Over current means over current protection. Three-stage current protection refers to the current flow rate off protection (the first section), limited current flow rate off protection (the second section), fixed time overcurrent protection (the third section), a set of overcurrent protection mechanism formed by mutual cooperation.
1 section, near the short circuit 0 seconds trip, generally protected to the bus side line outlet of a distance;
section 2, with a time limit of about 0.3-0.5 seconds tripping, generally protects the whole line, there may be a little extension;
section 3, with N second delay trip, is generally set according to avoid the maximum load current, protecting the whole line including a large part of the next stage, depending on the specific situation.
The fault of the line or equipment in the power supply system may cause short-circuit current. The short circuit current is much larger than the normal operation of the line, which need not be explained too much. The current value and the duration of the current value are measured by the current transformer. When the setting value is reached, the trip signal is output. This is the basic principle of overcurrent protection.
The closer the fault current is to the power point, the greater the short-circuit current is.
Over-current protection, also known as quick break protection. The current setting value of this protection is very large, and there is no setting time. In OTHER words, as soon as this current value is reached, the protection device must be immediately activated (the actual reaction speed is in the millisecond level)! However, in order to ensure the selectivity of protection (the fault of the lower level line cannot cause the protection action of the upper level), quick-break protection does not protect the full length of the line. So despite its name, quick break protection is not the main protection of the line!
Overcurrent second stage protection. The current setting value of the protection is smaller than a period, and there is also a setting time. After the line current reaches the set value and lasts for a period of time, the protection action. The current setting value of the overcurrent second stage protection must ensure the protection of the full length of the line, but also extended to the first half of the next stage of the line. The second stage protection is the primary protection of the current line and serves as the far backup protection of the next level line.
Overcurrent three stage protection. The current setting value of protection is smaller than that of the second section, and the time is longer than that of the second section. The three-stage protection should not only ensure the full length of the line, but also ensure that it is longer than the over-current second-stage protection. The three-section protection is the backup protection of the line and acts as the distant backup protection of the next level of line (or even the level after that).