Page 304 - Programmable Logic Controllers, Fifth Edition - Mobile version

L1
Input module N

2-wire
proximity
switch

Bleeder
resistor
Figure 13-5 Heat-shrinkable wire identification sleeves. Common
Source: Courtesy Tyco Electronics Ltd. Leakage
current
Figure 13-6 Bleeder resistor connection for input sensors.
• Low-level signal conductors such as thermocou-
ples and serial communications should be run as
shielded twisted pair and routed separately. L1 N
• A fiber optic system, which is totally immune to all Output
kinds of electrical interference, can also be used for module
signal wiring.
High-
An important part of a PLC installation is clearly iden- L1 impedance
tifying each wire to be connected and the terminal to load
which it is connected. A reliable labeling method, such
as the heat-shrinkable wire identification sleeves shown
in Figure 13-5, should be used to label each wire. Wir- Bleeder
ing connectors for input/output modules usually include resistor
spaces for labels used for identifying each I/O address and
device connected. Proper wire and terminal identification Leakage
current
will simplify the installation and aid in troubleshooting
and maintenance. Figure 13-7 Bleeder resistor connection for a high-
impedance output.

13.3 Leaky Inputs and Outputs Leakage current may also occur with the solid-state

Many electronic devices with transistor or triac outputs switch used in many output modules. Problems similar to
exhibit a small leakage current even when in the off state that encountered with input modules can be created when
that may need to be considered when they are connected a high-impedance load device is used with these modules.
to PLC input modules. This so-called leakage is typi- For example, a PLC output might supply a sound alert
cally exhibited by two-wire proximity, photoelectric, and device as illustrated in Figure 13-7. In this case the leak-
other such sensors. Often, the leaky input will only cause age current could cause continuous false or intermittent
the module’s input indicator to flicker. However, a large operation. A resistor can be connected as shown to bleed
enough leakage current can activate the input circuit, cre- off this current. An isolation relay could also be used to
ating a false input signal. solve this type of problem.
A common solution to the problem of leaky input cur-
rent is to connect a bleeder resistor across or in paral- 13.4 Grounding
lel with the input, as shown in Figure 13-6. The bleeder
resistor acts as an additional lower resistance load, which Proper grounding is an important safety measure in
allows the leakage current to flow through the lower re- all electrical installations. The authoritative source on
sistance path. Typically a 10 to 20 kΩ resistor is used to grounding requirements for a PLC installation is the
solve the problem. National Electrical Code. The NEC specifies the types

PLC Installation Practices, Editing, and Troubleshooting Chapter 13 285

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