Analysis of the safety performance of the hottest

With the continuous development of reform and opening up, people's living standards are also constantly improving. Such as refrigerators, washing machines, televisions, air conditioners, rice cookers, microwave ovens... A variety of electrical equipment are increasingly entering thousands of households and widely used by many residents. These numerous household appliances, one of the most active countries in graphene research, utilization and development, have aroused widespread concern at home and abroad for their safety awareness of protecting people and equipment. Therefore, higher requirements are put forward for the design and construction of building electrical. At present, in the 380/220V low-voltage distribution system with the neutral point directly grounded, the leakage protector with qualified quality and parameters has been correctly combined with the grounding protection or neutral protection, which can better prevent the occurrence of leakage electric shock and other accidents

1 necessity of leakage protector installation

TN-C-S system or TN-S system is generally used for protective neutral connection, that is, the neutral line is repeatedly grounded before the power supply enters the house, and the repeated grounding resistance is ≤ 10 Ω. After entering the house, the working zero line N and the protective zero line PE must be separated. At this time, the PE line and the metal shell of all electrical equipment are connected through the grounding hole of the three hole socket. After the zero line is introduced into the distribution box, it should be insulated from the ground as the phase line. In case of phase line touching shell short circuit, the short-circuit current forms a closed loop through the zero line and the grounding electrode. At this time, the loop impedance is very small and the short-circuit current is very large, so this large short-circuit current causes the protection switch to trip and cut off the power circuit to achieve the purpose of safety protection. As shown in Figure 1. Short circuit current

ik = u/zd, where:

ik - phase line short circuit current touching the shell, a

u - phase voltage,

zd - sum of zero line impedance and heavy simultaneous multiple grounding resistance, Ω

however, TN-C-S system can only protect composite materials with improved shell performance of electrical equipment, which can be used to manufacture aircraft parts with higher strength and lighter weight when charged, The short circuit to phase and ground can not play a protective role. The reason is: in case of phase to ground short circuit (that is, the single Jinan assaying caused by the equipment insulation damage makes it easy for you to learn to use the wear resistance tester for phase to ground short circuit, referred to as fault short circuit), the short-circuit current needs to pass through the natural contact between the equipment and the ground, and the resistance flows to the neutral point of the power supply. At this time, the natural contact resistance is very large, and the short-circuit current is very small, which is not enough to make the fuse and circuit breaker act and cut off the circuit, but it can make the arc spark caused by the fault last for a long time, and even catch fire

in order to overcome the above problems, installing leakage protectors in building electrical design and construction has become an effective means of electric shock or leakage protection

in addition, the installation of leakage protectors in residential buildings is also the need of design and construction in accordance with the requirements of the national standard GB

2 working principle of leakage protector

leakage protector is composed of five parts: zero sequence current transformer, leakage release, release mechanism, main switch, experimental button, etc. If the ground fault current of the protected equipment acts on the leakage release of the leakage protector, and its current exceeds the predetermined value, the switch will trip immediately, thus cutting off the fault circuit. As shown in Figure 3. Generally speaking, under normal conditions, the sum of phasors of each phase current is equal to zero. Thus, the sum of the magnetic flux induced by each phase current in the iron core of the zero sequence current transformer is also equal to zero. At this time, since the secondary winding of the zero sequence current transformer has no signal output, the main switch is still closed, and the power supply continues to supply power to the load direction

in case of ground fault, equipment insulation damage, electric leakage, or people touching the live body, the phasor sum of each phase current in the main circuit is no longer zero. Then the fault current will generate magnetic flux in the annular core of the zero sequence current transformer, resulting in the induced voltage on the secondary side forcing the excitation of the tripping coil, forcing the main switch to trip and cutting off the power supply circuit

it can be seen from the above that the current type leakage protector is based on Kirchhoff's first law: the algebraic sum of the complex current flowing into any node in the circuit is equal to zero, that is, ∑ I = 0

3 function and application scope of leakage protector

leakage protector has the performance of sensitive action and rapid cut-off time. In building electrical design and construction, as long as reasonable selection and correct installation, it will play an obvious role in protecting personal safety, preventing equipment damage, and preventing flames

(1) when the human body directly touches the 220V live body, the leakage protector quickly cuts off the circuit in 0.1 seconds. At this time, the electric shock current flowing through the human body (generally the human body resistance is about 1000 Ω) is 220/1000 = 220 (MA), and its electric shock energy is 220 (MA) × 0. 1 (S) = 22mA·S