1.1 Introduction
In typical industrial applications, the machines and equipment involved require high electrical ratings, like high voltages and high currents. Different machineries have varying voltage and current ratings. These ratings should be followed in order that the equipment operates properly whenever needed. However, undesirable conditions, like voltages and currents above the prescribed ratings (usually called as faults), are not totally unavoidable. In these conditions, damages on the machinery might occur which can cause production issues and problems, and may even be hazardous to humans. These conditions can be avoided by integrating protective circuitry within the electrical systems.
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Two of the more common technologies for fault protection on electrical systems are the fuse, and the circuit breaker. The main principle of these protective components is that whenever currents above their ratings occur they become open circuits in order to disconnect the system from the unwanted currents. Therefore, the harmful currents could not cause any damage to the delicate equipment within the system.
This experiment explores the operating levels of two overcurrent protection technologies: the fuse, and the circuit breaker. They are tested for different current levels, and the time it takes for them to ‘fuse’ or turn into an open circuit. The measured values will be tabulated appropriately for comparison among the variants and between the two types of protective devices.
1.2 Aim
1.3 Objectives
This experiment aims to quantify the fusing time of the fuse and the 3 circuit breakers at different levels of current. The outputs of the experiment are the following:
1. Table of current levels versus fusing times of the 3 circuit breakers.
2. Table of current values versus fusing time of the fuse.
3. Graphs of current versus fusing time of the breakers and the fuse.
The results are evaluated with regards to the operating properties of the protective device. Specifically, the fusing time and the range of current values are the two properties upon which the performances of the protective devices are evaluated. Ideally, it is expected that the relationship between the time and fault current for a device to trip is inverse. This relationship will be observed in the experimental results.
2.1 Theory: Fuse
A fuse is an electrical device that is basically a wire that melts or disconnects when subjected to a large amount of current . An image of fuses of different sizes is shown in the following figure:
Figure 1. Fuses of different sizes
The fuse is usually connected in series from the power source to the system that is being supplied with power. With this circuit configuration, the fuse isolates the system in the occurrence of overcurrent so that the delicate components within the system would not be damaged by high levels of current. The following figure shows the circuit diagram of a typical fuse set-up (fuse is s-shaped component):
Figure 2. A fuse connecting the power source to the load
During normal current conditions, the fuse acts as a short circuit that simply connects the power source to the load. When a fault current occurs, the fuse melts due to thermal energy of the fault current; then, the fuse effectively turns into an open circuit, isolating the load from the power source. Because of this, the load will be safe from the harmful effects of overcurrent. However, the load will temporarily stop its operation until the fuse is replaced by a new one.
Typically, the rating of the fuse should be compatible with the rating of the system it would protect. The goal is that at normal rating, the fuse would function without overheating, and at currents above this rating, the fuse rapidly melts such that no fault current could reach the system .
The following are some important terminologies regarding fuses :
Current rating of fuse element (CR) - the current that the fuse element can normally carry without overheating or melting. This current rating is the critical level because at levels greater than this, the fuse would start to heat up and this could possibly escalate to faulty current levels.
Rated minimum fusing current (RMFC) – the minimum current at which the fuse element melts, and thus disconnects the circuit protected by it. At this current level, the fuse is converted into an open circuit.
Fusing factor (FF) - the ratio of minimum fusing current to the current rating of the fuse element. Or simply,
FF=RMFCCR
Since RMFC is always greater than CR, the fusing factor is always greater than 1. Smaller fusing factor generally translates to greater probability of overheating and melting. And with greater probability, the fusing time also shortens because not much thermal energy would be needed to ‘fuse’.
The following graph shows current values in the fuse when a faulty current is detected:
Figure 3. Current levels in the fuse on the occurrence of a fault
Prospective fault current – the maximum current obtained when the fuse is replaced by an ordinary conductor of negligible resistance. The fault current usually would have a large first loop, but the thermal energy gathered in the fuse is already enough to melt the fuse element even before the peak of the loop. The RMS value of the first loop of fault current is known as the prospective fault current.
Cut-off current – the maximum value of fault current actually reached before the fuse melts. The current at point ‘a’ in figure 3 is actually the cut-off current.
Pre-arcing time – the time it takes from the beginning of the occurrence of the fault current up to the moment the cut-off current is reached. This value is typically small.
Arcing time – the time between cut-off and the instant the arc ends. At this point, the fuse can only be replaced with a new one.
Total operating time – the sum of pre-arcing time and arcing time. The total operating time for a fuse is significantly shorter than that of a circuit breaker.
I2t rating – a measure of the amount of current being limited by the fuse at a particular period of time.
Ideally, the fuse should disconnect at the moment the current reaches harmful levels. In reality, it would take some time for the fuse to heat up and melt before the connection breaks. Within this period of time, the fault current is still being fed to the load. Therefore, the goal is to minimize this fusing time such that minimal amounts of harmful current would reach the load. In this experiment, it should be observed that the fusing current is inversely proportional to the fusing time.
The following figure shows the fusing current versus fusing time graphs of different-rated fuses:
Figure 4. Fusing Current vs Fusing time for Different fuses
The graphs show an inverse relationship between fault currents and fusing time. Also, as expected, the rated values of the fuses are directly proportional to the current levels needed for the fuse to melt.
2.2 Theory: Circuit Breaker
A circuit breaker is a switch that can work manually or automatically which serves as a protective device against overcurrent or overvoltage . An image of a typical circuit breaker is shown in the following figure:
Figure 5. Circuit Breaker
The basic operation is similar to the fuse: short circuit during normal operation, open circuit in the event of faulty currents. The main difference is that the circuit breaker is a switch: it turns on during operating conditions, and it turns off during undesirable conditions. In contrast, a fuse is destroyed in the event of an overcurrent. Thus, it needs to be replaced with a new fuse in order that the system could function back to normal again.
The circuit breaker itself is simply a switch (manual action). To be able to detect fault currents, additional circuitry should be added in the overcurrent protection system (a relay system, converting the circuit breaker into automatic action).
The choice between the fuse and the circuit breaker are usually based on the following criteria :
Economic value – a fuse is much cheaper than an equivalent circuit breaker. The economic value of the two options only even out in bad cases where several fuses need to be replaced very frequently in a short period of time. Nevertheless, the initial purchase remains favorable for the fuse.
Size – fuses are smaller than circuit breakers. Circuit breakers are very applicable in robust applications. Fuses are usually more desirable in applications where several system components need individual overcurrent protection.
Maintenance – in the case of overcurrent, fuses need replacement; circuit breakers just have to be switched back ON to operate again. For applications that could not allow additional time on replacement of devices, circuit breakers are more desirable. Circuit breakers can be manual or automatic in their activation. For automatic conditions, some additional equipment might be needed.
Reaction time – fuses are normally faster than circuit breakers. The conductor inside a fuse has relatively low melting point, making it faster to react at fault current situations. Circuit breaker reaction time may depend on several factors including the mechanism involved in switching, the sensitivity of the device with respect to fault currents, etc.
Current limiting – fuses are relatively smaller than circuit breakers. Hence, fuses have an inherent current limiting effect as compared to relatively larger breaker contacts that allow more current.
Therefore, design trade-offs between the two types of protection should be decided by the engineer before finalization of the choice of device.
3 Lab Report
This section discusses the procedure implemented in the experiment.
3.1 Reminders
This is a high current laboratory experiment. The following reminders are observed:
- The Hi-Current source is capable of delivering a large current: caution is advised.
- It is suggested that the conductor be wound thrice through the current probe to ensure it registers. Current values will therefore have to be divided by three.
- Ensure that the switches in the Circuit Breaker Rig are set so that
- Only one circuit breaker is operating
- The rig is not short circuited
3.2 Starting Point
It is suggested that the Circuit Breaker Rig be connected to the Hi-Current Source, with the current probe and Fluke meter connected appropriately.
- Measure the time taken for each circuit breaker to operate when current is at (say) 100%, 90%, 80% etc.
The following figure displays the graphs of currents versus fusing time of the circuit breakers and the fuse:
Figure 6. Current Versus Fusing Time of the Protective Devices
The plots show generally that as the current decreases, the fusing time increases. This trend is similar to the graph presented beforehand.
In the case of the circuit breakers, the trend of the tripping time is more or less increasing with respect to decreasing current levels. However, this trend is observed to be inconsistent at the 90% level, wherein the tripping time is higher than that of the 100% and 80% levels. This inconsistency is also observed at the 60% level for breaker 1 and at the 70% level for breaker 2, wherein the tripping time become lower with respect to decreasing current levels. Circuit breaker 3 has the lowest tripping times compared to the other two. Circuit breaker 1 is observed to be the slowest of the 3 breakers in terms of tripping time. Therefore, at the current levels used in the experiment, circuit breaker 3 achieved the best performance. The next best option is circuit breaker 2, while the worst case is circuit breaker 1. These results show that these 3 circuit breakers have different ratings. Moreover, inconsistencies were present in the operating levels of the circuit breakers.
In the case of the fuse, the fusing time was observed to increase as the current level decreases. Therefore, the inverse proportion which was proposed beforehand was achieved by the fuse. At 40%, there is no fusing time measured probably because the current level is below the rated minimum fusing current of the fuse. This means that at this current level (and below), the fuse will operate normally without overheating or melting. Hence, the fuse has an effectively wider range of current values at which it could operate as compared to the circuit breakers in the experiment.
The reaction time of the fuse was observed to be a lot faster than any of the breakers in the experiment. This is more desirable as mentioned beforehand. When it comes to the range of operating values, the fuse is observed to be more flexible because at current levels 40% and below, the fuse function normally without melting while the circuit breaker will eventually trip at a later time.
5 Conclusions
The fuse and the circuit breaker as a means of overcurrent protection were tested in this experiment. The fuse was found to operate at a faster reaction time and at a more flexible range of current values. The fuse exhibited inverse proportionality between the fusing current and the fusing time. The circuit breaker yielded different tripping times, and slightly inconsistent trend of current levels versus tripping times.
The results of the experiment show that the fuse operates better than the circuit breaker in terms of reaction time and range of fault current values. However, other properties should be considered, like economic value and size of device, before choosing which one is more appropriate to be used in an application. Nevertheless, both devices showed effective overcurrent protection at occurrences of fault currents.
The results and methods used in this experiment can possibly be used as a deciding tool on what type of device should be used as an overcurrent protection in a given application. The criteria of fusing time and range of current values can be accounted for. By testing the fuse or the circuit breaker, the fusing current versus fusing time table can be generated. With this table, the protective device can be evaluated if it is appropriate for the given application in terms of reaction time and range of current values. Choosing the right reaction time would ensure the minimal amount of faulty current to be fed to the protected system. Choosing the right range of current values with respect to the expected current values in the application would ensure the stability of the system in terms of current level.
6 References
Electrical Circuit Breaker | Operation and Types of Circuit Breaker. (2014). Retrieved October 7, 2014, from Electrical4u: http://www.electrical4u.com/electrical-circuit-breaker-operation-and-types-of-circuit-breaker/
Fuses. (2014). Retrieved October 2014, 7, from All About Circuits: http://www.allaboutcircuits.com/vol_1/chpt_12/4.html
Let-Thru Current and I2t. (2014). Retrieved October 7, 2014, from Mersen: http://ep-us.mersen.com/fileadmin/catalog/Literature/Application-Guidelines/ADV-P-Application-Information-Let-Thru-Current-and-I2t.pdf
Sahib, O. (2014). Power System Protection: Fuses. Retrieved October 7, 2014, from http://www.engineering.uodiyala.edu.iq/uploads/depts/power/teacher%20lectures/protection%204%20stage/Fuses.pdf
