Calculating Short Circuit Currents (SCC)

In my last blog post (which was much too long ago), I wrote that the National Electric Code (NEC) requires the equipment be “selected and coordinated to permit the circuit protective devices used to clear a fault to do so without excessive damage to the electrical equipment of the circuit” [1].  To achieve a safe condition requires that equipment connected to the electrical system have short circuit current rating (SCCR).

To ensure that the equipment is being installed such that the calculated or tested SCCR is sufficient, a short circuit current (fault current) study must be performed.  Short circuit current (SCC) studies can be conducted through hand calculations, or software simulations.  Software simulations are the main method for conducting short circuit current studies.  While there are a number of vendors of this software, EasyPower, SKM, and ETAP seem to be the most used.

To aid engineers or installers when an SCC is not readily available, one can approximate the SCC to determine whether or not the SCCR of the equipment is suitable for the point of application.  The approximation of the SCC can be conducted by referencing the parameters of the closest upstream transformer and conducting some simplistic calculations.

The SCC calculation at the secondary of the transformer can be determined by the power rating of the transformer (P), the full-load secondary current of the transformer (I), the line-line voltage at the secondary of the transformer (V) and the transformer impedance (Z) [2].  The power rating is in kVA, the current is in amperes (A), the voltage is in volts (V), and the transformer impedance is in percent.

The full-load secondary current (I) of the transformer can be calculated by EQ1.

                                                     I = P / (V * SQRT(3))       EQ1

The SCC can be calculated by EQ2

                                                           SCC = I / Z    EQ2

 

As an example, suppose that we were going to install an industrial control panel on circuit that was supplied from a 500 kVA transformer with a voltage of 480 V, 3W+G, and an impedance of 3 %.  Using EQ1 and EQ2 yields a short circuit current of 20.05 kA.  To properly apply the industrial control panel, the equipment is required to have an SCCR that is greater than 20.05 kA. 

 

Common values of equipment SCCR are 5 kA, 10 kA, 14 kA, 22 kA, 25 kA, 30 kA, 42 kA, 50 kA, 65 kA, 85 kA, 100 kA, 125 kA, 150 kA, and 200 kA [3].   

 

An industrial control panel for this application would need to have an SCCR of at least 25 kA.  The SCCR can be calculated using UL508A Supplement SB.

 

This is a simplistic calculation that does not take into account the parameters associated with electrical conductors.  Also, as the power rating (P) of the transformer increases, the available SCC increases.  As the impedance (Z) transformer increases, the available SCC decreases.

Ensuring the equipment is properly applied is the responsibility of engineers and installers.  The NEC requires that equipment have an SCCR that is equal to or greater than the SCC at the point of installation.  Appling an industrial control panel or any other electrical equipment with an SCCR that is less the SCC at the point of installation will compromise the safety of nearby personnel.

REFERNCES.

1. National Fire Protection Association (NFPA).  National Electric Code, NFPA 70-2011, Quincy, MA USA
2. McKeown, D., Simple Methods for Calculating Short Circuit Current without a Computer, Retrieved 2012 August 10, Available [on-line] at http://www.geindustrial.com/publibrary/checkout/Short%20Circuit?TNR=White%20Papers|Short%20Circuit|generic.
3. Underwriters Laboratories (UL).  Standard for Safety Industrial Control Panels, UL 508A, Northbrook, IL USA

What is SCCR and Why is it Needed

From an electrical safety perspective, the past couple of months have been interesting.  Whether from recent posts of properly applying Industrial Machines or Industrial Control Cabinets, the number of electrical safety training programs that I have taught, or the meetings and phone calls, there are questions around short circuit currents, fault-currents, short circuit current ratings (SCCR) and why they are important.

Short circuit current and fault current are often used interchangeably.  However, there are some differences.  A short circuit current is an abnormal condition where two or more conductors of opposite polarity (or ground) are connected together and where the only limiting impedance is that associated with the resistive and inductive parameters associated with the electrical system [1].  Fault current is any abnormal current that flows in an electrical circuit [2].  Fault current can be from an unintended short circuit, transient current, overload current, or other abnormal condition.

Short circuit current rating (SCCR) refers only to equipment or systems and is a parameter associated with the connection to the electrical system.  SCCR is defined as “The prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptable criteria” [3]. 

When installing electrical equipment or systems (including machines), the National Electric Code requires that equipment be “selected and coordinated to permit the circuit protective devices used to clear a fault to do so without excessive damage to the electrical equipment of the circuit” [3].  Equipment that is Listed and applied within their Listing requirements are properly applied [3].

Depending on the equipment or machine, SCCRs can be determined through testing or through calculations.  Switchboards, switchgear, panelboards, transfer switch equipment, surge protective devices (SPDs) and the like require testing to obtain their SCCR.  The SCCR for industrial machines and industrial control cabinets can be calculated using Underwriter Laboratories Standard for Safety, Industrial Control Panels, UL 508A; although these calculations are a result of known practices from tested results.

Listing agencies (like UL, ETL, TUV, etc.) that conduct testing to determine the SCCR of equipment are doing so to determine if the equipment is “safe” for installation and operation at the SCCR.  Within product safety standards, Listing agencies have predetermined pass/fail criteria.  While the pass/fail criteria vary slightly standard to standard, common examples include [4]:

• Emission of flame, molten metal, glowing or flaming particles through any openings (preexisting or created as a result of the test) in the product.
• Charring, glowing, or flaming of the supporting surface, tissue paper, or cheesecloth.
• Ignition of the enclosure
• Creation of any openings in the enclosure that result in accessibility of live parts, when evaluated in accordance with the accessibility of live parts
• Loss of structural integrity to a degree that the equipment collapses or experiences such displacement of parts that there is a risk of short-circuiting or grounding of current-carrying parts.
• Opening of an external leads or conductors.

To ensure that the equipment or machine is installed in a manner that reduces electrical hazards, the device must have a short circuit current rating (SCCR) that is equal to or greater than the short circuit current at the point of installation.  Equipment that has a SCCR less than the short circuit current at the point of installation increases the probability of electrical hazards (both Shock and Arcing Hazards), is inconsistent with standards, and can result in injuries to employees or personnel.

Part of engineering equipment or a system includes not only ensuring that it functions properly, but also ensuring that it is installed in a manner that does not create hazardous conditions.  Knowing what the short circuit current at the point of installation and the SCCR of the equipment is one step in determining if the equipment is properly applied.

 

1. Wikipedia.  Short Circuit Current.  Retrieved 2013 March 23.  Available [on-line] at http://en.wikipedia.org/wiki/Short_circuit
2. Wikipedia.  Fault Current.  Retrieved 2013 March 23.  Available [on-line] at http://en.wikipedia.org/wiki/Fault_(power_engineering)
3. National Fire Protection Association (NFPA), National Electric Code, NFPA 70-2011, Quincy, MA USA
4. Underwriters Laboratories (UL), Standard for Safety, Surge Protective Devices, UL 1449 3^rd edition, Northbrook, IL USA

 

Installation of Industrial Control Panels

This month, the topic is the Installation of Industrial Control Panels.

An industrial control panel is an assembly of two or more components consisting of one of the following:

1. Power circuits components only; e.g. motor controllers, overload relays, fused disconnect switches, and circuit breakers
2. Control circuit components only, e.g. pushbuttons, pilot lights, selector switches, timers, switches, control relays
3. A combination of power circuit and control circuits [NFPA 70-2011]

Industrial control panels do include the controlled equipment, but does include all terminations, wiring, enclosures, and other ancillary devices required to connect the components [NFPA 70-2011].

The minimum installation requirements for an industrial control panel is that it meets the requirements detailed in the National Fire Protection Association (NFPA), National Electric Code, NFPA 70-2011, Article 409, Industrial Control Panels.  There are two types of requirements within NFPA 70-2011, Article 409: Installation (marking), and Construction requirements. 

An industrial control panel is required to have a label on the enclosure with the following information:

1. Manufacturer’s name, trademark, or other descriptive marking
2. Supply voltage, number of phases, frequency, and full-load current for each incoming supply
3. Short-circuit current rating (SCCR) of the industrial control panel using an approved method, unless the industrial control panel only contains control circuit components
4. Electrical diagram number(s) or the number of the index to the electrical diagrams
5. Enclosure type
6. If the industrial control panel is intended for a service entrance application, it shall be marked accordingly
7. Industrial control cabinets supplied by more than one power source shall be marked

Industrial control panels are also required to have specific components within the industrial control panel analyzed for proper and safe operation.  This includes:

1. Overcurrent protection (internal or external)
2. Equipotential bonding of all grounded parts
3. Proper wiring space and wiring utilization
4. Proper spacings between adjacent components
5. Proper disconnecting means for all motor loads

Industrial control panels for use in the US are not required to be Listed or Approved by an OSHA Nationally Recognized Testing Laboratory (NRTL), but it is a good idea.  As the reader will note, there are minimal operational or product safety requirements defined in the NFPA 70-2011, Article 409.  From a product safety standpoint, NFPA 70-2011, Article 409 does not have requirements regarding leakage current, that the internal components are used in accordance with their operating limitations, or that the industrial control equipment does not create a safety hazard when exposed to normal or abnormal operating conditions.

Evaluation of an industrial control panel to product safety standards will reduce the potential of the equipment presenting a product safety hazard.  The most notable safety standard for an industrial control panel in the US is Underwriters Laboratories (UL), Standard for Industrial Control Panels, UL 508A.  However, UL 508A is in a transition phase where it will soon be obsolete.  To harmonize with international standards, UL will be moving to the UL, Low-Voltage Switchgear and Controlgear, UL 60947 series of standards.  There will be a number of standards for specific components used for industrial control panel and auxiliary equipment applications. 

After 2012 January, all industrial control panels will be evaluated to UL 60947 unless the manufacturer request’s the industrial control panel be evaluated to UL 508A.  After 2017 January, all industrial control panels must be evaluated to UL 60947 to maintain Listing or Approval.

To ensure the best installation of an industrial control panel, the equipment should be Listed or Approved to UL 508A or UL 60947 by an NRTL and installed in accordance with the requirements of NFPA 70-2011, Article 409.  In addition, a Risk Assessment should be conducted to ensure that operational and safety requirements not identified in the standards have been accounted for.

For more information, please contact me by commenting on this blog or sending me an e-mail.

 

Installation of Industrial Machines

As I noted in my 2012 December blog, for 2013 I intend to focus more on installation issues.  The first topic for 2013 is on Industrial Machines.

Industrial machines are defined as a power-driven machine that is used to process material by cutting; forming; pressure; electrical, thermal, or optical techniques; lamination; or a combination of these processes, and can include associated equipment used to transfer material or tooling, including fixtures, to assemble/disassemble, to inspect or test, or to package [NFPA 79-2012].  Industrial Machines come in many different sizes, shapes, operating characteristics and incorporate a variety of components including power, control, monitoring, signaling, heating, movement and measurement.  Industrial Machines encompass a large category of equipment, but they do not include hand-held or portable devices.

For equipment that is installed in the US, the manufacturer should have their Industrial Machines evaluated to Electrical Standard for Industrial Machinery [NFPA 79].  If the Industrial Machine is intended for use outside of the US, many countries have adopted Safety of machinery – Electrical Equipment of Machines – Part 1: General Requirements [IEC 60204-1] as the standard used to evaluated Industrial Machines. 

While evaluation to consensus standards such as Electrical Standard for Industrial Machinery [NFPA-79] is not mandatory in the US, installers are required to meet the minimum requirements detailed with the National Electric Code [NFPA 70].

The National Electric Code has three main requirements for Industrial Machinery.  The first requirement is that the Industrial Machine be provided with a permanent nameplate.  The nameplate shall have the following information:

1. Supply voltage, number of phases, frequency, and full-load current
2. Maximum ampere rating of the short-circuit and ground-fault protective device
3. Ampere rating of largest motor
4. Short-circuit current rating (SCCR) of the machine industrial control panel
5. Electrical diagram number(s) or the number of the index to the electrical diagrams

The second requirement addresses the requirements for the supply conductors and the overcurrent protective device.  These requirements are:

1. The supply conductor shall have an ampacity that is the sum of:
1. 125 percent of all resistive heating loads
2. 125 percent of the highest rated motor load
3. 100 percent of all connected loads
2. The Industrial Machine shall have a disconnecting means
3. The Industrial Machine shall have overcurrent protection, whether contained integral or external to the device

The third requirement is that the Industrial Machine shall not be installed where the available fault current exceeds the short-circuit current rating as marked on the nameplate of the Industrial Machine.

Installing an Industrial Machine to the requirements of the National Electric Code [NFPA 70] will limit the potential for overcurrent conditions to damage the facility.  To provide personnel protection from normal and abnormal operating conditions of the Industrial Machine, Electrical Standard for Industrial Machinery [NFPA-79] provides more comprehensive requirements.  Some of the requirements include:

• Disconnect and overcurrent protection
• Grounding and bonding
• Power system interactions
• Internal and external conductor sizes, colors, and routing
• Control system performance and safety
• Enclosure types, openings, spacings, and guarding
• Labeling and documentation

For more information, please contact me by commenting on this blog or sending me an e-mail.

Electrical & Product Safety Observations For 2012

It is hard to believe, but 2012 is almost over.  Last November, I wrote on the Electrical and Product Safety blog about the top safety mistakes that I observed in 2011.  Instead of looking at mistakes in 2012, I am going to look at both “good” and “not so good” observations related to electrical and product safety.

In the not so good category, my observations are follows:

·        Not designing or purchasing equipment that minimizes electrical hazards during operation or maintenance (repeat from last year)

·        Not using basic statistical techniques to determine equipment safety and reliability (repeat from last year)

·        Not using basic statistical techniques to identify vendor performance – more of quality problem than a safety problem, but still an important observation

·        Not fully understanding the design parameters associated with equipment

·        Organizations struggling with defining and implementing regular maintenance of electrical infrastructure equipment

·        Organizations struggling with management of change processes

 In the good category, my observations are as follows:

·        More people are aware of hazards associated with working on or near exposed live (energized) equipment

·        More people are asking for their equipment to be analyzed by safety experts

·        More people are participating in various type of webinars or classroom training associated with all aspects of safety

·        More organizations are conducting shock and arc flash hazard analysis of their electrical equipment

·        ElectricalProductSafety.blogspot.com has had a large increase in readers

In 2013 I plan on branching out from the requirements of NFPA 70E into other items associated with electrical and product safety.  I plan on looking at the design of equipment and applications.  Some of the topics will examine requirements from standards while others will be “best practices”. 

As you start thinking about your electrical and product safety goals for 2013, I would like to leave with one thought from a manufacturing colleague of mine from many years ago.  That thought is “What gets measured, gets done”. 

When you make you safety goals for 2013 (or anytime), make sure that these goals can be measured and do not be afraid to display your metrics.

Have a Safe and Happy New Year!

 

Shock & Arc Flash Analysis of DC Electrical Systems

Most concepts of electrical safety revolve around AC electrical systems.  While AC electrical systems are the most prevalent in the workplace, DC electrical systems are being used in more applications and equipment, e.g. photovoltaic (PV) systems, within UPS equipment, control cabinets, industrial machines and telecommunication systems.

In the most recent edition (2012) of the Standard for Electrical Safety in the Workplace, NFPA 70E, information to protect qualified employees from the shock and arc flash hazards associated with working on exposed live (energized) circuits in DC systems is starting to be addressed.  As noted in Table 130.4(C)(b), the limited, restricted, and prohibited approach boundaries for workers working near or on exposed live (energized) electrical conductors in direct current (DC) electrical systems are provided [1]. 

For DC electrical systems operating at less than 100Vdc, the limited, restricted, and approach boundaries are not specified.  For DC electrical systems operating at 100 Vdc to 300 Vdc, the limited approach boundary is 42 inches, while the restricted and prohibited approach boundary is to simply avoid contact.  For DC electrical systems operating at 301 Vdc to 1,000 Vdc, the limited approach boundary is 42 inches, the restricted approach boundary is 12 inches, and the prohibited approach boundary is 1 inch.

While shock hazard boundaries are now identified, information associated with arc flash boundaries, incident energy associated with DC electrical systems is not specified differently than AC electrical systems.  Additionally, the common electrical simulation tools do not have the capabilities to calculate the arc flash boundaries and incident energy for DC electrical systems.

The literature on arc flash hazards associated with DC electrical systems is limited [2, 3, 4].  While limited, this research data combined with an additional understanding of how DC electrical systems are rectified from AC electrical systems will add to the understanding of the shock and arc flash hazards for a DC electrical system [5].  When combining literature, interpolation of overcurrent protective devices, an understanding of the DC electrical system parameters, and equipment, an engineer can create a mathematical model that will adequately represent the arc flash boundaries and incident energy for a DC electrical system.

Once the shock and arc flash hazard analysis for electrical equipment connected to the DC electrical system has been evaluated, the equipment is required to be labeled in accordance with NFPA 70E and ANSI Z535.4.

For more information on electrical and product safety, please comment on this blog or send me an e-mail. 

References:

1.     National Fire Protection Association (NFPA), Standard for Electrical Safety in the Workplace, NFPA 70E – 2012, Quincy, MA USA.

2.     Ammerman, R.F., Gammon, T, Sen, P.K., and Nelson, J.P. (2010), DC-Arc Models and Incident-Energy Calculations.  IEEE Transactions on Industry Applications, Volume 46, Number 5.

3.     Doan, D.R. (2010).  Arc Flash Calculations for Exposures to DC Systems.  IEEE Transactions on Industry Applications, Volume 46, Number 6.

4.     Fontaine, M.D. and Walsh, P (2012).  DC Arc Flash Calculations – Arc-in-open-air & Arc-in-a-box – Using a Simplified Approach (Multiplication Factor Method).  IEEE Paper No. ESW2012-25.

5.     RailCorp (2010).  Rectified Transformer & Rectifier Characteristics.  Document EP 03 00 00 01 TI, Version 3.0, Issued May 2010.

 

Electrical Safety - When Can You Work on Energized Circuits?

Technicians and maintenance personnel often need to work on exposed live (energized) circuits to troubleshoot or perform maintenance on electrical systems or equipment. 

The Occupational Safety and Health Administration (OSHA) and the National Fire Protection Associated (NFPA) provide guidance on working on exposed live (energized) circuits.  OSHA 29 CFR 1910 requires that work on electrical systems or equipment shall be de-energized unless de-energizing the circuits could create a greater hazard or if it is infeasible to de-energize the circuits.  The requirements identified in NFPA 70E, Standard for Electrical Safety in the Workplace, Articles 130.1(A)(1) and 130.1(A)(2) are similar to the OSHA requirements.

There are at least three classifications of electrical circuits that if they were de-energized could create a greater hazard to other people: emergency circuits, legally required standby circuits, critical operation power system (COPS) circuits.  Emergency, legally required standby, and COPS circuits are defined by NFPA 70, National Electrical Code, in Articles 700, 701, and 708 respectively.  Emergency circuits are identified as those systems that provide safe exit and communication within a building or other areas associated with life safety (e.g. patient rooms, operating rooms, or other areas within a hospital) [1].  Legally required standby circuits are identified as those systems that if stopped could result in general public safety hazards (e.g. sewage treatment facilities, pollution abatement systems, chemical processes, etc.) [1].  COPS circuits are identified as those systems that vital to national security, the economy, and public safety [1].  De-energizing emergency, legally required standby, or COPS circuits could endanger other personnel.  Therefore, technicians and maintenance personnel are allowed to perform work on these systems.

Optional standby systems, and unclassified systems are other types of electrical systems.  However, these systems in general do not pose a greater hazard to other personnel if they are de-energized like emergency, legally required standby or COPS systems.

When working on exposed live (energized) circuits an electrical hot work permit, risk assessment, and job safety meeting are required prior to conducting the work on exposed live (energized) circuits [2].  Items to be considered in the risk assessment are work practices that could result an inadvertent de-energizing of the electrical system due to human error or equipment malfunctions.

There are a number of work practices where it is infeasible for qualified employees to de-energize the electrical system or equipment regardless of their classification.  This includes troubleshooting, measuring, and adjustment of equipment.  Troubleshooting and measuring of electrical systems or equipment often involves measuring voltage, current, power, phase or other electrical quantities that cannot be observed without electrical energy present.  Adjustment of servos, indicators and similar devices also require electrical energy.

When qualified employees are conducting troubleshooting, measurement, or adjustment of electrical systems or equipment in the presence of exposed live (energized) circuits, a risk assessment and a job safety meeting are required prior to conducting the tasks.  As aforementioned, a risk assessment that evaluates the inadvertent de-energizing of the electrical system due to human or equipment malfunctions should be considered.

1.  National Fire Protection Association (NFPA). National Electric Code, 2011.
2.  National Fire Protection Association (NFPA). Standard for Electrical Safety in the Workplace, 2012.