Analysis of unnecessary operations

While the operations of ELCB caused by the intended faults, such as earth leakage, electric shock and ground fault, are considered to be normal operations, the operations caused by other factors, such as surge and induction, are unnecessary operations (nuisance operations or nuisance trips). Many users may think that ELCB operates without reasons and causes troubles. This section analyzes the unnecessary operations of ELCB and describes the correct selection of ELCB.

Classification of ELCB operations

The operations are classified as shown below.

Details of operations

(1) Normal operations

The normal operations of ELCB refer to the intended operations of ELCB. The major causes of the operations

are shown below.

a) Deterioration of machine insulation

Mainly, machines, such as washing machines, using water and machines, such as presses, receiving high impact.

b) Deterioration of wiring insulation

Mainly, joints and terminals of temporary electric circuits.

c) Nonconforming work

Ground fault caused by cables damaged during work and disconnection.

d)Careless handling

Electric shock caused by submerging and ground fault caused by surge and drop of foreign substances.

(2) Defects of ELCB

Some parts of circuit breakers may be deteriorated and corroded to cause troubles, but the earth leakage detection module causes less defects. Wear of the electromagnet switching mechanism may cause unstable switching.

Mitsubishi ELCB have sufficiently improved durability and can be used without concern for these problems. Besides these defects, circuit breakers with low equilibrium characteristics may operate at the start of motor. If the characteristics of the ZCT used in ELCB are inferior or the magnetic shield effect for the ZCT is low, the equilibrium characteristics of the ZCT are deteriorated by the influence of the residual current, and the circuit breaker may operate improperly because electromotive force is generated on the secondary winding of ZCT when the motor starting current (several t imes the ful l - load cur rent ) f lows in the samemAnner as when an apparent ground fault occurs. As the bus-bar current increases, the absolute value of residual current becomes larger. Therefore, it is necessary to pay attention to the influence of residual current on a circuit with large load current. Care must be taken for circuits with unshielded ZCTs.

The residual current characteristics of ZCT vary depending on the coremAterial, conductor position and winding. It is not allowed that the circuit breakers on general circuits are operated incorrectly by the ZCTs. If a low-quality core is used or the ZCT shielding effect is insufficient, malfunction may be caused. For the ZCTs of Mitsubishi ELCB circuit breakers, high-quality Ni-based permalloy having good residual magnetic characteristics is used, and the outer surfaces of ZCTs are covered with a high-quality material having good magnetic characteristics for hydraulic magnetic shielding. Therefore, the influence of residual current on them can be minimized, and they will not malfunction. When unbalanced current occurs on a load, theoretically, ELCB will not operate. However, if a ZCT with low residual current characteristics is used, malfunction may be caused. malfunction at the start of motor or under unbalanced loading is caused owing to improper equi l ibr ium characteristics of ZCT which are based on the residual current characteristics. It is necessary to use the product of a reliable manufacture.

(3) Improper current sensitivity

When the current sensitivity of ELCB is too high for the normal leakage current of a circuit, ELCB operates unnecessarily. This trouble is caused by improper selection of current sensitivity. In most cases, circuit leakage current is caused by earth floating capacitance of electric wire. However, some electric furnaces and sheathed heaters decrease in insulation resistance at high temperatures although they have sufficient insulation resistance at low temperatures, and it may take time to reveal the cause of the operation. In addition to the leakage current in the steady state, the transient leakage current at the switching or start may activate ELCB. The transient leakage at the start is caused through the capacitance to the winding frame because the potential distribution on the winding at the start differs from that during operation.

(4) Operation caused by surge

For the surge caused by secondary shift of induced lightning in distribution line, fig. 9. 3 shows a circuit for test for nonoperation at lightning impulse. When a circuit is affected by induced lightning surge, high voltage is applied to the distribution device through the electric line. At this time, the electronic parts of ELCB may malfunction to trip ELCB or may be damaged to disable ELCB. ELCB for service entrance may be influenced by this high voltage. Special care must be taken for this influence. The magnitude and frequency of surge voltage carried by the induced lightning significantly vary depending on the region also in Japan. Statistically, in most cases, the surge voltage is 5kV or less. Although large induced lightning surges corresponding to 6 to 7kV have been recorded several times a year, it is generally allowed to think the surge voltage is about 5kV. Mitsubishi ELCB use electronic parts which have sufficient  characteristics to cope with such phenomena. However, in many cases, as shown in Fig. 9. 4, the switching surge generated when the circuit is opened or closed by the inductive load switch S is not a single pulse unlike induced lightning surge, but a continuous pulse. Since the non-operating performance for continuous pulse is different from that for single pulse, it is necessary to improve the performance for continuous pulse. Mitsubishi ELCB have sufficient non-operating performance for continuous pulse and can be used reliably. However, it may be helpful to be well acquainted with this property.

Fig. 9. 3 Test circuit for non-operation at lightning impulse

Fig. 9. 4 Switching surge

 a) The switching surge is not always caused when the current of inductive load is large. It occurs rather easier when the load current is low, 1 to 2A, and the load inductance is large. Attention must be paid to magnetic coil loads, such as magnetic switch coil current.

b) The magnitude and repetition of surge depend on the performance of the switch S shown in Fig. 9. 4. If the switch S causes chattering or a vacuum switch having too high breaking performance is used, surge is easily generated. Therefore, as the switch S, a device with less chattering and high current cutting performance is favorable. General magnetic relays are regarded as relatively useful.

c) To prevent switching surge, it is effective to add an arc reduction device, such as C or R, between the contacts of switch S or install a surge absorber on the load side.

Since the distribution line and load device have capacitance to the earth (earth floating capacitance), the charge current flowing through the earth floating capacitance may instantaneously increase when switches are closed under adverse conditions with non-simultaneous operations of contacts upon closing of switches and the abovementioned surge voltage, and, if the charge current exceeds the rated non-operating current value, ELCB may operate. Electric circuits have various degrees of capacitance to the earth, but zerophase current is not generated in the steady state if the capacities of the phases are well-balanced. However, switching surge is caused owing to contact chattering, the voltage phase will be disturbed, high-frequency voltage will occur, the impedance by the earth floating capacitance will decrease, and excessive charge current will flow, and, as the result, electromotive force will be generated on the ZCT secondary winding to activate ELCB. Therefore, a filter is provided on the ZCT secondary side to prevent the thyristor from responding to extremely short-time output of ZCT secondary winding owing to the surge voltage, and a surge absorbing circuit is installed to protect the electronic components against excessive leakage and large ground fault current. Most of Mitsubishi MCCB are provided with DPDC surge discriminating circuits for discriminating the ground fault current and the transient leakage current caused by surge to improve the performance to prevent unnecessary operations and will not malfunction on general circuits.

(5) Operation caused by loop circuit (circulating current)

As on parallel circuits connected on the load side as shown in Fig. 9. 5, the branch currents of each phase at the right and left branches are not always equal to each other. For example, when the current of phase A is divided into 11A and 9A, current of 1A is continuously flowing in this loop. This circulating current causes ELCB to operate. Therefore use of two ELCB in parallel must be avoided. If earth leakage is detected on the ground wire of each transformer in the case of parallel operation of two transformers as shown in Fig. 9. 6, circulating current will flow through the ground wire owing to unequal division of current, thereby causing ELCB to operate. As a method to avoid this, the circuit may be configured as shown in Fig. 9. 6 (B).

Fig. 9.5 Parallel circuit

Fig. 9. 6 Parallel operation of transformers

 6) Operation caused by induction

A circuit contains a loop circuit as stated in (5) is easily affected by induction. That is, if the loop shown in Fig. 9. 5 is considered to be a loop antenna, the ZCT primary winding is connected to the antenna and easily generates induction. As an example, assume that the loop area is 1m2, and a 200A heavy current source is located at a distance of 5m (see Fig. 9. 7). When the circumference of a circle with a radius of 5m is the magnetic path length, the magnetic field strength H (AT/m) is:

This value is sufficient to activate ELCB with current sensitivity of up to 500mA. Actually, since the power supply is not isolated only in one direction, the influence of induction will be lower than the above calculation result. For example, when a single-phase power supply is used and other phases are at a distance of 5.2m from the power supply, the circulating current I’ is 0.2/0.5 of the above value: I’ = 0.04A. This value is sufficient to activate ELCB with current sensitivity of 30mA. As stated above, loop circuits are unfavorable from the viewpoint of induction. It is desirable to avoid such circuits.

Fig. 9. 7 Loop antenna

If a ZCT is installed on the solid line in the figure when a common ground wire is used as shown in Fig. 9. 8, the primary conductor of the ZCT forms a loop. To avoid this, it is desirable to install the ZCT on the dashed line. Induction can be caused also on the input circuit of an earth leakage relay. Therefore, the lead wires between the relay and ZCT must be twisted.

Fig. 9. 8 Detection of ground fault by ground wire

(7) Operation caused by improper connection

Simple errors, such as failure in passing the neutral line to the ZCT in Fig. 9. 9, can occur. In the case of Fig. 9. 9, ELCB will operate owing to single-phase load current.

Fig. 9. 9 Improper connection of 3-phase 4-wire circuit

The TN-S system has the same circuit configuration as shown above. Care must be taken when using the system. To the contrary, if a line not to be passed through the ZCT is passed through the ZCT, the circuit breaker may not operate when earth leakage occurs (see Fig. 9. 10). Therefore, do not pass the common ground wire through the ZCT. The TN-C system corresponds to this.

Fig. 9. 10 Improper connection of common ground wire

If grounding work has been performed for the metallic conduit or metallic shield of cable on the power supply side of the ZCT position as shown in Fig. 9. 11, ELCB may not normally operate when leakage occurs in the metallic conduit. In this case, move the ground wire to the load side of the ZCT, or install the ZCT in a place other than the metallic conduit. If this is difficult, it is necessary to remove the metallic parts from the area of installation of the ZCT or take other proper measures. If grounding is provided in two places, on the power supply side and the load side, in Fig. 9. 11, a loop as the primary conductor of ZCT is formed by the metallic conduit, ground wires and earth, and the circuit breaker may malfunction owing to the induction stated in (6).

Fig. 9. 11 Grounding for metallic conduit

(8) Operation caused by improper grounding

The wire of a single-phase grounding delta circuit as shownin Fig. 9. 12 should not be grounded also on the load side. In Fig. 9. 12, part of the load current is divided to I' T by the voltage drop of the electric circuit on the grounding side, and ELCB is operated. (In addition, in Fig. 9. 12, the circuit breaker may not operate even if current leaks from the motor.)

Fig. 9. 12 Improper double grounding

When the purchased power-private power line is switched, the neutral line must be switched simultaneously. If the line is not switched simultaneously as shown with the dashed line in Fig. 9. 13, the neutral line return current IN will be divided into IN and I’N shown in the figure, and ELCB on this circuit will operate.

Fig. 9. 13 Simultaneous switching of neutral line

On machines (electronic calculators, numerically-controlled machine tools, etc.) using electronic circuits, filters are often used as measures against noise on the electronic circuits. If a line filter is used as shown in Fig. 9. 14, current flows as shown with the dashed line to activate ELCB. To avoid this, an insulating transformer should be used in the power supply unit of the electronic machine. On home-use audio equipment of auto transformer or transformer-less type, part of the return current I2 leaks through the chassis earth as shown in Fig. 9. 15 to activate ELCB.

Fig. 9. 14 Line filter

Fig. 9. 15 Chassis earth of transformer-less machine

On a system using a ZCT to detect zero-phase current for detection of ground fault, care must be taken for connection of neutral line. The neutral line on a single-phase 3-wire or 3-phase 4-wire circuit must pass through the ZCT without fail. In addition, the neutral line must be insulated from the earth, and the neutral line of each system must be electrically independent. When a ZCT is installed at each of the branches A and B in Fig. 9. 16, it is necessary to configure the circuit so that the neutral lines are not electrically connsected directly or indirectly through the earth. If they are connected correctly, the current at the branch A is i1a + i1b + i1c + i1N = 0, and that at the branch B is i2a + i2b + i2c + i2N = 0. No electromotive force occurs in each ZCT, and the ZCT will detect ground fault current when ground fault occurs. However, if the neutral lines at the branches A and B are connected through the earth, i1a + i1b + i1c + i1N = i2a + i2b + i2c + i2N = i0N unless they are in full balance, and the ZCTs will detect current of i0N and operate even when ground fault current does not flow. As stated above, for detection of ground fault, if the connection of neutral lines is improper, troubles can occur. Therefore, wiring work for ground fault detection shall be performed more carefully compared to that without ground fault detection.

Fig: 9.16

(9) Operation of sound circuit upon occurrence of ground fault of branch circuit

Circuits may be configured as shown in Fig. 9. 17, and ELCB not only on the ground fault circuit, but also on the sound circuit may operate. This can be prevented by using ELCB with current sensitivity appropriate to the leakage current caused by earth capacitance.

(10) Operation owing to overload or short circuit

It is normal that circuit breakers with overload and short circuit elements operate upon occurrence of short circuit. Most of ELCB are designed both for these factors, and you may forget that they can operate upon occurrence of overload or short circuit. Furthermore, since the equilibrium characteristics of ELCB

Fig. 9. 17 Operation on sound circuit owing to earth capacitance

are restricted, even those for ground fault may operate at large overload or short circuit. However, in this case, you will notice such a large overload or short circuit.

(11) Environmental conditions, such as vibration, impact and high temperature

The influence of these conditions on ELCB can be considered to be almost identical to that on MCCB. Although the heat resistance of electronic circuits is often regarded as unreliable, Mitsubishi ELCB have a sufficient margin for the rating of parts and use parts which can withstand high temperatures and is containing temperature compensation circuits to stably operate at various ambient temperatures.

(12) Operation caused by carrier phone

If ELCB is installed on an electric circuit where a carrier phone unit for communication through power line is installed, ELCB may malfunction. Since the carrier phone unit is a unit which forcibly inserts high frequency signals (normally, 50 KHz to 400 KHz) between electric circuit and earth as shown in Fig. 9. 18, ELCB detects the high frequency signals as leakage current and malfunctions.

Whether or not ELCB malfunctions depends on the magnitude of high frequency signal and the high frequency characteristics and rated current sensitivity of ELCB.

To prevent this malfunction, it is effective to use ELCB whose current sensitivity for high frequency current has been intentionally reduced. For determination of the specifications, consult me n personal.

 

Fig. 9. 18 Example of installation of carrier phone unit

 (13) Operation caused by radio wave

In the case where there is a large-output broadcasting station, taxi radio station or amateur radio station near a circuit with ELCB, ELCB may operate unnecessarily if the strength of radio wave, frequency, weather, landform and wiring method adversely affect them. Particularly, when signals from a portable transceiver are received near ELCB, high magnetic field strength is generated, and unnecessary operation can be easily caused.

Generally, portable transceivers are used in frequency bands of 27/28MHz, 50/60MHz, 150MHz, 400MHz and 90MHz, and their output is about 0.5 to 5W. Mitsubishi ELCB have been confirmed to cause no unnecessary operations when var ious commercial ly avai lable transceivers with output of 5W transmit signals at a distance of 1m from ELCB. If it is expected that a stronger magnetic field will be generated, house ELCB in an iron box, and ground the box. The unnecessary operations can be prevented by installing a capacitor of hundreds to thousands pF as shown in Fig. 9. 19.

Fig. 9. 19 Installation of capacitor for prevention of unnecessary operations

(14) Operation caused by inverter

Since these high frequency components are constantly carried by the earth floating capacitance, ELCB may operate unnecessarily when the earth floating capacitance is increased. (Fig. 9. 20).

 Therefore, when using a general ELCB on an inverter circuit, it is necessary to select one with current sensitivity lower than usual to prevent unnecessary operations. To ensure high-sensitivity detection of ground fault on an inverter circuit and realize stable detection of ground fault on the primary and secondary sides of the inverter, it is suitable to use ELCB applicable to higher harmonics and surge which is hardly affected by high frequency components so that it can be used for inverter. ELCB shall be installed on the inverter primary side. Never install it on the secondary side.

Fig. 9. 20 Circuit with inverter

(15) Others

As the electronics technologies are increasingly applied to load devices, in many cases, surge absorbers are installed in the devices or on the electric circuits to protect the devices from surge. The surge absorbers connected to the earth, which discharge surge to the earth, generate large transient leakage current although for a short time and may cause unnecessary operations of ELCB (Fig. 9. 21). Most of ELCB are provided with DPDC surge discriminating circuits for discriminating ground fault current caused by faults, such as insulation failure, and the transient leakage current caused by surge to improve the performance to prevent unnecessary operations even if surge absorbers are installed between them and the earth.

Fig. 9. 21 Transient leakage current caused by surge absorbers

Summary of unnecessary operations

The unnecessary operations of ELCB are described above. We have taken all possible measures against unnecessary operations caused by ELCB. Therefore, they will not cause any problem under normal working conditions. Almost all unnecessary operations are caused by factors of circuits.

Most of unnecessary operations can be prevented by taking special care when wiring and connecting ELCB and selecting their installation locations and current sensitivity. It is desirable to carefully examine the earth floating capacitance to the earth and the rated current sensitivity at the stage of design.