1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-09a71 Center Point of Contact: KSCa71 Submitted by: Wil HarkinsSubject: Electrostatic Discharge Control in GSE Practice: Reducing the risk of Electrostatic Discharge (ESD) during Space Transportation Systems (STS) processi
2、ng requires an understanding of the problems and proper reliability practices or controls for various conditions/situations. When conductive materials are present, proper bonding and grounding techniques are essential. When non-conductive materials (i.e., wood, paper, glass, plastics, etc.) are pres
3、ent, these materials must either be eliminated or properly selected or treated for the particular application and use. Another important factor in ESD control is the percent relative humidity (RH) which should be monitored and regulated/controlled, where feasible.Programs that Certify Usage: This pr
4、actice has been used on the Space Shuttle Program, Launch Processing at the kennedy Space Center.Center to Contact for Information: KSCImplementation Method: This Lesson Learned is based on Reliability Practice number GSE-3008 from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practice
5、s for Design and Test.Benefit:Proper control of electrostatic discharge can significantly reduce the possibility of:Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-a71 Inadvertent ignition of solid propellants, explosives and flammable/combustible ma
6、terials,a71 Inadvertent actuation of electronic devices/systemsa71 Personnel shock or injury anda71 Ground Support Equipment (GSE) hardware/equipment damage that could lead to flight hardware damage.Implementation Method:Electrostatic controls are implemented at KSC to reduce the effects of ESD duri
7、ng the following operations:a71 General STS and Payload Operations - The percent relative humidity in an operational area is recorded every four hours prior to the start and during operations involving exposed solid propellants, open flammable/combustible fluid systems and category A Electro-explosi
8、ve Devices (EED) when the Faraday (electrically conductive shield) cap is removed or firing circuits to EEDs are exposed. At or below 50% RH, bonding and grounding measures are verified. In addition, non-conductive materials in the area and personnel not wearing personnel grounding devices are check
9、ed with an electrostatic meter to ensure voltages greater than 350 volts are not present. Electrostatic scanning, not exceeding one hour intervals, is performed during the operation when at any time: a72 additional personnel, equipment or hardware are introduced into the immediate area,a72 the RH de
10、creases, ora72 the handling of non-conductive materials is required.At or below 30% RH, operations involving exposed solid propellant, except Solid Rocket Booster (SRB) segments, or open flammable/combustible fluid system and category A EEDs (with Faraday caps removed or firing circuits exposed) are
11、 not permitted. In the Orbiter Processing facility (OPF), the RH is controlled to within 40 - 50% thus minimizing the effects of ESD.a71 STS SRB Processing - Segment processing may continue below 50% RH using the following guidelines, segment processing is not permitted at or below 10%: a72 Between
12、50% and 30% RH the above requirements apply.a72 With proper approval, operations may continue at RH levels below 30% down to 10%. In addition to the above requirement, electronic scanning is required to to be accomplished at 10 minute intervals if the propellant is exposed, and at 30 minute interval
13、s if the propellant is covered.a72 At no time will operations continue on the segments with propellant exposed and a Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-potential of 350 volts or greater is measured on the segment case, propellant, or any
14、 equipment/personnel within five feet of exposed propellant.a72 When the segment and rings with shipping covers are installed (propellant protected by a Faraday cage), the following guidelines apply: 1. A reading of one kilovolt or less on the case is acceptable and work may continue.2. A reading of
15、 greater than one kilovolt, but less than four kilovolts on the case requires all segment processing to cease and personnel will stand back four feet. An electrostatic scan will be repeated every five minutes and recorded. If the reading of one kilovolt or up to four kilovolts exists for 30 minutes,
16、 connection to facility ground will be verified. If this connection is open, then the ground will be reconnected thru a resistor (1 Meg ohm +/-20%). Processing may continue when the electrostatic scan indicates less than one kilovolt.3. An electrostatic reading of four kilovolts or greater requires
17、all work to stop. All personnel will evacuate to a location 500 feet from the processing area. Designated personnel may re-enter for electrostatic scan. This scan is repeated not less than every 15 minutes. No attempt to check grounds will be performed. If, after an hour, readings still exceed four
18、kilovolts, then a separate ground with a resistor (1 Meg ohm +/- 20%) in line may be connected to another ground point on the segment case. Work may resume when a reading of one kilovolt or less is obtained and all grounds have been rechecked.a71 Launch Processing System (LPS) Operations - LPS opera
19、tors coming in contact with the LPS hardware subsystems are required to wear approved grounding wrist straps during test and launch processing operations in environmental conditions below 45% relative humidity. LPS system, maintenance, or support personnel coming in contact with low voltage LPS hard
20、ware Line Replaceable Unit (LRU) / Components are required to wear approved grounding wrist straps at all times. LPS / Checkout, Control and Monitor Subsystem (CCMS) console stations are equipped with approved terminated ground cables containing quick connect / disconnect type wrist strap snaps.Syst
21、ems, maintenance or support personnel will connect the grounding lead clip to a conductive surface on the LPS grounded hardware structure.Testing stations are available for proper go/no go testing of the approved grounding cable and wrist straps.a71 Logistics - The effects of ESD on static sensitive
22、 Line Replaceable Units (LRUs) during transportation and storage is alleviated with the use of transparent electrostatic-shielding bags. This packaging scheme affords static protection against both external static fields and internal triboelectric charge. Provided by IHSNot for ResaleNo reproduction
23、 or networking permitted without license from IHS-,-,-All programmable read-only memories (PROMs) are transported and stored in static-shielding tubes or approved foam and shielding bags for distribution to installation in LPS hardware sets. These tubes protect against triboelectric charging and dir
24、ect discharge yielding a dynamic response rating of less than 1 millisecond.All integrated circuits (ICs) for temporary staging are placed (leads inserted) on crosslink, noncorrosive, high density, conductive foam. This foam ensures that all device leads are kept at the same potential, thereby prote
25、cting the device from ESD damage.a71 Other - inspection, testing and repairs of static sensitive electronic components or LRUsare performed at approved ESD work stations. These work stations, designed to bleed off static charges, utilizes static dissipative table mats electrically connected to a sta
26、tic dissipative floor mat which is connected to ground thru a series resistor (1 Meg ohm +/-20%). The operator connects a wrist strap, to a single point ground, to make himself equal to the same potential as the components that are on the table, thus, prohibiting static discharge on conductive compo
27、nents.Technical Rationale:Fundamentals of Electrostatics - To insure successful implementation of proper guarding against harmful effects of ESD, familiarity with the concepts of electrostatics and ESD is necessary. Here, the basic elements are briefly discussed. For more extensive discussion, refer
28、 to books listed in the references. The main basic elements of ESD are&colon. electric charges, electric field, electrostatic potential, capacitance, charge generation and charge removal.Electric Charges - Experiments have shown that when two dissimilar objects are brought into contact with each oth
29、er and then separated, the two objects become charged. The classic example is the rubbing of a silk cloth on a glass rod where the charge is visually evident when the charged objects cause the hair on your arm to stand up or cause small bits of paper to be attracted to the charged objects. If two co
30、rks are charged by the glass rod, the corks will repel each other because of like charges. Conversely, if one cork is charged by the glass rod and the other is charged by the silk (opposite charges), the corks will attract each other. The charge on these objects is due to an excess or deficiency of
31、electrons on their surfaces. These charges are referred to as static electricity. This is because they can remain stationary on an object for substantial long periods of time. It is to be noted that this sudden transfer of charge from one body to another (by oppositely charged bodies being brought i
32、nto close proximity) is called electrostatic discharge or the better known acronym “ESD“. Lightning is a very high energy form of ESD. The interactions between electric charges are described by Coulombs law: F=kq1q2/r. Where F is the magnitude of the force, k is the proportionally constant, q1& q2ar
33、e the charges on the object and r is the distance between them.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Electric Field - describes the influence of an isolated charge on other charges in its vicinity. This electric field is the direction and m
34、agnitude of the force exerted by the charge on a unit charge at any point in its environment. The field strength (E) of an object is force(F)/charge(q). A good example of electrostatic field is when a person generates a charge on a balloon and then holds it above his head to make his hair stand up.
35、The balloon never touches the hair, but an electrostatic field has been created.Electrostatic Potential - the amount of energy (work) per unit charge required to move a charge from one point to another in a field. The movement of the charge (q) along a distance (X) requires work (W). Thus the equati
36、on W=FX and, substituting the forementioned, W=qEX or W/q=EX . This work per unit charge is the potential difference expressed in volts. Thus the formula E=V/d, where E is the electric field intensity, V is the electric potential and d is the distance between the potential voltages. Ideally, we want
37、 to make sure that all charges remain at the same level or same potential so that no ESD (zap) occurs.Capacitance - the ratio between the charge on two plates and the potential difference between them, or, C= Q/V. In the case of two parallel plates this capacitance C=eA/d where A is the area of the
38、plates and d is the distance between the plates (potential voltages) and, e is the dielectric constant. Where typical capacitance (C) values for humans ranges from 50 to 500 picofarads (pf) and dielectric constants of 1 (air and insulators) to 10 (insulators). The energy stored in a capacitor is giv
39、en by the expression E= 1/2CV2or 1/2Q2/C.Charge Generation - charging that occurs as a result of contact and frictional motion is referred to as triboelectricity. To describe, whenever two materials (one must be an insulator) are brought together and then separated, there will be a flow of electrons
40、 from one material to the other. The material giving up the electrons becomes positively charged while the material accepting the electrons becomes negatively charged.Charge Removal - dissipation of electrostatic potential with the use of soft grounding, static dissipative materials and air ionizers
41、.Listed below are some of the more common materials and their polarity (+ or -) in the triboelectric series.Asbestos Acquires a more positive chargeGlassHuman Hair Nylon Wool Aluminum Paper Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Polyurethane
42、 Cotton Neutral reference point Wood Steel Sealing Wax Hard Rubber Mylar Epoxy Glass Nickel, Copper, Silver Brass, Stainless Steel Acrylic Polystyrene Foam Polyurethane Foam Polyester Polyethylene Polypropylene PVC (vinyl)Teflon Silicone Rubber Acquires a more negative chargeWhen the charged object
43、is an insulator, the charge can last for extended periods of time. This is true because insulators are poor conductors of electricity. Conversely, when the charged object is a conductor, the charge will decay rather quickly. The factors affecting the magnitude of the rate of charge generation are:a7
44、1 Relative position in the triboelectric series.a71 Intimacy of contact (proximity of the materials).a71 Coefficient of friction between materials.a71 Rate of separation.The factors affecting the magnitude of a rate of discharge (dissipation) are:a71 Conductivity of the materials.a71 Relative humidi
45、ty.a71 Moisture on the surfaces of the materials.a71 Rate of recombination.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Some typical electrostatic potential (volts) generated for typical events with various levels of relative humidity (RH) are res
46、pectively:Walking across a carpet 35,000 volts 10% RH15,000 volts 40% RH 7,500 volts 50% RHWalking across a vinyl floor 12,000 volts 10% RH 5,000 volts 40% RH 3,000 volts 50% RHPulling tape quickly from its roll 10,000 volts 30% RHESD is not normally a concern when the relative humidity is greater t
47、han 50% RH because moisture in the air will act as a high resistance bleeder. This will then dissipate voltage potentials on the surface before they can build up to a level of approximately 350 volts and result in ESD. A few materials such as teflon, vinyl, etc. do not absorb moisture and therefore,
48、 will not bleed off readily even in environments above 50% RH and should be avoided where ESD is a concern. Environments below 50% RH require special attention for selection of and use of tapes, plastic films and electrostatic flooring material. Operations below 30% RH should be carefully assessed a
49、nd avoided when possible. At levels below 30% RH, additional precautions shall be employed (e.g., air ionizers, humidifiers). Voltages, especially on large surfaces, should be dissipated using a high resistance resistor (1 Meg ohm +/- 20%) in series with the ground wire until the charge is eliminated before going directly to ground.Note: Surface resi
