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B.6 Noise Suppression at the Source
Introduction
Generally speaking, when good wiring practices are used with well- designed digital electronic equipment,
no further noise protection is necessary. However, in some severe electrical environments, the magnitude of
the electrical noise is so great that it must be suppressed at the source. In most control cabinets, the main
sources of noise are motor starters, contactors, relays, and switching gear. For this reason, many
manufacturers of these devices supply ''surge suppressors'' which mount directly on the noise source (for
example, on the coil of a control relay or motor starter).
For those devices that do not have accessory ''surge suppressors,'' resistance-capacitance (RC) circuits
and/or voltage limiters such as metal varistors may be added when and where needed. This can be broken
down into two categories, namely inductive loads (for example, a relay switch in series with a relay coil)
and contacts.
Inductive coils
Metal Oxide Varistors (MOVs) are recommended for transient suppression in inductive coils. An MOV is
connected in parallel with the coil and is as close as physically possible to the coil (see Figure Appendix B-
2). MOV devices (listed in Table Appendix B-2) are recommended for general purpose applications.
Table Appendix B-2 lists part numbers for recommended MOV devices.
Part Number
Maximum AC
Energy Pulse Rating
Supplier (General Electric)
12/00
Table Appendix B-2 MOV Devices
30732481-501
130V
10 Joules
V130LA10A
VPR & VRX – User Manual
Appendix B
30732481-502
275V
15 Joules
V275LA15A
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