White Paper – May, 2007
Parallel connected “Sine Wave Tracking” (SWT) surge suppressors and series connected “Active Tracking  ” filters (ATF) both have their place in the “Total Copper
Wire Protection” scheme. In order to achieve the most effective applications, it is important to first look at how both types of systems are connected to the facility’s power grid. Figure 1 shows connection diagrams for both types of systems. The parallel connected device is tapped off the load side of a service panel. Typically, a dedicated circuit breaker in the service or branch panel is used as the means of connection. On the other hand, the series connected filter, also wired to the load side of the service panel, is directly connected in-line with the protected equipment (typically hard wired with no quick means of disconnect). To understand why one type might be the better choice, for a particular application, it is
Parallel Connected SPD
Ø's
N
G
Service
Panel
To
"Protected"
Panel or
Loads
SPD
Ø's
N
G
Service
Panel
Series Connected Filter
Active
Tracking
Filter
To
"Protected"
Panel or
Loads
Figure 1.
Connection Diagrams necessary to now look at each from a component level.
Parallel Connected Devices:
Parallel surge protective devices (SPDs) generally contain components that clamp and divert transients away from the load. The technologies most commonly used are gas tubes, silicon avalanche diodes (SADs) and metal oxide varistors (MOVs). In this paper, MOVs are going to be the primary component discussed. MOVs are voltage sensitive components that begin to conduct current when a transient voltage condition exists on the line.
The diagram, in figure 2, shows the inside of a typical parallel surge protection device. The MOVs are attached across, or in parallel to, the incoming wiring from the panel. The length of the surge paths a, b, and c are dependent on the distance between the SPD and the panel it is connected to. This distance has a direct effect on the performance of the device. The greater the distance, the greater the amount of “let-through” voltage the loads are going to experience. Because parallel SPDs perform in this manner, installation rules require the SPD to be mounted as close as possible to the panel that it is protecting (general rule of thumb is less than five feet away). Often, this can be a difficult task for a contractor. Additionally, even if the contractor mounts the SPD as close as possible to the connecting panel, the lead length may have adverse effects on the desired performance.

Figure 2.
Parallel-Connected Components
Series Connected Filters:
“Active Tracking Filters” (ATF) were originally designed to protect sensitive equipment from highfrequency noise. Emerson Network Power Surge
Protection now offers these products with parallel and series components that provide high-energy transient protection as well as filtering.
Emerson Network Power Surge Protections ATFs use a low-pass circuit to eliminate high-frequency noise. Low-pass or L-C filters are the most popular type of circuit used for transient reduction and consist of series inductors, capacitor and resistors. ATFs are load dependent, which means that the series inductors located on each phase and neutral conductor, are sized to handle the maximum current draw on the line.
These inductors together with the capacitors and resistors form a circuit capable of absorbing a large bandwidth of noise. The ATF’s parallel, high-energy, clamping components are similar to that discussed in the previous section. Surge components (MOVs) are mounted directly across the lines with no additional lead length (Figure 3). The a, b and c surge paths are as short a possible allowing virtually all of the surge to get to the MOV. This minimizes the amount of let-though voltage and maximizes the circuit performance.
The components shown in figure 3 perform the following functions. Starting from the left side of the diagram, the series inductors block voltage and smooth current. Next, capacitor/resistor networks shunt and absorbs normal mode noise. Finally, the parallel MOVs clamp high-energy common mode (L-G) and normal mode (L-N) transients. This combination of staged components offers the tightest clamping voltage ratings in the industry.
Figure 3.
Series-Connected Components
Sine Wave Tracking:
“Sine wave tracking” is an industry wide term that refers to a device that contains parallel connected components as their sole means of transient protection. The majority of these products are parallel connected to the service or distribution panel. A “sine wave tracking” device has a threshold at which it clamps high-energy spikes (figure 5). This threshold, which is commonly called the unit’s clamping voltage, is based on three criteria:
1.
The nominal system voltage
2.
The size of the pulse
3.
The length of leads
For example, a 120V parallel device that is subjected to 6kV, .5A pulse at 6 inches outside the case will typically have a clamping voltage of 400 to
600V.
Additional benefits of parallel connected SPDs are as follows:
1.
Usually lower price than series connected SPDs.
2.
Because they contain no series components they generally have smaller overall size.
3.
Easy installation, power to equipment does not need to be interrupted for servicing.
Disadvantages of parallel connected SPDs include:
1.
Performance is at the mercy of the installer, because clamping voltage is dependent on lead length.
2.
Let-through voltage at 0 ° point is significantly more than at 90 ° on sine wave.
3.
High frequency noise may not be attenuated by the
SPD, thus leaving sensitive equipment unprotected.

“
Figure 4.
“Sine Wave Tracking” Performance
Active Tracking  ” Filters:
“Active Tracking  ” is a Emerson Network Power
Surge Protection patented term describing devices that contain series components. The devices are connected in series with the critical industrial loads they are protecting. Active tracking filters have the ability to attenuate high and low level transients at any point on the sine wave. This is accomplished by the addition of an
L-C circuit to the existing parallel components.
Figure 5 shows how high-energy transients and high-frequency noise are converted once they pass through the filter and to your sensitive equipment
.
benefits
1.
Converts potentially damaging high frequency noise into clean reliable ac power.
2.
Lowest possible clamping voltage because modules are directly across lines.
3.
Performance of device does not depend on installer.
Disadvantages
1.
Load dependent therefore may be substantially higher in cost than parallel connected devices.
2.
Power to equipment must be cut for installation and servicing.
Figure 5.
“ActiveTracking” Performance
Parallel (SWT) vs. Series (ATF):
A major difference between a parallel connected
“Sine Wave Tracking” device and a series connected
“Active Tracking Filter” is the way they respond to high frequency noise. High frequency (also known as
EMI/RFI) noise typically falls within the range of 10kHz to 150MHz. Figures 6 and 7 are EMI/RFI sweeps of a parallel and series connected devices. The X-axis of each graph indicates the frequencies injected into the device. The Y-axis represents the amount of attenuation, input versus output, measured in decibels.
The higher the dB value, the better the filter. The plot in figure 6 show a narrow curve or “notch” of 80dB maximum at the approximately 1MHz. As one can see, the parallel connected device is providing very limited protection at frequencies other than 1MHz. On the other hand, the plot in figure 7 shows a deep “bath tub” shaped curve reaching 90dB attenuation over a wide range of frequencies. Comparison of the two graphs clearly shows that the ATF design provides much more protection from high frequency noise then the SWT.

Figure 6.
EMI/RFI Sweep
Parallel Connected
Figure 7.
EMI/RFI Sweep
Series Connected
Conclusion:
After reading this paper the following questions regarding transient protection should be able to be answered.
1.
How are series and parallel devices connected within a facility?
2.
What types of components are used in a parallel TVSS?
3.
What type of components are used in a series filter?
4.
What protection does a TVSS provide?
5.
What protection does a ATF provide?
6.
What is the major difference in protection between a TVSS and ATF?
During the procurement process the decision will need to be made between purchasing a series filter or parallel SPD. Answering this final question should make this decision easier.
Are you trying to protect against the distruction of equipment, the disruption of machines or both?
If protection of equipment against destructive highenergy transients is required, then a parallel connected device is likely to be the most logical solution. On the other hand, if sensitive equipment is being disrupted by high frequency noise, then a series connected filter may be the right choice. Sometimes power quality equipment is installed before any problems arise, and it is not known what types of problems exist. In this case the staged approach is suggested, where both parallel and series connected devices are used within the facility.
References:
Cole, B. (May, 1994)
Theoretical Analysis of an L-C, Low-Pass Filter.
Control Concepts Corporation, Binghamton,
NY
Control Concepts. (1999)
Industrial Strength Protection Catalog
Binghamton, NY :Author
.

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