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The best overall applications should match the specifications of the Surge Protective Device (SPD) to the installation parameters, requiring evaluation not only of the surge protective device and the susceptibility of the protected equipment, but also the electrical environment of the facility.

Susceptibility is often referred to when describing the ability of an installation to be affected by power surge events. Susceptibility can encompass much more than geographic location. It can also be determined by the electrical system size and location as well as its configuration. Susceptibility is generally categorized by Exposure Level, typically listed as “high, medium or low.”

The first area of susceptibility to be considered is the Electrical System Size. In general, the larger the electrical system, the more exposure to surge events because there is more equipment connected to generate a surge and also less impedance to resist an incoming event from an outside source. A high exposure would be 2500 to 6000 amperes, a medium exposure would be 600 to 2000 amperes, and a low exposure would be below 400 amperes.

Another area of susceptibility usually addressed early in the evaluation process is Geographic Location. It is well documented that geographic locations play a role in frequency and probability of lightning strikes. By using an isokeraunic and a ground flash density map, the likelihood of sustaining an AC surge event due to lightning can be charted.

The Distribution System Configuration, which defines how power is distributed from the secondary of a transformer to a specific load, is sometimes overlooked. It not only addresses the transformer configuration (High Resistance Ground, WYE, Delta, Split Phase, etc.) but also Available Short Circuit Current, insulation ratings, required protective modes as well as the protected load configuration.

With regard to equipment protection and system susceptibility, Equipment Location Categories are documented in surge suppression, but are not as well defined. These locations basically refer to Categories A, B and C as listed in IEEE C62.41.1. Experienced users of these categories will realize there is not a distinct definition for each category and the boundary line is “unclear.” As with the distribution system configuration, each installation location should be evaluated. For the purpose of system susceptibility, these categories have been divided into four areas:

  • Service Entrance – This application is considered to be on the secondary side of the main disconnect as it applies to SPD systems under UL1449 3rd Edition. Service Entrance equipment, which has the highest level of susceptibility within the distribution system due to location, can be affected by internal as well as external sources and has the greatest exposure to lightning events, requiring lightning surge protection.
  • Distribution – Considered to be the point at which the power system is divided to supply connected equipment, Distribution is located at the service entrance in some instances; in others, it is actually fed from the secondary of an isolation transformer. If the system Distribution is not at the service entrance, the susceptibility level normally decreases as there is less exposure to external and lightning events.
  • Branch Circuits – Sometimes described as “Sub-Distribution,” are supplied from a distribution source without an isolation transformer. A typical application is a circuit breaker within a larger distribution panel supplying a smaller branch panel in a location away from the main distribution panel. The distribution system configuration and the voltage rating of the branch panel are the same as the distribution source. Only the current rating is lower and there is some distance of cabling between the two. The impedance as a result of this additional cabling will tend to reduce the susceptibility of the branch circuit.

DOWNLOADS (pdf files):

Site Risk Assessment Spreadsheet [TSI 0119]

Electrical power quality problems cost the United States industries over $100 billion annually
80% of voltage surges and transients are caused by changes in electrical demand, inside the user's facility