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Improving Process & Equipment Availability

SITE SHIELD 3G RISK ASSESSMENT: Facility Coordination of Multiple Surge Protective Devices (SPDs) & Locations


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

  • Susceptibility is often referred to when describing the probability of an installation to be affected by surge events. Susceptibility can encompass geographic locality, electrical system size and location as well as its configuration.
  • The first area of susceptibility is the Electrical System Size. In general, the larger the electrical system, the more exposure to surge events.
  • It is well documented that Geographic Locality plays a role in the frequency and probability of lightning strikes. By using both an isokeraunic and a ground flash density map (see Site Shield Planning Guide), the likelihood of sustaining a surge event due to lightning within that locality can be charted.
  • The Distribution System Configuration defines how power is distributed to a specific load. It addresses the transformer configuration (High Resistance Ground, WYE, Delta, Split Phase, etc.), Available Short Circuit Current, and the protected load configuration.
  • With regard to equipment protection and system susceptibility (Equipment Location Categories), each installation location should be evaluated. For the purpose of system susceptibility, these categories have been divided into three areas:
  • Service Entrance, Switchgear/Switchboard, Automatic Transfer Switch – This application is considered to be on the secondary side of the main disconnect.
  • Distribution, Sub-Distribution is the point at which the power system is divided to supply connected equipment and is located at the service entrance in some instances; in others, it is actually fed from the secondary of an isolation transformer.
  • Branch Circuits 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.


  • Installation. The surge protection installation should always be as close as physically possible, as cabling impedance can add as much as 100V per foot to the Voltage Protection Rating “VPR” (recommend 3 feet or less of total lead length).
  • The Grounding System is another factor, as the majority of transient events occur in the common mode. A faulty or high impedance grounding system can render an SPD system ineffective. Most grounding installations of critical equipment call for the grounding system impedance to be 5 ohms or less.
  • Equipment Criticality is a vital consideration. Based on the significance of the protected equipment, the perceived susceptibility of the power distribution system may increase. Critical equipment may require an evaluation at a higher susceptibility due to the requirement of little or no down time. In these locations, normal selections based on exposure levels and equipment location categories may be increased for the equipment being protected.
  • Although not always considered a part of the installation environment, Features and Benefits are an important aspect of the application process. Along with exposure levels and equipment location categories, features and benefits may drive the selection of a particular model or system. Features or benefits promoting serviceability, performance, inter-connectability and available system information may influence the overall selection. The installation environment may dictate certain features, e.g., an installation in a high exposure environment with extremely critical equipment may require a disconnect switch as well as field replaceable suppression elements. This provides the means to be back online quickly in the event of a suppression system fault.

Download a worksheet (pdf) or contact THOR SYSTEMS to perform an on-site evaluation to determine the best solution for your environment.


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