Surge Protection Devices Guide

20th Nov 2020   |   Buying Guide   |   Distribution Division

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SPD TERM DEFINITIONS

• Nominal system voltage

  A should match the utility system voltage where the device is to be installed

• MCOV

  The Maximum Continuous Operating Voltage, this is the maximum voltage the device can withstand before conduction (clamping) begins. It is typically 15-25% higher than the nominal system voltage.

• VPR

  Voltage Protection Rating. A rating per the latest revision of ANSI/UL 1449, signifying the “rounded up” average measured limiting voltage of an SPD when the SPD is subjected to the surge produced by a 6 kV, 3 kA 8/20 µs combination waveform generator. VPR is a clamping voltage measurement that is rounded up to one of a standardized table of values. The standard VPR ratings include 330, 400, 500, 600, 700, etc. As a standardized rating system, VPR allows the direct comparison between like SPDs (i.e. same Type and Voltage).

• Nominal Discharge Current (In)

  is the peak value of current, through the SPD having a current waveshape of 8/20 where the SPD remains functional after 15 surges. The peak value is selected by the manufacturer from a predefined level UL has set. I(n) levels include 3kA, 5kA, 10kA and 20kA and may also be limited by the Type of SPD under test. Type 1 = 10kA or 20kA. Type 2 = 3kA, 5kA, 10kA or 20kA. SPD manufacturer selects the current to test their product with. SPD subject to 15 surges at chosen current, 1 minute apart with rated voltage applied between surges.

• SCCR

  Short Circuit Current Rating. The suitability of an SPD for use on an AC power circuit that is capable of delivering not more than a declared rms symmetrical current at a declared voltage during a short circuit condition. SCCR is not the same as AIC (Amp Interrupting Capacity). SCCR is the amount of “available” current that the SPD can be subjected to and safely disconnect from the power source under short circuit conditions. The amount of current “interrupted” by the SPD is typically significantly less than the “available” current.

How to specify Surge Protection Devices

General rule of thumb assumptions

A. Average design lifespan of electrical equipment should be roughly 30 years, thus SPD selection should fall within this design parameter.


B. Need to specify appropriate SPD with highest “Nominal Discharge Current (In) rating” available (i.e. 10kA or 20kA). Additionally look at where SPD is to be placed, (ie. Service entrance, sub-panel, etc) to specify Type 1 (service ent.), or Type 2 (sub panel) or Type 3 (point of use). Highest (In) rating provides.


C. Need to specify appropriate SPD with highest “Peak Surge Current” rating – provided by SPD manufacturer. (ie. 50kA, 100kA, 160kA, 250kA, etc) . . . generally higher the number, good indicator SPD will have longer life.


D. Need to specify appropriate SPD with high “Short Circuit Current Rating” (SCCR), in order to match system available short circuit current level.


E. Need to specify appropriate SPD with “Maximum Continuous Operating Voltage” that allows enough “space” so normal voltage fluctuations do not exceed the MCOV rating of the SPD. (ie. 120V system=150VMCOV, or 600V system=700MCOV).

Looking at SPD, you would want

1. Industry experience has shown that SPD with peak surge current ratings of over 200kA – 300kA can last life of electrical equipment they are connected to (in high lightning area, or high surge area)


2. Typical sub-panel level, SPD with peak surge current rating of 100kA – 160kA provides necessary protection & good life span


3. Below sub-panel level (control panels or point of use SPD [powerbars]) peak surge current rating of 50kA – 100kA provides necessary protection & good life span