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A short circuit, also known as a fault, occurs when energized conductors inadvertently come into contact with one another or another unintentional conductive path. Short circuits often allow dangerous amounts of current to flow through a circuit, far greater than what they are designed for. Overcurrent protective devices like breakers and fuses have to be used to clear the faults and ensure safety. The 2019 edition of NFPA 70B chapter 9 covers the requirements of studies like short circuit analysis. In order for an electrical system to be capable of reliably and efficiently transferring power from point A to point B, the conductors and transformers that make up the pathway from the source (say a gas power plant) to the load (say the dishwasher in your house) have to have a very low impedance. The load by comparison, will have a much higher impedance. By Ohm's Law, this means that the load is usually what determines how much current flows. Usually, this is great, but in a short circuit event the low impedance of the system turns the problem upside-down. Let's say the insulation on the wires leading up to your dishwasher breaks down after years of use. The wires then come into contact with one another and create a short circuit. Now, the load is no longer what is limiting the amount of current, but the transmission system itself. By Ohm's law, since the impedance is low and the voltage of the source has remained the same, the current will increase (often drastically)! The diagrams below show an example of a fault as viewed from a circuit perspective. The model below is based on the infinite bus assumption. What this means is that there has been no additional impedance included between the source and the first transformer. This assumption may seem unreasonable, but it's often very useful and will always yield conservative results. Moreover, the diagram below is for a three-phase fault. Instead of assuming that a single wire has broken down its insulation and come into contact with some other conductive pathway, we are assuming that all phase conductors have had their insulation broken down and are now in contact with one another. This condition will generally yield the highest fault currents in a system. Example Circuit for Fault Current Calculations The phase of the fault current can be important for more sophisticated applications, but we'll worry only about the magnitude of the current here. Applying Ohm's Law to the circuit above, the magnitude of the fault current |I | can be calculated: |I |= |Vn | / | Zt + Zc | Where: Vn is the secondary source voltage from line to neutral in Volts. Even if a source were delta-connected, the line-line voltage divided by √(3) is still applicable. Zt is the transformer positive sequence impedance in Ohms Zc is the conductor impedance in Ohms The transformer positive sequence impedance is often given as a percentage (and is often referred to as simply "the transformer's impedance"). We can convert the percentage impedance of the transformer to a value in Ohms by understanding the basis for that percentage. The conversion is: Zt = Z% ( VL ^ 2 / S ) Where: Zt is the transformer impedance in Ohms, usually assumed to be purely reactive (purely inductive) Z% is the transformer nameplate impedance percentage VL is the nominal line-line voltage rating of the transformer being referenced to (the secondary voltage for infinite bus short circuit current calculations) S is the three-phase apparent power rating of the transformer Infinite bus symmetrical fault calculations only make sense when applied to the secondary of a transformer. If we tried to calculate the infinite bus fault current before the transformer, the result would be...well, infinite. There would be no impedance between the voltage source and the fault location and Ohm's law would say to expect an infinitely large current flow. Of course, we know this is not real. In reality, there will be source impedances from the transmission system, upstream transformers, generators themselves, and more. The example below shows how these calculations can be applied to find the fault current with limited information on a project. Asymmetrical Fault Current - Discussions this far have been surrounding the symmetrical fault current, the current that flows in a three-phase fault as a steady-state condition. For a short period of time after the fault, though, the fault current can actually be much higher than the symmetrical value. The asymmetrical fault current is the sum of the symmetrical fault current and a transient DC offset. This offset decays exponential just like in an RL circuit. The rms asymmetrical fault current is given by: Ia = Is √( 1 + 2 e^( -2 R t / L ) ) Where: Ia is the asymmetrical fault current Is is the symmetrical fault current e is Euler's constant R is the system resistance in Ohms L is the system reactance in Henries t is the time in seconds after the fault event This equation can be re-written to utilize a time basis of cycles and an X/R ratio instead of the resistance and inductance as well. The results are the same: The maximum asymmetrical rms fault current is equivalent to 173% of the symmetrical fault current. Depending on the system's particular impedance and the time at which the fault occurs, the actual maximum effective asymmetrical fault current could be much closer to the symmetrical fault. A Plot of the Asymmetrical Fault Current and its Components Interpretation - The symmetrical and asymmetrical fault current each have their place in a good design. It is advisable that conductors be sized based to withstand the symmetrical fault current, since that is the standard method used in ICEA and NEC documents. Meanwhile, the system overcurrent protection scheme should be designed accounting for both the symmetrical and asymmetrical fault current. We need the system to clear a fault as quickly as possible, regardless of whether it is symmetrical or has an offset. Overcurrent devices are often rated in terms of their symmetrical fault current and are tested at a particular X/R ratio. If the X/R ratio of the system exceeds the tested value, then additional derating is required to ensure the design will operate safely. Equipment is often rated for duty based on symmetrical fault currents with special caveats for maximum X/R ratios. Example: Compute the symmetrical fault current seen on the secondary (low voltage) of a 6.9kV : 480V transformer with a nameplate apparent power rating of S=100kVA and a nameplate impedance of Z=4%. What is the peak rms asymmetrical fault current possible? Solution: For this example, we are neglecting any secondary conductor impedance and just focusing on the transformer. This means we first have to compute the transformer impedance. Referenced to 480V, the transformer impedance is: Zt = .04 * ( 480^2 / 100000 ) = .09216 Ohms Then, the fault current magnitude is given by the effective line-neutral voltage and Ohm's law: I = ( 480 / √(3) ) / (.09216) = 3005.6A The peak asymmetrical fault current is equivalent to √3 times the symmetrical fault current: √3 * 3005.6 = 5205A

Overview - AC power systems operate at a nominal frequency. In the United States, this is 60 Hertz. In other parts of the world 50 Hertz is also common. Figure 1, below shows what a typical 120V voltage waveform looks like when plotted with a frequency of 60 Hz. Notice that the peak of the waveform is actually higher than 120V. It's common to refer to voltages by their root-mean-square (RMS) values. For sinusoids, the RMS value is just the peak value divided by √2. Figure 1: A 120V Power Supply in the United States But why is the waveform a sinusoid? Well, there a several good physical and mathematical reasons for that. First, the physical reason: a sinusoid is a waveform that will naturally come out of a generator when rotated with a constant frequency. This means that getting something other than a sinusoid out of a generator may require a less-than-ideal design. Second is the mathematical reason: Sinusoids are what we call "eigenfunctions" of linear systems. What does that mean? Basically, if we feed a sinusoidal waveform into our electrical power systems (cables, transformers, etc.) we get sinusoids out. This is convenient to design around and easy to engineer. Other Waveforms - This begs the question, what happens if we put something other than a sinusoid into our power system? What if, for instance, we used a square wave (like in Figure 2) for the basis of our power systems? Figure 2: A Square Wave Voltage Source If the electrical world were made out of pure resistances, then the results would be just like our sinusoids (square wave in, square wave out). However, the real world is full of inductances and capacitances that distort waveforms as they pass through. Only a sinusoid will exhibit steady-state behavior that is still sinusoidal after passing through a system with inductances and capacitances. After passing through a transformer ( a component with a high inductance), our output waveform may look very similar to Figure 1! Figure 3: A Square Wave Input to a Transformer Leads to a Sinusoidal Output The Fourier Series - Why does this happen though? It turns out that any periodic waveform can be written as the sum of sinusoids. For something like the square waveform above, it will require an INFINITE number of sinusoids added together with different frequencies and magnitudes to approach the square wave shape. Figure 4 is the exact formula that is required to get a square with with a peak value of 1/-1. Figure 4: The Fourier Series for a Square Wave Notice that each term in the sum decreases in amplitude as the frequency increases. This makes sense; otherwise producing a square wave voltage (even with a slow frequency) would be really hard to do and require something to oscillate REALLY quickly! Additionally, notice that the frequency of the sinusoids is always a multiple of some fundamental frequency. This is very important to understand. It means that the dominant frequency, the frequency of the square wave itself, is the lowest frequency in the system. This explains why the square wave going into a transformer comes out looking sinusoidal. The higher frequency components of the square wave are more drastically attenuated by the transformer's inductance, leading to an effect we call filtering: Higher frequency terms from our Fourier Series disappear while lower frequency terms remain. If we passed through a capacitive system, the reverse would be true: high frequency terms would remain and lower frequency terms would disappear. Combining resistors, capacitors, and inductors in various combinations can create all kinds of elaborate and useful filters. We could (and many mathematicians do) write elaborate Fourier series equations for all kinds of waveforms. However, that's not that important to understand the big picture. The key takeaways are as follows: A periodic waveform can always be written as the sum of sinusoids The fundamental frequency of a waveform is the lowest frequency component in the system. All higher frequency components are multiples of this frequency. Inductances and capacitances create filtering on periodic waveforms, with certain frequencies experiencing greater attenuation than others. Practical Applications - So now that we know about Fourier Series, how does this help us in the real world? To understand that, let's return to WHY our power systems use sinusoidal sources for AC power: predictability and quality in design. However, a lot of components in our power systems are not so linear. Things like rectifiers, that convert AC to DC, or inverters, that convert DC to AC, do not work so nicely. They produce undesirable sinusoidal terms of higher frequency, known as harmonics. Harmonics distort our sinusoidal AC waveforms and can lead to undesirable impacts on our power system. Some examples of the problems caused by harmonics: Induced voltages in otherwise safe equipment Improper tripping of relays and protective devices Overheating of equipment And these are only at the lowest level. These problems can easily cascade into much bigger issues. IEEE 519 sets standards for harmonic content (ratios of harmonic voltage and current levels to fundamental levels) that must be met for designs. Many times, manufacturers will also adhere to IEEE 519 standards for products. Engineers need to know when harmonic are likely to occur in a power system, the level of harmonics produced by equipment, and ways to mitigate their effects. Generally, this means designing filters and placing them in strategic locations in the power system. A power system without the right harmonic mitigation strategy can be bad news for everyone involved!

In North America, there are three main electrical codes: The National Electrical Code (NEC), used in the United States Norma Oficial Mexicana (NOM), used in Mexico The Canadian Electrical Code (CEC), used in Canada While NOM and NEC are essentially the same document with a translation between Spanish and English, the CEC is a different animal. Before assuming anything between these codes, it is always the right answer to check. But, for a quick guide to major differences, here you go: Routing in Tray: In Canada, conductors in tray are generally required to have armor when routed in cable tray. There are exceptions to this for certain installations, but this is the reason why armored cable (particularly TECK cable) is so common in Canadian electrical power systems. Bundled Conductors in Tray: In the United States, we don't have to derate our conductor ampacity for routing in a common tray. This means that a large cable tray filled with 20+ conductors is treated similarly to a single 3-conductor circuit routed in conduit. In Canada, this is not the case. Per the CEC, cable tray is considered the same as any other raceway and the example of 20+ conductors in tray would require a .5 derate for bundling. You could potentially be looking at double the cable. Terminations: Terminations are one of the most confusing parts about the NEC. The ampacity of a conductor is required to be limited based on the termination temperature rating at the equipment. However, this is about all the NEC says on the matter. The CEC is much clearer. Per the CEC, a 5 foot segment extending out from the equipment must maintain its temperature at or below the equipment termination temperature rating. This means that terminations are not exempt from derating (which they arguably seem to be in the NEC). Ampacity Tables: The NEC has an entire section devoted to ampacity tables for medium voltage. The CEC, on the other hand, has only one ampacity table for both low voltage and medium voltage conductors aboveground. Underground conductors have some additional tables that may be used. This difference can be both good and bad, as it simplifies ampacity considerations but generally leads to lower medium voltage conductor ampacities than predicted by NEC tables. Temperature Derating: The NEC permits a temperature derating factor greater than 1 based on ambient temperature. This means that ampacity of conductors can be increased above the values shown in NEC tables where the ambient temperature is cooler than the reference temperature of the table. In the CEC, the temperature derating factor must be 1 or less. Improvements to cable ampacity beyond CEC tables are not permitted. Bonding Jumpers: The NEC requires that equipment grounding conductors installed for parallel runs in separate raceways must all be fully sized, meaning that each EGC is sized for the full upstream breaker rating from Table 250.122. In the CEC, EGCs are known as bonding conductors, and when installed for parallel runs in separate raceways the size of the bonding conductor is permitted to be divided (in terms of area) across the parallel runs. This means a lot less metal is required for CEC installations. This is far from an exhaustive list of the differences in design requirements for the CEC and NEC, but these are some of the most impactful. Conductors and overcurrent protection must always be designed in accordance with the codes and standards put forth by the authority having jurisdiction.

Your project is almost built! Raceways are installed and cables are pulled. What now? Do we terminate the cables and walk away? Not quite... After initial construction, we're still left with commissioning. Commissioning is the process of verifying the quality of an electrical installation prior to full energization. What are the activities involved with commissioning? It depends on the installation. Let's start off with a simple installation, like in a home. In this case, after wires are pulled the next step is usually just to terminate, do a visual check of the installation, and check out the wiring to make sure it works as intended. That's some really minimal commissioning. What about on a bigger project? What then? Here are some usual commissioning activities employed on larger projects: Continuity Testing: Continuity testing checks a circuit to make sure that the connection between two points is electrically continuous. In other words, the impedance between those two points, as measured by an Ohmeter, should be very low. Digital multimeters generally possess a continuity function designed specifically for continuity testing. The continuity test makes an audible noise when continuity is established. Insulation Resistance Testing: Insulation resistance testing, also known as Megger Testing, measures the resistance between a phase (current-carrying) conductor and its insulation. This testing can be performed on equipment or circuits. Megger testing can be performed with special multimeters at low voltage levels. Medium and high voltage equipment and cables require dedicated equipment for this purpose. The results of an insulation resistance test should be on the order of MegaOhms, with approximately 1 MegaOhm of resistance per 1 kV of nominal voltage level. Hipot Testing: An abbreviation for "high-potential" testing, Hipot tests use a high voltage to measure the ability of a conductor at medium or high voltage to withstand surges. Hipot tests are a PASS/FAIL test. A leakage current is measured when the high voltage is applied between phase conductors, and, if the leakage current is too high, the test fails. Hipot tests used to be performed using DC voltages, but this would lead to static charge built up on cables due to higher cable capacitance at medium and high voltage (from shields, etc.). Now, Hipot tests are done with very-low-frequency AC voltages, known as VLF testing, to mitigate this issue. Partial Discharge Testing: Partial discharge (PD) testing takes the concepts in a Hipot test one step further. A PD test applies a high voltage and assess the leakage characteristic of the cable across its entire length. PD tests tell the tester where failures are, providing more detail than a Hipot test.

The most important skill for an engineer isn't math or computer programming. It's not even technical. More important than anything else is an engineer's ability to communicate, particularly through writing like specifications and emails. No matter how brilliant you might be, your knowledge is useless if it can't be conveyed to others. How will you convince project stakeholders that your idea is as great as it seems? How will you win people to your cause? How will you market your product to consumers? Without proper written communication, none of this is possible. So, here are my tips: Know what you're talking about. This is the most important tip you can give any engineer. The market is always changing and new technology is always emerging. Just make sure that you really know what you're talking about when you start writing. If you're feeling unconfident, reach out to others for help! Make sure your audience will know what you're talking about. Know your audience! Suppose you want to buy a transformer. If you're talking to the manufacturer's engineer directly, they will likely understand and be interested in discussing all kind of details. On the other hand, if you're talking to your project finance team about the same transformer, there's a good chance that they won't understand the same discussion. Likewise, you may not understand exactly what the project finance team is doing! Respect other people's knowledge areas and levels and plan your writing accordingly. Don't put too little or too much information. This tip is pretty subjective, but it's also really important. Think about your audience. If you're talking to a busy executive, try to stick to the point and don't get into all the details. They just need to know what's actionable. On the other hand, if you're trying to give instructions to a newly hired employee, that employee will likely want all the detail you can provide. Use complete sentences. It might sound obvious, but using complete sentences is probably the most important writing tip out there. It's easy to get in a rush and write up an abbreviated version of exactly what you want to say. Just remember, notes that make sense to you will rarely make sense to others if they aren't written in complete sentences.

Lightning strikes on power systems are common. A storm is overhead and lightning attempts to find the easiest pathway to ground. This could be through a transmission line or tower, through a lightning rod and down conductor, or through a number of more unconventional paths. When lightning strikes, there's a possibility that serious damage can occur. Even with the best protection systems, lightning is famously unpredictable (think "lightning never strikes the same spot twice"). So, what can we do? Figure 1: A Lightning Storm Near an Overhead High Voltage Transmission Line What is Lightning? - Well, the first and easiest thing we can do is understand just what lightning is. This might sound obvious, but in order to protect against lightning strikes to our electrical system, we need to know how to model the strike itself. Lightning is modeled with an 8/20 microsecond current wave. This means that the lightning rises to its maximum current value in 8 microseconds and returns to half its peak value in 20 microseconds. The IEEE definition of this waveform uses a pair of exponentials, but for high level design it's suitable to use a triangular wave shape (See Figure 2 below). With a pulse this quick, usual steady-state power system analysis is far from suitable. Changes in current values on the order of microseconds means that the dominant frequencies in the system are on the order of MegaHertz. The effectives impedance from capacitances in the power system are no longer trivial (since capacitive reactance is proportional to the inverse of the source frequency) and the effective impedances from inductances are massive (since the inductive reactance is proportional to the source frequency). In order to analyze the effect of a lightning strike in detail, it's necessary to know the values of stray capacitances in the system (transformer winding to ground, cable to ground, etc.). Moreover conductors and buses usually need to be modeled as transmission lines to capture the transient behavior of the pulse moving through them, including voltages developed from surge impedance (also known as characteristic impedance) of the line. Unfortunately, all of this detail can be difficult to obtain and may significantly affect the accuracy of results. Figure 2: The 8/20 microsecond Lightning Current Waveform The 8/20 microsecond waveform describes the shape of the lightning strike, but what about the magnitude? According to the National Weather Service, a median lightning strike is about 30,000 Amps with a Norton Equivalent resistance of about 10 kiloOhms. IEEE standards provide additional estimates on the value of strike currents and the probability distributions. Figure 3: Model of a Lightning Strike in a Circuit Basic Lightning Insulation Level (BIL) - Although we model lightning as a current source, the bigger problem is usually the voltages developed from lightning. Conductor withstand for such a short duration is generally not an issue, but when the current passes through the system's impedance to ground, overvoltages will develop in accordance with Ohm's Law. They will be brief but put incredible strain on the electrical system. Equipment is rated to withstand these events by having proper electrical insulation, designed to not break down over short time frames of immense electric field strength. The ability to withstand these induced voltages is characterized by a piece of equipment's basic lightning insulation level (BIL). Various IEEE standards prescribe the minimum requirements of BIL based on the system operating voltage and the type of equipment. The NEC also lists blanket minimum requirements for BIL on systems up to 230kV (See Article 490 of the 2020 Code). As long as the lightning-induced voltage is below the equipment BIL rating, the equipment should be protected. Surge Arresters - Having the minimum BIL for a piece of equipment isn't usually sufficient protection. Without a low impedance path to ground, overvoltages will develop on equipment and may exceed the system's limitations. Surge arresters solve the problem of providing a low impedance path from phase to ground without compromising normal operation. A surge arrester can be thought of as a nonlinear resistor (also known as a varistor). During typical operating voltages, the resistance to ground is so high that the system behaves like normal. However, the surge arrester offers a very low resistance when a large current passes through, limiting the voltage developed across the arrester to something on the order of 2-3x the arrester's continuous operating voltage rating. Figure 4: Surge Arrester Curve Example Generally, surge arresters are applied between the phase conductor and earth. On an ungrounded system, such as a delta or ungrounded wye, the surge arresters need to have a maximum continuous operating voltage (MCOV) equal to the line-line voltage of the system. This is because a single-line-ground fault in one phase can lead to elevated voltages in other phases due to reference point shifting. On solidly grounded systems, the MCOV must be equal to or greater than the line-neutral voltage of the system. NEC Article 242 in the 2020 Code lists these requirements and more. IEEE C62.11 provides even more detailed information. Summary - With the right surge arrester and BIL we'll go a long way to making sure our equipment is protected, even if there is a lightning strike. But, as mentioned at the beginning of this article, lightning is very unpredictable and there's always a possibility that a strike with a fast wavefront (higher induced voltage on inductances) and a large peak current (higher induced voltages on resistances) will lead to excessive overvoltages. There's no way to protect a system 100%, but it's possible to bring it close!

General - Motors are magnetic devices. They rely on the interaction between magnetic fields to create rotation. Before this rotation takes place and the magnetic fields are in their steady-state condition, a large value of current will normally flow into the motor. This is called "inrush". If we don't properly account for inrush in our designs, there could be two different types of problems. First, if the inrush current is too high, we may see excessive voltage drop in the system. This dip in voltage could affect the performance of other loads and lead to unintended system failures. Alternatively, if there is too much impedance and the motor receives a substantially reduced starting voltage, the motor may take too long to start. Or worse, the motor may fail to overcome its own inertia and will stall out. Motor Starting Voltage Drop - In order for a motor to be able to turn over during starting, there needs to be sufficient voltage at the motor terminals, even when the locked-rotor-current (inrush current) is initially pulled in. Imagine a system with a very high impedance . If the motor is switched into service and attempts to draw the inrush current to magnetize, there will be a problem. The voltage will be so low that the motor isn't getting the power it needs. How low is too low? Motor manufacturers will provide engineers with the minimum starting voltage. 80% of nominal voltage is a typical minimum value. Likewise, the inrush current (locked-rotor current) can be taken from a motor datasheet to perform this voltage drop calculation. Locked rotor current is typically around 6.5x the full load current value. Motor Starting Inrush Reduction Methods - What happens if inrush causes the motor voltage to dip below its minimum rating? Do we have to redesign the entire system? No! There are ways that we can reduce inrush that don't require substantial redesign. Here's a quick sampling: Upsize motor feeder/branch circuit conductors - The voltage drop flowing through these conductors is directly related to their impedance. Larger conductors can significantly reduce this impedance, yielding improved voltage drop. The tradeoff of this is cost and installation effort. Reduce the impedance of upstream transformers - Ideal transformers don't affect the amount of real and reactive power flowing through them, but real transformers do. Practical transformers have a certain amount of impedance, a predominantly inductive reactance that causes voltage drops across transformers. Larger transformers can usually be specified with lower impedances (to a certain extent). The tradeoff of this is an increased fault current coming from the transformer. Use a more sophisticated starting method - The discussion of inrush effects up to this point has been based on the use of "full voltage" starting. Full voltage starting consists of closing a contactor, switch, or breaker to energize a motor with the full line-to-line voltage of the source. There are alternative ways to energize a motor, including soft starters and wye-delta starters. Soft starters use electronic components to start a motor with a smooth transition from zero voltage to rated voltage, controlling the current that enters and preventing excessive voltage drop. Soft starters are the most expensive motor starting device and can produce harmonics due to their nonlinear behavior. Wye-delta starters configure a motor to start based on line-neutral voltage across the motor windings until a certain state is reached. Then, the winding is switched to delta, to provide full, rated voltage. A Wye-delta starter is like a soft starter with only 2 discrete steps. They are less expensive than a wye-delta, but less sophisticated.

General - Overcurrent protection (OCP) devices are designed to protect conductors and electrical equipment from dangerous amounts of current flowing through them when something goes wrong. They do this by “tripping”, opening up the electrical circuit so there is no longer a conductive pathway for current to flow. There are two main types of overcurrent protection devices, circuit breakers and fuses. The big difference is this: fuses are used up after one overcurrent event and breakers are reusable. Article 240 of the National Electrical Code is all about overcurrent protection devices. The schematic symbols for breakers and fuses are here for reference: Overcurrent Protection Device Electrical Symbols Low Voltage - Breakers and fuses are rated according to three key parameters: the voltage rating, the ampere rating, and the interrupting rating. The voltage rating is the system voltage the equipment is designed for. For low voltage equipment, OCP devices can be used in systems with voltages at or below the OCP device rating. The ampere rating describes the amount of current that an OCP device can safely carry. For low voltage overcurrent protection devices, the ampere rating is usually the same as the trip rating. For example, a low voltage 30A breaker is designed to open up (trip) at no more than 30A flowing through it. The interrupting rating is the maximum value of current that the OCP device can safely interrupt. This value is usually on the order of kiloAmps. The interrupting rating must be coordinated with the available short circuit current to ensure that the system is adequately protected. The interrupting rating and the trip rating both show up on the overcurrent protection device’s trip curve. A trip curve is a graph showing the clearing time vs. current flowing through the OCP device. The long-time trip rating is the value of current that just barely is high enough to trip the OCP device. The instantaneous trip rating is the value of current which leads to the minimum time delay for clearing a fault (nothing can really happen instantaneously). Conceptual Trip Curve Diagram Medium Voltage - At higher voltages, the OCP device ratings mentioned above take on a slightly different meaning. Medium Voltage Circuit Breaker, Photo Courtesy Eaton Medium Voltage Fuse, Photo Courtesy Eaton Per NEC 240.100, medium voltage breakers must be controlled with a overcurrent relay elements and current transformers. Medium voltage breakers usually have an ampere rating that corresponds to their maximum allowable current that can pass through continuously. However, the trip setting of the relay, the value of current which the breaker opens at, could be adjusted down much lower than the device rating. In fact, the entire trip curve of a medium voltage breaker can be customized, including the long-time and instantaneous trip settings. Medium voltage fuses also often behave differently than their low voltage counterparts. MV fuses possess an ampere rating like medium voltage breakers. The rating corresponds to the allowable continuous current the fuse can carry safely, but does not correspond to the trip rating of the fuse. Medium voltage fuses usually trip at values over 200% of the ampere rating. As a result, overload protection is almost never included in medium voltage fuses. Withstand - Overcurrent protection devices don’t operate instantaneously, as seen in the trip curves above. In the time between a fault occurring and the OCP device operating, damage can occur to equipment and conductors in the path of the fault. The amount of fault current that can safely pass through for a defined period of time is known as withstand. Electrical equipment withstand ratings are provided by the manufacturer, usually for a specified period of time per an industry standard. Conductor withstand can be calculated in accordance with Table 240.92(B) in the 2020 NEC. A good design will ensure that equipment and conductors don’t have their withstand ratings exceeded during a short circuit event. Withstand is different from ampacity. Ampacity describes how much current a conductor can carry indefinitely without exceeding a particular temperature. Withstand describes how much current a conductor can carry for a brief period of time without exceeding the insulation’s maximum temperature rating. Sizing Devices and Conductors - Coordinating the trip rating of overcurrent protection devices, the expected load current, and the ampacity of conductors is necessary to achieve installations that are safe and user-friendly. If the trip rating is too close to the expected load current, OCP devices will frequently activate unnecessarily, a problem known as “nuisance tripping”. If the trip rating exceeds the conductor ampacity under normal operation, then the design may not be safe. For low voltage systems, the standard relation required to ensure protection is: A' > O > e I Where: A’ is the ampacity of the conductors in Amperes (after all derating) O is the ampere rating of the overcurrent protection device e is a unitless multiplying factor I is the expected load current in Amperes The value of e varies throughout the code. For most loads that run continuously (3 hours or more at their expected value), e = 1.25. For most loads that do not run continuously, e = 1. For special equipment referenced by name in the NEC, the relevant article should be checked to determine the correct value of e. The NEC actually gives a little bit of leniency on the definition above. For OCP devices rated less than 800A, the ampere rating of the overcurrent device is actually allowed to be larger than the ampacity of the conductors by up to 1 standard size. If the OCP is rated greater than 800A, then the conductors need to follow the relation above strictly. On medium voltage systems, the relationship above is not applicable. Instead, conductors and equipment just have to be protected against short circuit and overload per engineering supervision. Conductors still need to have an ampacity greater than their load current to ensure that they won't overheat. However, the ampacity of the conductors can be less than the trip setting of the OCP device. The design conditions for medium voltage can be summarized as: A'> e I and d A' > O > e I Where d is a multiplier on the conductor ampacity as described in NEC 240.100.

Determining the ampacity of conductors is never easy. There are a lot of factors to consider, including derating and overcurrent protection. This article shows how to design a circuit for a practical medium voltage example. Example: A feeder circuit serves 100A of continuous load and 50A of noncontinuous load at 15kV. The circuit utilizes single conductor aluminum cable pulled in a triplexed configuration. Routing near the source and load termination points is via aboveground conduit in a 40 Celsius ambient. Routing in the middle of the circuit is via underground conduit in a 25 Celsius ambient. The conduit is rated for a contact temperature of 105 Celsius. Determine the required conductor size and overcurrent protection device rating. Solution: The first step is to identify the required load current for sizing when terminating, I'. This current is based on NEC Article 215's requirement to size to 125% of the continuous load and 100% of the noncontinuous load before adjustment factors. I' = 1.25 * 100 + 50 =175A Next, we have to determine the relevant ampacity table from NEC Article 311 (in the 2020 edition of the Code). Conductors are routed in aboveground conduit near their termination points, which means that 311.60(C)(74) is applicable. Before any adjustment factors, the 90 Celsius ampacity column requires a minimum of 2/0 AWG Aluminum. We must also evaluate the ampacity for conditions-of-use throughout the circuit. Since the conduit is rated for 105 Celsius, we will assume that industry-standard MV105 cable is used and we can begin our derating from the 105 Celsius ampacity value. Feeder circuits are required to carry 100% of the continuous and noncontinuous load after derating. In this case, that is 150A. Since medium voltage systems aren't required to be protected at their ampacity, no additional multiplication factors on the sizing current are required. When routed aboveground in a 40 Celsius ambient, there is no additional derating required. Referencing the 105 Celsius column, the minimum conductor size required per 311.60(C)(74) is 1 AWG Aluminum. 311.60(C)(78) is the ampacity table for underground conduit routing with (3) single insulated aluminum conductors. When routed in underground conduit in a 25 Celsius ambient, we must derate for ambient temperature. The applicable derating factor is .97 using the standard equation. A 1/0 AWG Aluminum conductor can carry 160A at 105 Celsius after derating. Comparing all of the conditions-of-use and the termination requirements, the terminations represent the worst-case scenario. 2/0 AWG Aluminum is required. The overcurrent protection device needs to be rated for a minimum of 175A per the continuous and noncontinuous load. If the device is rated for operating at 100% of it s ratings, this minimum becomes only 150A. The actual device size can be much larger, as long as engineering calculations show that withstand and overload are properly accounted for in the protection system.

At the highest level, we could break electrical circuits into 2 categories: branch circuits and feeder circuits. Branch circuits describe the conductors from the last overcurrent protection device to the final load/source. For example, the conductors from a 15A breaker in your house's main panel to a receptacle would be considered a branch circuit. The current flowing through a branch circuit is entirely decided by the a single piece of equipment, whether that is a receptacle, a solar module, a motor, or something else. Feeder circuits are the conductors upstream of branch circuits. A panel could have branch circuits supplying various loads: motors, heaters, lights, receptacles, etc. The conductors that supply the panel have to consider ALL of these loads operating together. So how do we size feeder circuits and branch circuits? Branch circuits are easy, and the sizing requirements for conductors and overcurrent protection devices are discussed in a variety of other articles on Breaker & Fuse. Feeder circuits are more interesting. The load on a feeder circuit for conductor sizing is based on the sum of the following: 125% of the continuous load (loads that run for 3 hours or more continuously) 100% of the noncontinuous load 125% of the largest individual motor load 100% of the remaining motor load The overcurrent protection is sized based on the sum of the following: 125% of the continuous load 100% of the noncontinuous load The largest overcurrent protection device rating required by any individual motor 100% of the remaining motor load Overload protection isn't required at the feeder level. Motors will be protected from overload at the branch circuit level. Why don't we size the feeder circuit to carry 125% of all motor loads, not just the largest? Motor overload protection is accomplished via a dedicated relay system and short circuit protection is via a device with a much higher long-time trip rating. The result is that the systems won't be prone to nuisance tripping like a typical continuous load. If several motors will be started simultaneously, additional consideration should be made for increased ampacity.

General - Motors are a common type of load used to run fans, pumps, and more. They convert electrical energy to mechanical energy, an extremely broad class of applications common in the industrial space. Motors are magnetic devices. When a voltage is applied across the terminals of a motor, current rushes in and creates a magnetic field. This magnetic field then interacts with other magnetic components of the motor to create motion. Below is an example of a typical motor circuit one-line diagram. Each component of the circuit below must be carefully chosen to ensure that a motor is adequately protected. Typical Motor Circuit Motor Operation - During initial energization of a motor, a large current known as inrush will flow through the system for a brief period of time. Inrush current is the maximum value to flow through the motor and can be over 20x higher than the typical operating load current. Motors are marked with an indicating letter that describes this behavior. Further details can be found in NEC Table 430.7(B) of the 2020 Code. This current will drop considerably once the motor is able to begin moving and create an opposing voltage from the alternating magnetic field inside. Motor load currents are much lower than the inrush value and are provided by manufacturers as part of the motor datasheet. The National Electrical Code has a number of tables at the end of Article 430 that provide sample full load current values. Testing of motors is done with reference to the mechanical load supplied. The full-load current refers to the current flowing through the motor when supplying its rated mechanical torque. This current is generally the maximum operating current expected. The no-load current refers to the current that flows when the motors is allowed to rotate but has no external mechanical load to support. The no-load current is usually lower than the full-load current. The locked rotor current is the current that flows then the motor is prevented from rotating. If the motor is not allowed to rotate, no opposing voltage is created and the current that flows will be very large. Motors need to be protected from this current, but conductors don't need to be sized to carry this value continuously. Overload Relay - Per the National Electrical Code (and good engineering design practice!) motors are required to be protected against overload. Overload protection can be provided via a standard device like a breaker or a fuse, but for typical molded case circuit breakers and fuses this usually doesn't work well. These devices are prone to tripping during startup of the motor since the inrush current is much larger than the operating current. A dedicated overload relay can be used to protect the motor instead. This overload relay isn't intended to open up the circuit during a high-current fault. The relay is only used to open up the circuit for currents slightly above standard operating values. The overload relay should be coordinated to open up based on manufacturer information. Limits to how high the overload relay can be set are provided in the NEC, but in no circumstance can it be higher than 125% of the full load current. Overload protection is only required on the branch circuits feeding motors, and not on upstream feeder circuits. Overcurrent Protection Device - Overload relays keep a circuit and motor protected against currents that exceed the motor's recommended operating conditions. Overcurrent devices protect against fault currents. Overcurrent devices should be sized to coordinate the trip curve against the available fault current and motor inrush. For smaller installations, trial-and-error approaches may be employed, as long as the settings do not exceed the limitations set by NEC Article 430, based on the motor type. For large facilities or motors with critical applications, it is imperative that a detailed analysis of trip curves be completed. The image below is an example of how trip curves of overcurrent devices should be coordinated with relevant motor parameters. Motor Trip Curve Overcurrent Coordination Example Disconnecting Means - Motors need to be able to be switched in and out of service. Per the 2020 NEC, low voltage motor disconnecting means must be capable of opening up the circuit while carrying the full load current of the motor. If a circuit breaker is used as the overcurrent protection and/or overload device, this may be suitable as a means of disconnecting. Medium voltage systems are not required to be able to break a motor circuit while running at full load. Putting It All Together - The image below is of an integrated motor starter. Devices like these are common, as they combine all of the elements listed above with more sophisticated controls and monitoring. Instead of needing separate fuses, switches, and relays to all be installed and coordinated, the designer can specify a single device. Where several motors are required to be served at a single voltage level, it is common to use a motor control center (MCC). MCCs are enclosures designed with large numbers of motors starters to facilitate motor starting. MCCs are commonly used in industrial and generation facilities, where motors are used all over the place. An Integrated Motor Starter Motor Conductors - The requirements for sizing conductors to motors are straightforward. Single motor circuits (motor branch circuits) are required to be sized for 125% of the full load ampere rating of the motor. Multi-motor circuits (Feeder circuits) are required to carry 125% of the largest motor full load ampere rating and 100% of the remaining motor full load ampere ratings. The NEC is full of additional exceptions that lead to reduced ampacity requirements in special cases. Why not 125% of all motor loads? Motors are equipped with overload protection at the branch circuit level, so conductors can be protected closer to their ampacity than with a standard thermal overcurrent element. 125% is still required for the largest motor to account for inrush currents leading to increased temperatures over steady-state operation. In other words, this ampacity requirement assumes that two motors will not be experiencing inrush conditions simultaneously.

What's the difference between an electrical engineer and an electrician? I can't tell you how many times I've heard that question. It's an important question though, especially for those in the electrical industry or for those planning their career. Both jobs are essential for society and continue to be in high demand. If you're looking for a short answer: electricians install electrical power systems and engineers design them.However, I would hardly consider that answer satisfactory.The distinction between electrical engineers and electricians can sometimes be subtle. Electrical engineers and electricians work together closely. The best electrical engineers will work with their electrical construction counterparts to ensure a design is buildable. Likewise, the best electricians will consult with their engineer when questions or unforeseen challenges arise. Do electrical engineers always have to do the design? No-In the residential world, master electricians have authority to determine wire size, overcurrent device ratings, etc. per the requirements of the National Electrical Code. In this capacity, electricians are doing both the design and installation. Outside of the residential space, it is common for jurisdictions to mandate that electrical engineers complete the design for electrical systems and sign off (stamp) the design. These larger projects often require detailed calculations and careful planning to complete, the specialty of an electrical engineer. Electricians are then responsible for executing the design by installing raceways, conductors, overcurrent protection devices, and more. An area of overlap can be in electrical commissioning, the end stages of installation where a system is checked out for functionality and safety. Experienced electrical engineers and electricians both work in commissioning. They may perform and interpret results of cable testing, such as insulation resistance testing, high potential tests, and more. Likewise, both electrical engineers and electricians can check out relays and control systems to make sure they work as expected. A big difference is education and training. To become an electrical engineer, you must graduate from an ABET-accredited institution with a bachelor's degree in an engineering field. You must then complete four years of work experience and take two exams before becoming a licensed Professional Engineer. Requirements to become an electrician vary by state. In Kansas, electricians start as an apprentice and must complete 2 years of work experience before they can sit for the journeyman exam. To become a master electrician, they must complete 2 years of experience as a journeyman and take yet another exam. Both career paths are excellent and offer continued growth throughout your career. Should you choose to enter the electrical industry, you really can't choose wrong.