The way in which SPDs are connected can have a large impact on the overall protection provided
Bryan Cole
Introduction
Surge protective devices (spds) or transient voltage surge suppressors (TVSS) are used to reduce the impact of overvoltage transients on the electrical distribution networks found in various industries. Of course, the efficacy of these devices will affect the overall protection of the distribution network; how these devices are connected can have a large impact on the overall protection provided by an SPD.
Different types of configurations are used to connect SPDs to the electrical distribution network. The first is a parallel connection, which is commonly referred to as a one-port configuration. The second type is a series connection, which is referred to as a two-port configuration. There is also a type of configuration that is called a “Kelvin connection.” The configuration of a Kelvin connection is very similar to that of a two-port connection, but with small differences that will be explained.
Transient Parameters
Overvoltage transients are generated by a number of sources. These include natural events, such as lightning and electrostatic charge buildup, and man-made sources including switching utility surges, the opening of overcurrent protective devices, or the energizing or collapse of motor loads. Overvoltage transients vary significantly from event to event. Some transients are quite brief, while others are characterized by long durations. In general, all transients have fast rise times in comparison to the currents of AC electrical power. While overvoltage transient waveforms vary, there are some standard waveforms developed by the Institute of Electrical and Electronic Engineers (IEEE) that have been used for many years to evaluate SPDs.1 SPDs that have been evaluated using these waveforms have performed well in protecting electrical distribution networks.
Of the standard waveforms, the most common transient used to evaluate SPDs is the 8/20 µs current waveform. The 8/20 µs current waveform is an exponentially decaying transient with a rise time of 8 µs and a duration to one-half peak of 20 µs. A graphical representation of the 8/20-µs current waveform is shown in Figure 1. The 8/20-µs current waveform has a fast rise time and a medium duration time.
A port TVSS (or SPD) is defined as “an SPD having provisions (terminals, leads, plugs) for connection to the AC power circuit, but no provisions (terminals, leads, receivers) for supply current to AC power loads.”2 A one-port connection is a parallel connection. Figure 2 shows a one-port or parallel-connected SPD. In this connection type, there is just one conductor per mode. In this example, only the line and neutral conductors are shown, but an all-mode, three-phase wye system would require that all modes be connected to the electrical distribution network using conductors for line 1, line 2, line 3, neutral, and ground.
There are advantages and disadvantaging associated with using a one-port SPD. The main advantage of a one-port SPD is that it is connected in shunt with the load. Consequently, the SPDs:
Are independent of the steady-state current rating of the supply.
Are independent of the steady-state and short circuit current rating of the load.
Can utilize separate overcurrent protection from the load.
Can be serviced without removing electrical power to the load.
A one-port SPD is typically connected to electrical distribution at the service entrance or the distribution panels. Article 285.6 of the National Electric Code requires that all SPDs must have a short circuit current rating that is equal to or greater than the point of application.3 To meet this requirement, a one-port SPD can be connected through a circuit breaker or fuse That provides proper overcurrent protection.4
The SPD is in parallel with all other loads on the same panel, which makes it one of many, so the only current drawn by the SPD is the current required to power indication circuitry—as well as the currents from any parasitic parameters of the overvoltage protection components. For most one-port SPDs, the current draw is less than 3 A per phase. Therefore, the SPD is connected to a specific breaker based on the conductor size required to meet the installation needs of the circuit breaker and the SPD, both of which are identified in the product installation manuals.
Because the SPD is connected in shunt with the load, it can be de-energized to allow for maintenance without removing the electrical power from the load. This option ensures that the load will remain operational regardless of the operational status of the SPD.
The main disadvantage of the one-port connected SPD is that the lengthy leads for the conductors can give rise to impedance that reduces the amount of protection provided by the SPD. The function of the SPD is to reduce overvoltage transients by current. If this reduction is accomplished using voltage-limiting components, e.g., metal-oxide varistors (MOVs), a let-through voltage of the SPD is created. For those devices that are applied in the U.S., Underwriters Laboratories (UL) issues a suppressed voltage rating (SVR) that is similar to the let-through voltage of the SPD.4 The SVR of the SPD can be used in determining the overall protection level of the installed SPD. Referring to Figure 2, the let-through voltage is identified as VSPD.
Once the overvoltage protection characteristics of the SPD are known, the voltage drop resulting from the conductors used to connect the SPD to the network must be determined. In Figure 2, the conductors are identified as VL.
The conductors that connect the SPD to the electrical distribution have resistive, inductive, capacitive, and conductance parameters. For most cases, the essential parameter of concern is inductance.
The way in which SPDs are connected can have a large impact on the overall protection provided
Bryan Cole
Introduction
Surge protective devices (spds) or transient voltage surge suppressors (TVSS) are used to reduce the impact of overvoltage transients on the electrical distribution networks found in various industries. Of course, the efficacy of these devices will affect the overall protection of the distribution network; how these devices are connected can have a large impact on the overall protection provided by an SPD.
Different types of configurations are used to connect SPDs to the electrical distribution network. The first is a parallel connection, which is commonly referred to as a one-port configuration. The second type is a series connection, which is referred to as a two-port configuration. There is also a type of configuration that is called a “Kelvin connection.” The configuration of a Kelvin connection is very similar to that of a two-port connection, but with small differences that will be explained.
Transient Parameters
Overvoltage transients are generated by a number of sources. These include natural events, such as lightning and electrostatic charge buildup, and man-made sources including switching utility surges, the opening of overcurrent protective devices, or the energizing or collapse of motor loads. Overvoltage transients vary significantly from event to event. Some transients are quite brief, while others are characterized by long durations. In general, all transients have fast rise times in comparison to the currents of AC electrical power. While overvoltage transient waveforms vary, there are some standard waveforms developed by the Institute of Electrical and Electronic Engineers (IEEE) that have been used for many years to evaluate SPDs.1 SPDs that have been evaluated using these waveforms have performed well in protecting electrical distribution networks.
Of the standard waveforms, the most common transient used to evaluate SPDs is the 8/20 µs current waveform. The 8/20 µs current waveform is an exponentially decaying transient with a rise time of 8 µs and a duration to one-half peak of 20 µs. A graphical representation of the 8/20-µs current waveform is shown in Figure 1. The 8/20-µs current waveform has a fast rise time and a medium duration time.
A port TVSS (or SPD) is defined as “an SPD having provisions (terminals, leads, plugs) for connection to the AC power circuit, but no provisions (terminals, leads, receivers) for supply current to AC power loads.”2 A one-port connection is a parallel connection. Figure 2 shows a one-port or parallel-connected SPD. In this connection type, there is just one conductor per mode. In this example, only the line and neutral conductors are shown, but an all-mode, three-phase wye system would require that all modes be connected to the electrical distribution network using conductors for line 1, line 2, line 3, neutral, and ground.
There are advantages and disadvantaging associated with using a one-port SPD. The main advantage of a one-port SPD is that it is connected in shunt with the load. Consequently, the SPDs:
Are independent of the steady-state current rating of the supply.
Are independent of the steady-state and short circuit current rating of the load.
Can utilize separate overcurrent protection from the load.
Can be serviced without removing electrical power to the load.
A one-port SPD is typically connected to electrical distribution at the service entrance or the distribution panels. Article 285.6 of the National Electric Code requires that all SPDs must have a short circuit current rating that is equal to or greater than the point of application.3 To meet this requirement, a one-port SPD can be connected through a circuit breaker or fuse That provides proper overcurrent protection.4
The SPD is in parallel with all other loads on the same panel, which makes it one of many, so the only current drawn by the SPD is the current required to power indication circuitry—as well as the currents from any parasitic parameters of the overvoltage protection components. For most one-port SPDs, the current draw is less than 3 A per phase. Therefore, the SPD is connected to a specific breaker based on the conductor size required to meet the installation needs of the circuit breaker and the SPD, both of which are identified in the product installation manuals.
Because the SPD is connected in shunt with the load, it can be de-energized to allow for maintenance without removing the electrical power from the load. This option ensures that the load will remain operational regardless of the operational status of the SPD.
The main disadvantage of the one-port connected SPD is that the lengthy leads for the conductors can give rise to impedance that reduces the amount of protection provided by the SPD. The function of the SPD is to reduce overvoltage transients by current. If this reduction is accomplished using voltage-limiting components, e.g., metal-oxide varistors (MOVs), a let-through voltage of the SPD is created. For those devices that are applied in the U.S., Underwriters Laboratories (UL) issues a suppressed voltage rating (SVR) that is similar to the let-through voltage of the SPD.4 The SVR of the SPD can be used in determining the overall protection level of the installed SPD. Referring to Figure 2, the let-through voltage is identified as VSPD.
Once the overvoltage protection characteristics of the SPD are known, the voltage drop resulting from the conductors used to connect the SPD to the network must be determined. In Figure 2, the conductors are identified as VL.
The conductors that connect the SPD to the electrical distribution have resistive, inductive, capacitive, and conductance parameters. For most cases, the essential parameter of concern is inductance.
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