Hitachi NJI-350B - Manual (Page 248)

Chapter 10 PLC Installation, Mounting, Wiring
10-3
10.2 Wiring
(1) Separation of the power system
The power supplies include power for the MICRO-EH main unit/power for the I/O signals/power for general
equipment. These power supplies should be wired from separate systems as much as possible.
When these power supplies are supplied from one main power source, separate the wiring with a transformer or
similar device, so that each power supply is a separate system.
NF
100 V AC
to
240 V AC
NF
NF
Main power supply
Power for the PLC unit
Transformer
Transformer
NF: noise filter
Power for I/O signals
Power for general equipment
Figure 10.3 Example of power system diagram
(2) Regarding fail safe
1] Construct an interlock circuit external to the MICRO-EH.
When the MICRO-EH’s power is turned on or off, the inputs/outputs of the MICRO-EH may not temporarily
operate normally due to the time lag of the power supply of the MICRO-EH’s main unit, the external power
supply of the MICRO-EH’s expansion unit, and the external power supply (especially DC power supply) for
the MICRO-EH’s I/O signals, as well as the difference in their startup times.
Thus, either turn on the power to the expansion unit first, or turn on the power to both the base unit and
expansion unit simultaneously. Also, be sure to turn on the external power supply (especially DC power supply)
for the MICRO-EH’s I/O signals before turning on the MICRO-EH.
Additionally, a problem in the external power supply or a malfunction in the MICRO-EH’s main unit may
cause abnormal operations. To prevent such problems from causing abnormal operations of the entire system,
and from the viewpoint of creating a fail-safe mechanism, construct such circuits as an emergency stop circuit,
protective circuit and interlock circuit external to the MICRO-EH for the sections that may result in mechanical
damage or accident if abnormal operations occur.
2] Install a lightning arrester
To prevent damage to the equipment as a result of being struck by lightning, it is recommended that a lightning
arrester be installed for each MICRO-EH’s power supply circuit.
The MICRO-EH detects a power failure from a voltage drop in the internal 5 VDC power supply. For this
reason, when the load in the unit's internal 5 VDC system is light, 5 VDC is retained for a long period of time
and operations may continue for more than 100 ms. Thus, when an AC input unit is used, an off-delay timer for
coordinating with the internal 5 VDC system is required to avoid erroneous input since the AC input signal
turns off more quickly than the internal 5 VDC system.