Direct Answer
A certain SPD fails again and again, several times a year — is it poor SPD quality or a selection/coordination problem? The answer: repeated SPD breakdown is usually not "poor quality" but a too-small discharge capacity (the lightning energy in this area exceeds the SPD capability), failed inter-stage energy coordination (the energy is all pressed onto this stage), or an overlong lead with a too-high residual voltage. The diagnosis should check three directions — the lightning-current amplitude at this point, the front/rear-stage coordination, and the installation leads — instead of continuing to replace it with the same model.
At a switch room, the SPD was replaced three times a year, always with the same model. After connecting a lightning-current monitor, it was found that the flash amplitude at this point repeatedly exceeded the SPD's rated discharge — the capacity was too small, and the lightning energy broke the MOV down again and again. After upgrading the discharge level, there was no more damage. The key distinction: repeated breakdown is "a system problem, not a single-part problem" — measure the amplitude at this point first, check the coordination and the lead residual; treat the cause by raising the capacity/improving the coordination/shortening the leads. Replacing with the same model is treating the symptom, not the cause.
Why:
- The amplitude dimension: does the measured amplitude here exceed the SPD discharge; - The coordination dimension: has the inter-stage energy coordination failed (the energy pressed onto one stage); - The lead dimension: an overlong lead → a too-high residual voltage; - The treatment: raise the capacity, improve the coordination, shorten the leads — not repeated replacement.
Key insight: "Check three dimensions for repeated SPD breakdown: the lightning-current amplitude, the inter-stage coordination, the lead residual — replacing with the same model treats the symptom, not the cause."
At a plant, the front stage was normal but the rear stage always failed first: the coordination check found failed inter-stage energy coordination, with the energy all pressed onto the rear SPD. After adjusting the coordination, there was no more damage. Lesson: the broken one is not necessarily the bad one — diagnose before replacing; when the coordination fails, the same model will break again.
1. Look at the Measured Lightning-Current Amplitude First
Step one: is the discharge capacity too small?
Why the amplitude first: - The lightning energy at this point exceeds the SPD capability; - The MOV is broken down again and again; - Replacing the same model breaks again; - The amplitude is the first diagnostic dimension.
How to look: - Measure the amplitude at this point with a lightning-current monitor; - Compare with the SPD's rated discharge (In)/maximum discharge (Imax); - Repeatedly exceeding → the capacity is too small; - Count the flash frequency and the amplitude distribution.
The treatment: - Upgrade the discharge level (larger In/Imax); - Select by the lightning energy of this area; - Confirm which LPZ this point belongs to (connecting to KD-0165); - Speak with data, not guesswork.
Key insight: "Measure the amplitude here first and compare it with the SPD discharge — repeated exceeding means it is too small; upgrading the discharge level is the right treatment."
2. Check the Inter-Stage Energy Coordination
Step two: is the energy all pressed onto this stage?
Why check the coordination: - Multi-stage SPDs should discharge by stages; - Failed coordination → the energy pressed onto one stage; - That stage fails repeatedly; - Replacing the same model breaks again.
How to look: - See which stage is damaged; - The discharge and distance between the front/rear stages; - The front discharges the large energy, the rear the residual; - On failure, the front/rear stages do not cooperate.
The treatment: - Adjust the inter-stage distance and parameters; - Raise the front-stage discharge; - Match the rear to discharge the residual; - Coordinate by the energy-distribution principle.
Key insight: "Check which stage is damaged and whether the stages cooperate — energy pressed onto one stage is a coordination failure; adjust the coordination, not the same model."
3. Check the Lead Length and the Residual Voltage
Step three: is the lead too long?
Why check the lead: - The lead inductance drops voltage under the lightning current (connecting to KD-0172); - An overlong lead → a too-high residual voltage; - The SPD terminal residual exceeds the equipment withstand (connecting to Up/Uw); - Repeated breakdown/damage.
How to look: - Measure the SPD lead length; - The shorter the lead, the lower the residual; - Estimate the L·di/dt drop; - Whether the routing is reasonable.
The treatment: - Shorten the leads (L-type/Kelvin connection); - Optimize the routing and reduce the loop; - Keep the terminal residual within the equipment's Uw; - Optimize the lead and the coordination together.
Key insight: "The lead inductance drops voltage under the lightning current — an overlong lead raises the residual above the equipment withstand; shortening the lead is the right treatment."
4. Check Whether the Back-Up Protection Mis-Trips or Refuses
Step four: is the protection failing first?
Why check the back-up: - The back-up at the SPD front end (connecting to KD-0185); - Mis-trip → the SPD loses its discharge path; - Refusal → the fault current continues; - Both accelerate SPD damage.
How to look: - Check the back-up action records; - Whether it mis-trips on a strike; - Whether it refuses on a short; - The SCB/fuse state.
The treatment: - Replace with a matched dedicated SCB; - Check the dual condition (breaking + withstand); - Monitorable state; - Protection coordinated with the SPD parameters.
Key insight: "A back-up mis-trip takes away the discharge path and a refusal lets the fault continue — check the action records and replace a mismatched one with a dedicated SCB."
5. Treatment: What to Change after the Three-Dimensional Diagnosis
Step five: how to implement after the diagnosis.
The diagnosis order: - ① Measure the amplitude at this point; - ② Check the inter-stage coordination; - ③ Check the lead length; - ④ Check the back-up protection; - Check in order and record the data.
The treatments: - Exceeding the amplitude → upgrade the discharge; - Failed coordination → adjust the inter-stage coordination; - Overlong lead → shorten the lead; - Mismatched protection → replace with a dedicated SCB.
Practice verification: - Keep monitoring after the retrofit; - Online with the FS/ESM SPD monitor; - Review the strike-event records; - No more repeated damage means it is good.
Key insight: "After the three-dimensional diagnosis, treat the cause: raise the capacity, improve the coordination, shorten the leads, replace the SCB — keep monitoring after the retrofit; no more repeated damage means it is good."
6. Product Implementation: Monitoring and Diagnosis as One
WeiWuLian products manage SPD diagnosis:
- FL Lightning-Current Monitor: measures the amplitude at this point; - FS/ESM SPD Monitor: SPD state/leakage/events online; - Inter-stage data: front/rear action records; - Closed-loop diagnosis: amplitude-coordination-lead-protection data.
The design principle in one sentence: "Measure the amplitude, check the coordination, measure the leads, verify the protection — a four-dimensional data loop; repeated damage no longer relies on guessing."
7. Common Misjudgments
1. "Replace it with the same model when broken": ignoring the root cause. Result: it breaks again. Correct: diagnose the three dimensions first. 2. "It is poor quality": ignoring the system problem. Result: a new one breaks too. Correct: check the amplitude/coordination/leads. 3. "Look only at the discharge": ignoring the coordination. Result: energy pressed onto one stage. Correct: check the inter-stage coordination. 4. "Ignore the leads": ignoring the residual. Result: a too-high residual breaks it down. Correct: shorten the leads. 5. "Done after installation": no monitoring data. Result: no one knows why it broke. Correct: monitor online and record the events.
Scope and Limitations
This article explains the root-cause diagnosis of repeated SPD breakdown (the four-dimensional positioning of lightning-current amplitude, inter-stage coordination, lead residual and back-up protection, with the treatments), applicable to troubleshooting and understanding repeated SPD damage. The exact parameters and coordination follow GB/T 18802.11 and the manufacturers' data; this article explains only the diagnostic logic and judgment basis. All cases are anonymized (a switch room, a plant) and do not refer to any specific customer.
