The magnitude of the surge current is dependent on its source. Lightning strikes, for example, that can in rare occurrences contain current magnitudes exceeding several hundred thousand amps. Within a facility, though, internally generated transient events will produce lower current magnitudes (less than a few thousand or hundred amps).
Since most SPDs are designed to handle large surge currents, one performance benchmark is the product’s tested Nominal Discharge Current Rating (In). Often confused with fault current, but unrelated, this large current magnitude is an indication of the product’s tested repeated withstand capacity.
From IEEE Std. C62.72: The Nominal Discharge Current Rating exercises an SPD’s ability to be subjected to repetitive current surges (15 total surges) of a selected value without damage, degradation or a change in measured limiting voltage performance of an SPD. The Nominal Discharge Current test includes the entire SPD including all surge protective components and internal or external SPD disconnectors. During the test, no component or disconnector is permitted to fail, open the circuit, be damaged or degrade. In order to achieve a particular rating, the measured limiting voltage performance level of the SPD must be maintained between the pre-test and post-test comparison. The purpose of these tests is to demonstrate the capability and performance of an SPD in response to surges that in some cases are severe but might be expected at the service equipment, within a facility or at the installation location.
For example, an SPD with a nominal discharge current capacity of 10,000 or 20,000 amps per mode means the product should be able to safely withstand a transient current magnitude of 10,000 or 20,000 amps a minimum of 15 times, in each of the modes of protection.
End Of Life Scenarios
From IEEE Std C62.72: The greatest threat to the long-term reliability of SPDs might not be surges, but the repeated momentary or temporary overvoltages (TOVs or “swells”) that can occur on the PDS. SPDs with an MCOV – that are precariously close to the nominal system voltage are more susceptible to such overvoltages which can lead to premature SPD aging or premature end-of-life. A rule of thumb that is often used is to determine if the MCOV of the SPD is at least 115% of the nominal system voltage for each specific mode of protection. This will allow the SPD to be unaffected by the normal voltage variations of the PDS.
However, aside from sustained overvoltage events, SPDs can age, or degrade, or reach their end-of-service condition over time due to surges that exceed the SPDs ratings for surge current, the rate of occurrence of surge events, duration of the surge, or the combination of these events. Repetitive surge events of significant amplitude over a period of time can overheat the SPD components and cause the surge protective components to age. Further, repetitive surges can cause SPD disconnectors that are thermally activated to operate prematurely due to the heating of the surge protective components. The characteristics of an SPD can change as it reaches its end-of-service condition – for example, the measured limiting voltages can increase or decrease.
In an effort to avoid degradation due to surges, many SPD manufacturers design SPDs with high surge current capabilities either by using physically larger components or by connecting multiple components in parallel. This is done to avoid the likelihood that the ratings of the SPD as an assembly are exceeded except in very rare and exceptional instances. The success of this method is supported by the long service life and history of existing SPDs installed that have been designed in this fashion.
With regard to SPD coordination and, as stated with regard to surge current ratings, it is logical to have an SPD with higher surge current ratings located at the service equipment where the PDS is most exposed to surges to aid in the prevention of premature aging; meanwhile, SPDs further down-line from the service equipment that are not exposed to external sources of surges might have lesser ratings. With good surge protective system design and coordination, premature SPD aging can be avoided.
Other causes of SPD failure include:
- Installation errors
- Misapplication of a product for its voltage rating
- Sustained over-voltage events
When a suppression component fails, it most often does so as a short, causing current to begin flowing through the failed component. The amount of current available to flow through this failed component is a function of the available fault current and is driven by the power system. For more information on Fault Currents go to SPD Safety Related Information.