IEEE C62.42.1-2016 pdf free.IEEE Guide for the Application of Surge-Protective Components in Surge Protective Devices and Equipment Ports- -Part 1: Gas Discharge Tubes (GDTs).
Gas discharge tubes consist of two or more metal electrodes separated by a small gap and held by a ceramic or glass cylinder. Figure 4 shows a two-electrode GDT.
The cylinder is filled with a noble gas mixture. When sufficient voltage is applied to the electrodes, gas sparkover occurs into a glow discharge mode and finally a low-voltage arc condition when sufficient current is available. When a slowly rising voltage across the gap reaches a value determined primarily by the electrode spacing, gas pressure. and gas mixture, the turnon process initiates at the sparkover (breakdown) voltage. Once sparkover occurs, various operating states are possible, depending upon the external circuitry. These states are shown in Figure 1. At currents less than the glow-to-arc transition current, a glow region exists. At low currents in the glow region, the voltage is nearly constant: at high glow currents, some arrester types may enter an abnormal glow region in which the voltage increases. Beyond this abnormal glow region the tube impedance decreases in the transition region into the low-voltage arc condition. The arc-to-glow transition current may be lower than the glow-to-arc transition current. The (IDT electrical characteristic, in conjunction with the external circuitry, determines the ability of the gas tube arrester to extinguish after passage of a surge, and determines the energy dissipated in the arrester during the surge.
If the applied voltage (e.g., transient) rises rapidly, the time taken t’or the ionization/arc formation process may allow the transient voltage to exceed the value required for breakdown in the previous paragraph. This voltage is defined as the impulse breakdown voltage and is generally a positive tunction of the rate of rise of the applied voltage (transient).
A single chamber three-electrode (iDT has two cavities separated by a center ring electrode (see Figure 5). The hole in thc center electrode allows gas plasma from a conducting cavity to initiate conduction in the other cavity, even though the other cavity voltage may be below the sparkover voltage.
Because of their switching action and rugged construction, gas tubes esceed other voltage limiting surge protective components in current-carrying capability. Many gas tubes intended for telecommunication applica. tions can easily carry surge currents as high as 10 kA. g120; further, depending on design and size of the gas tube, surge current values of >100 kA. J2O can be achieved.
The construction of gas discharge tubes is such that they have very low capacitance, generally less than 2 pR.
This allows their use in many high frequency circuit applications.
Whcn GDTs (gas discharge tubes) operate, they may generate high frequency radiation, which can influcncc sensitivc clcctronics. It is therefore wise to place (IDT circuits at a ccrtain distance from the electronics. The distance depends on the sensitivity of the clectronics and how well thc clcctronics arc shielded. Another method to avoid thc cllcct is to place the GDT in a shielded cnclosure.
5. Characteristics
In terms of voltage limiting pcrliirmancc, a GDT has ftur key paramcters sparkoser vollage, glow voltage. arc voltage, and dc holdover voltage (see Figure 6).
5.1 GDT sparkover voltage
5.1.1 DC and impulse sparkover voltgage
The maximum value of limiting voltage depends on the surge voltage rate of rise. Figure 7 shows a typical relationship between the (1DT & sparkover and the impulse, fast rate of voltage rise, sparkover of a GDT. In this example, the minimum 1000 V/is sparkover voltage of 575 V occurs with a 150 V dc sparkover voltage GLYI. The much lower voltage 75 V de sparkover (il)T has a 1000 V4is sparkover voltage of 700 V—nine times higher than the dc value. Where fast rising transients occur. often the 150 V GL)T will be more eftective thana75VGDT.
Figure 7 shows absolute values of voltage. In terms of relative voltage increase, then this factor continuously decreases with increasing voltage, being: 9 at 75 V.4 at ISO V. 2.6 at 3(X) V and 2.1 at 6(X) V. For example. two ISO V (iDTs in series would have a 10(X) V4is sparkover of 1150 V. but a single 300 V GDT would have a 1000 V sparkover of 775 V. Two or more series connected GDTs will always have a net 1000 V/1.s sparkover voltage higher than the equivalent single T. These numbers are just for demonstration ofGL)T characteristics and may vary for different designs.
5.1.2 Sparkover voltage stability
GDTs have inherent end-of-service mechanisms. By selecting the appropriate GDT for an application, the desired service life can be achieved such as 30 years of service life for applications is network telecommunications equipment. Prime indicators ofend-of-serv ice are changes to the insulation resistance and sparkover voltage. Figure X shows the measured change in sparkover voltage with number of impulses for heavy duty 500 A. 101000 (JI)Ts. This shows the importance of’ suppliers giving users assurances of not only day pertirmance. but also stability over life.
System designers would like GDTs with dc-to-impulse sparkover voltage ratios as low as possible. In 2000. as a response. some GDT formulations with lower sparkover voltages were introduced. The downsides to such ftrmulaiions were often higher arc voltages, more ac power loss, and shorter life times compared to standard (11)1’s. Where such faster wear out, or shorter-lived, GDTs are installed, the end users may need to introduce (DT replacement maintenance programs.IEEE C62.42.1 pdf download.