Transformer tcc curve explained

A time current curve TCC plots the interrupting time of an overcurrent device based on a given current level. These curves are provided by the manufacturers of electrical overcurrent interrupting devices, such as fuses and circuit breakers.

transformer tcc curve explained

These curves are part of the product acceptance testing required by Underwriters Laboratories UL and other rating agencies. The shape of the curves is dictated by both the physical construction of the device as well as the settings selected in the case of adjustable circuit breakers.

The time current curves of a device are important for engineers to understand because they graphically show the response of the device to various levels overcurrent. The curves allow the power systems engineer to graphically represent the selective coordination of overcurrent devices in an electrical system.

Modern power system design software packages, such as EasyPower, SKM Power Tools, and Etap, contain graphical libraries of curves to allow the power system engineer the ability to plot, analyze, and print the curves with minimal effort compared to the previous methods used when coordinating a power system. Current is shown on the horizontal axis using a logarithmic scale and is plotted as amps X 10 X.

Time is shown on the vertical axis using a logarithmic scale and is plotted in seconds X10 X. The light blue curve is a switchgear feeder circuit breaker curve. The violet curve is the switchgear main circuit breaker curve. The red curve is the transformer primary fuse curve. The orange curve is the transformer damage curve. The green curves are the cable damage curves. Each one of these items will be explained.

The system represented by this curve is well coordinated and adequately protected from damage. It also has minimal arc flash hazard category ratings due to low instantaneous circuit breaker trip values. The one line diagram see Figure 2 and TCC plot show a typical hypothetical industrial power system.

There is a utility delivery point with power supplied at a medium voltage level in this case 4, Vwhich feeds the primary side of a 2. The V secondary side of the transformer feeds a piece of low voltage power switchgear utilizing draw out low voltage power circuit breakers for the main and feeder circuit breakers.

The TCC plot also displays the transformer and cable damage curves. Interpreting the damage curves is fairly straight forward. Operating conditions overcurrent protection must be kept to the left of the damage curve to guarantee no permanent damage is done to the transformer or cable in question.

Operating conditions that allow operation to the right of the damage curve subject the device in question to currents that can cause permanent irreversible damage, a shortened lifespan, and possible catastrophic failure.

Therefore, the overcurrent and circuit breaker coordination schemes must take this into account during the initial design phase. There are two transformer damage curves shown in orange on Figure 1 — one is dashed, the other is solid.

The solid damage curve is unbalanced and takes into account a de-rating factor for transformer winding type and fault type. The transformer inrush current is also plotted a single point on the TCC diagram. Again, as part of the initial design, the transformer inrush current must be to the left of the transformer primary fuse curve, otherwise the fuse will open when the transformer is energized.

Understanding time current curves: Part 3

There are three cable damage curves shown in green on Figure 1. There is one curve for each cable represented on the one line diagram. As part of the initial design, the overcurrent interrupting device must limit the fault current to left of the damage curve to prevent permanent damage.

The damage curves for the cables are dependent on size, insulation type and raceway configuration. This post was written by David Paul. David is a Principle Engineer at MAVERICK Technologiesa leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries.Continued from Part 2 …. Now that the basics of TCCs have been explained, a review of coordination is in order.

Our sample curves to coordinate will consist of an MCC with main A fuses, a 1,A feeder circuit breaker and the switchgear 3,A main circuit breaker.

transformer tcc curve explained

In the uncoordinated system there is overlap of the circuit breaker trip curves, and in some instances the main circuit breaker will trip before the feeder circuit breaker. The main fuse in the MCC is also uncoordinated. While the fuse is not required, it is included in this example because it is typical of an industrial installation. The purpose of the fuse is to provide current limiting to increase the short circuit withstand rating of the MCC bus.

For example purposes, it is assumed the MCC fuse is required and the cable feeder sizes cannot be changed. These assumptions make the example case here realistic as there are often constraints, such as this found in real world coordination problems in industrial facilities.

In the coordinated example the main and feeder circuit breakers are selectively coordinated and the main circuit breaker provides adequate protection for the power transformer. Coordination between the MCC main fuse and the feeder circuit breaker was also improved.

Coordination improvements to these circuit breakers included the following changes:. There is overlap of the MCC main fuse and the feeder circuit breaker time current curves for long term low level overloads. This overlap could be eliminated if a larger long time pickup setting was used in the feeder circuit breaker. Increasing this setting would then require the upsizing of the feeder cable to maintain conformance with the National Electric Code. Coordination involves tradeoffs and selections that require engineering experience and judgment to find the most optimal settings.

In many real world cases it is impossible to coordinate all possible cases. As such, engineering judgment is required to coordinate the most likely scenarios and create the most reliable system. Additionally, Arc Flash hazard category reductions generally result in diminished selective coordination.

Conversely, improved coordination may result in increased arc flash hazard categories in some cases. In the above example Arc flash hazard category ratings for both the uncoordinated and the coordinated cases were unchanged even with improved coordination. It is possible to achieve these optimized results through the use of engineered selections.

It is for these reasons that the selection of overcurrent device ratings and settings be left to power system engineers experienced in industrial power systems. This post was written by David Paul. David is a Principle Engineer at MAVERICK Technologiesa leading automation solutions provider offering industrial automation, strategic manufacturing, and enterprise integration services for the process industries.

MAVERICK delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, business process optimization and more. Get the latest updates on the Coronavirus impact on engineers. Click Here. Understanding time current curves: Part 3 The final installment of a three-part series about time current curves TCCs reviews the coordination of sample curves and the importance of coordination.

By David Paul February 4, Continued from Part 2 … Now that the basics of TCCs have been explained, a review of coordination is in order.Time-current curves are used to show the amount of time required for a circuit breaker to trip at a given overcurrent level.

Time-current curves are typically shown on a log-log plot.

transformer tcc curve explained

Figures along the horizontal axis of the curve represent the continuous current rating In for the circuit breaker, figures along the vertical axis represent time in seconds.

To determine how long a breaker will take to trip: find the current multiple of In at the bottom of the graph. Next, draw a vertical line to the point where it intersects the curve and then draw a horizontal line to the left side of the graph to find the trip time.

Most curves have an information box that will define which circuit breaker the curve applies to. This information box may also contain important notes from the manufacturer such as the allowable deviation from trip times. In thermal magnetic breakers, a thermal overload occurs when a bi-metal conductor inside the circuit breaker deflects after becoming heated by the load current, de-latching the operating mechanism and opening the contacts.

The larger the overload, the faster the bi-metalic strip will heat up and deflect to clear the overload. In electronic circuit breakers, the long-time function L simulates the effect of a thermal bi-metal element. Once picked up, the circuit breaker will trip after the time specified by the long-time delay adjustment has been achieved. The lower portion of the time-current curve displays the short circuit response of the circuit breaker. In electronic circuit breakers, the Instantaneous I function simulates the magnetic characteristic of a thermal-magnetic circuit breaker.

2011 wrx body kit

This is achieved through the microprocessor which takes samples from the AC current waveform many times a second to calculate the true RMS value of the load current. Instantaneous tripping occurs with no intentional time delay. Some electronic circuit breakers may be equipped with a Short-time function S which gives the circuit breaker a delay before tripping on a significant overcurrent.

This allows for selective coordination between protective devices to ensure that only the device nearest to the fault open, leaving other circuits unaffected see circuit breaker cooridnation below.

The I 2 t characteristic of the short time function determines the delay type. This is similar to the long time function except with a much faster delay. I 2 t OUT provides a constant delay, usually 0.

Fuses - Time/Current Curves

Circuit breakers equipped with zone interlocking on short delay with no restraining signal from a downstream device will have the minimum time band applied regardless of setting, this is sometimes referred to as the maximum unrestrained delay.

When the instantaneous function is disabled, a short-time delay override is used to instantaneously trip circuit breakers in the event of a significant short circuit. This is called the short-time withstand rating and is represented on the trip curve as an absolute ampere value.

Like the long-time function, the ground fault G element consists of a pickup and delay setting. When a phase-to-ground fault occurs, the sum of the phase currents are no longer be equal because the ground fault current returns through the ground bus.

In a 4-wire system a fourth CT is installed on the neutral bus to detect this imbalance. When a current imbalance occurs, the circuit breaker will pick up if the magnitude exceeds the ground fault pickup setting. If the breaker remains picked up for the time specified by the ground fault delay, the circuit breaker will trip. Ground fault protection is sometimes supplied with an I 2 t function which operates under the same principle as short-time delay. Ground fault protection requires the least energy to trip the circuit breaker, often times with trip values set well below the long time pickup setting.

Whirlpool energy smart water heater error e02

When testing the overload or short circuit function of a circuit breaker, the ground fault protection will need to be disabled or "moved out of the way" for other functions to operate. Use of the manufacturer's test kit or rewiring the neutral CT input is the preferred method of primary-injection testing on a low voltage circuit breaker with ground fault protection, otherwise two poles can be connected in series to provide balanced secondary currents to the trip unit.

Time-current curves are essential for the proper coordination of circuit breakers.A time current curve TCC plots the interrupting time of an overcurrent device based on a given current level. These curves are provided by the manufacturers of electrical overcurrent interrupting devices, such as fuses and circuit breakers. These curves are part of the product acceptance testing required by Underwriters Laboratories UL and other rating agencies. The shape of the curves is dictated by both the physical construction of the device as well as the settings selected in the case of adjustable circuit breakers.

The time current curves of a device are important for engineers to understand because they graphically show the response of the device to various levels overcurrent. The curves allow the power systems engineer to graphically represent the selective coordination of overcurrent devices in an electrical system.

Modern power system design software packages, such as EasyPower, SKM Power Tools, and Etap, contain graphical libraries of curves to allow the power system engineer the ability to plot, analyze, and print the curves with minimal effort compared to the previous methods used when coordinating a power system.

Current is shown on the horizontal axis using a logarithmic scale and is plotted as amps X 10 X. Time is shown on the vertical axis using a logarithmic scale and is plotted in seconds X10 X.

The light blue curve is a switchgear feeder circuit breaker curve.

Nad c388 bluos

The violet curve is the switchgear main circuit breaker curve. The red curve is the transformer primary fuse curve. The orange curve is the transformer damage curve. The green curves are the cable damage curves. Each one of these items will be explained. The system represented by this curve is well coordinated and adequately protected from damage.

It also has minimal arc flash hazard category ratings due to low instantaneous circuit breaker trip values. The one line diagram see Figure 2 and TCC plot show a typical hypothetical industrial power system.

There is a utility delivery point with power supplied at a medium voltage level in this case 4, Vwhich feeds the primary side of a 2.

The V secondary side of the transformer feeds a piece of low voltage power switchgear utilizing draw out low voltage power circuit breakers for the main and feeder circuit breakers. The TCC plot also displays the transformer and cable damage curves.

Interpreting the damage curves is fairly straight forward. Operating conditions overcurrent protection must be kept to the left of the damage curve to guarantee no permanent damage is done to the transformer or cable in question.

Operating conditions that allow the operation to the right of the damage curve subject the device in question to currents that can cause permanent irreversible damage, a shortened lifespan, and possibly catastrophic failure. Therefore, the overcurrent and circuit breaker coordination schemes must take this into account during the initial design phase. There are two transformer damage curves shown in orange in Figure 1 — one is dashed, the other is solid.

The solid damage curve is unbalanced and takes into account a de-rating factor for transformer winding type and fault type. The transformer inrush current is also plotted a single point on the TCC diagram. Again, as part of the initial design, the transformer inrush current must be to the left of the transformer primary fuse curve, otherwise, the fuse will open when the transformer is energized.

There are three cable damage curves shown in green in Figure 1. There is one curve for each cable represented on the one line diagram. As part of the initial design, the overcurrent interrupting device must limit the fault current to the left of the damage curve to prevent permanent damage.

The damage curves for the cables are dependent on size, insulation type, and raceway configuration. This post was written by David Paul.Cable damage curve must be above the maximum fault current at 0.

Set below the transformer damage curve.

Funny inappropriate sentences

Set at or below cable ampacity. Set below cable damage curve. Cable damage curve must be above the maximum fault current at the CB total clear curve. SKM disclaims any responsibility and liability resulting from the use and interpretation of this information. Introduction The proper selection and coordination of protective devices is mandated in article To fulfill this requirement an overcurrent coordination study is required. The electrical engineer is always responsible for this analysis.

It is an unfortunate fact of life that many times the engineer who specified and purchased the equipment will not set the devices. Therefore, compromises are inevitable. There are three fundamental objectives to overcurrent coordination that engineers should keep in mind while selecting and setting protective devices. Life safety requirements are met if protective devices are rated to carry and interrupt maximum available load currents, as well as, withstand and interrupt maximum available fault currents.

Life safety requirements are never compromised. Protection requirements are met if overcurrent devices are set above load operating levels and below equipment damage curves. Feeder and transformer damage curves are defined in applicable equipment standards. Motor and generator damage curves points are machine specific, and are normally provided in the vendor data submittal package. Based on system operating and equipment sizing practices equipment protection is not always possible.

Selectivity requirements are met if in response to a system fault or overload, the minimum area of the distribution system is removed from service. Again, based on system operating and equipment selection practices selectivity is not always possible.

The purpose of the phase overcurrent relay is to allow for full use of the transformer, and to protect the transformer and cable from overloads and faults. To accomplish this, the relay curve should be to the right of the transformer FLA rating and inrush point, and to the left of the transformer and cable damage curves and the cable amp rating. Suggested margins are listed below that have historically allowed for safe operation of the transformer and cable while reducing instances of nuisance trips.

Time Dial let-thru current 1. Set at or above low voltage main device. The fuse characteristics are plotted on a phase TCC along with the transformer and feeder damage curves.

Device Coordination Refresher - Part 2, TCC Curves

The purpose of the fuse is to allow for full use of the transformer, and to protect the transformer and cable from faults. To accomplish this, the fuse curve should be to the right of the transformer inrush point and to the left of the cable damage curve.

Circuit Breaker

Typically the fuse will cross the transformer damage curve. The secondary main device provides overcurrent protection for the circuit.There are at least six basic adjustable tripping settings functions you really should understand in order to fully understand how circuit breaker actually works. All these adjustable functions actually shape the time-current curve of a circuit breaker and allows proper tripping according to the network parameters and also the proper coordination between upstream and downstream devices.

Note that modern circuit breakers MCCB, ACB mostly have an electronic tripping unit which is much more advanced comparing to these explained here, but the basics are the same, very same. Continuous current [Amps]. Long-time delay causes the breaker to wait a certain amount of time to allow temporary inrush currentssuch as those encountered when starting a motor, to pass without tripping.

The adjustment is from 2. As shown below, the long-time delay effects the position of an I 2 T slope. Short-time pickup is used for selective tripping. Short-time pickup is adjustable from 1. Short-time delay, used in conjunction with short-time pickup, controls the time involved in postponing a short-time pickup trip. Just a quick question — what exactly does The adjustment is from 2. This information is very complete and helps me to have a knowledge of how to adjust parameters correctly.

The information is given is very useful for us. Thank you.

transformer tcc curve explained

Please also post similar technical information detail. Truly appreciate your time spending to share knowledge. I have problem to start 3. As per Mechanical documnet the Motor Alignment everything looks ok. Please advice …. T Breaker tripped on earth fault what was the reasons?

Great information. I am very thankful to you if you send me the all breaker trip setting level as per different ampare load for data center to fallow the IEC.

I tested an electronic insulated case breaker the other day and the current setting was adjustable from. That your answer : To create PDF out of any technical article. Thank you Edvard, I have been looking for this info for long. I shall inquire about this in detail soon.Log In. Thank you for helping keep Eng-Tips Forums free from inappropriate posts. The Eng-Tips staff will check this out and take appropriate action.

Click Here to join Eng-Tips and talk with other members! Already a Member? Join your peers on the Internet's largest technical engineering professional community. It's easy to join and it's free. Register now while it's still free! Already a member? Close this window and log in. Are you an Engineering professional? Join Eng-Tips Forums! Join Us! By joining you are opting in to receive e-mail.

Promoting, selling, recruiting, coursework and thesis posting is forbidden. Students Click Here.

English level test pdf

Related Projects. When looking at MV transformer protection I curious what is the correct method for determining weather or not the transformer damage curve is protected. Must we look at the transformer damage curve on a TCC in oder to determine if the transformer is adequately protected on the primary? If so must the damage curve lie completely above the primary fuse on the TCC?

When we are looking at the damage curve are we only looking at it from a fault perspective or is it used for an overload perspective as well. For example from an overload perspective if the damage curve falls below the primary fuse on the TCC but is still above the secondary protective device curve is the transformer considered adquately protected from an overload standpoint?

In this case how would we confirm weather or not it was protected from primary faults or through faults? The primary fuse must be below the transformemr damage curve to protect the transformer on through faults.

If there is a fault between the transformer terminals and the low voltage breaker, the primary fuse msut clear it before the transformer is damaged.


Thoughts to “Transformer tcc curve explained

Leave a Reply

Your email address will not be published. Required fields are marked *