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Surge & Lightning Protection Tools

Advanced SPD Selection Calculator

Select a preliminary SPD type, connection arrangement, Uc or Ucpv range, discharge-duty level, and backup-protection checks.

Advanced SPD selection method

Advanced SPD selection starts with installation duty and connection mode, then checks continuous operating voltage, protection level, discharge ratings, prospective fault current, and backup protection.

Continuous-voltage screening
Uc ≥ Uworking × Km
Effective protection check
Ueffective = Up + ΔUleads < Uw,equipment
Short-circuit coordination
SPD Isccr/SCCR with specified backup device ≥ prospective fault current

where:

Uc
Maximum continuous operating voltage for an AC SPD[V]
Ucpv
Maximum continuous operating voltage for a PV SPD[V DC]
Uworking
Maximum normal voltage across the selected protection mode[V]
Km
Screening margin for voltage variation, earthing, and operating conditions
Up
SPD voltage protection level from the datasheet[kV]
ΔUleads
Additional voltage created by surge current through connecting leads[kV]
Uw,equipment
Equipment impulse withstand voltage[kV]

The selected SPD must be a tested product configuration for the actual earthing system and backup protective device. The calculator intentionally does not invent exact In, Imax, Iimp, or fuse values without product-family data.

How to select an SPD

Start with system voltage across each protection mode, earthing arrangement, installation point, lightning exposure, supply route, prospective short-circuit current, and equipment impulse withstand. These inputs determine the starting Type, connection arrangement, and voltage-rating screen.

  1. Identify Type 1, Type 2, or coordinated Type 3 duty.
  2. Choose the earthing-specific connection arrangement.
  3. Check Uc or Ucpv.
  4. Coordinate Up with equipment withstand and lead length.
  5. Verify discharge ratings and backup protection.

Uc and Ucpv selection

Uc or Ucpv must remain above the maximum continuous voltage expected across the SPD protection mode, including normal system variation and earthing behavior. Selecting it too low risks premature failure; selecting it unnecessarily high can weaken practical protection coordination.

Type 1, Type 2, and Type 3 duties

Type 1 addresses lightning-current duty at the installation origin, Type 2 addresses distribution-level induced and switching surges, and Type 3 provides coordinated local protection near sensitive equipment. A Type 1+2 product combines tested duties but still requires correct placement and coordination.

Protection level and backup device

Effective protection includes the SPD Up value plus voltage contributed by connection leads. Confirm In, Imax, Iimp where applicable, thermal disconnection, status indication, Isccr or SCCR, the exact permitted backup fuse or breaker, conductor routing, pole arrangement, and manufacturer wiring diagram.

Assumptions

  • The entered voltage is across the SPD protection mode, not automatically the system line-to-line label
  • AC frequency is 50 or 60 Hz
  • The output is a product-family screening result rather than an IEC compliance certificate

Important Warnings

  • TT, IT, PV, and special neutral arrangements require the exact manufacturer connection diagram and failure-mode coordination.
  • Lead length can add substantial residual voltage; keep SPD connections short and follow the manufacturer's routing instructions.

FAQ

Why should I not select an SPD by kA alone?

SPD type, Uc or Ucpv, Up, earthing arrangement, installation point, backup protection, short-circuit conditions, and lead length can be more important than a headline maximum-discharge-current value.

What is the difference between In, Imax, and Iimp?

In is a nominal discharge-current test reference, Imax is a maximum discharge-current rating for Type 2 context, and Iimp is an impulse-current rating associated with Type 1 lightning-current duty.

What does 3+1 mean for an SPD?

It normally describes phase-to-neutral protection modules plus a separate neutral-to-PE spark-gap or protection path, commonly considered for TT arrangements according to the product design.

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