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41187_700188.pdf Electrostatic Discharge Sensitive (ESDS) Equipment Susceptibility To Welding Generated Electromagnetic Fields Dr. T. R. Anderson Craig F. Derewiany
Naval Undersea Warfare Center Naval Undersea WarfareCenter
Division Newport Division Newport
1176 Howell Street 1176 Howell Street
Newport, RI, USA Newport, RI, USA Michael D. ObaraNaval Undersea Warfare Center
Division Newport
1176 Howell Street
Newport, RI, USA
Abstract: Concerns regarding damage to electrostatic discharge sensitive (ESDS) equipment, as a result of electromagnetic fields generated by welding processes have been raised. The concern that electromagnetic fields generated during the welding process could damage ESDS equipment is quite real. Guidance for welding in the vicinity of ESDS equipment was required by shipbuilders in order to avoid expensive and time-consuming removal of ESDS equipment prior to welding.In this
article, the damage to ESDS equipment from welding generated electromagnetic fields is evaluated.Analysis
showed that welding in the vicinity of ESDS equipment is not anticipated to cause damage to internal electronics. Guidelines were developed for welding in the vicinity of ESDS equipment. Current trends in ship construction requires the shipbuilder to install shipboard electronics very early in the construction phase and to utilize large (structural) subassemblies populated with electrostatic discharge sensitive (ESDS) electronics. These design practices necessitate welding very near to ESDS equipment, systems and cables. Damage to ESDS equipment, as a result of electromagnetic fields, becomes a real concern. The concern that electromagnetic fields generated during the welding process could damage ESDS equipment is quite real.PHYSICAL MODEL
Determination of the electromagnetic fields requires knowledge of the geometry of the welding cables. The possible configurations are limitless. A determination was made that the cables from the welder would be run parallel and would open to approximately a one square meter (1 .O m2> loop in the location of the weld. The equivalent model is illustrated in Figure 1. The welding arc is assumed to be at a comer of the loop. Figure 1. Equivalent Model of Welding Cables. Magnetic FieldOur model assumed the
welding system is a square current loop in the xy plane and one square meter in size. We calculated the magnetic field at some arbitrary point p along the z-axis using the Biot-Savart Law: dz ,u,,i dlxr =--4n IA3
The result is:
(1) (2)0-7803-5015--t/98/$10.00 0 1998 IEEE 36-l
where: ,UQ = permeability of free space i = current, AmperesI= length of side, meters
z = position along z-axis, meters The maximum magnetic field we take at &I to obtain the worst case result: (3) Evaluating this result with a welding current of 2000 amperes and l=l.O meter, we obtain 5.66 Gauss at the center of the loop.Electric Field
,u = permeability of the medium, Henries per meter i = current, AmperesS = area of the loop, square meters
r = distance from the loop, meters Utilizing a welding current i of 2000 Amperes, a frequency of10 kHz, and a loop area of 1.0 m2 yields an electric field
intensity of -25 volts per meter ("/,,,) at a distance of 1.0 meter from the loop, or -1075 "/,,, at a distance of 6 inches.Magnetohydrodynamic Model of rht? Welding Arc
We have taken the arc to be in the comer of the circuit loop. The arc current is in cylindrical coordinates. We have modeled the magnetic field from the arc in the first approximation as the solution to steady state hydrodynamics. We have solved the following system of equations in steady state magnetohydrodynamics. We now need to determine the electric field intensity from welding circuit. The square loop model for the welding circuit The equation of continuity: is electrically small (area=l.O m') based on the -100 microsecond rise and fall times for the welding current [I]. Stutzman and Thiele give the electric field for an electrically small loop (k-&I) as [2]: The momentum transport equation: (4) -L&L=~C dr (9)
W -3Reducing (5), to obtain the maximum electric field intensity, where p is the magnetic pressure and J is the current along the
let B = 90 "and neglect e'@ yields: arc. The components of Ohm's law for a plasma becomes the set of three equations:E= fi@S l+j - -- y$"
r c 1 (5) &I.!L1dP --- r C ne drRecall: E, = 0 (11) (12)
and when: Ampere's law becomes:1 --+A
(P > r the maximum electric field intensity is: (14) Solving this system of equations for the angular component of the magnetic field yields: where: f= frequency, Hertz 365and:
2n a2 B. =-J,- rka c r
The welding arc radius a is taken equal to 0.159 cm (which is typical) and distance to the center of the plane to be: r = Jz meters (because of the diagonal distance to the center). The quantity c is the speed of light. The current density is: i J, =1 7m" where the current i is taken to be 2000 amps continuity. Substituting these values into (12): (17) by charge (18) However this result is in Gaussian units. Converting this toMKS units yields:
BO=Y - r 7c (1%This yields:
BB =1.0~10-~T=lOG (20)
This is the magnetic field contribution to the center of the loop from the welding arc taken in the steady state magnetohydrodynamic approximation. We plan on solving the arc radiation in the transient magnetohydrodynamic model in the near future. We feel that the steady state magnetohydrodynamic model will give reasonable results.Induced Electromotive Force (EMF)
Next we need to examine the induced electromotive force (emf) in a typical circuit card from both the electric and magnetic fields. For the case of the magnetic field we will useFaraday
' s Law of Induction: v= A! - dt (21) which reduces to:V = 0.406flA (22)
where:V = induced voltage, microvolts
f= frequency, HertzB = magnetic flux density, Gauss
A = effective pickup loop area, square inches
The welding literature states that typical rise and fall times of the welding current is -100 microseconds which corresponds to a frequency of 10 kHz. Typical circuit card is eight inch by eight inch (64 square inches). Substituting these values into (5) and using a magnetic flux densityB of 15.66 G (5.666 +
10 G) yields an induced voltage of 4.1 volts.
For the case of the electric field we must determine the current induced into a cable shield. Martin shows the current induced in the shield of a 2.0 meter long shielded cable, illuminated by a uniform 10 "/,,, field to be -32 microamperes at 100 kHz [3]. Scaling the current for an electric field intensity of 1075 "/,,, yields an induce shield current of 3.4 milliamperes.We now utilize the surface transfer impedance:
z 1dV =-- t i dz to determine the open-circuit voltage formed between the shield and the conductors within the shield. Integrating (22): yields: where:V= open-circuit induced voltage, volts
i = total current flowing in the shield, Amperes2, = surface transfer impedance, Ohms per meter
I = cable length, meters
Now using a nominal surface transfer impedance Z, of 0.01 "i,,, and a cable length I of 2.0 m yields an open-circuit voltage of69 microvolts.
PRECAUTIONARYGUIDELINES
Welding in the vicinity of ESDS equipment is not anticipated to cause damage to ESDS electronics. As an added precaution, the following guidelines should be followed during welding: i. Welding is permissible in the vicinity of, but not closer than 6 inches to, ESDS equipment provided that the 366equipment is not opened and/or ESD sensitive parts, circuit cards, etc. are in protective packaging. Welding will not be allowed in equipment spaces/compartments when ESDS equipment is open and/or ESDS parts, circuit cards, etc. are removed from their protective packaging. ii. Welding directly on ESDS equipment (i.e. cabinets, mounting plates, etc.) is not permissible without prior approval. . . . 111. Equipment cabling shall be connected to ESDS equipment or metal/static dissipative covers shall be in place over any open connectors/receptacles during welding, iv. Cables connected to ESDS equipment will be considered ESDS. Cabling terminated (connected) on one end to ESDS equipment shall have ESD protection in place over open connectors during welding. v. Welding cables, including welding equipment power cables, shall not be permitted within 6 inches of ESDS equipment. vi. Welding cables, including welding equipment power cables, shall not be run with any other cables. These cables will not be allowed in shipboard cable banks. The following requirements should be levied against equipment manufacturers to ensure that equipment damage is not of concern during equipment installation or welding: i. The equipment shall meet the requirements of DOD- STD-1686 and DOD-HBK-263 [4], [5]. shielding effectiveness of a cabinet should be of the same magnitude as the cable. The electromagnetic shielding provided by the equipment cabinet and cables will further reduce the effects of the welding generated electromagnetic fields. The electric fields, therefore, are not considered to be problematic.
REFERENCES
[l] American Welding Society. Welding Handbook. Sixth edition, 1961. [2] Stuzman, Warren L., Garry A. Thiele.Antenna Theory
and Design.New York: John Wiley and Sons, 1981.
[3] Martin, Albert R. "An Introduction to Surface TransferImpedance
" . Technology, July 1982. [4] MIL-STD-1686A, Electrostatic Discharge Control Program for Protection of Electrical and Electronic Parts,Assemblies and Equipment, 8 August 1988.
[5] DOD-HBK-263, Electrostatic Discharge Control Handbook for Protection of Electrical and Electronic Parts,Assemblies and Equipment, 2 May 1980.
[6] MIL-STD-461C, Electromagnetic Emission andSusceptibility
Requirements for the Control of
Electromagnetic Interference, 4 August 1986.
[7] Whittlesey, A. C., J. M. Lumsden. "Electric Welding Hazard To Spacecraft Electronics". Proceedings of the 1981 International Symposium on Electromagnetic Compatibility,August 1981.
ii. The equipment shall comply with the MIL-STD-461C [8] Sutton, George W., Arthur Sherman.Engineering
requirement RS02, Radiated Susceptibility, Magnetic Magnetohydrodynamics. New York: McGraw-Hill, 1965.
Induction Fields, Spikes and Power Frequencies [6].