Electrical noise is the result of more or less random electrical signals getting coupled into circuits where they are unwanted, i.e., where they disrupt information-carrying signals. Noise occurs on both power and signal circuits, but generally speaking, it becomes a problem when it gets on signal circuits. Signal and data circuits are particularly vulnerable to noise because they operate at fast speeds and with low voltage levels. The lower the signal voltage, the less the amplitude of the noise voltage that can be tolerated. The signal-to-noise ratio describes how much noise a circuit can tolerate before the valid information, the signal, becomes corrupted.
Noise is one of the more mysterious subjects in power quality, especially since it must be considered with its equally mysterious twin, grounding. To lessen the mystery, there are two key concepts to understand:
- The first is that electrical effects do not require direct connection (such as through copper conductors) to occur.
- The second concept is that we can no longer stay in the realm of 60 Hz. One of the benefits of 60 Hz is that it's a low enough frequency that power circuits can be treated (almost) like dc circuits.
Coupling MechanismsThere are four basic mechanisms of noise coupling. It pays to understand them and how they differ one from the other because a lot of the troubleshooter's job will be to identify which coupling effect is dominant in a particular situation.
1. Capacitive CouplingThis is often referred to as electrostatic noise and is a voltage-based effect. Lightning discharge is just an extreme example. Any conductors separated by an insulating material (including air) constitute a capacitor - in other words, capacitance is an inseparable part of any circuit. The potential for capacitive coupling increases as frequency increases (capacitive reactance, which can be thought of as the resistance to capacitive coupling, decreases with frequency, as can be seen in the formula: XC = 1/2pfC).
2. Inductive CouplingThis is magnetic-coupled noise and is a current-based effect. Every conductor with current flowing through it has an associated magnetic field. A changing current can induce current in another circuit, even if that circuit is a single loop; in other words, the source circuit acts as a transformer primary with the victim circuit being the secondary. The inductive coupling effect increases with the following factors: (1) larger current flow, (2) faster rate of change of current, (3) proximity of the two conductors (primary and secondary) and (4) the more the adjacent conductor resembles a coil (round diameter as opposed to flat, or coiled as opposed to straight).
Magnetic fields are isolated by effective shielding. The material used must be capable of conducting magnetic fields (ferrous material as opposed to copper). The reason that a dedicated circuit (hot, neutral, ground) should be run in its own metal conduit when possible is that is in effect magnetically shielded to minimize inductive coupling effects.
Both inductive and capacitive coupling are referred to as near field effects, since they dominate at short distances and distance decreases their coupling effects. This helps explain one of the mysteries of noise - how slight physical repositioning of wiring can have such major effects on coupled noise.
3. Conducted NoiseWhile all coupled noise ends up as conducted noise, this term is generally used to refer to noise coupled by a direct, galvanic (metallic) connection. Included in this category are circuits that have shared conductors (such as shared neutrals or grounds). Conducted noise could be high frequency, but may also be 60 Hz.
Common examples of connections that put objectionable noise currents directly onto the ground:
- Sub-panels with extra N-G bonds
- Receptacles miswired with N and G switched
- Equipment with internal solid state protective devices that have shorted from line or neutral to ground, or that have not failed but have normal leakage current. This leakage current is limited by UL to 3.5 mA for plug-connected equipment, but there is no limit for permanently wired equipment with potentially much higher leakage currents. (Leakage currents are easy to identify because they will disappear when the device is turned off).
4. RFI (Radio Frequency Interference)RFI ranges from 10 kHz to the 10 s of MHz (and higher). At these frequencies, lengths of wire start acting like transmitting and receiving antennas. The culprit circuit acts as a transmitter and the victim circuit is acting as a receiving antenna. RFI, like the other coupling mechanisms, is a fact of life, but it can be controlled (not without some thought and effort, however).
RFI noise reduction employs a number of strategies.
Signal GroundingTo understand the importance of "clean" signal grounds, let's discuss the distinction between Differential Mode (DM) vs. Common Mode (CM) signals. Imagine a basic two-wire circuit: supply and return. Any current that circulates or any voltage read across a load between the two wires is called DM (the terms normal mode, transverse mode and signal mode are also used). The DM signal is typically the desired signal (just like 120V at a receptacle). Imagine a third conductor, typically a grounding conductor. Any current that flows now through the two original conductors and returns on this third conductor is common to both of the original conductors. The CM current is the noise that the genuine signal has to overcome. CM is all that extra traffic on the highway. It could have gotten there through any of the coupling mechanisms, such as magnetic field coupling at power line frequency or RFI at higher frequencies. The point is to control or minimize these ground or CM currents, to make life easier for the DM currents.
MeasurementCM currents can be measured with current clamps using the zero-sequence technique. The clamp circles the signal pair (or, in a three-phase circuit, all three-phase conductors and the neutral, if any). If signal and return current are equal, their equal and opposite magnetic fields cancel. Any current read must be common mode; in other words, any current read is current that is not returning on the signal wires, but via a ground path. This technique applies to signal as well as power conductors. For fundamental currents, a ClampMeter or DMM + clamp would suffice, but for higher frequencies, a high bandwidth instrument like the Fluke 43 Power Quality Analyzer or ScopeMeter should be used with a clamp accessory. Transients should be distinguished from surges. Surges are a special case of high-energy transient which result from lightning strikes. Voltage transients are lower energy events, typically caused by equipment switching.
They are harmful in a number of ways.
Transients can be categorized by waveform. The first category is "impulsive" transients, commonly called "spikes", because a high-frequency spike protrudes from the waveform. The cap switching transient, on the other hand, is an "oscillatory" transient because a ringing waveform rides on and distorts the normal waveform. It is lower frequency, but higher energy.
CausesTransients are unavoidable. They are created by the fast switching of relatively high currents. For example, an inductive load like a motor will create a kickback spike when it is turned off. In fact, removing a Wiggy (a solenoid voltage tester) from a high-energy circuit can create a spike of thousands of volts! A capacitor, on the other hand, creates a momentary short circuit when it's turned on. After this sudden collapse of the applied voltage, the voltage rebounds and an oscillating wave occurs. Not all transients are the same, but as a general statement, load switching causes transients.
In offices, the laser copier/printer is a well-recognized "bad guy" on the office branch circuit. It requires an internal heater to kick in whenever it is used and every 30 seconds or so when it is not used. This constant switching has two effects: the current surge or inrush can cause repetitive voltage sags; the rapid changes in current also generate transients that can affect other loads on the same branch.