SPD surge current calculation (surge protection device)

A SPD (surge protection device) must conduct high-intensity, short-duration currents, diverting them from the part of the installation that it must protect. After actuation during a surge, the DPS must return to its initial state of high impedance, when ideally it does not conduct any current.  Surge currents are divided into: 

• Discharge impulse current for class I DPS tests: IIMP corresponds to the peak value of a discharge current passing through the SPD with a specific charge transfer Q and a specific energy W/R during a specified time. Normally IIMP  corresponds to a wave in the form 10/350 µs;
• Nominal discharge current for DPS class II tests: IN corresponds to the peak value of a current passing through the DPS with an 8/20 µs waveform. 

Very important for defining the surge current for the DPS is differentiating its technology. SPDs based on spark gap or diodes conduct an inrush current infinitely many times without degrading. Therefore, they are specified with just one chain.  Varistor-based DPS (Figure 1) conduct inrush current a few times until they degrade. That's why they are specified with two currents: 

IMAX → Maximum current, corresponds to the current value that the DPS (varistor) can conduct twice before completely losing its useful life. 

IN → Nominal current, corresponds to the current value that the DPS (varistor) can conduct 20 times before completely losing its useful life.

Therefore, when specifying a DPS that uses a spark gap or diode, simply indicate the intensity of the current that it can conduct. But when specifying a DPS that uses a varistor, it is necessary to indicate the value of the nominal current and the value of the maximum current, or at least not just write I = X kA, but rather IN = X kA or IMAX =X kA. 

In addition to the impulse current and the nominal surge current, there are other currents that will be mentioned here for information only:

• Subsequent current IF: peak current supplied by the electrical supply network and passing through the DPS after a discharge current pulse. This current can be considered something similar to a leakage current. It is not calculated because it is something undesirable, and it is up to the DPS to interrupt it;
• Rated load current IL: maximum effective value of the permanent current that can supply a resistive load connected to the protected output of a DPS. The rated load current is typically specified for Class III SPDs because they are in series with the protected equipment (Figure 2). So if we want to protect a television, for example, IL of DPS class III on the television socket will be equal to the current consumed by the TV multiplied by a safety factor. 

Example: Class III DPS to protect a notebook  I notebook = 5A   I L DPS = 5A X 1.1 = 5.5A

Impulse current calculation for class I DPS

Class I SPDs conduct a portion of the lightning current that tends to enter the building.

• Determine SPDA level 

Level I = 200 kA Level II = 150 kA Level III / Level IV = 100 kA  Half of the current that goes down through the SPDA captors to ground returns to the installation through a remote grounding system, power, signal, metal pipes (Figure 3).

Current returning to the installation:

• 50% of the current returning at Level I = 200 kA / 2 = 100 kA 

Level II = 150 kA / 2 = 75 kA  Level III / Level IV = 50 kA  Each class I DPS needs to conduct the portion of the current that returns through it. Therefore, considering only the energy SPDs, the current that returns is divided by the number of phases and the neutral. The neutral, even when grounded, conducts the surge current through it.   Example 1. Three-phase system with grounded neutral at the input, in a building with SDPA level I protection: 

• Level I = 200 kA 
• Return to the building: 100 kA 

Phases with neutral will conduct:

• 100 kA / 4 = 25 kA 

So we need three class I DPS with IIMP = 25 kA. Since the neutral is grounded at the input, we will not need a class I DPS for the neutral.  If the option is a DPS class I spark gap, its current will be 25 kA.  If the option is a DPS class I varistor, its current will be IN = 25 kA (10/350µs) or IMAX = 25 kA (10/350µs).    In the first case IN = 25 kA (10/350 µs) and IMAX = 50 kA (10/350 µs).   In the second case IN = 12.5 kA (10/350 µs) and IMAX = 25 kA (10/350 µs).  For signal DPS, the calculation would be more complex because there are numerous types of signal DPS. Experience shows that it is not necessary to calculate the signal DPS current, because each manufacturer offers one or two maximum DPS class I models for signal. So, it makes no sense to calculate the current as, for example, 8.7 kA (10/350 µs), if the models used have a current specification of 15 kA (10/350 µs). 

Current calculation for DPS class II or III

The induced currents that may appear for a class II or III SPD depend on numerous factors, such as: 

• Original lightning current intensity;
• Design of cables within the installation;
• Distance between the point of impact and the frame where the DPS is.

Therefore, what is done is to estimate that the surge currents have an intensity between 1 kA (8/20 µs) and 10 kA (8/20 µs). From a practical point of view, the recommendation is that the following values should be used for class II or III DPS:

• DPS with spark gaps and diodes (used in class III DPS): enough IN;
• DPS with varistors, normally those used as class II: we need to clearly indicate whether we specify IN or IMAX, even better if we indicate both;
• It must be very clear that the current value does not depend on the type of SPD (spark gap, varistor or diode). The current value is what will appear on the board that must be protected during the occurrence of the surge.

General recommendations

Class II DPS

Small facilities (shops, offices, apartments, etc.). Places where cables are relatively short in length, small induction loops:  IN = 5 kA or 10 kA (8/20µs) – if it is a varistor IMAX will be double.  Larger installations (industries, houses, shopping centers, industries). Locations where cables are relatively long and have significant induction loops:   IN = 20 kA (8/20µs) – if it is a varistor IMAX will be double. 

Class III DPS

For class III DPS, a nominal current of I would be sufficientN = 5 kA, maximum IN = 10 kA.