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In this multipart series we will cover surge and transient protection for all types of signal, control and power lines. My goal is to help the reader understand the basic principles and be able to assess the capabilities of the types of protection available on the market today. One of the most important skills you should learn from this series is the ability to ask the right questions and evaluate the answers received.
Some of the topics we will cover include:
The most important part of this series is YOU. It is through your feedback that I can best understand your needs and can help you develop the tools you need in this area.
Surges and transients are a fact of life in residential, commercial, and industrial electrical systems. The NEC recognizes this and provides some guidance on the connection of these devices. Surges and transients can have a number of sources. These sources are both external and internal. External sources include lightning, power system switching, faults, inductive loads and other causes. Internal sources include fluorescent lighting, inductive load switching (starting and stopping of motors especially) faults, welding and other causes. To begin this series let us look first at the NEC requirements in Article 280 for surge arresters and then at Article 285 for transient voltage surge suppressors (TVSSs).
Secondary surge arresters are those devices that are designed for connection to the wiring system after the distribution transformer but prior to any overcurrent protection. These devices are normally at system phase to ground voltages of less than 1kV. When used on circuits of less than 1 kV they must be listed, rated for the available fault current and rated at or greater than the maximum continuous phase-to-ground power frequency voltage available at the point of application. Where used, the surge arrester is required to be connected to each ungrounded conductor. The grounding conductor for the arrester elements can be connected to one of four items:
Devices listed as surge arresters may also be connected on the load side of the overcurrent protection. Some devices are listed as both surge arresters and as transient voltage surge suppressors.
Silicon Carbide type surge arresters are also permitted by Article 280 on circuits at system phase to ground voltages of 1 kV or greater. We will not discuss these higher voltage arresters at this time but may come back to them if there is sufficient interest in the feedback.
NEC article 285 governs the use of TVSSs permanently installed on premises wiring systems. In residential wiring systems such devices are normally wired into the service entrance equipment. In commercial or industrial locations, additional TVSSs may be installed at other locations such as branch panels to shunt both remnants of externally generated surges and internally generated surges back to their source. TVSSs are only permitted on circuits of 600 volts or less and on the load side of the overcurrent device. They must be listed, normally to UL-1449 version 2, and rated for the maximum continuous phase-to-ground power frequency voltage available at the point of application and for the available fault current. TVSSs are not permitted to be installed on ungrounded systems, impedance grounded systems, or corner grounded delta systems unless they are specifically listed for use on these systems. TVSSs are permitted between any two conductors - ungrounded conductor(s), grounded conductor, grounding conductor.
The NEC points out that the conductors used to connect the surge arrester or TVSS to line or bus and to ground must be no longer than necessary and avoid unnecessary bends. These conductors must be a minimum of #14 AWG copper or #12 AWG aluminum. Where bend radiuses, fill and clearances permit, a larger wire size is preferable. Additionally, the NEC mandates that the grounding conductors must not be run in metal enclosures unless the conductor is bonded to both ends of the enclosure. This requirement ensures minimum inductance in the interconnection wiring.
In our next segments we will begin to look at how surges and transients are generated, how they impact our facilities, basic operations of surge arresters and TVSSs and the individual components they employ. We will also consider the impact of inductance on suppression operations.
Lightning and Transient Protection, Grounding,
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