The basics of device circuit breakers | Why device circuit breakers ?
1.4 System availability
System availability reflects the actual versus scheduled production time for a machine or system expressed as a percentage . System downtime or failure has a direct negative impact on availability . Production processes are being optimized more and more because every second that can be saved counts . All of the steps in a production process repeat cyclically . Ideally , this means that any optimization adds up over time .
It is especially important to be able to permanently monitor system status in order to keep an eye on all of the relevant indicators . If a fault can be detected prior to a failure , a timely response can be made – planned maintenance saves time and money .
Having the right protection in the system also plays an important role in maximizing system availability . If a fault is present , whether it is an overload or a short circuit , it must be detected quickly and shutdown must occur within a few milliseconds if possible . Devices connected in parallel are shut down along with it . For this reason , it is important to structure the protection as finely as possible in order to avoid compromising parallel loads . This is also referred to as parallel selectivity .
Selectivity Selectivity can be subdivided into absolute and conditional selectivity . The safest form is absolute selectivity . This is the only way to ensure that the tripping ranges of two safety devices
+ F1
10A
F10 1A F11 1A F12 1A
Fig . 5 : A is the backup fuse and B is the safety equipment for the individual circuits
do not overlap in the overall progression and that only the immediately upstream fuse trips in the event of a fault . With conditional selectivity , there are overlaps in the characteristic curves . This can lead to a situation where the safety equipment and the upstream backup fuse trip , or even a situation where only the upstream backup fuse trips ( Fig . 5 ).
Parallel selectivity Parallel selectivity describes how parallel circuits affect each other . If a fault is present , it must be shut down . Other system parts , on the other hand , should continue to run undisturbed , provided the process permits . Since a voltage dip involves all parallel circuits , the shutoff must take place in an appropriately selective manner . If parallel load circuits are affected , there is no selectivity .
Serial selectivity Selectivity can also be serial . However , this requirement arises primarily from the fuse domain or from building engineering . In this case , the superordinate fuse must
A B
have a value that is high enough that only the next higher fuse in the current path is tripped . This results in better fault containment and increases the line protection . The superordinate value must be at least 1.6 times the nominal . As a general rule , the fuse is designed two fuse steps higher , since the standard values for a fuse will then meet the factor of 1.6 . What is important here is that the characteristic curves are mutually selective .
8 PhoENix CoNtACt