Flash & Burn

Experts agree that air is the cheapest and most commonly used form of electrical insulation. If a water leak or some other form of contamination creates a conductive path between phases/neutral and/or ground, the air insulation will break down and produce a short-circuit arc. During the life of the equipment, other insulating materials can break down and fail, also producing an arc and short-circuit current. Workers who take voltage measurements or perform other work on energized equipment can also unintentionally bridge or short-out conductors in the equipment, creating a short circuit (Photos 1 and 2).

It's also important to examine how the utility system influences short-circuit current. The amount of impedance between the source and the short-circuit location has a direct effect on the amount of short-circuit current that will flow during a fault. If the utility increases circuit conductor size, replaces the service transformer with a larger unit, or installs a new generating station near the customer, the available short-circuit current will increase. If little impedance exists between the source of power and the location of the postulated fault, the resulting short circuit can be very large, possibly more than 100,000A.

Why is a short circuit dangerous?

A short circuit always involves the flow of uncontrolled current that isn't restrained by the normal load resistance. A short circuit, whether a short-circuit arc or bolted fault, provides a much lower resistance to current flow than typical loads. The resulting overcurrent condition will normally exceed the load current rating of conductors, transformers, and other equipment through which the current flows. This increased flow of current quickly heats the conductors and equipment, since heating is a function of current squared.

If the short circuit involves an arc in air, the arc produces intense heat that can exceed 20,000°F. This temperature will vaporize conductors, insulation, and other nearby materials. The byproducts of this process include ionized gas, which is conductive and will perpetuate the arc. As copper vaporizes, it expands by a factor of about 67,000. This rapid expansion will result in near-explosive forces on any nearby equipment or workers.

The National Electrical Code, IEEE 1584 Guide for Performing Arc Flash Hazard Calculations, and NFPA 70E Electrical Safety Requirements for Employee Workplaces refer to this phenomenon as arc flash and provide guidelines for protection from, and calculation of, arc flash energy. Because of the intense heat and destruction produced by an uncontrolled electrical arc, it's important to de-energize the circuit as quickly as possible after a short circuit.

What is short-circuit protection?

A common misconception is that fuses and circuit breakers will prevent short circuits or equipment failure. In reality, these protective devices are reactive and only operate after a failure has initiated. The real job of overcurrent protective devices is to limit the damage and effect of a short circuit. They minimize the damage at the point of failure, minimize or prevent injury, prevent damage to other equipment, and minimize the extent of the resulting power outage. If they're designed and adjusted to act very quickly, only a small amount of damage will occur as a result of the fault energy.

If there's excessive short-circuit current, protective devices like fuses and circuit breakers are designed, tested, and rated to interrupt specific maximum levels of this current. If the available short-circuit current exceeds the rating of the protective device, the device is likely to fail catastrophically when it attempts to interrupt a fault. Such a failure would result in downtime and extensive repair, and it could unnecessarily expose personnel to injury (Photos 3 and 4).

• Operating voltage, often referred to as electrical pressure

• System impedance or the resistance to current flow

In simple form, the equation for determining short-circuit current is derived from Ohm's Law and is expressed as I=E÷Z, where I is current, E is voltage, and Z is impedance. The voltage used in the calculation is the rated operating voltage of the circuit. The impedance value used is the sum of all the equipment and conductor impedances from the source(s) of power to the point in the circuit where the short circuit is postulated. Since the voltages, impedances, and resulting currents are vector quantities, these calculations can become very complex. Most engineers now use commercially available software to model the system and perform these calculations to conform with the IEEE Brown Book (ANSI/IEEE 399 Standard, Recommended Practice for Power Systems Analysis).

The importance of “coordinating” breakers and fuses.

The diagram of a simple electrical system resembles a tree-like configuration. The main power source corresponds to the tree trunk, and the primary feeder circuits and branch circuits correspond to large and small tree branches. To minimize damage and the extent of the power outage, breakers and fuses are located at strategic points in the system — usually at the main power entrance and the start of each primary and branch circuit.

If the fault occurs near the end of a branch circuit, the fuse or breaker immediately upstream from that fault should open before any other protective devices do, which would limit the resulting power outage to only the portion of the circuit downstream of the protective device. Similarly, if the fault occurs on a primary feeder, the fuse or breaker for that feeder should open before any other upstream protective devices. Selecting and setting the time-current characteristics of protective devices so they'll operate in this manner is called “coordination.”

When the branch breaker and main breaker aren't coordinated, the main breaker will trip when a fault occurs on a small branch circuit, exposing the entire facility to a complete power outage. Conversely, if the branch breaker were coordinated with the upstream breakers and fuses, only the branch breaker immediately upstream of the fault should trip.

Circuit breaker, relay, and fuse operating characteristics are graphically represented by time-current curves. These protective devices are typically designed to interrupt the current more quickly for higher current values and slower for lower current values. For example, a bolted fault is interrupted more quickly than an overload.

Each protective device has a unique curve or set of curves that manufacturers and engineers use to represent its operating characteristics. These curves are a plot of operating time vs. current level. From these curves, you can tell how long it will take for the protective device to interrupt at any value of current.

Although fuse manufacturers offer a variety of fuse types, each with its own curve shape and current rating, fuses are non-adjustable devices. If a different operating characteristic or current rating is needed, you must replace the fuse with a more compatible type. Smaller molded case breakers typically aren't adjustable either and must similarly be replaced if a different operating characteristic or trip value is necessary.

Most relays and electronically controlled breakers, however, are designed with considerable flexibility. They offer a wide range of field-adjustable trip settings and operating curves. Breaker curves provided by manufacturers show a band of operating times for any given current. The width of each breaker curve is a result of the following factors:

• Manufacturing tolerances that produce slight variations for each breaker

• The small but discrete amount of time for a relay or breaker to sense the fault or overload

• Time for the breaker mechanism to move (opening the contacts), and time for the current to extinguish

Consequently, the characteristic curves for breakers are shown with a certain width to indicate the minimum and maximum operating time.

Similarly, fuse curves are shown with a distinctive width that's a result of manufacturing tolerances, which produce slight variations for each fuse. In addition, there is a small but discrete amount of time between initial melting of the fuse link(s) and extinction of the current.

As explained earlier, it's important to coordinate protective devices by choosing a main fuse or breaker with slower operating characteristics than the feeder breakers. You would also want to select feeder breakers with slower operating characteristics than the branch breakers, and so on.

In general, the protective device furthest downstream should have the lowest trip setting (in amperes) and be the one that operates fastest for a given current level. The study engineer will typically plot and overlay the characteristics of each protective device to confirm the sequence in which they'll operate and to confirm that there is adequate margin between the operating times of each.

To select the appropriate fuse and breaker/relay settings, it's necessary to perform a short-circuit and coordination analysis for the electrical system. The process begins with a computer model of the system based on a single line diagram. Equipment and conductor impedances, operating voltage, load values, starting currents, equipment ratings, and interrupting characteristics of the protective devices must be included in the model. The short-circuit calculation will identify any interrupting equipment that may be inadequately rated for the available short-circuit current. Using the results of the computer model analysis of the system, it's then possible to choose optimum time-current settings for relays and breakers and plot the results. Engineers use the following general concepts when making these determinations:

• The fault current or overload should always be interrupted by the first protective device upstream — on the source side — of the fault location.

• Normal transformer inrush current and motor starting current should never cause a protective device to operate.

• Overcurrent devices should interrupt the current as quickly as possible after an overload or short-circuit occurs.

After the coordination study is complete and a summary report has been issued, field engineers will use the study results to make the appropriate breaker/relay adjustments and test the breakers/relays to confirm that they operate as intended.

What about special cases?

A fact in all fields of study is that rules come with exceptions. Below we explore special cases and how to best handle them.

• What if there is an automatic transfer switch?

  If the system involves one or more transfer switches, the short-circuit and coordination studies must consider various possible operating parameters, such as source and load configurations, for these switches.

• What if there is an emergency generator?

  If the system includes one or more emergency generators, it will have a similar number of transfer switches. Each of the possible operating scenarios must then be considered when performing the short-circuit and coordination studies.

• What if the system is supported by a UPS or back-up battery?

  An uninterruptible power supply requires special attention when performing a short-circuit and coordination study. Since a UPS and battery represent a load during normal operation but a source during utility outage situations, the protective devices must be sized for either condition. Manufacturer recommendations must be considered.

• What if there is co-generation?

  Co-generation can present special difficulties since available fault current can be high with a generating unit connected directly to the system. Often the addition of a co-gen facility will require that protective equipment be replaced with higher rated equipment.

When weighing the myriad factors that can affect plant availability and production, it's critical not to overlook the pivotal role played by short-circuit and coordination studies. They can prevent unplanned outages, eliminate workforce accidents, and extend the life of equipment. When placed in the context of the overall facility operation, they can make the difference between performance and disaster. Considering the fact that the cost of a short-circuit and coordination study is typically a small fraction of the electrical system cost, it's a wise investment that can pay dividends in the form of increased safety and availability.

Vahlstrom is director, technical services for Electro-Test, Inc. in San Ramon, Calif.

Sidebar: Do You Have the Data for a Study?

To perform a short-circuit and coordination study, you'll need the following information:

• A single line diagram of the electrical system

• Data from the utility, including available fault current, operating voltage, and specifics regarding the utility's protective equipment at the point of service, such as manufacturer, model, time/current settings, or fuse rating

• Specifics for each protective device in the electrical system, including manufacturer, model, available time/current settings, and short-circuit interrupting rating

• Impedance and rating of each transformer

• Conductor specifics, including lengths, sizes, and types of all overhead lines, bus ducts, and cables

Sidebar: Additional Studies

You can perform several other valuable engineering studies to improve the safety, efficiency, and reliability of your electrical distribution system. They can be performed with essentially the same information used for short-circuit and coordination studies.

• Arc-flash evaluation.

  IEEE 1584 and NFPA-70E provide guidelines for calculating the incident energy produced by an electric arc. An arc-flash calculation will determine the available arc fault exposure at equipment locations within a facility. This information will provided workers with the information they need to select the appropriate level of PPE required to work on any piece of energized equipment.

• N+1 reliability study.

  A single-point-of-failure or redundancy study can be performed using the information collected for a short-circuit and coordination study. N+1 refers to a normal plus one redundant path for supplying critical loads. In some facilities, the electrical system has been designed to N+2 criteria. This provides for the continued supply by the third path when one path is out for maintenance and a second path fails.

• Probabilistic risk assessment (PRA) reliability study.

  IEEE 493 provides guidance and data for performing a risk assessment for electrical systems. The standard provides data collected from industrial facilities, including probability of failure and mean time to repair for typical electrical equipment, generators, and utility feeders. Using these methods and data, it's possible to analyze an electrical system and calculate the probability of failure and predicted average annual downtime. If actual data is available for the facility involved, this data can be used in the calculations and more accurately predict number and length of outages per year. PRA and its resultant data can be especially helpful in comparing the predicted reliability or availability that would result from alternate electrical system designs.

• Voltage profile.

  Using the model prepared for performing short-circuit and coordination studies, the study engineer can also calculate the probable band of voltages within which the facility will operate. These calculations consider the impedances of the system and operation of various loads at the facility.

• Load flow analysis.

  Using load data and the information collected to perform a short-circuit and coordination study, a load flow analysis can be created that will calculate load currents and voltage levels for various operating conditions. This can be especially helpful in determining tap settings for transformers and when considering or evaluating voltage correction alternatives.

• Motor starting analysis.

  Short-circuit and coordination information can also be helpful in calculating system voltages that will occur during motor starting conditions. As discussed above, such an assessment can be especially helpful in determining tap settings for transformers and when considering or evaluating voltage correction alternatives.

• Harmonic current and voltage assessment.

  The information gathered for a short-circuit and coordination study can also be used to perform a harmonic current and voltage assessment of the electrical system. The results can help evaluate alternative corrective actions such as installation of filters or re-configuration of circuits.

• Power factor correction.

  Poor power factor can result in costly rate penalties imposed by the utility. It can also contribute to overload and overheating of conductors and transformers. The information gathered for a short-circuit and coordination study can be used to assess alternative locations and sizes of any needed power factor correction equipment.