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The How's and Why's of IG Wiring

Aug 1, 2008 12:00 PM, By Thomas M. Gruzs, Emerson Network Power

Sidebar: What's Electrical Noise?

Common-mode noise is any unwanted signal that is common to all circuit conductors simultaneously. The other form of noise is normal-mode noise (also known as transverse or differential-mode noise), which is any unwanted signal that exists between the circuit conductors.

In AC power systems, the difference in potential between neutral and ground is one form of common-mode noise, because any change in neutral potential relative to ground also affects all of the other power circuit conductor potentials to ground. Another more troublesome form of common-mode noise is the differences in ground potentials throughout an electrical system.

When multiple electronic devices are interconnected by way of control, data, or communication cables, any difference in ground potentials between the interconnected pieces of equipment is common-mode noise to the control, data, or communication circuits. It's virtually impossible to keep all of the (chassis) ground potentials of distributed electronic devices at the same potential under all possible circumstances. Therefore, manufacturers design some level of common-mode noise immunity into electronic devices that are intended to be interconnected. Furthermore, the surge suppression, wiring, shielding, and grounding of the building electrical system (including the control, data, and communication cabling) can have a pronounced effect on the levels of common-mode signals to which the electronics are exposed.

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