Surge Protection Device (SPD)
Choose Uc, SPD type and check effective protection level against equipment withstand.
Step-by-step
- Enter valid values to begin.
Screening check only. U0 is phase-to-earth voltage. Minimum Uc uses 1.1×U0 (TN, TT) or 1.1×√3×U0 (IT) as typical IEC 61643-12 / IEC 60364-5-53 guidance. Impulse withstand voltages follow IEC 60664-1 Table F.1 by voltage class, typed from memory: verify against the standard. The 1 kV/m lead allowance is a rule of thumb (about 1 µH/m × di/dt), not a standard value. Type ratings shown are typical minimums. It does not assess TOV, coordination, backup OCPD sizing or risk per IEC 62305-2.
Surge Protection Devices: Uc, Up, Type and Lead Length
Core Engineering Principles
An SPD sits idle across the line until a surge arrives, then conducts and clamps. Voltage-limiting devices (varistors, diodes) change resistance gradually and leave a modest residual voltage. Switching devices (spark gaps) crowbar to near zero and carry big impulse current, but can follow with mains current if the design is careless. We choose on four numbers. Uc is the maximum continuous operating voltage and must exceed the highest sustained voltage the line sees. Up is the voltage left while the SPD conducts. In and Imax are 8/20 µs discharge ratings, and Iimp is the 10/350 µs lightning rating for Type 1.
Datasheet Up is not what your equipment sees. Each metre of lead adds roughly 1 µH, and a fast surge drops about a kilovolt across it. We use 1 kV per metre as a rule of thumb, not a standard value. Add it to Up and keep the sum under about 80% of the equipment impulse withstand Uw. Stages need coordination: Type 1 at the service entrance, Type 2 at the board, Type 3 at the load, with cable between them, or the downstream device absorbs energy meant for the upstream one. Always fit the manufacturer’s backup fuse, because a worn varistor fails short-circuit.
Up,eff = Up + 1 kV/m × L (rule of thumb) ≤ 0.8 × Uw
Short-circuit: ISCCR ≥ Iavailable
NEC & Standard References
IEC 61643-11 defines low-voltage SPD tests and Types 1, 2, 3; IEC 61643-12 covers selection, including Uc by earthing system. IEC 62305-4 and IEC 60364-5-53 cover protecting installations and where SPDs go, and IEC 60664-1 gives the impulse withstand categories behind Uw. UL 1449 lists Types 1, 2, 3 and 4, which do not map one-to-one to IEC types, and NEC Article 242 covers overvoltage protection. Check the edition your authority has adopted.1. Uc,min = 1.1 × 230 = 253 V, standard pick 255 V.
2. Board location gives Type 2, typically In ≥ 20 kA, which we meet.
3. Up,eff = 1.5 + 0.5 × 1 = 2.0 kV.
4. Category III on the 300 V class gives Uw = 4 kV, limit 0.8 × 4 = 3.2 kV, margin 1.2 kV.
5. 25 kA against 10 kA available passes.
Stretch the leads to 2 m and Up,eff becomes 3.5 kV, failing the limit with the same device.
- Long leads. Looped pigtails cost more protection than the choice between two Up grades. Keep them short.
- No backup OCPD. At end of life the SPD shorts and can burn the enclosure without its fuse.
- Uc too low. Tight Uc runs hot on swells and neutral loss and ages fast. Check TOV withstand.
- One stage only. A lone board device does not handle lightning current on a structure with an LPS.
- IT systems. After a first earth fault, line-to-earth voltage rises toward line-to-line, so TN Uc values fail.