CA0000042 AECL, Chalk River Laboratories, Chalk River, Ontario, Canada KOJ 1J0
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SHIELDING AND FILTERING TECHNIQUES TO PROTECT SENSITIVE INSTRUMENTATION FROM ELECTROMAGNETIC INTERFERENCE CAUSED BY ARC WELDING W. Kalechstein CA0000042 AECL, Chalk River Laboratories, Chalk River, Ontario, Canada KOJ 1J0 discussed may be applied in the construction of ABSTRACT temporary shielding enclosures for arc welding to Electromagnetic interference (EMI) caused by arc reduce concern over possible EMI to nearby I&C welding is a concern for sensitive CANDU® equipment. The work complements previous instrumentation and control equipment, especially investigations that are summarized below. start-up instrumentation (SUI) and ion chamber instruments used to measure neutron flux at low A literature search and discussions with personnel at power. Measurements of the effectiveness of simple nuclear power plants and with EMI consultants in shielding and filtering techniques that may be applied Canada and the US has shown that arc-welding EMI to limit arc welding electromagnetic emissions below in instrumentation is a common problem. the interference threshold are described. Shielding Administrative controls are widely used to avoid arc configurations investigated include an arrangement in welding near potentially sensitive equipment [1]. At 7 which the welding power supply, torch (electrode some plants, the use of high-frequency (HI ) holder), interconnecting cables and welder operator stabilization is prohibited for arc welding near were housed in a single enclosure and a more practical sensitive equipment. configuration of separate shields for the power supply, cables and operator with torch. The two configura- Guidelines on electromagnetic compatibility (EMC) tions were found to provide 30 dB and 26 dB recently published by the Electric Power Research attenuation, respectively, for arc welder electric-field Institute (EPRI) advise that arc welding be avoided in emissions and were successful in preventing EMI in rooms containing in-service EMI-sensitive safety SUI set up just outside the shielding enclosures. equipment, but may be permitted, provided the arc Practical improvements that may be incorporated in welding equipment is contained in shielded enclosures the shielding arrangement to facilitate quick setup in [2, 3]. However, the authors were not able to identify the field in a variety of application environments, any implementation guidelines for shielded while maintaining adequate EMI protection, are enclosures, nor any plants that have applied shielding discussed. to arc welding operations. INTRODUCTION An experimental investigation of EMI effects in SUI operating in proximity to an industrial arc welder of a Electromagnetic interference (EMI) caused by arc type used at some CANDU stations was performed. welding is a concern for sensitive CANDU The investigation showed a low level of disturbance instrumentation and control (I&C) equipment. Arc that has little effect on the SUI output when the arc welding has caused interference with startup welder is operated in direct current (DC) mode without instrumentation (SUI) and normal ion-chamber-based HF stabilization [1]. neutronic instrumentation on a number of occasions. Although the problem is most severe when the reactor is at low power and the flux signal is very small, Measurements of arc welder electric and magnetic field there is concern about possible EMI effects whenever emissions showed strong emissions in the frequency arc welding near sensitive equipment is considered. range 0.25 to 10 MHz for welder operation in This often results in restrictions on or deferral of alternating current (AC) mode, with or without HF maintenance work involving arc welding. stabilization, and DC mode with HF stabilization. Strong emissions were found whenever HF stabilization was selected, even in the absence of a This paper describes simple shielding and filtering welding arc, indicating that significant emissions techniques to mitigate electromagnetic emissions originate in the welding power supply. In contrast, from arc welding and measurements of the welder operation in DC mode with HF stabilization effectiveness of these techniques. The techniques disabled was found to cause peak electric and magnetic AECL-11872, COG-97-430-I. CANDU® is a registered trademark of Atomic Energy of Canada Limited (AECL). 268 field emissions that are more than 100 times lower rolled steel sheet, steel screen (expanded metal), and are not far above the ambient level [4]. aluminum foil and EMI tape. A number of recommendations were made to help As shielding structures and sensitive equipment are avoid EMI caused by arc welding, including avoiding typically located in the near field of strong arc-welder the use of AC welding and HF stabilization [1]. emissions, the attenuation of electric and magnetic However, as the above are necessary for certain welds, fields needs to be evaluated separately. The shielding and use of HF stabilization allows superior welds to effectiveness of aluminum and steel sheet of practical be made, the above restrictions sometimes present thickness is expected to exceed 145 dB for both problems for maintenance operations and are not a electric and magnetic fields, while aluminum foil is fully satisfactory solution to the problem. expected to provide minimum shielding effectiveness of 120 dB for electric fields and 55 dB for magnetic SHIELDING ENCLOSURES FOR ARC fields, in the frequency range of interest [5]. WELDERS The shielding effectiveness for screen material is Requirements expected to be somewhat lower than for solid sheet. Nevertheless, shielding effectiveness of a practical The requirements for a shielding enclosure for arc shielding enclosure made of any of the above welding are as follows: materials, including screen, is expected to be limited by radiation from conducted emissions and leakage must provide adequate electric-field and magnetic- through apertures. field shielding, must provide for penetrations of the shield by Control of Conducted Emissions electrically-conductive structures such as the power line and work piece, without overly The test shielding enclosure constructed for the . compromising shielding effectiveness, welding power supply was penetrated by a 600 VAC, must allow the welder operator easy access to the single-phase power line and by the pipe being welded. work piece, admit light and provide for ventila- tion of gases evolved in welding, A power-line filter was installed in the power line must be easily constructed, using materials that near the entrance to the shielding enclosure to are easily brought to the site, and minimize high-frequency conducted emissions from must be grounded. the shielding enclosure. Such emissions give rise to electric and magnetic fields that reduce the Based on previous work, it is concluded that the effectiveness of the shielding enclosure. required shielding effectiveness (ratio of field strength in the absence of the shield to that in the presence of The power-line filter selected for the application was a the shield) is no more than about 60 dB for electric 7t-type, low-pass filter. The nominal insertion loss of fields and 40 dB for magnetic fields, and may be much the filter was at least 50 dB in the frequency range less. 0.25 to 10 MHz. The filter case and ground terminals were bonded to the input AC power ground, the The shielding effectiveness of a shielding enclosure is welder ground and the shielding enclosure. determined by the following factors: Conducted emissions from the shielding enclosures intrinsic shielding effectiveness of the shielding via the pipe being welded were minimized by (i) materials used, securely clamping the welder ground electrode cable to electric and magnetic field emissions from high- the pipe inside the shielding enclosure, (ii) frequency currents conducted along the AC power circumferentially bonding the pipe to the shielding line, pipes or other metal structures penetrating enclosure at each penetration, and (iii) bonding the the shielding enclosure, and welder ground and shielding enclosure. This provides leakage through seams, holes and other apertures low-impedance return paths for welding current in the shield. through the welder ground cable and shielding enclosure, and minimizes stray currents conducted along the pipe outside the shielding enclosure. Shielding Materials Test shielding enclosures were constructed of hot- 269 Bonding of Shielding Panels - the power-line filter, located on the floor just inside the door; To avoid high leakage through gaps between shielding - the welder foot control, which is moved inside panels, the maximum dimension of any holes in the when welding inside SE1; and shielding and gaps between shielding panels must be - the rod antenna (for measuring the electric field), small compared with a quarter wavelength. For supported in a horizontal position above the effective shielding, various sources recommend shielding enclosure on a cardboard box. limiting maximum dimensions of holes and gaps to values from 0.02 to 0.1 of a wavelength. As signifi- The floor of SE1 was made from two sheets of hot- cant arc welder emissions are limited to frequencies rolled steel sheet tack welded at several points. The below 10 MHz, even the more stringent criterion is sides of SE1 were made using existing structures used easily satisfied as it requires that the maximum to partition welding work areas. These structures dimension of holes and gaps be held to 0.6 m. consisted of hinged sections made of steel sheet welded to a tubular steel frame supported on casters. Tack welds and EMI tape were used to bond shielding One partition formed the front, left side and half the panels in the 2 shielding enclosures constructed. The rear wall of SE1 and another partition formed the EMI tape used is an embossed copper tape 0.035 mm other half of the rear wall and the right side. A gap in thick and 5 cm wide, having adhesive applied on the the rear wall roughly 0.4 m by 1.2 m was covered high points. When applied to a conductive surface with aluminum foil. The foil was bonded to the steel and pressed down, the copper makes contact with the panels and pipe using EMI tape and a pipe clamp.