SPD sizing: Type, Uc, Up and coordination Up ≤ Uw

What SPD sizing is

Sizing a surge protective device is not picking the highest-current model in the catalog: it is matching three voltages so that a transient overvoltage is clamped to a level the protected equipment can survive, while the device itself lives a long life on the system. Those three voltages are Uc (what the SPD must permanently tolerate), Up (what it lets through during a surge), and Uw (what the equipment can withstand). Get the relationship between them right and the installation is protected; get it wrong and either the SPD ages and fails, or the surge passes through to the equipment.

The most common field failure is an SPD that “burns out for no reason” — almost always because its Uc was chosen equal to the nominal voltage instead of above the real permanent stress, or because the connection leads were so long that the effective protection level climbed past the equipment withstand.

Continuous operating voltage Uc — set by the earthing

The first decision is Uc, the voltage the SPD endures continuously without conducting. It is governed by the earthing system, per IEC 60364-5-534:

• TN-S, TN-C-S, TN-C: the line-to-neutral mode sees up to 1.1·Uo, so Uc must be at least 1.1·Uo.
• TT (SPD line-to-PE, 3+1 arrangement): the L-PE SPD must withstand a temporary overvoltage from an upstream fault, so Uc must be at least 1.55·Uo — markedly higher than the TN value.
• IT (without distributed neutral): a first earth fault is not cleared and the healthy phases rise toward the line-to-line voltage, so the SPD must withstand √3·Uo.

The required value is then rounded up to the next commercial Uc in the standard series (150, 275, 320, 335, 350, 385, 420, 440, 460, 550, 690 V). For a 230 V system this lands on 275 V in TN, 385 V in TT (1.55·230 ≈ 357 V), and about 420-440 V in IT. Choosing Uc too low is the textbook cause of premature SPD ageing.

Equipment withstand voltage Uw — by overvoltage category

The target the SPD has to beat is Uw, the rated impulse withstand voltage of the equipment it protects. IEC 60364-4-44 (Table 443.2) assigns it by overvoltage category and voltage band:

The 120-240 V band uses lower values (4 / 2.5 / 1.5 / 0.8 kV). The category descends as you move from the origin toward the load, which is exactly why protection is staged: a Type 1 at the entrance brings the surge down, and a downstream Type 2/3 brings it under the lower Uw of the sensitive load.

The effective protection level — Up is not enough

The datasheet Up is measured at the SPD terminals, but the equipment is connected through leads. During a fast surge the inductance of those leads produces an extra voltage of roughly 1 kV per metre of connection (phase + PE). The voltage the equipment really sees is therefore:

Up_effective = Up + ΔU, with ΔU ≈ 1 kV/m · L_conn

This is why the connection must be kept ≤ 0.5 m: a careless 1 m loop turns a 1.5 kV SPD into 2.5 kV effective, enough to break coordination with a Category III (4 kV) board and certainly with a Category I (1.5 kV) load. A short, V-shaped connection is one of the cheapest ways to improve protection.

The coordination condition

Everything converges on a single inequality. The SPD coordinates with the equipment only when:

Up_effective ≤ Uw

and good practice adds a 20 % reserve to absorb ageing, tolerance and oscillation:

Up_effective ≤ 0.8·Uw

Rearranged, the largest SPD Up that still gives the 20 % margin is Up_max = 0.8·Uw − ΔU. If the effective level sits between 0.8·Uw and Uw, the device still protects but with little headroom — the cue to pick a lower-Up SPD, shorten the leads, or add a Type 3 stage near the load.

Choosing the Type — position and lightning exposure

The SPD Type (test class) follows from where it sits and whether a direct lightning current can reach it:

1. Service entrance, with an LPS or exposed overhead line → Type 1, tested with the 10/350 µs impulse current Iimp (12.5 kA/pole for LPL III/IV, 25 kA/pole for LPL I/II, from the IEC 62305-2 / NBR 5419-2 risk study). Only a Type 1 drains a direct lightning current; a Type 2/3 in that position fails on the first strike.
2. Entrance or distribution board, no direct-strike exposure → Type 2, tested with the 8/20 µs currents In (repetitive nominal) and Imax (single peak). This is the workhorse of the boards.
3. Next to sensitive equipment (≤ 10 m) → Type 3, fine protection, always downstream of a Type 1/2 and never alone at the origin.

A common, invalid combination the method flags as an error: a non-Type-1 SPD at an entrance with an LPS, or a Type 3 placed at the origin.

Short-circuit withstand and backup

A surge protective device also has to survive the power-frequency fault that may follow. Its short-circuit current rating Isccr must be at least the prospective fault current Icc at the installation point, and this rating is only valid together with the manufacturer-specified backup protection (gG fuse or breaker). The connection lead is sized at 6 mm² Cu for a Type 2 and 16 mm² Cu for a Type 1. Take Icc from the short-circuit study; without it, Isccr and the backup cannot be verified.

Practical design considerations

• Pin Uc to the earthing first: 1.1·Uo for TN (L-N), 1.55·Uo for TT (L-PE), √3·Uo for IT, rounded up to a commercial value.
• Read Uw from the category: IV at the origin, III at boards, II/I toward the load — and remember the voltage band.
• Never compare bare Up with Uw: add the lead drop and keep the connection ≤ 0.5 m.
• Aim for the 20 % margin: Up_effective ≤ 0.8·Uw leaves room for ageing.
• Match the Type to the position: Type 1 only where direct lightning can reach; Type 3 only near the load.
• Verify Isccr ≥ Icc with the specified backup.

Following this chain — Uc from the earthing, Uw from the category, the Type from the position, then the coordination Up_effective ≤ Uw and the short-circuit check — yields an SPD selection that protects the equipment and survives the system it sits on.