11.3 Molded Case Circuit Breakers (MCCB), Earth Leakage Circuit Breakers (ELCB) and
Magnetic Contactors (MC)
11-5
SELECTING PERIPHERAL EQUIPMENT
11.3 Molded Case Circuit Breakers (MCCB), Earth Leakage Circuit Breakers
(ELCB) and Magnetic Contactors (MC)
11.3.1 Function Overview
■ MCCBs and ELCBs* * With overcurrent protection
Molded Case Circuit Breakers (MCCBs) are designed to provide protection up to the inverter main circuit terminals
(three-phase input: L1/R, L2/S, L3/T) from overload or short-circuit, which in turn prevents secondary accidents
caused by inverter damage trouble.
Earth Leakage Circuit Breakers (ELCBs) function in the same way as MCCBs, and are connected to protect main
circuit wiring to the inverter, and for power supply switching.
Built-in overcurrent/overload protective functions protect the inverter.
■ Magnetic contactor
MCs can be used at both the power input and output sides of the inverter. MCs are used as described below. Use
them as necessary. When inserted in the output circuit of the inverter, MCs can also be used for switching the
commercial power supply for inverter drive motors.
At the power supply side
MCs at the inverter input side are used in the following cases.
(1) To forcibly cut off the inverter from the power supply with the protective function built into the inverter, or with
the external signal input.
(2) To stop the inverter operation in an emergency when unable to input the stop command due to circuit trouble.
(3) To cut off the inverter from the power supply when the MCCB inserted in the power supply side cannot cut it
off for maintenance or inspection purpose If using for this purpose only, it is recommended that you use an
MC capable of turning the MC ON/OFF manually.
Avoid frequent ON/OFF operation of the magnetic contactor (MC) at the input side (primary side);
otherwise, the inverter failure may result.
The frequency of the MC's ON/OFF should not be more than once per 30 minutes. To assure 10-
year or longer service life of the inverter, it should not be more than once per hour.
If frequent start/stop of the motor is required, use [FWD]/[REV] terminal signals or the / keys
on the inverter keypad.
At the output side
MCs at the inverter output side are used in the following cases.
Prevent externally turned-around current from being applied to the inverter power output terminals ([U], [V], and
[W]) unexpectedly. An MC should be used, for example, when a circuit that switches the motor driving power
supply between the inverter output and commercial power lines is connected to the inverter.
If a magnetic contactor (MC) is installed at the inverter output side (secondary side) for switching
the motor to a commercial power or for any other purposes, it should be switched on and off when
both the inverter and motor are completely stopped. This prevents the contact point from getting
damaged due to a switching arc of the MC. Do not equip magnetic contactors with main circuit
surge suppression units (Fuji Electric SZ-ZM□, etc.)
Applying a commercial power to the inverter's output (secondary) circuit breaks the inverter. To
avoid this, employ an interlock to ensure that the magnetic contactors at the commercial power
supply side and inverter output side do not turn ON at the same time.
(2) Drive more than one motor selectively by a single inverter.
(3) Selectively cut off the motor whose thermal overload relay or equivalent devices have been activated.
Driving the motor using commercial power lines
MCs can also be used to switch the power supply of the motor driven by the inverter to a commercial power supply.
Select the MC so as to satisfy the rated currents listed in Table 11.2-1, which are the most critical RMS currents for
using the inverter (see Table 11.3-1). For switching the motor drive source between the inverter output and
commercial power lines, use the MC of class AC3 specified by JIS C8325 in the commercial line side.
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