Why can't TN-C use 2p circuit breaker?

tn-c Why not 2p Circuit breaker ？

I will answer the question in two parts . One is about TN-C Overview of grounding system , The second is to answer the main question .

1.TN-C Overview of grounding system

Let's look at the picture below ：

chart 1：TN-C The single-phase distribution system at the end is equipped with 1P Incoming switch of

Look at the picture 1, The main incoming switch of the power distribution system at the end QF2 by 1P The switch of , It is particularly noted that the leakage protector passes through the live line and the zero line at the same time , Is this connection right ？

Let's look at electrical equipment 1, The switch above it QF3 It's also 1[P Of , A leakage protector is connected below it RCD. We noticed the electrical equipment 1 The shell of is connected to the zero line , We call it protective neutral connection .

Now electrical equipment 1 There was a fire line collision accident , That is, electric leakage accident , Leakage flows through the zero line and returns to the power supply . Let's go from the picture 1 We can see clearly in , Leakage accident is actually a short circuit between live line and neutral line , The leakage current is very large , So the switch QF3 Will perform trip protection .

Be careful 1： There are three kinds of TN Characteristics of the system （TN-C、TN-S、TN-C-S）, Therefore, national and international standards include TN The grounding system is called large current grounding system .

chart 1 in , Due to electrical equipment 1 The shell of is protected from neutral , Electrical equipment 1 After electric leakage, its shell potential is near zero potential , Therefore, it will not have a strong electric shock effect on the human body .

Let's look at the picture again 1 Electric equipment in 2, It also has an electric leakage accident ,QF4 The switch will certainly perform protection .

In the process of leakage protection , Electrical equipment 1 Leakage switch in the branch RCD Is it useful? ？

We see ,RCD Zero sequence current transformer （ Is the ellipse in the figure ） It crossed the live line and the zero line at the same time , Their current is equal in size and opposite in direction ,RCD Zero sequence current transformer core （ Magnetic core ） There can be no magnetic flux in ,RCD It will not perform leakage protection action .

Be careful 2： stay TN-C Use leakage protector in grounding system RCD, It is useless for the protection of leakage accidents . Therefore , The specification States ,TN-C There is no need to install leakage protector in the grounding system .

We noticed a fact , The zero line must ensure its zero potential , To ensure that the protective neutral electrical equipment shell maintains zero potential . If QF1 use 4P The switch of , perhaps QF2 use 2P The switch of , As long as the zero line contact of the switch has a poor contact , The zero line potential will rise or drift , We can see that the shell potential of electrical equipment will change from high to low , This is very detrimental to the safe use of electricity , Even dangerous .

Be careful 3： The exact name of the neutral line is the protective neutral line PEN. Since the zero line is grounded at the power transformer , Even if the three phases in the system are unbalanced , But the potential of the zero line is still near zero . The neutral line must be grounded at multiple points , To ensure its zero potential . This is the national standard and specification GB50054《 Code for design of low voltage power distribution 》 Mandatory provisions in .

Be careful 4： To ensure the zero potential of the zero line ,GB50054 Regulations TN-C Do not use in the grounding system 2P and 4P switch .

Let's see TN-C Leakage switch in grounding system RCD Protection of human body . Let's look at the picture below ：

chart 2：TN-C In the grounding system RCD working principle

chart 2 Medium RCD It is three-phase , single-phase RCD The working principle is similar .

chart 2 In the left figure of the figure, there is a fire wire touching the shell accident inside the electrical equipment , The fault current returns to the power supply through the zero line . Because the three-phase current is balanced , So figure 2 The three-phase leakage protector in the left figure RCD Can't move . Specially , single-phase RCD So it is with .

chart 2 On the right , The human body touches A Phase conductor and electric shock , The electric shock current returns to the power supply through the earth rather than through the zero line , At this time, the leakage protector RCD It can induce leakage and take protective measures . Single phase leakage protector is similar .

Be careful 5： thus it can be seen , stay TN-C The leakage protector in the grounding system shall not touch the shell inside the equipment （ Single phase earth fault ） Implement protection , But it can protect human body from electric shock .

2. Answer the main question

Let's look at the problem description of the subject ：

Self built house , There are two wires entering the house, one live wire and one zero wire , There is no grounding bar at the distribution box , Should be tn-c System , I've seen it for two days , All say tn-c Out of commission 2p Switch and leakage protection , Why? ？

1 If it is used at home 2p Circuit breaker , In case of a short circuit, disconnect the live line and the zero line at the same time. Aren't the electrical appliances in the home dead ？

answer ：

About TN-C The grounding system shall not be used 2P and 4P switch , I have already answered , No more repetition .

2 Use leakage protector , If there is leakage, won't the leakage protector be triggered to disconnect the live line from the zero line after touching ？

answer ：

There are three specifications of leakage protector , According to the leakage action threshold current, it can be divided into ：10 To 50mA, It is used to protect human body from electric shock ;50 To 100mA, It is used for both personal electricity safety protection and fire protection ;100mA above , Used for electrical fire protection caused by leakage , Namely fire protection .

The leakage protector itself cannot disconnect the line , The circuit breaker must be actuated （ air switch ） Perform line protection operation . in addition , If the circuit breaker is 2P Of , You can cut off the phase line and neutral line （ Note that it is not the live line or the zero line , With live line and zero line TN-C Earth leakage protector shall not be used in grounding system ）.

3 tn-c System , If the ground wire is led out by the grounding electrode , If the ground wire is bad , Will there be any harm ？

answer ：

The self introduced ground wire is called TT Grounding system . If the ground wire is not done , Effective electric shock protection will not be achieved .

emphasize ： National standard GB16895 Specified in the , The maximum electric shock voltage that the human body can withstand is 50V, We call this voltage human shock safety voltage . Definition of safety voltage of electric shock to human body , see GB/T 18379《 Voltage range 》:

. chart 3： Definition of electric shock voltage of human body —— Voltage range

Many people think 36V Voltage is the electric shock safety voltage of human body , It's actually wrong .36V Voltage is the national standard GB/T 156《 Standard voltage 》 Defined , It is the supply voltage , It is not the electric shock safety voltage of human body .

For household power distribution system and enterprise power supply system ,50V Safe voltage is the basic basis for our design of grounding system and leakage protection .