4-4-3
Selecting Protective Devices
This section describes how to select protective devices (e.g., breakers and fuses) to protect against
short circuits and overcurrents in external circuits.
Overcurrent is the current that flows when an excessive load is connected and one of the following
ratings is exceeded.
Unit Item for rating Rated value
CPU Unit NX Unit power supply capacity 10 W max.
Current capacity of power supply termi-
nal
4 A max.
Additional NX Unit Power
Supply Unit
NX Unit power supply capacity Refer to the NX-series System Units
User
’
s Manual (Cat. No. W523).
Current capacity of power supply termi-
nal
Additional I/O Power Supply
Unit
Maximum I/O power supply current
Current capacity of I/O power supply ter-
minals
Selecting Protective Devices
Consider the following items when you select protective devices.
• Protective device specifications (breaking/fusing, detection characteristics, steady current value,
etc.)
•
Inrush current when power is turned ON
• Inrush current when connected external devices turn ON and OFF*1
*1. Refer to 4-3-4 Restrictions on Inrush Current for ON/OFF Operation on page
4-20 for information on the in-
rush current when connected external devices are turned ON and OFF.
For the breaking/fusing time, use protective devices that meet the conditions in the following table.
l
For Unit Power Supply
Current Breaking/fusing time
6 A 1 min max.
12 A 15 s max.
21 A 5 s max.
30 A 2.5 s max.
l
For I/O Power Supply
The following values apply when the current capacity of power supply terminal is 10 A.
Current Breaking/fusing time
14 A 1 min max.
28 A 9 s max.
56 A 1.5 s max.
70 A 0.8 s max.
The following values apply when the current capacity of power supply terminal is 4 A.
4 Designing the Power Supply System
4-25
NX-series NX502 CPU Unit Hardware User's Manual (W629)
4-4 Selecting External Power Supplies and Protective Devices
4
4-4-3 Selecting Protective Devices
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