The correct use of leakage switch

In lighting and other low-voltage circuits, leakage switches are essential low-voltage switching appliances. When the conductive part (housing) of the electrical equipment has a fault voltage and generates a certain value of leakage current, it can automatically cut off the power supply, thereby effectively preventing electric shock accidents.

1 Principle of operation When the electrical equipment is in normal operation, the current in the phase line and N (zero line) enters the zero-sequence current transformer is equal, and the direction is opposite. The magnetic flux generated by the two currents in the transformer cancel each other, and the secondary side of the transformer does not. Induced current signal output, the switch maintains the conduction state; when the device appears relative housing leakage fault, the magnetic flux generated by the phase current in the transformer is greater than the magnetic flux generated by the N-line current, residual flux appears in the zero-sequence current transformer and Induce the current signal, after the amplification, drive the switch actuator to trip, cut off the power.

It can be seen that the leakage switch plays a protective role and the residual flux is indispensable in the zero-sequence current transformer, that is, the phase current is greater than the zero-line current. To meet this condition, the installation wiring must be correct, otherwise it will not achieve the expected results.

2 Correct installation of leakage switch Install leakage switch, must master the grounding type and difference of low-voltage power distribution system, and then correct wiring.

(1) TN system (protective zero-connected type in which the neutral point is directly grounded in the power system). In principle, the outer casing of the electrical equipment is connected with the PE wire (protected neutral wire) or the PEN wire? protection, and works with the neutral wire? For the entire system If it is difficult to use TN, a local TT system may be used after installing the leakage switch.

The grounding mode of the TN system should meet:

EsIa ≤ U0(1)

Where: Zs - impedance of ground fault circuit Ω;

Ia - ensure that the protective appliance automatically trips the current A within the specified time;

U0 - Relative nominal voltage V.

(2) The TT system (protective grounding type in which the neutral point is directly grounded in the power system) is a protective earthing device that does not share the neutral point grounding of the electrical equipment.

The grounding method of the TT system should meet:


Where R is the ground resistance of the enclosure

3 Common Installation Errors During the installation of the leakage switch, the following kinds of spurious errors are prone to occur. The analysis is as follows:

(1) In the TN system, the PEN lead is not positioned properly.


I-I' = 0 (4)

In the formula: I - enter the phase current of the leakage switch;

I' - N line current entering the leakage switch;

I1 and ΔI are the operating current of the load and the leakage current to the case, respectively.

From equation (4), there is no residual magnetic flux in the zero-sequence current transformer of the leakage switch. Even if the ΔI exceeds the protection operation value, the leakage switch cannot be tripped, so that the leakage protection function is not achieved;

(2) In the TN system, earth is only used as ground protection.

The load case is neither zeroed nor grounded. It is only installed on the earth with electrical equipment, or it is temporarily inserted into the earth as a ground electrode. If the grounding resistance is too large to satisfy formula (2), even if the voltage displayed on the enclosure exceeds 50V, the leakage breaker cannot be tripped.

(3) In the TN system, the PEN or N line device after the earth leakage switch is repeatedly grounded.



In the formula I2 - I1 repeated grounding current.

It can be seen that when the load is earth faulted, the residual magnetic flux generated by the zero-sequence current transformer in the leakage switch is larger than the magnetic flux generated by the actual leakage current ΔI, which easily causes the leakage switch to malfunction.

In the same way, it can be used to introduce errors when PE and N lines are separated and N is grounded repeatedly.

(4) In the TN system, PE and N lines are separated and connected again.

This type of situation occurs most often in projects where the natural conductor in a building is a PE line.

Set load 1 no fault, its operating current is not involved in the analysis:

1 Three-phase load balancing, UN = 0, when the load current 2 fault current â–³ I I = I1 + â–³ I = I1 + â–³ I '+ I2 = I' + â–³ I' (7)


In the formula, I2, â–³ I' - â–³ I of the sub-current, â–³ I = I2 + â–³ I 'is visible, â–³ I' < â–³ I residual current generated in the zero-sequence current transformer of the leakage switch is less than the load fault current â–³ I. The magnetic flux that should be generated will make the leakage switch likely to cause bucking and will not provide protection;

2 Three-phase load is unbalanced, UN ≠ 0, Load 2 is not faulty or the fault current ΔI is small, and the potential on the load case is smaller than UN:



In the formula I2 - I1 the partial current?ΔI'=△I+I2

It can be seen that, ΔI'>ΔI, the residual magnetic flux generated in the zero-sequence current transformer of the leakage switch is greater than the magnetic flux actually generated by the load fault current ΔI, so that the leakage switch may be mistakenly tripped.

4 Application Scope In the engineering design, it is first necessary to understand which cases are suitable for installing the leakage switch. According to the design specifications of the relevant industries in the country, the following electrical equipment and places must be installed with leakage switches on the distribution lines:

(1) Electrical equipment installed in humid, corrosive and other harsh environments;

(2) Provisional electrical equipment for temporary use;

(3) In-room socket circuits of hotels, restaurants and hostels;

(4) Outlet circuits in institutions, schools, enterprises, and residential buildings;

(5) Water lighting equipment for swimming pools, fountains and baths;

(6) Power supply lines and equipment installed in water;

(7) Electrical medical devices that are in direct contact with the human body in hospitals;

The electric leakage device shall be installed on the electric device or place that will cause accidents or major economic losses in the event of electric leakage cutoff. Such as:

(1) Access lighting and emergency lighting in public places;

(2) Fire-fighting elevators and equipment for ensuring the safety of public places;

(3) Power supply for fire fighting equipment, such as fire alarms, fire pumps, and fire exit lighting.

5 Leakage switch parameter selection Select leakage switch, should pay attention to the following parameters selected:

5.1 Rated voltage and current value According to the three-phase or single-phase power supply of the equipment to be protected, the rated voltage is 380V or 220V. The rated current of the leakage switch shall be greater than the total load current of the line.

5.2 The rated operating current value is referred to the relevant specifications and can be selected according to the following data:

(1) Hand-held consumer equipment is 15mA;

(2) The equipment used in the harsh environment or humid place is 6 ~ 10mA;

(3) Medical electrical equipment is 6mA;

(4) The power equipment for the construction site is 15 to 30mA;

(5) The home appliance circuit is 30mA;

(6) Distribution switchgear distribution of electric plates, etc. is 100mA;

(7) Prevent 300mA from electrical fire.

6 Check of leakage switch In order to guarantee the installation quality of leakage switch, the electrification test of the selected leakage switch should be conducted. The general test is to check if the self-jump of the leakage switch is normal (according to its own test button), but the conditional value should be checked for the operating value, especially for places where the leakage current is less demanding, it should be sent to the relevant unit for verification.

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