the standard choice for a 230/400 V installation |
| 230/400 V, three phase (TN) | 230 V | 275 V | Protection is line-to-earth, so the three-phase line voltage does not raise the step |
| 127 V, single phase | About 127 V | 275 V (lowest step) | The lowest step in the range already sits above the to-earth voltage; Uc must clear the floor, not match it exactly |
| Higher-stress sites | Fault or earthing raises the voltage | 320 V / 385 V / 440 V | Where the earthing arrangement or the supply leaves more live-to-earth voltage to withstand |
The trap is quoting Uc from the line-to-line voltage. A three-phase 400 V board does not need a 400 V-step SPD; it needs a device whose Uc clears the 230 V line-to-earth value, because that is what the phase conductor sees against the earthed neutral.
TN and TT: why earthing changes the answer
The earthing arrangement is the second decision, and it is the one that most often turns a “correct” Uc into an under-specified one. In a TN system the supply neutral is earthed at the source, so a phase-to-earth fault pulls the faulty conductor close to earth potential, and the to-earth voltage stays bounded. In a TT system the installation has its own earth electrode and the source neutral is not bonded to it locally, so a fault can raise the voltage between live parts and the installation’s earth by the full supply voltage until the protective device clears it.
That is why TT installations and exposed-earth sites are commonly specified a step higher – 385 V where a TN board on the same grid uses 275 V. It is also why we ask grid voltage, frequency, phase count and earthing arrangement before quoting an SPD. The same reasoning runs through the earlier guide on SPD Type 1, 2 and 3.
Type 1, Type 2 and 1+2
Uc is only the first column. The type of SPD is chosen by where the surge energy comes from and where the device stands in the installation.
| Type | Where it sits | What it is for |
|---|---|---|
| Type 1 | Main incoming, where a lightning protection system or an overhead feed exists | Handling the partial lightning current that travels on the incoming conductors |
| Type 2 | Distribution boards, downstream of the incoming position | Clamping the switching and induced surges that reach equipment |
| Type 1+2 | Main incoming where both duties must be met by one device | Combining the direct-current class with the distribution-board class |
Mixing the types up is a specification error, not a preference. A Type 2 device cannot be asked to carry a partial lightning current, and a Type 1 device deep in the installation does nothing for the equipment behind it.
The NU6 ranges
We quote two NU6 families for this duty. NU6-II is a Type 2 device for 230/400 V systems, with Uc steps of 275, 320, 385 and 440 V, a 255 V neutral-to-earth module, a nominal discharge current across 20, 30 and 50 kA and a maximum discharge current across 40, 65 and 100 kA on the 8/20 microsecond waveform, with a voltage protection level from about 1.5 to 2.4 kV. It is a pluggable cartridge with a condition indicator and an optional remote-contact version.
NU6-I+II is the combined Class I+II device for the same 230/400 V systems, with Uc steps of 275, 385 and the 255 V neutral figure, a 10/350 microsecond impulse current of 12.5 kA, nominal discharge current of 25 kA and maximum discharge current of 50 kA, and a voltage protection level at or below about 1.5 to 1.8 kV. It is declared to IEC/EN 61643-11 and carries published CE, CB, RoHS and VDE documents.
Both families are listed on the SPD, changeover and isolator page. Certificates are stated per exact ordering code, because the ratings differ by module.
Where the SPD goes, and its back-up
An SPD must have a means of disconnection so that a failed device cannot leave the installation without supply, and it must be coordinated with the protection upstream. The usual arrangement is a Type 1 or 1+2 device at the main incoming position and a Type 2 device at a sub-distribution board, with an upstream fuse or breaker sized from the manufacturer’s coordination table rather than picked by eye. Keep the connecting leads short and straight, and bond them to earth properly. The breaker that feeds the board is chosen for the installation; see miniature circuit breakers for the final-circuit side.
For 60 Hz and dual-voltage destinations the Uc decision is unchanged by frequency, but the supply question is not: a Saudi site may be 127 V or 230 V, 60 Hz, and the to-earth voltage decides the step. That dual-system case is set out in Saudi Arabia: 60 Hz, SASO and SABER, and the wider grid map in voltage and frequency by country.
Sizing in four steps
- Find the voltage to earth at the point the SPD protects – not the line-to-line voltage.
- Move up to the Uc step that clears about 110% of that value; add margin where the earthing arrangement is TT or the site is exposed.
- Choose the type by position and by whether a lightning protection system or overhead feed is present: Type 1, Type 2, or 1+2.
- Size the back-up from the coordination table and keep the connecting leads short.
FAQ
Q: Should Uc be based on the line-to-line or line-to-earth voltage?
A: Line-to-earth. The SPD protects a conductor against earth, so a three-phase 400 V board is specified from the 230 V to-earth value, landing at the 275 V step.
Q: Why does a TT system need a higher Uc than a TN system on the same grid?
A: In TT the source neutral is not bonded to the installation’s local electrode, so a fault can raise the voltage between live parts and earth until the protective device clears it. That extra stress is met with a higher Uc step, commonly 385 V where a TN board would use 275 V.
Q: What Uc do I use on a 127 V system?
A: The lowest step in the range already sits above the 127 V to-earth voltage, so the 275 V step covers it. Uc is a floor to clear, not a value to match exactly.
Q: Does frequency change the SPD selection?
A: Not the Uc decision, which runs off the voltage to earth. Frequency matters for the contactors and meters elsewhere in the panel, and for confirming the supply details before quoting – which is why the grid is one of our four standard questions.
Q: Does an SPD need a fuse in front of it?
A: It needs a means of disconnection and coordination with the upstream protection, sized from the manufacturer’s table. An SPD that cannot be isolated cannot be safely replaced, and one that is coordinated wrongly may not clear.
Related: SPD Type 1, 2 and 3 explained · SPD, changeover switches and isolators · 230 V breakers in 110-127 V markets · Voltage and frequency by country · Saudi Arabia: 60 Hz, SASO and SABER · Request a quotation