SPD backup-protection rules
Backup protection must follow the exact SPD datasheet and withstand the installation fault current.
In,backup ≤ In,backup,max(SPD)Icn,backup ≥ Ikwhere:
- In,backup
- Backup fuse or breaker rating[A]
- Icn,backup
- Backup device breaking capacity[kA]
- Ik
- Prospective short-circuit current[kA]
A generic device is not interchangeable with the manufacturer-approved backup device.
How to use the SPD Backup Fuse Selection Calculator
Screen an SPD backup fuse or MCB rating from the SPD datasheet maximum backup protection and prospective fault current. Start by choosing the operating mode, then enter values from nameplates, measurements, or project documents in the units shown. The result updates immediately and keeps intermediate quantities visible for review.
- Confirm the system and calculation mode.
- Enter measured or manufacturer-rated values.
- Review the primary result and intermediate metrics.
- Continue with the related protection or equipment-selection checks.
Calculation method and validation
The equations and symbol definitions above show the complete calculation basis. Inputs are checked before calculation so zero, negative, incompatible, or physically impossible combinations are flagged instead of silently producing a misleading result.
How to interpret the result
Use the main result as a starting value and read every supporting metric. Maximum backup value comes from the exact SPD datasheet No generic fuse or breaker can substitute for manufacturer coordination approval A passing or recommended result means the entered mathematical conditions are satisfied; it does not certify the complete installation or a particular product combination.
- Maximum backup value comes from the exact SPD datasheet
- No generic fuse or breaker can substitute for manufacturer coordination approval
Selection limits and next checks
Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions. If the SPD manufacturer specifies a particular device or no external backup device, follow that instruction. Compare the output with the current applicable standard, local rules, exact VIOX product datasheet, environmental conditions, protection coordination, and the engineer's project requirements before procurement or installation.
Continue with advanced spd selection calculatorContinue with fuse sizing calculatorContinue with short circuit current calculator
Assumptions
- Maximum backup value comes from the exact SPD datasheet
- No generic fuse or breaker can substitute for manufacturer coordination approval
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- If the SPD manufacturer specifies a particular device or no external backup device, follow that instruction.