Selection of ELCB( Earth Leakage Circuit Breaker)

Before selecting rated current sensitivity

 The most important factor for selection of ELCB is the rated current sensitivity. The rated current sensitivity should be determined examining various conditions of environment of use of electricity. However, also legal restrictions and measures against unnecessary operations should be taken into consideration. This section describes the factors to be examined in the selection.

9. 1. 1 Legally regulated places

Technical standards for electrical equipment, Labor Safety and Health Regulations and Interior Wiring Regulation specify the rated current sensitivity for some places. When installing ELCB in such a place, select the rated current sensitivity according to the regulations.

9. 1. 2 Selection for prevention of electric shock

For protection from electric shock, in principle, ELCB and protective grounding should be used, and the contact voltage should be reduced to low voltage. Therefore, if two bare live parts are touched even when ELCB is installed, the contact voltage caused by current passing the human body exceeds the allowable contact voltage regardless of protective grounding, and a shock hazard may be caused. ELCB is a device to provide protection against indirect contact described in IEC 60364-4-41. Protection against indirect contact can be provided by appropriately selecting ELCB rated current sensitivity based on the contact voltage and the resistance value of protective grounding. Generally, since the rated current sensitivity is not regulated, it is determined by the formulas (1) and (2) in 6. 4. 2. However, the rated current sensitivity should be selected from 15, 30, 100, 200 and 500mA, and the protective grounding resistance should be controlled to prevent the contact voltage from exceeding the allowable value.

9. 1. 3 Consideration of constant leakage current

From the viewpoint of protection coordination, it is necessary to confirm that unnecessary operations are not caused by constant leakage current originating from earth floating capacitance. Particularly, when selecting a highsensitivity model, it is necessary to take measures, for example, reducing the wiring length and increasing the distance from the earth, to reduce the constant leakage current.

9. 1. 4 Procedures for selecting rated current sensitivity

The three factors, rated current sensitivity, protective grounding resistance value and constant leakage current on electric circuit, have a relationship with one another. From the viewpoint of electric shock protection, the relationship between rated current sensitivity and protective grounding resistance value is critical, and, from the viewpoint of prevention of unnecessary operations, the relationship between rated current sensitivity and leakage current cannot be ignored. Fig. 9. 1 shows the relationship among them.

Fig. 9. 1

The factor to be defined first depends on various conditions. Generally, it is better to determine the relationship between rated current sensitivity and protective grounding and check for possibility of unnecessary operations caused by constant leakage current. It is not favorable, from the viewpoint of electric shock protection, to select a model with low rated current sensitivity for reasons of large leakage current on electric circuit except for unavoidable cases.

9.2 Selection of rated current sensitivity

As a rule, the rated current sensitivity shall be selected based on the theories. However, since the theories contain many assumptions, it is necessary to select the rated current sensitivity referring to practical field data and experiences and observing the regulations if necessary. The actual selection procedures are described below.

9. 2. 1 Selection for electric shock protection

(1) The fundamental concept of prevention of risk of electric shock in the case of indirect contact is reduction of contact voltage value to reduce the passing current to the human body. Therefore, there is a close relationship between rated current sensitivity of ELCB and protective grounding resistance value of device. If the allowable contact voltage (Table 6. 4 in 6. 4. 2 in “Outline of ELCB”) is determined according to the electrical environment conditions, the rated current sensitivity can be theoretically calculated by the following formula.

Actually, the protective grounding resistance value cannot be controlled finely. Therefore, the rated current sensitivity is selected from 30, 100, 200 and 500mA, and protective grounding work is performed to sufficiently meet the formula (1).

From the viewpoint of protection coordination, it is necessary to confirm that unnecessary operations are not caused by constant leakage current originating from earth floating capacitance. Particularly, when selecting a high-sensitivity model, it is necessary to take measures, for example, reducing the wiring length and increasing the distance from the earth, to reduce the constant leakage current.

9. 2. 2 Selection based on leakage current

In many cases, there is some leakage current on electric circuits even if the insulation resistance is normal because

earth floating capacitance exists between wire and earth. This leakage current can be approximately calculated if the wire type, wire size and circuit length from ELCB installation point to load device are determined. It is necessary to determine the rated current sensitivity to prevent unnecessary operations of ELCB owing to this leakage current.

(1)   Total leakage current on Δ connection 3-phase 3W 200V electric circuit

a) Calculation of leakage current from wire

Determine the length of electric circuit, wire type and size on the load side of ELCB, and calculate the leakage current from Attached Tables 4 to 8 in Appendix 10.

b)     Calculation of leakage current from motor

Determine the motor capacity and the number of motors, and calculate the leakage current using the value shown

in the “leakage current at start” column in Attached Table 11 in Appendix 10 for the number of motors to be simultaneously started (generally, 10% of total number of motors are selected starting from that with the largest capacity) or in the “leakage current during operation” column for other motors. Leakage current from almost all machines, such as air conditioners and machine tools, using motors, may be calculated based on the motor capacity.

c)     Leakage current from fluorescent lamp

When lamp is installed directly on steel frame (also when metallic fittings are used): 0.1mA/unit When lamp is installed on wood or concrete: 0.002mA/unit

(2)   Calculation of leakage current on electric circuits by other wiring methods

Determine the leakage current by multiplying the leakage current value obtained in 9. 2. 2 (1) “Total leakage current on Δ connection 3-phase 3W 200V electric circuit” by the multiplying factor shown in Table 9. 3.

(3)   Inrush current on capacitive circuit

To prevent unnecessary operations, it is important to examine not only the constant leakage current caused by earth floating capacitance of the line, but also the transient leakage current caused by switching surge generated when switches (Magnetic Contactors, MCCBs, etc.) are opened and closed. As an example, the ratio of steady-state value to transient value for a circuit shown in Fig. 9. 2 is determined.

im and ôo in Fig. 9. 2 (b) are obtained by the following formulas.

 Assign the values shown in Fig. 9. 2 (a) to these formulas, and the following results can be obtained.

Since the steady-state leakage current Ig in Fig. 9. 2 (a) is R

im/Ig = 1670.

Since current of 1670 times the steady-state current flows to a circuit, it is necessary to examine whether or not ELCB will operate at this pulse current.

As is evident from this example, on a capacitance circuit, even if the constant leakage current is 0.754mA, current of 1670 times (1.26A) the steady-state leakage current may flow in a transient state depending on conditions. The resistance to the transient current varies depending on the manufacture and product. Mitsubishi circuit breakers with rated current sensitivity of 30mA will not operate if the capacitance is lower than the earth floating capacitance In this case, the constant leakage current Ig is equal to 

and unnecessary operations will not occur at a leakage current of 5.2mA or less. However,it is generally desirable that the leakage current is less than 1/10 of the ratedcurrent sensitivity to allow some margin.

(4)    Selection of rated current sensitivity

After determining the constant leakage current by the above method and further examining the transient inrush

current, finally determine the rated current sensitivity. Table 9. 4 shows the relationship between constant leakage current (the insulation resistance to the earth is generally negligible, and the capacitance has a larger influence) and rated current sensitivity.

(5) Example of calculation and selection of leakage current

a) Conditions

1. Steel-framed single story plant

2. 3-phase 3W 200V Delta connection electric circuit

3. Electric devices

Machine tools (motor capacity of 0.75kW or less): 30 units

2.2kW compressor: 2 units

220W fluorescent lamp: 30 pcs.

4. Wiring with 600V vinyl-coated 14mm2 wire, circuit length 50m, 2mm2 X 100m, 1.6(Dia) x 1km

5. One ELCB installed onmAin circuit

b)  Leakage current

As shown in Table 9. 6, the total leakage current is approx. 13mA.

 c)  Rated current sensitivity

The rated current sensitivity is 200mA from Table 9. 3. In this case, the use of one ELCB with a sensitivity of 30mA on the main circuit is insufficient. It is necessary to install one circuit breaker on each of the power circuits of compressors and machine tools and lighting circuits.