LinkLine: Summer 2015

An Overview of Power System Studies

By Joe I. Moreland, PE, CSI (GA PE#26321)

Performance of power system studies is commonly delegated to the Contractor for both Design-Build and Design-Bid Build contracts. For the latter case, the accuracy of certain studies requires characteristics of actual installed equipment and circuits generally not known during the design phase when competition between multiple manufacturers is permitted. The purpose of this article is to provide a brief overview of three studies most commonly performed:

Short-Circuit Study

Purpose:  Determine the maximum available fault current for selection of equipment with appropriate short circuit current ratings.

NFPA 70 (NEC) Requirements:

Study Methodology is described in IEEE 551 – Recommended Practice for Calculating Short-Circuit Currents in Industrial and Commercial Power Systems (IEEE Violet Book).

Study Reports:  Typically identify at each bus location:

Protective Device Coordination Study

Purpose:  For selection of protective devices and associated settings required to achieve (or optimize) selective coordination. Time-current curves of protective devices installed in series are overlaid on log-log scale graphs to evaluate which protective devices may trip for current values up to the maximum short circuit current. Generally the goal (which might not be fully achievable) is to select devices and trip settings such that the device closest to the fault will trip first while maintaining adequate protection for equipment and conductors.

NFPA 70 (NEC) Requirements:

Study Methodology is described in applicable portions of:

Study Reports typically include:

Arc Flash/Shock Risk Assessment

Purpose:  The term 'risk assessment' is defined in NFPA 70E as "an overall process that identifies hazards, estimates the potential severity of injury or damage to health, estimates the likelihood of occurrence of injury or damage to health, and determines if protective measures are required." An arc flash risk assessment determines if an arc flash hazard (defined as “a dangerous condition associated with the possible release of energy caused by an electric arc”) exists and if so, determine safety-related requirements for personnel protection. A shock risk assessment is usually performed in conjunction with an arc flash risk assessment.

NFPA 70 (NEC) Requirements:

110.16 requires electrical equipment likely to require examination, adjustment, servicing, or maintenance while energized to be marked, warning qualified persons of potential electric arc flash hazards. Accompanying informational notes cite the following references:

  1. NFPA 70E for guidance in determining severity of potential exposure, planning safe work practices, arc flash labeling, and selecting personal protective equipment.
  2. ANSI Z535.4 for guidelines for the design of safety signs and labels for application to products.

NFPA 70E requires that the arc flash/shock risk assessment be updated when a major modification or renovation takes place, and be reviewed at intervals not exceeding 5 years.

Study Methodology is contained in NFPA 70E – Standard for Electrical Safety in the Workplace, which includes various incident energy and arc flash boundary calculation methods in Informative Annex D. The one most commonly used is IEEE 1584 – Guide for Performing Arc Flash Hazard Calculations.

IEEE 1584 includes equations for calculating the arcing short circuit current, which is then used to calculate the incident energy at a particular working distance. Equations are also provided for calculating the arc flash boundary, defined as the distance from a prospective arc source within which a person could receive a second degree burn from an electrical arc flash.

Study Reports typically provide information necessary for the production of warning labels containing specific information that can be used by a worker to select appropriate personal protective equipment (PPE). Reports may also include recommendations for reducing the incident energy at locations where the calculated maximum incident energy exceeds a certain level.

NFPA 70E requires the nominal system voltage, arc flash boundary and at least one of the following to be included on the label:

Specifying Power System Studies in SpecLink

Performance of all three of the power system studies described above may be specified in BSD’s Section 26 0573 – Power System Studies, which has recently been completely revised and updated. A more in-depth guide to these studies and how to specify them in SpecLink can be found here, on the Supporting Documents pages of BSD’s website.