Protecting Against Slow Overcurrent

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In all these examples the problem is caused by the increase in normal operating current to the point where damage can occur. Circuit breakers can sense either the heat or magnetic forces as they increase beyond the maximum safe level, and fuses and overloads can sense increased heat that the overload creates. Separation of these devices will cause a set of contacts or a conducting element to open anytime the current increases above the safe level. This presents a problem with some loads, such as motors that have a very large inrush current when they start.

Figure 1 shows a graph of the inrush current caused by a 208 volt, 5hp motor starting. Notice that the motor will draw 16.5 A at full load. This motor will draw up to 99 A when it starts, which is six times the amount of full-load current. This presents a problem in protecting against overcurrents because a circuit breaker or fuse that is sized to protect the motor during full-load current (16.5 A) would trip when the motor is started, and if they are sized to allow the motor to start (99 A), they won't provide adequate protection when the motor is running at full-load current.


Above: Fig. 1 A graph of inrush current for a 208 volt, 5hp motor.

Several solutions to this problem are available. One of them is a motor starter with heaters and overloads, and another is inverse-time circuit breakers. Each of these devices provides several minutes of time delay before they trip and take the motor off line. The theory of their operation involves allowing small overcurrents to exist for up to 4 or 5 minutes, and allowing larger overcurrents to exist for less than 10 seconds. These times are based on the amount of time a specific overload condition can exist before a motor begins to sustain damage. The devices must be sized properly to provide adequate protection, and may be adjusted slightly once they are installed. The only problem that remains with the inverse-time circuit breaker and overloads is that they can't sense a short circuit and open the circuit fast enough to provide interruption capacity.

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