Direct Answer

When several SPDs are installed in stages along the distribution, who discharges first and who takes how much energy when a surge arrives? The answer: multi-stage SPDs form a natural coordination through lead inductance and their respective voltage protection levels — the upstream stage (near the service entrance, with large discharge capacity) discharges most of the energy first and reduces the residual voltage to what the downstream stage can withstand; the downstream stage handles only the remainder. The typical signature of failed coordination is that the small-capacity SPD near the equipment fails first, meaning all the energy was pushed onto the downstream stage.

A distribution room installed two stages: a Type-1 SPD at the main board (Iimp 25 kA, 10/350 μs) plus a Type-2 SPD at the equipment end (In 20 kA, 8/20 μs). After a thunderstorm season, inspection found the Type-1 intact and the Type-2 burnt out. The investigation showed the two stages were only 0.5 m apart with long leads, so no energy coordination was established — the surge reached the downstream stage before the upstream could fully discharge, and the small-capacity Type-2 could not take it. After increasing the separation (per code ≥ 5 m or adding a decoupling inductor) and shortening the leads, coordination was restored. The key point: energy coordination decides "who acts first and who takes how much" — upstream takes the bulk, downstream backs it up; failed coordination means the downstream fails first.

Why:

  • Upstream discharges first: the Type-1 SPD near the service entrance has large discharge capacity (Iimp class) and releases most of the lightning current;
  • Downstream handles the remainder: the Type-2 SPD (In class) clamps the residual further to protect the equipment port;
  • Lead inductance acts: the inter-stage distance/lead inductance makes the upstream operate first and "holds" the impact energy at the line side;
  • Failed-coordination signature: the small-capacity SPD near the equipment fails first, meaning coordination was never established.

Key insight: "Energy coordination means upstream acts first and downstream backs it up — achieved by staged discharge capacity plus lead inductance, not by simply stacking SPDs."

A factory once installed three SPDs within 3 metres "to be safer", only to have the one nearest the equipment fail repeatedly, again and again after replacement. After reconfiguring by energy coordination (Type-1 Iimp + Type-2 In + proper separation), the damage stopped. Lesson: more SPDs is not safer — mis-coordinated stages make the downstream fail first; coordination matters more than quantity.

1. What Energy Coordination Solves: Who Discharges, Who Clamps

Step one: why coordination is needed.

The essence of the problem:

  • Every SPD stage must share the surge discharge;
  • Without coordination, the energy can concentrate on one stage;
  • A small-capacity downstream stage takes large energy → fails first;
  • The equipment end loses protection (a failed SPD exposes the port).

Coordination goals:

  • The upstream (Type-1) discharges most of the energy;
  • The residual voltage stays within the equipment withstand;
  • The downstream only handles the "remainder";
  • All stages age evenly; none fails prematurely.

Two roles:

  • Discharge stage: near the service entrance, large capacity (Iimp class, 10/350 μs);
  • Clamp stage: near the equipment, low residual voltage (In class, 8/20 μs);
  • Optional intermediate stage: a compromise of capacity and residual voltage;
  • Roles are set by discharge capacity and installation position.

Key insight: "Coordination solves 'who acts first, who backs up' — Type-1 discharges, Type-2 clamps; only then the downstream does not fail first."

2. How Lead Inductance Works in Inter-Stage Coordination

Step two: how the stages "naturally coordinate".

Lead inductance:

  • The wiring between an SPD and the bus has inductance;
  • Surge current changes fast (large di/dt), producing a voltage drop across the inductance;
  • That voltage drop makes the upstream SPD experience the voltage first and conduct first;
  • Shorter and straighter leads mean less inductance (clearer coordination).

Natural coordination mechanism:

  • When the surge arrives, the Type-1 SPD operates first due to the inductance voltage drop;
  • Type-1 discharges most of the current and the residual drops;
  • Type-2 sees an already-reduced surge;
  • With sufficient inter-stage distance, Type-2 never operates before Type-1.

Engineering points:

  • Keep the Type-1-to-Type-2 distance per code (generally ≥ 5 m or per calculation);
  • When the distance is insufficient, add a decoupling inductor (to emulate inductance);
  • Keep leads short, straight, and avoid coiling;
  • Include the lead length in the coordination calculation.

Key insight: "Lead inductance is the 'natural decoupler' — Type-1 acts first, Type-2 second; when the distance is short, add a decoupling inductor."

3. The Division of Labour Between Iimp and In

Step three: what the two parameters govern.

Iimp (Type-1, 10/350 μs):

  • Direct-strike waveform (the strong impact of direct/induction strikes);
  • Large discharge capacity (kA level, e.g., 12.5/25/50);
  • Installed at the main distribution/service entrance (discharge stage);
  • Takes most of the energy discharge.

In (Type-2, 8/20 μs):

  • Induction-strike/switching-surge waveform;
  • Relatively smaller capacity (e.g., 20/40 kA);
  • Installed at sub-distribution/equipment end (clamp stage);
  • Clamps the residual further to the equipment withstand.

Coordination relationship:

  • Type-1 Iimp discharges → residual feeds Type-2;
  • Type-2 In clamps → the equipment-port voltage meets the limit;
  • Insufficient Type-1 parameters → energy pushes to Type-2 (Type-2 fails first);
  • Type-2 too small → cannot take the Type-1 residual (premature failure).

Key insight: "Iimp handles the large direct-strike energy; In clamps at the equipment end — mismatched parameters break the coordination."

4. Failure Signatures of Failed Coordination

Step four: how to recognise failed coordination from the symptoms.

Typical symptoms:

  • The small-capacity SPD near the equipment fails first (the most typical);
  • Type-1 intact while Type-2 burns out repeatedly;
  • Surge records at the equipment end show high residual voltage;
  • A replaced Type-2 fails again within a short period.

Why "the downstream fails first":

  • Insufficient inter-stage distance → Type-2 operates before Type-1;
  • Type-2 capacity too small → cannot take the full energy;
  • Type-1 parameters insufficient → discharge incomplete;
  • Leads too long/coiled → abnormal inductance, broken coordination.

Troubleshooting order:

  • First, check the failure location (Type-1 vs Type-2);
  • Second, measure the inter-stage distance (per code or not);
  • Third, check the lead length/routing;
  • Fourth, verify the parameter pairing (Iimp/In matched or not).

Key insight: "Downstream fails first = the direct signal of failed coordination — first check the distance and parameters, not just replace with a more expensive SPD."

5. Distance/Capacity Pairing Principles in Engineering

Step five: how to configure at design time.

Distance principles:

  • Keep the Type-1-to-Type-2 distance per code (generally ≥ 5 m);
  • Add a decoupling inductor when the distance is short (selected by calculation);
  • Route leads short, straight, and parallel (reduce inductance);
  • Keep each stage's leads roughly equal in length (balance).

Capacity pairing:

  • Size Type-1 Iimp to cover the "expected direct-strike energy";
  • Size Type-2 In to cover the "remainder after the Type-1 residual";
  • Stage ratings decrease stage by stage (Type-1 > Type-2 > Type-3);
  • The equipment-end SPD voltage protection level ≤ equipment withstand (matching UP values).

Verification methods:

  • Perform the coordination calculation per code;
  • Verify with measured lightning/surge records (FL/monitors);
  • Check each stage's status periodically (communication reporting);
  • Re-run the coordination calculation after retrofits.

Key insight: "Distance + capacity + leads — all three configured together; energy coordination is a calculable, verifiable design item."

6. Product Implementation: Intelligent Lightning-Protection Online Monitoring

WeiWuLian products bring energy coordination into monitorable practice:

  • FSS Intelligent Surge Protective Device: status indication + thermal disconnection, making coordination status visible;
  • YSE DM Power Arrester: selected by Iimp/In staging, implementing the coordination design;
  • Monitoring platform: stage status + surge records, early detection of coordination failure;
  • FS/ESM Monitor: leakage-current trends, degradation visible before the next strike.

The design principle in one sentence: "Type-1 discharges, Type-2 clamps, monitoring verifies — energy coordination is no longer a 'how many stages' stacking exercise but a calculable engineering item."

7. Common Misjudgments

  1. "More SPDs is safer": stacking quantity without coordination. Result: downstream fails first. Correct: configure the stage count and capacities by energy coordination.
  2. Arbitrary inter-stage spacing: distance too short. Result: Type-2 operates before Type-1. Correct: per code ≥ 5 m or add a decoupling inductor.
  3. Ignoring lead length: leads too long/coiled. Result: abnormal inductance, broken coordination. Correct: short, straight, parallel routing.
  4. Replace without investigating: Type-2 fails, replace it. Result: the coordination problem remains and it fails again. Correct: check the distance and parameters first.
  5. Ignoring parameter pairing: Iimp/In mismatched. Result: Type-1 cannot hold or Type-2 cannot take it. Correct: decrease stage by stage, match the voltage protection level.

Scope and Limitations

This article explains the mechanism of multi-stage SPD energy coordination, the role of lead inductance, the division between Iimp and In, and the failure signatures of failed coordination, applicable to SPD staged design and lightning-damage troubleshooting in distribution systems. The quantitative coordination calculation (inter-stage distance, decoupling inductor, discharge allocation) must follow IEC 61643-11, GB/T 18802.11 and related codes; the specific parameters depend on the expected system lightning current and equipment withstand, completed by professional design. All cases are anonymized (a distribution room, a factory) and do not refer to any specific customer.