It's only a circuit breaker. Yet there is enough complexity and confusion when it comes to specifying circuit protection that many engineers are designing equipment with too little or too much protection. Under protected circuits leave equipment vulnerable to damaging electrical surges. Over protected circuits add cost and can lead to nuisance tripping....

1. 12 most common mistakes when
specifying circuit protection
The Dirty Dozen

2. 11Specifying the wrong
circuit breaker type

3. Circuit Breaker
Type
• Thermal
• Thermal-Magnetic
• Magnetic
• High Performance

4. 22Specifying to avoid
nuisance tripping

5. • Engineers oversize fuses
• Rating is the maximum current
consistently maintained
• In-rush currents: delay should match
duration of in-rush currents
Avoid
Nuisance Tripping

6. 33Failure to provide
spacing in design

7. Provide
Spacing
• 1 mm space: non-temperature-
compensated thermal circuit
breakers
• Effects sensitivity of the bimetal
trip mechanism
• Derate to 80% of their normal
amperage rating if need to touch

8. 44Specifying the degree
of protection

9. Degree of
Protection
IEC 529 / EN 60529
• Drip-proof
• Ignition protection
• Water splash protection
• Dust-proof

10. 55Selecting the Correct
Actuation

11. Selecting
Actuation
Manual release Push-to-reset
Push-PushPush-Pull
Rocker Toggle
• Select by location
• Need for illumination
• Need for human operator safety
• Convenience
• Consequences of accidental
engagement.

12. 66Failing to use circuit
breakers as on/off
switches

13. • Reduce components
• Less consumption of space
• Reduce wiring
• Increased protection over
ordinary switches
• Reduce installation time
On/Off
Switches

14. 77Specifying the wrong
type of terminal

15. Screw
Stud
Plug-in
Blade
PC Board
• Can simplify installation and
replacement
• Screw terminals more secure;
suited for high current and
high-vibration
• Quick connect used up to 25A
Quick Connect.110
Quick Connect .250 Combination
Selecting
Terminals

16. 88Specifying a fuse
instead of a circuit
breaker

17. Fuses Vs.
Circuit Breakers
Circuit breakers
• Quickly reset
• Minimum downtime
• Stable performance over time
• More options
• Combined with other components
• Precisely matched to loads
• 100% tested
Fuses
• No assurance that replacement
will be properly rated
• Fuses age
• Trip characteristics change
• Lead to nuisance tripping
• Increases downtime
• Cannot be tested

18. 99Specifying for a high
vibration environment

19. • Thermal circuit breakers highly
tolerant of shock and vibration
because of mechanical latch
• Magnetic vulnerable to vibration
High
Vibration

20. 1010Failure to derate

21. • Rate for 100% of the load
• Change in temperature affects load
Failure
To Derate

22. 1111Derating when it is
not necessary

23. Derating When
Not Needed
• Thermal circuit breakers sensitive to
ambient temperature
• Trips at higher amperage in a cold
environment
• Trips at lower amperage in a hot
environment
Thermal circuit breakers track the
performance needs of the system

24. 1212Over specifying
interrupting capacity

25. Interrupting
Capacity
Icn (Normal Interrupting Capacity)
Highest current breaker
interrupts repeatably (3x minimum,
per IEC934 / EN60934 PC2)
UL1077
(or IEC934 / EN60934 PC1)
maximum current a breaker
safely interrupts at least one
time without causing a fire
hazard.

26. 12 most common mistakes when
specifying circuit protection
The Dirty Dozen