The right choice of protective device ensures safe operation of electrical systems and high system availability.
Circuit breakers protect current distribution cables in buildings or systems. It is only in the event of a short circuit in a termination device that they switch off to protect the power supply line in the event of overload. The circuit breakers have a high switching capacity of 6 kA upwards.
As the last protection stage for termination devices, thermomagnetic and electronic circuit breakers offer the most effective short circuit and overload protection. If individual loads or small function groups are protected individually, then unaffected system parts can continue to work in the event of an error, insofar as the overall process allows.
If a new circuit is installed, appropriate protection for the termination device provided must be sought immediately. During installation, cable lengths and conductor cross sections must also be observed. The cables must be designed for the expected operating current, but also to be able to deal with any potential overload and short-circuit currents. Within the scope of graded protection of system areas, the selectivity between the individual fuses or protective devices should be retained. This ensures higher system availability as only the faulty circuit is switched off.
It is advisable to make device circuit breakers easily accessible when installed in control cabinets, so that they can be switched on again easily and without problems after tripping. In addition, a control cabinet should not be overpopulated, in order to prevent the power supply unit from overload. Furthermore, a sufficient air flow and cooling process should be ensured. In this way, incorrect tripping can be prevented.
The demands placed on optimum device protection vary depending on the area of application. Device circuit breakers therefore work with a wide range of technologies: electronic, thermal, and thermomagnetic. The differences are in the tripping technology and shutdown behavior used. Characteristic curves clearly illustrate the switch-off characteristics of the various device circuit breakers.
Device circuit breakers are selected based on the nominal voltage, nominal current, and, if required, the starting current of a termination device. The expected error situation (short circuit or overload) then determines the appropriate shutdown behavior.
|Tripping time in the case of overload||Tripping time in the case of short circuit||Your application is optimally protected in the event of|
|Thermal circuit breakers||Suitable||Unsuitable||
|Thermomagnetic circuit breakers||Suitable||Ideal||
|Electronic circuit breakers||Ideal||Ideal||
Tripping characteristics help you in finding the right protective device depending on the application. They indicate the operating range of the protective devices limiting the current in a current/time characteristic curve.
The different types of protective device feature operating ranges varying in size. Conventional fuses with fuse wires are ranked among the oldest safety equipment.
In essence, the form and thickness of the fuse wire determine the nominal current, for which the fuse is used. Modern miniature circuit breakers and device circuit breakers, which we are currently discussing, can be developed with high precision for a particular tripping characteristic.
The various circuit breakers respond in different ways to external temperature influences. For device circuit breakers with thermal tripping in particular, the ambient temperature must be observed.
A temperature factor is used to determine the correct switch-off time. This is multiplied by the relevant current/time characteristic curve values. This results in the final value.
The table illustrates typical values. An ambient temperature of 23°C has been used as the default value. The factor is 1 in this case. If the ambient temperature is lower, tripping is delayed. The factor is then below 1. Higher temperatures ensure faster tripping. The factor is then above 1.
|Circuit breaker versions||-20°C||-10°C||0°C||+23°C||+40°C||+60°C|
thermomagnetic circuit breaker
Thermal miniature circuit breaker
Thermal circuit breaker
The internal resistance of a protective device is either specified as a resistance value in ohms or as a voltage drop in millivolts.
Ideally, there should be low internal resistance, thereby causing the power dissipation in the circuit breaker to be reduced. It is therefore ideally suited to circuits with low nominal voltage.
The following tables illustrate the typical voltage drop values and the internal resistance of various device circuit breakers.
|Typical voltage drop||1 A||2 A||3 A||4 A||5 A||...|
|Electronic circuit breakers||140 mV||100 mV||120 mV||100 mV||130 mV|
|Thermal miniature circuit breaker||<150 mV||<150 mV|
|Typical internal resistances||0.1 A||0.5 A||1 A||2 A||3 A||4 A||5 A||8 A|
|5 Ω||1.1 Ω||0.3 Ω||0.14 Ω||0.09 Ω||0.06 Ω||≤ 0.02 Ω|
|81 Ω||3.4 Ω||0.9 Ω||0.25 Ω||0.11 Ω||0.07 Ω||≤ 0.05 Ω|
When device circuit breakers are mounted in rows with simultaneous current load, a mutual thermal effect occurs. This corresponds to an increased ambient temperature and results in fast switch-off of the circuit breakers.
The circuit breakers can be correctively dimensioned in such a way that they are only loaded with 80 percent of the circuit breaker nominal current under normal operating conditions. This would compensate for temperature influences and optimize the shutdown behavior.
By the planning stage, the requirements for a power supply unit in terms of capacity for future extensions have normally already been defined. This is because the demands placed on a power supply unit are constantly on the rise. The compact design and increasing performance capabilities, for instance, are particularly important to 24 V DC power supply units in industrial applications.
Power supply units must match the power requirement of the termination devices to be connected. Furthermore, no more than 80 percent of the nominal current should be planned for, in order to guarantee a reliable short-circuit current in the event of an error. If the selected power supply unit is too small or the connection value is too high, then this can result in undervoltage. This causes entire system components to fail and the manufacturing process to be interrupted.
A number of power supplies feature Selective Fuse Breaking Technology, also known as SFB. This provides six times the nominal current for a few milliseconds. This current reserve enables protective devices to reliably trip in the event of an error. Together with thermomagnetic device circuit breakers with SFB Technology, they form a reliable unit which guarantees maximum system availability.
A redundant power supply significantly increases availability and productivity. Neither connection errors, short circuits nor voltage dips in a primary supply branch affect the output voltage. This is particularly useful for sensitive processes and key system parts.
In a redundant system, the power supplies are decoupled from one another. Redundancy modules assume this task, equipped with a wide range of features. For example, the load can be optimally distributed to both power supplies in error-free operation. Depending on the design, the input voltage and output current can be continually monitored. If one power supply fails, the other takes over immediately.
Redundantly installed supply cables prevent line fault on the path between the redundancy module and the load. The application example illustrates the redundant design from the power supply through to protection with the device circuit breaker board. Equipped with double power terminal blocks, the board enables two supply cables to be connected.