SECTION 26 0573 – POWER SYSTEM STUDIES
- General Description
- Scope of Studies
- Data Collection
- Short-Circuit Study
- Protective Device Coordination Study
- Arc Flash and Shock Risk Assessment
- How to Specify
The MasterFormat title for Section 26 0573, currently “Overcurrent Protective Device Coordination Study” changes to “Power System Studies” in 2016. This section describes the power system studies required to be performed by the Contractor or his consultant. Delegating performance of studies in this manner is common 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 three studies most commonly performed are:
- Short-Circuit Study
- Protective Device Coordination Study
- Arc Flash/Shock Risk Assessment
These studies may each be specified independently. A Protective Device Coordination Study is typically performed after a Short Circuit Study. An Arc Flash/Shock Risk Assessment is typically performed after both a Short-Circuit and Protective Device Coordination Study.
Studies may be performed for new construction and modifications/additions to an existing electrical distribution system to determine criteria for the selection and adjustment of specified equipment and associated protective devices. Studies may also be performed strictly for analysis of an existing electrical distribution system documenting existing conditions and identifying potential problems and/or upgrade opportunities. The analysis typically extends from the utility source to each piece of equipment involved, including all parts of the system affecting calculations being performed (e.g. fault current contribution from motors).
Alternate operating modes should be evaluated since the base case representing normal operating mode may not always result in worst case conditions. Possible operating modes to consider include:
- Utility/generator as source (or in parallel).
- Main/tie breakers open/closed.
- Alternate utility configuration (e.g. utility transformer off line).
- Maintenance settings, which might be established as part of an electrical safety program for adjustment of protective devices in order to reduce arc energy prior to performing work.
- Future configurations.
A single-line diagram is likely to be included in the construction documents unless the scope of studies is limited to an existing electrical distribution system with no modifications or additions. The study preparer can use this as a starting point for making a more comprehensive single-line diagram including data such as actual feeder lengths and manufacturer-specific protective device characteristics. Such information is often not available during the design phase. Assumptions can be made for a short-circuit study performed during the design phase. This would yield conservative results that could be used to determine short circuit current ratings. However, information representing actual installed conditions is essential for performing accurate protective device coordination studies and arc flash/shock risk assessments.
Utility source data is used to calculate short circuit currents at various locations in the system and to evaluate coordination between utility primary protective devices and downstream protective devices. The “Utility” is most often the Utility Company providing electric service, but it could also be the Owner if the power to the project is obtained from a source under the Owner's control, such as a power plant for a campus.
Generators are sources of available fault current, which is limited by the generator impedance and the impedance of the circuit between the generator and the short circuit.
Motors stop delivering torque to the load and become a source of fault current during a short circuit, which is limited by the motor impedance and the impedance of the circuit between the motor and the short circuit. The motor type (e.g. induction, synchronous) affects how quickly the fault current contribution decays.
Transformer characteristics affecting the available fault current include:
- Transformer kVA Rating: Transformers with larger kVA ratings let through more fault current than transformers with smaller ratings.
- Transformer Percent Impedance: Transformers with a smaller percent impedance let through more fault current than transformers of the same kVA rating with a larger percent impedance.
Protective device and protective relay time-current curves are used to evaluate coordination between protective devices and to determine clearing times for calculating the incident energy released in an arc flash event.
- Thermal magnetic circuit breakers typically have two tripping functions, a thermal inverse time tripping element for overload protection and a magnetic instantaneous tripping element for short circuit protection. Larger breakers (typically 225A and larger) may have an adjustable instantaneous trip setting. Some breakers may also have interchangeable trip units.
- Electronic trip circuit breakers with adjustable trip response settings can provide increased flexibility in improving selective coordination by allowing the time-current curve to be shaped. Circuit breaker trip functions are often described by a combination of the abbreviations L, S, I, and G (L = Long time, S = Short Time, I = Instantaneous, G = Ground Fault). Long time pickup may also be called the continuous ampere rating. Some breakers may have zone selective interlocking making it capable of communicating with other electronic trip circuit breakers and external ground fault sensing systems to control short time delay and ground fault delay functions for system coordination purposes.
- Cartridge type fuse class designation establishes dimensions and certain performance characteristics within a broad range. The performance of fuses of the same class may vary significantly among manufacturers in terms of let-through current and energy, coordination with downstream devices, and physical features. Fuses may be "time-delay" or "fast-acting, non-time-delay." Common fuse classes include Class R (RK1 and RK5), Class J, Class L, and Class T.
Conductor impedance affects the available fault current at a given location (the greater the impedance, the lower the fault current). Characteristics affecting conductor impedance include:
- Conductor Size: Smaller conductors have a greater impedance than larger ones.
- Conductor Material: Aluminum conductors have a greater impedance than copper conductors of the same size.
- Number of Conductors per Phase: Multiple parallel conductors have a smaller equivalent impedance than a single conductor of the same size.
- Raceway Type: Raceways with magnetic properties (e.g. steel) have a greater impedance than raceways without magnetic properties (e.g. PVC, aluminum).
- Feeder Length: Longer conductors have a greater impedance than shorter ones.
Purpose: Determine the maximum available fault current for selection of equipment with appropriate short circuit current ratings.
NFPA 70 (NEC) Requirements:
- 110.9 and 110.10 require equipment to have ratings sufficient for the available fault current.
- 110.24 requires service equipment in other than dwelling units to be marked with the maximum available fault current and the date calculations were performed.
Study Methodology is described in IEEE 551 – Recommended Practice for Calculating Short-Circuit Currents in Industrial and Commercial Power Systems (IEEE Violet Book).
Additional information on these procedures can also be found in applicable portions of:
- IEEE 141 – Recommended Practice for Electrical Power Distribution for Industrial Plants (IEEE Red Book).
- IEEE 242 – Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book).
- IEEE 399 – Recommended Practice for Industrial and Commercial Power Systems Analysis (IEEE Brown Book).
Worst-case fault currents are calculated based on a 3-phase bolted fault condition, which assumes all three phases are shorted together with zero impedance. Consideration is given to conditions that may result in maximum available fault current, including maximum utility fault currents, maximum fault current contribution from motors, and known operating modes. For grounded systems, line-to-ground fault currents are usually also calculated.
Study Reports: Typically identify at each bus location:
- Calculated maximum available symmetrical and asymmetrical fault currents (both three-phase and line-to-ground where applicable).
- Fault point X/R ratio.
- Associated equipment short circuit current ratings.
The report should identify whether equipment short circuit ratings are fully rated or series rated systems. For series rated systems, protective devices are tested in series and listed in combination for a certain rating. NFPA 70 (NEC) 240.86 includes requirements for application of series ratings, including limitations on motor contribution and 110.22 includes requirements for labeling of equipment utilizing series ratings.
The report should also identify locations where the available fault current exceeds the equipment short circuit current rating (as specified or as installed in the case of existing installations), along with recommendations.
Purpose: For selection of protective devices and associated settings required to achieve (or optimize) selective coordination.
NFPA 70 (NEC) Requirements:
- 517.26 requires the life safety branch of essential electrical systems for health care facilities to meet requirements of Article 700, which includes selective coordination (see 700.28 below).
- 620.62 requires selective coordination for circuits serving multiple elevators.
- 645.27 requires selective coordination for critical operations data systems.
- 695.3 requires selective coordination for fire pumps with feeder sources in multi-building campus-style complexes.
- 700.28 requires selective coordination for emergency systems.
- 701.27 requires selective coordination for legally required standby systems.
- 708.54 requires selective coordination for critical operations power systems (COPS). 708.52 also requires selectivity for ground-fault protection systems.
Study Methodology is described in applicable portions of:
- IEEE 242 – Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book)
- IEEE 399 – Recommended Practice for Industrial and Commercial Power Systems Analysis (IEEE Brown Book).
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. Selective coordination is often not possible when protective devices utilize short circuit current series ratings because the upstream device will usually trip first, but this is not necessarily always the case.
Study Reports typically include:
- Time-current coordination curves plotted on log-log scale graphs representing each portion of the system.
- Fixed and adjustable characteristics for each protective device with available ranges and recommended settings.
The report should also identify cases where either full selective coordination or adequate protection is not achieved, along with recommendations.
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 (as referenced in NFPA 70E 130.5) 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 (as referenced in NFPA 70E 130.4) 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 the following references:
- NFPA 70E for guidance in determining severity of potential exposure, planning safe work practices, arc flash labeling, and selecting personal protective equipment.
- ANSI Z535.4 for guidelines for the design of safety signs and labels for application to products.
Despite improved clarity in the standards, there remains debate among design professionals as to whether or not an arc flash study is required. Since NFPA 70E is only referenced in the NEC by informational note, during construction authorities having jurisdiction often permit the use of generic warning labels to satisfy the marking requirement. Furthermore, NFPA 70E clarifies that it is the responsibility of the owner/employer to ensure requirements are met. Because it is widely accepted that compliance with NFPA 70E is essential for an employer to meet OSHA requirements for employee safety, the real question is less about whether or not a study is required, and more about whether or not the responsibility is included in the specifier's scope of work. 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 are 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 less than the aforementioned bolted short circuit current because it includes the impedance of an air gap. Tables are included for typical gap distances based on the voltage and type of equipment. Factors are also introduced to account for differences in arcs occurring in open air or in a box (e.g. equipment enclosure). The arcing current is then used to calculate the incident energy at a particular working distance, which may be adjusted to account for variation in the confidence of results for voltages at or below 1 kV. Finally, equations are 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 (where the incident energy level is above 1.2 cal/sq cm).
The results of the short-circuit study and protective device coordination study are the basis for the performance of the arc flash/shock risk assessment. While the maximum fault current determines the worst case condition for evaluating equipment short circuit current ratings, the worst case condition for arc flash incident energy calculations may occur at the minimum fault current. This is because the incident energy released in an arc flash event is determined by both the fault current magnitude AND the duration. The minimum fault current can take longer for an upstream overcurrent protective device to clear, possibly resulting in a greater arc flash hazard than a larger fault current that is cleared more quickly.
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).
NFPA 70E 130.5 requires the nominal system voltage, arc flash boundary and at least one of the following to be included on the label:
- Either the calculated available incident energy or arc flash PPE category selected from appropriate tables (but not both); note that "hazard risk categories" were removed from NFPA 70E beginning with 2015 edition.
- The minimum arc rating of clothing.
- The site-specific level of PPE.
Guidance for selection of PPE based on calculated incident energy is included in NFPA 70E Annex H. This includes arc-rated clothing specifically tested for exposure to an electrical arc discharge and assigned an arc rating (expressed in calories per square centimeter), which should be equal to or greater than the incident energy at the location. Flame-resistant clothing without an arc rating has not been tested for exposure to an electric arc, but all arc-rated clothing is also flame-resistant.
Start in POWER SYSTEM STUDIES under PART 1, progress to PART 2 (where applicable), then PART 3, and finish up with the remaining articles in PART 1.
In PART 1:
Begin by editing the scope according to the particular project requirements in article POWER SYSTEM STUDIES under paragraph “Scope of Studies.” Any known operating modes (e.g. utility as source, generator as source, utility/generator in parallel, bus tie breaker open/close positions, maintenance settings) can be listed here if desired. In addition to electrical safety programs established by the Owner, maintenance settings might be applicable if arc energy-reducing maintenance switching has been specified in other sections to meet arc energy reduction requirements of NEC 240.87, revised for 2014 edition to apply to any circuit breaker 1200 A or higher (previously applied only to circuit breakers with non-instantaneous trip).
Utility Company contact information may be included under paragraph “Data Collection” to facilitate collection of utility source data. Alternatively the source data, where known, could be listed here. However consideration should be given to the time period between the design phase and commencement of the study, which may be uncertain and subject to unforeseen delays. Requiring the study preparer to obtain the most up-to-date information ensures the data is based on the most recent utility configuration, which is subject to continual changes.
Include the paragraph “Existing Installations” where applicable. Procurement of services of a field testing agency or the equipment manufacturer’s representative to perform field data collection may be specified if desired. This paragraph may also be used to list any available existing data (e.g. drawings, previous studies) that may be relevant in the performance of studies. Alternatively, these items may be listed under PART 3 article ATTACHMENTS, if included.
Include the applicable studies to be performed.
Under paragraph “Arc Flash and Shock Risk Assessment”, the specifier may choose to clarify some key requirements for the study preparer’s calculations:
1. 125 kVA, 208 V Transformer Exception: IEEE 1584 (2002 edition) states that "equipment below 240 V need not be considered unless it involves at least one 125 kVA or larger low-impedance transformer in its immediate power supply." This was based on the test data at the time which suggested arc flash events involving lower short circuit currents at 208 V were unlikely to sustain long enough to result in a significant amount of incident energy.
In the past NFPA 70E (2009 edition) included an exception that indicated an arc flash hazard analysis was not required where the circuit was rated 240 volts or less, was supplied by one transformer, and the transformer supplying the circuit was rated less than 125 kVA. For the 2012 edition of NFPA 70E, that exception was replaced with an informational note "see IEEE 1584 for more information regarding arc flash hazards for three-phase systems rated less than 240 volts".
While this statement suggests that incident energy calculations may not be required for these circuits, NFPA 70E does not explicitly exclude these circuits from the arc flash risk assessment or from the requirement to be marked with a warning label that includes either the calculated incident energy or the PPE category selected from NFPA 70E tables.
If these circuits are not included in the calculations, then one has to determine what information to include on the warning label. In the past it has been common industry practice for many engineers to assume an incident energy of 1.2 cal/sq cm for these circuits (equivalent to Hazard Category 0, which has been eliminated from NFPA 70E for the 2015 edition). However, this assumed value is not specified in IEEE 1584. Furthermore, in the absence of a calculated incident energy, the only alternative explicitly supported in NFPA 70E is the selection of a PPE category from the appropriate tables, assuming parameters are within the stated limits. This would result in a PPE category of 1 for panelboards and other equipment rated 240 V and below (minimum arc rating of 4 cal/sq cm). The most conservative approach is to just include these circuits in the calculations, which is the default choice that is offered in the specification text. It is left to the specifier to use the fill-in for other solutions.
Work is being done towards development of a new version of IEEE 1584 that, based on new available test data, will most likely revise the cutoff from 125 kVA to something lower and might include an assumed value for this case (4 cal/sq cm has been proposed).
2. Two Second Maximum Clearing Time Assumption: IEEE 1584 suggests that "it is likely that a person exposed to an arc flash will move away quickly if it is physically possible, and 2 seconds is a reasonable maximum time for calculations". However, it goes on to caution that "sound engineering judgment should be used in applying the 2-second maximum clearing time, because there could be circumstances where an employee's egress is inhibited", citing "a person in a bucket truck" and "a person who has crawled into equipment" as examples.
3. Single-phase Systems: While the IEEE 1584 theoretically derived model is "intended for use with applications where faults escalate to three-phase faults", it suggests that "where single-phase systems are encountered, this equation will likely provide conservative results". The use of IEEE 1584 equations on single-phase systems, assuming three-phase service, is also supported by IEEE 1584.1- Guide for the Specification of Scope and Deliverable Requirements for an Arc-Flash Hazard Calculation Study in Accordance with IEEE Std 1584.
4. Multi-section Equipment: For multi-section equipment where the main device is installed in a separate compartmentalized section, ensuring that an arc in the feeder section will not propagate to the main device, many engineers will run separate incident energy calculations for the main and feeder sections. The incident energy at the main section, based on the clearing time of the upstream device (which could be the utility primary protective device), could be much greater than the incident energy at the feeder section, based on the clearing time of the main device. For this case, it is common to provide two separate arc flash hazard warning labels indicating the two different incident energy levels.
Under paragraph “Study Reports”, the specifier may adjust requirements for the arc flash/shock risk assessment that are dependent upon the calculated incident energy level:
- Identifying locations where the calculated maximum incident energy exceeds a certain level: NFPA 70E includes an informational note that "when incident energy exceeds 40 cal/sq cm at the working distance, greater emphasis may be necessary with respect to de-energizing when exposed to electrical hazards". This level is widely accepted as the upper limit of incident energy where it becomes too dangerous to work on live equipment. Commonly warning labels with red header and word "DANGER" are reserved for locations with incident energy above this level while warning labels with orange header and word "WARNING" are used elsewhere. The default specification text requires the report to identify locations exceeding this level.
- Including recommendations for reducing the incident energy at locations where the calculated maximum incident energy exceeds a certain level: Edit the choice to indicate the project goal for maximum incident energy. The default goal of 8 cal/sq cm is common, chosen to correlate with PPE category 2.
Include only if arc flash hazard warning labels are to be provided as part of the contract. Label information required by NFPA 70E is turned on automatically via linking. The incident energy is included by default since it will be calculated by the arc flash risk assessment, but additional information on PPE requirements could be provided as well. Though not required, limited and restricted approach boundaries for shock protection, which are independent of the arc flash boundary, are commonly included as well. Note that a third shock protection boundary "prohibited approach boundary" was removed from NFPA 70E beginning with the 2015 edition. Also coordinate with Section 26 0553 – Identification for Electrical Systems, which includes a choice between specifying generic arc flash hazard warning labels in that section or including a cross reference to Section 26 0573.
Include the INSTALLATION article only if arc flash hazard warning labels are specified under PART 2 and installation of labels is included in the contract.
The FIELD QUALITY CONTROL article is turned on automatically via linking to provide for verification and adjustment of equipment and protective devices for compliance with studies and recommended settings. Procurement of services of a field testing agency or the equipment manufacturer’s representative to perform field inspection, testing, and adjusting may be specified if desired.
Training pertaining to arc flash and shock hazards may be specified under PART 3 article CLOSEOUT ACTIVITIES if desired.
REMAINING ARTICLES IN PART 1:
SECTION INCLUDES: Corresponding studies will be activated via linking according to selections made under POWER SYSTEM STUDIES.
RELATED REQUIREMENTS: Will automatically include other sections cited within the specification text (except for standard Division 1 cross references). Other sections may be listed because they include items that might be expected to be found within this Section or include items important for the completion of the work that are not specified in an obvious location (e.g. isn’t obvious from the section title).
REFERENCE STANDARDS: Will automatically include standards cited within the specification text. If the Consolidated List of Citations option is active, cross sectional links (not visible in the links window) will activate the reference standard in Section 01 4219 – Reference Standards as well.
ADMINISTRATIVE REQUIREMENTS: Includes optional text for a pre-study meeting to determine operating modes and for scheduling requirements applicable to existing installations.
SUBMITTALS: Edit according to project requirements.
QUALITY ASSURANCE: Study preparer qualifications are turned on automatically via linking. It is important for studies to be performed under direct supervision of a professional engineer. Optional text specifies whether or not the study preparer may be employed by the equipment manufacturer or field testing agency depending on the level of impartiality desired. Qualifications for field testing agency may be included where applicable. Finally, acceptable software products for performing studies may be specified. If the Owner uses specific power systems analysis software, they might require the studies to be performed using the same product.
- NETA ATS - Acceptance Testing Specifications for Electrical Power Equipment and Systems, published by the International Electrical Testing Association, includes simplified procedures for performing each of the specified studies in Section 6 – Power System Studies.
- NEMA ABP 1, available for free download at www.nema.org, may provide useful information on selective coordination.
- IEEE 1584.1 - Guide for the Specification of Scope and Deliverable Requirements for an Arc-Flash Hazard Calculation Study in Accordance with IEEE Std 1584 may provide useful information on specifying performance of an arc flash/shock risk assessment.