Residual current devices characterized by simplicity and cost effectiveness holds a wide range of applications in the protection against electric shock that is one of the important detriments to the safety of human and the performances of electrical apparatus. This article puts the focus on the description of the basic structure and principles of residual current devices, particularly the effect of the earth current frequency and distortion on the mal- and miss operation of electrical apparatus is investigated, which is of reference value for the proper selection of electric apparatus.
Keywords: Residual Current Devices; Electric Shock Protection; Earth Current; Miss Operation; Mal Operation
Effects of the Earth Current Frequency and Distortion on Residual Current Devices
Electric shock is caused by flash strick as flashing lighting touch the household electrical wiring, or accidently broken wire touch electrified body , easily lead to electrical equipment damage and cause personal safety accident.
Protection against electric shock is a important method to prevent electric shock. There are several ways:Automatic power off, II equipment, Insulation equipment, electrical isolation; equipotential connection without GND.Automatic power off become the nomalest way to prevent electric shock because of it’s simpleness and operability.As a result, residual current devices with automatic power off technology are more and more widely used.
Residual current devices works by receiving signal,so it works also under other electromagnetic interference signal which called mal operation. When the power swith turn on, it comes out the impact signal cause to the mal operation.The sum of multi-branch electric leakage may lead to override maloperation.When neutral line iterative earthing may cause residual current devices comes to shunt miss operation. When the power leakage happens to the one used by residual current device, it comes miss operation.
As can be seen from the above analysis , the residual current devices have mal operation and miss operation situation in actual use. This article describes the basic structure and principles of residual current devices , focused on effects of the Earth Current Frequency and Distortion on Residual Current Devices in the protection against electric shock.
Most electrocution death and equipment damage accidents are caused by electric shock . Generally speaking, the electric shock accident basically means in terms of electric shock . In accordance with the state of the electrical equipment , electrical shock can be divided into direct contact and indirect contact type.The former is a normal state of electric shock when human touch the charged body,The latter is a shock occurs when people touch the accidently charged body under broken situation but no uncharged normally. Therefore , the former is known as electric shock of normal state , the latter is known as electric shock under breakdown conditions.
According to the way of how people body touch the charged object and the way of how the electric current go through people body, shock can be divided into single-phase electric shock, two-phase electric shock and step voltage electric shock.
single-phase electric shock is mean human body touch the electric conductor in earth or landing, so the partial body of human body touch the charged object lead to electric shock accident. And most shock accident belongs to such kind. The dangerous degree of single-phase shock is related to the operation mode of the electric network. Normally, the single-phase of landing electric net work is more dangerous than UNGND electric network. Two-phase electric shock means the accident that two places in human touch two-phase charged dody , it’s very dangerous. When charged body landing , it’s current go to earth lead to the voltage of GND earth decline. When people closed to earth electrode, the voltage appeared between feets is called step voltage which lead to step voltage electric shock accident. It may appears high step voltage in high voltage broken GND point or beside earth device when large current go through.
IEC 60479-1 defines the four region of A / t relationship , described the morbid physiological effects within each region of the body. As shown in Picture 1 , the horizontal axis represents the current figure flowing through the body , the vertical axis represents current sustained time. Every area shows:AC- 1 zone indicates no perception ; AC-2 area expressed perception ; AC-3 region indicates the presence of reversible effect of muscle contraction ; AC-4 zone may indicate irreversible effects ; AC-4-1 zone may represent cardiac fibrillation up to 5%; AC-4-2 zone represents cardiac fibrillation up to 50%; AC-4-3 zone represents cardiac fibrillation can be more than 50% ;
Each curve in following picture shows : Curve A represents the current perception threshold ; Curve B represents the muscle response threshold ; Curve C1 represents the 0% probability of ventricular fibrillation threshold ; Curve C2 represents a 5 % chance of ventricular fibrillation threshold ; C3 curve represents 50% probability of ventricular fibrillation threshold. Take Curve C1 as an example , when the current go through human body , if the current exceeds 30 mA, the human body is likely to be fatal electric shock unless the current can be cut off in a relatively short period of time.
Picture 1 Regional Rendering of Morbid Physiological Effects
2 Principle of Residual current device
Picture 2 Residual Current Device Appearence
Residual current devices can be divided into two categories: current type and voltage type . Current type is on the basis of error current movement and voltage type is based on error voltage movement. Since the voltage type has characteristics with bad prevention to outside interference ,poor stability and high production costs, such type of residual current devices has been basically elimiated. So the current residual current devices has the dominat position on the research and application of residual current devices in domestic market.
Residual current device used partial residual current from the electric circuit(usually called the residual current) as the movement signal, commonly used electronic component as the Intermediaries. It becomes more and more widely applied with high sensitivity and multiple function. As show in Picture 3, Residual current device is made of 3 parts:
Ferranti transformer (As shown in picture TR): Protected phase conductor L1 , L2, L3 and the neutral line N through the annular core compose transformer’s primary winding.the winding intertwining in the core compose the secondary winding. If no electric leakage, then the current vector sum which go through the three phase conductor and the neutral line is zero. No alternating magnetic field produced, no induced current produced by the secondary winding of transformer.If electric leakage occurs,then the current vector sum which go through the three phase conductor and the neutral line is not zero.The secondary winding of transformer produce induced current and give them to the tripping mechanism for further processing.
Tripping mechanism(As shown in picture WS）: includes amplifier , comparator, release and the main switch. The amplifier works for the induced current from the ferranti transformer. the comparator is used to compare the enlarged induced current and the rated residual action current, While the former is larger the release and the main switch work, automatically cut off the power of the error line to protect.
Test button(As shown in picture T):Regularly check residual current device is intact and reliable. Simulate electric leakage ways by pressing the test button to check if it is operating normally .
Picture 3 Residual Current Device Inner Structure
3.Effects of the Earth Current Frequency and Distortion
This section mainly analyzes the protection effects from the current frequency and distortion to the residual current device in the circuit with frequency conversion function.
Picture 4 Earth current waveform in frequency changing circuit
Frequency conversion is generally used to control the speed of squirrel cage electric motor. Figure 4 shows a frequency changing circuit schematic diagram with residual current device(RCD represent in figure 4). Frequency changing circuit is composed of 3 parts: the Rectifier (works for rectifying the waveform input from the power side), the intermediate DC circuit( used to filtering waveform output from rectifier)and the inverter(reverse intermediate DC circuit generates AC DC into the desired frequency). After alternating current through the frequency changing circuit , low frequency current change to the high frequency current. Similarly, the ground analog current frequency increases after go through the frequency changing circuit.
Under the condition of current frequency 50Hz , the perception current threshold is 0.5mA, that is, in such situation , human body has some sense when the current comes to at least 0.5mA. As can be concluded from Figure 1, 10mA cause human body lose muscle control and muscle contraction under 2s consitent current; 30 ~ 40mA lead to ventricular fibrillation and then life-threatening. As for higher frequency current threshold, the threshold of human perception, muscle response and ventricular fibrillation increase accordingly. Figure 5 shows the frequency graph with curve.
Picture 5 50-1000Hz threshold change coresponsding to frequency
When the frequency is 1000Hz , the ventricular fibrillation threshold goes from 30 ~ 40mA to 420mA. This is very important information , because if the ground current frequency higher than 50Hz, many RCD tripping current increase obviously.
In order to study the effects of ground current frequency to RCD tripping operation, people test several RCDs under the rated residual current ΔIn = 30 mA and 100mA. Figs. 6, 7 and 8 shows the test results under laboratory conditions.
Figure 6 ΔIn = 30mA the AC RCD tripping operation current change
Figure 6 shows the threshold change of the AC RCD tripping operation current and ventricular fibrillation according to the frequency change under ΔIn = 30mA circumstance. The solid line represents ventricular fibrillation threshold varies, dashed line indicates the tripping current varies from 1 ~ 1000Hz. From the figure, comes the conclustion that RCD tripping operation current is decided by ground currnt frequency. When the frequency is higher than 50Hz,tripping current is much smaller than ventricular fibrillation threshold , and therefore, shock occurs the body does not produce ventricular fibrillation in the test frequency range, so AC RCD can play normal protection.
Figure 7 ΔIn = 100mA , A RCD tripping current
Figure 7 shows under the ΔIn = 100mA circumstance ,the changing threshold of A RCD tripping current and ventricular fibrillation fllow the frequency. From the figure the conclustion is that tripping current is much higher than ventricular fibrillation. Therefore, in the test frequency range,the human body may cause ventricular fibrillation when electrical shock occurs, A RCD can not play such a normal electric shock protection.
Figure 8 ΔIn=100mA AC RCD current and ventricular fibrillation threshold
Figure 8 shows under the ΔIn=100mA circumstances, short-time delay AC RCD current and ventricular fibrillation threshold change follows the frequency.From the figure the conclustion is that tripping current is much smaller than ventricular fibrillation threshold, that is to say, within the frequency range from 100Hz to650Hz, human body won’t emerge ventricular fibrillation when electric shock occurs, short-time delay AC RCD can play a protection role to such nomal electric shock.
Figure 9 the effect of multiple harmonicwave to the RCD tripping current
The ground current in frequency changing circuit nomally emerge multiple harmonicwave, resulting in waveform distortion. This study shows: the higher of the harmonicwaves,the higher tripping current of RCD. Figure 9 shows under the circumstances,how the multiple harmonicwave affect the RCD tripping current, the actual operation current corresponding to fundamental wave is 65mA.
Study indicates that multiple harmonicwave of ground current may increace the actual operating current of the RCD.
Residual current device can effectively protect the personal safety and electrical equipment when electric shock accident occurs. However, it must be very cautious in choosing the RCD. The change of frequency and waveform in ground current affect RCD’s protective performance. This article study effects of the Earth Current Frequency and Distortion to tripping current, and the conclusion has a certain practical significance for the correct selection of Residual current devices.