A few years back a fatality in Western Australia garnered a fair bit of attention.
A person was electrocuted when working on a 415 volt movable hydraulic restricting machine that was plugged into a 415 volt socket outlet.
The active and earth conductors of the flexible supply cable were transposed at the plug-top connections and the metal frame of the machine became live.
This is not a rare occurrence. Just in WA alone, over a thousand cases of shock are reported every year.
Circuit protection devices are also required to protect wiring from overload currents, while allowing the maximum demand of the protected circuit.
Electrical hazards are often hidden and can be difficult to identify, such as a small hole in an extension lead or a power board damaged internally.
For decades now, E-T-A has been providing solutions for electrical overcurrent protection.
Drawing upon its experience over the past 60 years, E-T-A has drawn up “The 12 Most Common Mistakes of Specifying Circuit Protection for Equipment” to help make work environments safer.
1. Specifying the wrong circuit breaker type for the application
There are four choices of circuit breaker technology: thermal, magnetic, thermal-magnetic and high performance.
Magnetic circuit breakers are appropriate for printed circuit board applications and impulse disconnection in control applications. Thermal circuit breakers are appropriate for machinery or vehicles where high current in-rushes accompany the start of electric motors, transformers and solenoids.
Thermal-magnetic circuit breakers have a characteristic two-step trip profile that provides fast short-circuit protection of expensive electrical systems while minimising the risk of disrupted system operation.
High performance circuit breakers provide high interrupting capacity and excellent environmental specifications.
2. Specifying too high a rating in an effort to avoid nuisance tripping caused by in-rush or transient currents
Unlike a fuse rating, a circuit breaker rating tells you the maximum current that the circuit breaker will consistently maintain in ambient room temperature.
Nuisance tripping is often caused by in-rush currents associated with certain electrical components - primarily motors, transformers, solenoids and big capacitors. In such cases, the designer needs to specify a circuit breaker that has a delay.
3. Failure to provide spacing in design
A mere 1 mm spacing between breakers is all that is required. Without this tiny thermal gap, the circuit breakers can heat up and increase the sensitivity of the bimetal trip mechanism.
4. Over specifying or ambiguously specifying the degree of protection
When specifying, use the established standards as a measure, such as EN 60529/IEC 529, which defines the degree of protection of Electrical Equipment.
5. Selecting the correct actuation
Circuit breakers are reset manually by means of an actuator. The type of actuator will be determined by the location of the circuit breaker, the need for illumination, the need for human operator safety or convenience, and the consequences of accidental engagement.
6. Failure to consider using circuit breakers as on/off switches
The advantages of a combination device are a reduction in components, less consumption of panel space, reduced wiring and increased protection over ordinary switches.
7. Specifying the wrong type of terminal
Circuit breakers with plug-in style quick connect terminals simplify installation and replacement. Screw terminal connections are more secure and suited for high current and high-vibration environments. Quick connect terminals may be used for circuit breakers rated up to 25A.
8. Specifying a fuse when a circuit breaker would be better
Circuit breakers can be quickly reset, enabling the circuit to be restored with a minimum of downtime. Circuit breaker performance is relatively stable over time, but as fuses age, their trip characteristics change.
Circuit breakers have a variety of types and trip profiles, and therefore can be more precisely matched to loads and environment.
9. Specifying the wrong type of circuit breaker for a high vibration environment
Magnetic circuit breakers are particularly vulnerable to vibration; in contrast, a typical thermal circuit breaker is highly tolerant of shock and vibration.
If a magnetic circuit breaker is the best type for the application, its vibration resistance can be improved by using a push-pull style actuator. This type of actuator has a latching design.
10. Failure to derate
Some applications require a circuit breaker to operate continuously in either high or low temperatures. In these cases, follow the manufacturer's guidelines for derating.
11. Derating when it is not necessary
The performance of a thermal circuit breaker is sensitive to fluctuations in ambient temperature. It will trip at higher amperage in a cold environment, and it will trip at a lower amperage in a hot environment.
12. Over specifying interrupting capacity
Interrupting capacity is the maximum amperage a circuit breaker can safely interrupt. Circuit breaker manufacturers publish this specification along with the number of times the circuit breaker will perform this feat.
For example, E-T-A publishes two types of interrupting capacity specifications. One is called Icn, or normal Interrupting Capacity. Icn is the highest current the circuit breaker can interrupt (three times minimum, per IEC934/ EN60934 PC2). Icn gives a rough idea of circuit breaker quality. The other specification is UL 1077 interrupting capacity.
UL1077 interrupting capacity is the maximum current a circuit breaker can safely interrupt at least one time without causing a fire hazard.
To comply with various standards, engineers must specify circuit breakers with adequate interrupting capacity.
If you keep these tips in mind, it is easy to specify the right measure of circuit protection at the lowest cost. Start the selection process by working to truly understand your load. Then decide which type of circuit breaker is suited to your application. Avoid the common mistakes, and you will be rewarded with a reliable design.