An SPD can have a high surge-current rating and still be unsuitable for the available short-circuit current inside a distribution board or industrial control panel. This guide explains how panel builders and OEM buyers should verify SCCR, Isccr, upstream protection and the complete panel rating before approving a model.
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Risposta Veloce
The available fault current at the SPD installation point must not exceed the SPD’s declared SCCR or Isccr.
This comparison is valid only when the SPD is used at the declared voltage and with the fuse, circuit breaker, wiring arrangement and other conditions stated in its technical documentation.
What Do SCCR and Isccr Mean on an SPD?
The short-circuit current rating, or SCCR, states the maximum declared RMS symmetrical fault current at which an AC surge protective device is suitable for use at its declared voltage and under its evaluated conditions. [1]
IEC-oriented documentation may use Isccr. It represents the maximum prospective network short-circuit current for which the SPD is rated together with its stated upstream overcurrent protective device. [2]
Both values address short-circuit safety. They do not describe how much lightning surge current the SPD can discharge.
Project-side value
The available fault current comes from the electrical installation and must be determined at the actual SPD connection point.
Product-side value
SCCR or Isccr comes from the SPD marking, datasheet, installation instructions, certificate or applicable test documentation.
Approval condition
The product value is meaningful only when the documented voltage, protective device and installation conditions are followed.
Open the full-size diagram
SCCR, Isccr, Imax, In and Iimp Are Not Interchangeable
Several SPD parameters use kA. They describe different electrical events and answer different project questions.
| Parametro | What It Represents | Project Question | Do Not Confuse It With |
|---|---|---|---|
| SCCR | Short-circuit suitability at a declared AC voltage and under evaluated conditions. | Can the SPD be installed where this available fault-current level exists? | Imax, In or breaker interrupting rating. |
| Isccr | Maximum prospective short-circuit current for which the SPD is rated with the stated upstream OCPD. | Is the SPD suitable with the documented fuse or circuit breaker? | An unconditional stand-alone value. |
| In | Nominal discharge current, normally tested with an 8/20 μs waveform. | What nominal surge-current test duty has the SPD passed? | Power-frequency fault current. |
| Imax | Maximum discharge current, normally stated with an 8/20 μs waveform. | What maximum Type 2 surge current is declared? | SCCR or Isccr. |
| Iimp | Impulse discharge current, normally stated with a 10/350 μs waveform. | What Type 1 lightning-current duty is declared? | Available network fault current. |
| Su | Voltage protection level established under specified IEC test conditions. | What voltage level may appear at the SPD terminals during the test event? | A current rating. |
| VPR | UL-oriented voltage protection rating. | What limiting-voltage category appears in the applicable UL record? | IEC Up as an automatically identical value. |
Open the full-size diagram
Two common procurement mistakes
A 40 kA Imax SPD does not automatically have a 40 kA SCCR. A 100 kA SCCR does not mean that the SPD can discharge a 100 kA lightning surge.
SCCR vs AIC or AIR
SCCR and breaker interrupting capacity solve different safety questions. NEMA specifically distinguishes the SPD short-circuit current rating from the ampere interrupting capacity of an overcurrent protective device. [3]
SPD SCCR
States the fault-current level at which the SPD is suitable for use under its evaluated conditions.
Fuse or Breaker AIC / AIR
States the maximum fault current the protective device can safely interrupt at its rated voltage.
Both ratings must be adequate
A high-AIC breaker does not automatically increase an SPD’s SCCR. A high-SCCR SPD does not increase the breaker’s interrupting capacity.
Use the Available Fault Current at the Actual SPD Location
Available fault current can change between the transformer, main switchboard, feeder and downstream control panel.
Transformer impedance, source strength, cable length, conductor size, busbar impedance and upstream current-limiting devices can all affect the value.
Quick transformer-secondary estimate
For a three-phase transformer, first estimate the secondary full-load current:
Then estimate the symmetrical bolted-fault current at the transformer secondary terminals:
Illustrative example
A 500 kVA, 400 V three-phase transformer with 6% impedance has a full-load current of approximately 722 A.
Dividing 722 A by 0.06 gives an estimated transformer-terminal fault current of approximately 12.0 kA. [4]
This is only a screening estimate. It does not replace a formal short-circuit study and may not represent the value at a downstream SPD.
Do not use the breaker rating as the project fault current
A breaker marked 50 kA may be capable of interrupting that level. It does not prove that the installation actually has 50 kA available fault current.
Why the Backup Fuse or Circuit Breaker Matters
An Isccr value may depend on a specific upstream fuse or circuit breaker. The number should not be separated from the test and installation conditions under which it was established. [2]
If an SPD develops a low-impedance failure path, power-frequency current can flow through it. The coordinated internal and external protective arrangement must disconnect the failed SPD safely.
Ask the complete question
“Under which fuse, breaker, voltage and wiring conditions was this SCCR or Isccr established?”
Verify these OCPD details
- Fuse or breaker type
- Maximum permitted current rating
- Fuse class or breaker characteristics
- Interrupting capacity
- Branch or through-wiring arrangement
Equal ampere ratings do not prove equal protection
A 125 A fuse and a 125 A molded-case circuit breaker can have different clearing times, let-through energy and current-limiting performance.
A High-SCCR SPD Does Not Give the Complete Panel the Same SCCR
The SPD rating applies to the SPD under its evaluated conditions. The panel rating applies to the complete industrial control panel or electrical assembly.
UL identifies Supplement SB of UL 508A as an accepted method for determining industrial control panel SCCR. [5]
The assessment may include disconnect switches, fuses, circuit breakers, contactors, overload relays, motor starters, distribution blocks, terminals, busbars, drives and SPDs.
Do not copy the SPD value onto the panel nameplate
A 100 kA or 200 kA SCCR SPD does not automatically make the complete panel suitable for 100 kA or 200 kA available fault current.
Illustrative Project Selection Example
The example below shows why Imax cannot be used as a substitute for Isccr.
Illustrative Option A
- Uc275 V AC
- In20 kA
- Imax40 kA
- Isccr25 kA
- Backup Fuse125 A gG maximum
The 25 kA Isccr is lower than the 36 kA available fault current. The 40 kA Imax value does not correct this problem.
Illustrative Option B
- Uc275 V AC
- In20 kA
- Imax40 kA
- Isccr50 kA
- Backup Fuse125 A gG maximum
The 50 kA Isccr exceeds the 36 kA project value. Final approval still depends on the exact fuse, voltage, circuit, certificate scope and panel review.
How to Verify the Datasheet, Certificate and Installation Conditions
Use the exact model and configuration. Do not approve an SPD from one isolated number or from a certificate for a similar-looking model.
Open the full-size diagram
Define the project market
Confirm the destination country, applicable standard, required certificate and approval route.
Confirm the electrical system
Record the voltage, frequency, earthing arrangement and actual SPD installation point.
Obtain the fault-current value
Use the project short-circuit study or a calculation completed by a qualified responsible party.
Identify the exact SPD model
Confirm the full code, voltage suffix, pole arrangement, base and replaceable module.
Locate SCCR or Isccr
Determine whether the value applies to one module, one protection mode or the complete assembly.
Verify the upstream OCPD
Check the fuse or breaker type, rating, interrupting capacity and substitution restrictions.
Match the technical evidence
Compare the label, datasheet, installation instructions, certificate and test documentation.
Review the complete panel
Record the decision in the BOM, technical file, project submittal or sample approval record.
How LEEYEE Documents SCCR and Isccr
LEEYEE product and certification documents show that SCCR or Isccr must be matched to the exact series, voltage and model configuration.
Published first-party examples
- The LEEYEE certificate index lists LY1-C40M Type 2 models with an Isccr of 25 kA for the covered configurations.
- The certificate index also lists LY1-C40 and LY1-D20 series models with an Isccr of 25 kA within their stated certification scope.
- Il LY1-12.5/3(S) product specification table publishes model-dependent SCCR values. The exact value varies by voltage and product variant.
- The LEEYEE SPD catalogue lists backup-fuse data separately from short-circuit current ratings. Buyers should therefore request and verify both values.
Evidence does not transfer automatically between models
A certificate for one series or an SCCR shown for one voltage variant cannot be used as blanket evidence for every LEEYEE SPD.
SPD SCCR and Isccr RFQ Checklist
Complete project data helps the supplier identify technical conflicts before quotation, sampling or OEM approval.
- Destination country
- Required standard
- Certificato richiesto
- Panel application
- Tensione nominale del sistema
- Line-to-line voltage
- Line-to-neutral voltage
- AC frequency
- Earthing arrangement
- SPD installation point
- Available fault current
- Required panel SCCR
- Upstream fuse type
- Fuse rating
- Circuit-breaker type
- Breaker rating
- Interrupting capacity
- Required SPD Type
- Configurazione dei poli
- Required In or Imax
- Required Iimp
- Required Up or VPR
- Required SCCR or Isccr
- Remote-contact requirement
- Quantità d'ordine
- OEM label and packaging
Need to Verify an SPD for a Panel or OEM Project?
Send the system voltage, earthing arrangement, available fault current, upstream fuse or breaker, required SPD Type and certification requirement.
LEEYEE can help compare the requested parameters with the available product documentation. Final fault-current calculations, panel SCCR determination and project approval remain with the responsible engineer, panel builder or approval authority.
Domande frequenti
È un SPD Imax da 40 kA adatto dove la corrente di guasto disponibile è di 40 kA?
Non necessariamente. Imax descrive le prestazioni di corrente di picco. L'SCCR o Isccr dichiarato separatamente deve essere verificato rispetto alla corrente di guasto disponibile.
SCCR o Isccr devono essere superiori alla corrente di guasto disponibile?
Deve essere uguale o superiore alla corrente di guasto disponibile al punto di installazione del SPD, soggetto a tutte le condizioni di tensione, OCPD e installazione indicate.
Is SCCR the same as breaker AIC or AIR?
No. SCCR describes component or assembly suitability under short-circuit conditions. AIC or AIR describes the fault current an overcurrent protective device can safely interrupt.
Can an upstream breaker increase the SPD SCCR?
Only when the resulting combination and rating are supported by the applicable test, certification, listing or manufacturer documentation.
Does an internal thermal disconnector eliminate the backup fuse?
Not automatically. Follow the exact installation instructions and protective-device conditions for the selected SPD model.
Does a 200 kA SCCR SPD make the panel 200 kA SCCR?
No. The complete panel SCCR must be determined separately under the applicable panel or machinery procedure.
What should the buyer do if the backup-protection condition is missing?
Do not approve the SPD solely from an isolated SCCR or Isccr claim. Request the exact datasheet, installation instructions and applicable certification or test evidence.
