Application to concrete electric circuits

Branch circuits

Branch circuits are closest to devices and operators and, therefore, involve a high risk of electric hazard. Except for special cases, the use of high-sensitivity circuit breakers (30mA or so) is desirable. For outlets in western style bathrooms, vending machines, machines using midnight power and mobile and portable motor-driven machines, high-sensitivity high speed circuit breakers should be used. Generally, since branch circuits are shorter and have lower earth floating capacitance, the risk of unnecessary operations is low even if high-sensitivity circuit breakers are used. However, if the earth floating capacitance is increased, it is better to reduce the constant leakage current through improvement of branch wiring method and use high-sensitivity circuit breakers. Under some conditions, it is difficult to use high-sensitivity circuit breakers. In such a case, as stated in 9. 2. 2 “Selection for electric shock protection for general equipment,” use medium sensitivity circuit breakers with sensitivity of 100mA, 200mA or 500mA reducing the class D grounding resistance value to prevent the voltage from exceeding the allowable contact voltage.

Main circuits

ELCB installed on main circuits shall be capable of providing selective coordination with ELCB on branch lines and protecting the electric circuits from ground fault of the main lines. The most common and simplest method is to use medium sensitivity time delay ELCB, and this method is economical. Note that since the rated non-operating current of ELCB is 50% of the rated current sensitivity, the rated current sensitivity of ELCB for branch exceeds the rated non-operating current of ELCB for main line if the rated current sensitivity of ELCB for branch is too close to that of ELCB for main line, and selectivity cannot be obtained. Therefore, the current sensitivity of branch and that of main line shall be different desirably by 2.5 times. Table 9. 8 shows the relationship between rated current sensitivity of ELCB for branch circuit and that for main circuit.

Table 9. 8 Relationship between rated current sensitivity of ELCB for branch circuit and that for main circuit

Detection of ground fault by ground wire

One of the methods for protecting the whole electric circuit from ground fault is detection of ground fault by ground wire. As shown in Fig. 9. 26, only the ground wire of power supply is passed through the ZCT. When the circuit is broken, it is protected by the earth leakage relay combined with MCCB with SHT.

Fig. 9. 26 Ground fault protection by ground wire

Arc welder circuits

In many cases, an arc welder is moved in a working site during use. Therefore, there is a possibility that the workers may touch the insulated wires or movable cables on the primary side. In such a case, it is desirable to install ELCB. Therefore, ELCB are used in many cases. ELCB for arc welders must not malfunction with an instantaneous transient phenomenon at the start of arc welding. Therefore, some manufacturers separate the circuit breakers for arc welders from others. Mitsubishi ELCB of

Fig. 9. 27 Arc welder circuit

standard type can be used for arc welders. There are various kinds of welders, including arc welders and resistance welders. On welder electric circuits, as shown in Fig. 9. 27, the low-voltage circuit (primary circuit of welder transformer) is grounded, and the load circuit of the welder (secondary circuit of welder transformer) is insulated from the primary side. When ELCB is installed on this low-voltage circuit, ELCB can protect only the range to the primary side of the welder transformer from electric shock and cannot protect the secondary circuit because the circuit is insulated from the primary side. 

Fig. 9. 27 Arc welder circuit

For example, if insulation breakage occurs between the primary winding of the welder transformer and outer case, the “welder transformer / outer case / protective ground wire / earth / electric circuit ground wire / transformer” circuit will be formed as shown in Fig. 9. 27, and ground fault current will flow to operate ELCB. However, even if the welding rod or workpiece is connected to the earth, ground fault current will not flow to the electric circuit ground wire because the primary side and secondary side of the welder transformer are insulated.

Therefore, it is unnecessary to take into consideration the leakage current which will be generated if the workpiece is connected to the earth, and ELCB rated current sensitivity of 30mA is allowed. However, when one ELCB is installed for tens of welders or the wire between ELCB and welder is remarkably long, it may be desirable to install ELCB with medium sensitivity (200 or 500mA) in consideration of the earth floating capacitance. The secondary circuit of the arc welder transformer has a voltage of about 70V while welding is suspended and can cause electric shock. Therefore, measures against electric shock must be taken for the circuit. To prevent the electric shock, an electric shock preventive device should be installed. The electric shock preventive device keeps open the primary side of the welder transformer while welding is suspended. Therefore, there is no possibility of electric shock on the secondary side. During welding, the voltage between welding rod and workpiece is reduced to several V, and there is no risk of electric shock.

✦Points for selection

(1) Operation with overcurrent trip element

The instantaneous tripping current value must be set larger than the transient inrush current value of welder. The transient inrush current values of commercially available welders are 8 to 9 times. (2) Operation with earth leakage trip element Voltage may be generated on the secondary side of ZCT by the transient inrush current of welder, and a phenomenon similar to ground fault may occur. Mitsubishi MCCB have excellent resistance to such transient phenomenon (equilibrium characteristics), and even the standard models will not malfunction.

Fig. 9. 28 Overcurrent tripping characteristics of ELCB

(3) Rated current

Generally, calculate the rated current by the following formula in consideration of the use at the maximum output.

(4) Rated current sensitivity

Since the major purpose is protection from electric shock, it is recommended to select a circuit breaker with rated current sensitivity of 30mA. However, when the electric circuit is remarkably long, determine the rated current sensitivity carefully because a circuit breaker may malfunction owing to the earth floating capacitance of the electric circuit.

In this case, calculate the current sensitivity by the following formula.

Resistance welder circuits

Resistance welders are classified into several types according to voltage and capacity. All resistance welders used at 400VAC shall be provided with ELCB, and those used at 200VAC shall be provided with ELCB if they are water cooled and may be exposed to moisture. In the case where workers may touch insulated wires or movable cables of welders, ELCB shall be installed on the welders used at 200VAC and 400VAC. Fig. 9. 29 shows an example of a resistance welder circuit. Also on this circuit, ELCB can protect only the range to the primary side of the welder transformer from electric shock and cannot protect the secondary circuit. However, the voltage on the secondary side is normally 8V or so, and there is no possibility of electrocution at this voltage. (It is said that voltage of 25V or less is safe even in a sweating state.) Mitsubishi ELCB (NV225-WEP and NV400-WEP) have builtin timers and can protect circuits even from abnormal weld flow.

Fig. 9. 29 Resistance welder circuit

When welding is started, the timer detects the welder current and starts counting the time. If the welder current continues to flow after a lapse of the welding time set on the timer (continuous arc-through or abnormal weld flow), the built-in contact in the timer will close, the tripping coil will be excited, and the circuit breaker will automatically trip. In the case of normal weld flow, the welder current will be stopped within the welding time set on the timer, and the timer will be reset (the maximum reset time is 0.1 sec) and get ready for start of next welding. The welding time shall be set on the timer somewhat longer than the welding time set on the welder control unit. When earth leakage occurs, the ZCT will detect the leakage, the tripping coil will be excited through the leakage detector, and the circuit breaker will trip. At a welder circuit a large inrush current will flow owing to the transient phenomenon at the start of welding, and the ZCT may detects it as earth leakage and cause malfunction (the magnitude of inrush current at which the circuit breaker does not malfunction is indicated as the RMS value referred to as an equilibrium characteristic). Mitsubishi ELCB have an equilibrium characteristic improved by reinforced magnetic shield and will not malfunction. When short circuit occurs, the instantaneous trip device will function, and instantaneously the circuit breaker will trip automatically.

✦Points for selection

(1) Selection of rated flowing current

The rated flowing current is 225A or 400A. Select the current based on the thermally equivalent current of the welder primary current. Actually, the welder primary current varies depending on the welding conditions, such as the material and thickness of workpiece. However, the welder primary current determined based on the current value obtained from the rated capacity of welder can be used without problem. Since the rated capacity of welder is prescribed as input at a service factor of 50%, the thermally equivalent current Ie can be obtained by the following formula.

Where, P is the rated capacity of welder, V is the rated voltage of welder, and beta is the service factor and expressed by the formula beta = weld time/weld cycle.

For example, when the rated capacity is 100kVA and the rated voltage is 415V:

The rated flowing current is determined by allowing a margin of about 15% for this value in consideration of supply voltage fluctuation. Therefore, 170A 3 1.15 = 19A, and a circuit breaker with rated flowing current of 225A should be selected.

 

Table 9. 9 Table of selection of rated flowing current

Table 9. 10 Rated current sensitivity and maximum electric circuit length (m)

(3) Setting of instantaneous tripping current

The instantaneous tripping current must be determined in consideration of the maximum input current of welder and the inrush current at the start of welding. The maximum input current can be obtained from the standard maximum input of welder, but, when the welder secondary side is completely shorted, the maximum input current will be higher by about 30% than the current value determined from the standard maximum input. Therefore, the instantaneous tripping current Iinst in consideration of the inrush current at the start of welding can be determined by the following formula.

 

Table 9. 11 Examples of selection of instantaneous tripping current

Note: In the above table, the instantaneous tripping current values determined based on inverter welder specifications are shown. When selecting the instantaneous tripping current, it is necessary to ensure the coordination so that the surge current capacity of control element (thyristor stack) is not exceeded.

Table 9. 9 shows the relationship between welder rated capacity and rated flowing current of ELCB determined by the above calculation formulas.

(2) Selection of rated current sensitivity

The standard current sensitivity of ELCB for inverter welders is 30mA. If the electric circuit between ELCB and welder is increased, ELCB may malfunction under the influence of earth floating capacitance. It is necessary to install ELCB in such a way that the circuit length is less than the value shown in Table 9. 10. For cases where the circuit length is longer and circuit breakers with sensitivity of 30mA cannot be used, 100mA, 200mA and 500mA circuit breakers are available as special models.

 

Where, Pmax is the standard maximum input of welder, V is the rated voltage of welder, and K is the margin ratio for inrush cur rent and 1 to 1.5 for models with synchronous peak control, 1.4 to 3 for models without synchronous peak control and 2 to 6 for models with asynchronous soft start. Table 9. 11 shows examples of selection of instantaneous tripping current determined by the above formula.