Most design engineers who specify circuit breakers do not understand the basics of circuit protection, according to survey data from E-T-A Circuit Breakers. In a nationwide quiz of 882 design engineers, only 10% of respondents were able to correctly answer five simple questions about specifying circuit protection.
The poor results indicate that design engineers are failing to properly protect their designs, despite the fact that engineers think specifying a circuit breaker is easy.
E-T-A sent the multiple-choice quiz in April 2002 to 5,972 electrical and design engineers who specify circuit protection. Less than half of the 882 anonymous respondents correctly answered three out of the five questions, and 6% of respondents got no answers correct at all. In addition, the results suggest that four out of 10 engineers are specifying the wrong type of circuit breaker for their applications.
"Anyone who sits at the applications support desk of a circuit breaker manufacturer quickly learns that most engineers don''t understand how to properly specify circuit breakers," said Tony Bright, president of E-T-A Circuit Breakers. "There are many misconceptions about circuit protection that have persisted for decades," he said.
Of the concerns raised by the quiz, Bright said the most alarming is that 37% of the respondents are sizing thermal circuit-breaker protection at 200% of the load, leaving equipment underprotected against damaging overcurrents.
The most popular wrong answer in the quiz centered on thermal circuit breakers. Nearly half of respondents thought that a typical thermal circuit breaker tripped at 115% of its rating after one hour, but the correct answer is 140% of its rating. This statistic suggests that engineers are specifying circuit breakers with a higher-than-ideal rating to avoid nuisance tripping, which may expose circuits to harmful current peaks caused by inrush, transient currents, and prolonged low-level overloads.
A slim majority of respondents correctly answered questions concerning the different types of circuit breaker technology. In the quiz, 60% answered that thermal circuit breakers are the type most resistant to nuisance tripping and 58% answered that magnetic style circuit breakers are the type most susceptible to vibration.
Respondents split evenly on the question of when to use quick-connect terminals. Only half of the respondents knew that quick-connect terminals can be used on circuit breakers rated up to 25A.
Detailed Quiz Results
In the E-T-A Circuit Breaker IQ Quiz, respondents had to choose the best answer from among three or four choices. The five quiz questions and the results for each choice are shown below.
1. A typical thermal circuit breaker for equipment will trip within one hour at ____ % of its rated current?
A. 140% - 269 responses (30%)
B. 115% - 428 responses (48%)
C. 85% - 183 responses (20%)
Correct Answer: 140%. Thermal circuit breakers are tolerant of start-up surges and transients.
2. The current rating of a thermal circuit breaker at room temperature should equal ____?
A. 100% of the load - 405 responses (46%)
B. 200% of the load - 330 responses (37%)
C. 50% of the load - 145 responses (16%)
Correct Answer: 100% of the load. The circuit breaker rating may vary with ambient temperature.
3. Which type of circuit breaker performs best in high vibration applications?
A. Thermal - 535 responses (60%)
B. Magnetic - 118 responses (13%)
C. Hydraulic-Magnetic - 223 responses (25%)
Correct Answer: Thermal. Because of their latching mechanism, thermal circuit breakers are very tolerant of vibration.
4. Which type of circuit breaker is most susceptible to nuisance tripping?
A. Thermal-Magnetic - 50 responses (5%)
B. Thermal - 247 responses (28%)
C. Magnetic - 509 responses (58%)
D. Hydraulic-Magnetic - 66 responses (7%)
Correct Answer: Magnetic. Magnetic circuit breakers with hydraulic delays are less susceptible to nuisance tripping.
5. Quick Connect terminals should be used with circuit breakers up to ____ amp rating.
A. 12A - 378 responses (43%)
B. 25A - 447 responses (51%)
C. 200A - 54 responses (6%)
Correct Answer: 25A. Use screw terminal connections above 25A.
* Responses may not total 100% because of blank answers and rounding.
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