What are AFDD and AFD

 What are AFDD and AFD?

Arc fault protection devices (AFDD) are products installed by professionals in homes and similar places to reduce electrical fires on the load side. When an arc fault is detected, the AFDD disconnects the fault circuit.

Arc fault detector (AFD) is a detector used to detect arc faults in the protected electrical circuit. It disconnects the fault circuit by outputting a control signal.

Background of arc fault protection devices

As we all know, residual current protectors can achieve leakage protection and arc fire protection caused by relative ground by controlling zero-sequence current. However, in fact, residual current protectors, fuses or small circuit breakers cannot reduce the risk of electrical fires caused by series arcs or parallel arcs between live conductors.

Electrical fires mainly come from: "series arc" or "parallel arc" short circuits caused by poor contact or damaged insulation of electrical lines (mostly carbonization caused by aging of lines); grounding (arc) faults caused by damaged insulation of electrical lines to the ground; abnormal temperature rise caused by poor contact of electrical lines, improper layout of electrical equipment, and overload.

Among them, arc short circuits have become the first priority for electrical fire prevention because they are hidden and difficult to find. The temperature of each arc exceeds 5500℃, and the heat particles emitted by the high-intensity thermal arc are easily accumulated over time to ignite the insulation layer materials around the line, causing electrical fires.

When a series arc fault occurs, since no leakage current to the ground is generated, the residual current protector cannot detect this type of fault. Furthermore, the fault impedance of the series arc reduces the load current below the tripping threshold of the miniature circuit breaker or fuse. In the case of a parallel arc between the phase and neutral conductors, the current is limited to the impedance of the device. The most serious case is an occasional arc, for which conventional circuit breakers are not designed.

Product principle of arc fault protection device

AFDD generally consists of operating mechanism, contact system, tripping mechanism, test button, terminal block, frame and other general structures. Its characteristic structure also includes arc detection circuit, arc fault electronic identification circuit (including microprocessor). It is based on PCB hardware and preset protection algorithm to realize intelligent arc detection and fault arc identification.

AFDD protection process: (1) Arc detection. Monitor the arc in the circuit through advanced electronic technology. (2) Arc characteristic identification. Analyze the characteristics of the detected arc to identify whether it is a fault arc. In the manufacture of AFCI/AFDD, hundreds of possible operating states need to be tested and programmed into the arc characteristic filter to identify "normal" and "dangerous" arcs. (3) Protection characteristic matching analysis. (4) Cut off the circuit to achieve fault protection. When the arc fault protection characteristics are met, a tripping signal is issued to cut off the circuit.

AFDD detects the fault arc and triggers the tripping when the protection characteristics are met after analysis by the protection algorithm. Typical method: detect the load current, amplify the current signal and transmit it to the arc characteristic filter to determine whether the current signal frequency is greater than the power supply frequency and less than the power line communication frequency. The signal output by the filter is compared with the set arc current threshold value, and when it is greater than the current threshold value, it is added to the accumulator. AFDD regularly checks the output of the accumulator and triggers the trip when it exceeds the threshold.

AFDD is used in conjunction with RCBO or RCCB, or products with combined functions can provide home electrical fault protection with both arc and leakage fault protection.

The working principle of AFD is similar to that of AFDD, except that AFD cuts off the fault current by outputting a control signal to the main switch.

Arc detection and fault identification methods

AFDD uses electronic technology to identify arc status, and fault arc detection is a key link in arc fault protection. Research on arc and fault arc detection began in the United States in the late 1980s and early 1990s. Using the optical, thermal, acoustic and electromagnetic characteristics of arc discharge, the main arc detection and fault identification methods are:

(1) Based on arc waveform characteristics: Identify arc faults by judging whether the current waveform derivative and the cumulative arc cycle exceed the set threshold.

(2) Based on arc high-frequency energy mutation: Identify arcs by detecting energy mutations in the high-frequency part of the current signal, and identify arc faults by detecting the number of arcs.

(3) Use high-frequency wavelet transform: Sample the load current at high frequency, calculate the non-zero-crossing discrete wavelet coefficients, and determine whether the threshold is met together with the low-frequency current zero-crossing signal.

(4) Use Fourier transform: Use short-time Fourier transform to analyze the changes in the fundamental component, odd and even harmonic components of the sampled signal, and extract and judge the characteristics of series arc faults.

(5) Time-frequency analysis: Based on the difference in the delay decay time of high and low pulses reflecting the sudden change of current, the time domain above and below the threshold is used as the basis for judgment.

(6) High-frequency signal comparison: By judging whether the periodically generated high-frequency current is different from the normal switching arc, and detecting whether the spectrum range is different from the ordinary high-frequency harmonics generated by power electronic devices and other applications.

(7) Arc wavelength switching: The ultraviolet light in the received arc light is converted into visible light, which is converted into a trigger signal by the photoelectric converter.