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13.4.1. Electrical Low Voltage Electromechanical Circuits Design Requirements
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13.4.1.1. All solid rocket motor ignition circuits and other high hazard ordnance systems using low voltage initiators shall provide an electromechanical safe and arm (S&A) device.
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The term high hazard refers to specific catastrophic events such as the inadvertent firing of a solid rocket motor or actuation of a destruct system that could result in multiple fatalities, typically threatening more than just the ordnance technicians handling the hazardous item, and/or "total" destruction of high value hardware such as the payload, launch vehicle, or facility.
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13.4.1.2. EED ordnance systems other than solid rocket motor ignition circuits and other high hazard ordnance systems shall provide two independent circuit interrupts such as “enable” and “fire” switches in the power side of the initiator and one safe plug that interrupts both the power and return side.
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A key consideration in providing inhibits in an ordnance circuit is that they be both valid and independent. Valid means that the inhibits reside in the direct current path for firing the EED, not in the control circuit used to change the status of an inhibit. For example, if a two-inhibit compliance approach is to close two control circuit relays to close a single firing line relay, it is not compliant because there are not two valid inhibits. In other words, the single firing line relay is the only inhibit. Independent means a singular action to remove a singular inhibit. Two inhibits is possible; for example, two open relays in a firing line. However, if a single command removes both inhibits, (for example, closes both relays), then the inhibits are not independent. In other words, there are not two independent inhibits. A concept that is often overlooked is that inhibits are not independent if a single failure can negate both inhibits.
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13.4.1.3. The safe plug shall provide interruption of the circuit after the “enable” and “fire” switches and as close to the end item ordnance as possible.
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13.4.1.4. The final electrical connection of an EED to the firing circuit shall be as close to the EED as possible.
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13.4.1.5. EEDs shall be protected from electrostatic hazards by the placement of resistors from line-to-line and line-to-ground (structure). The placement of line-to-structure static bleed resistances is not considered to violate the single point ground requirement as long as the parallel combination of these resistors is 10 kilo-ohms or more.
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13.4.1.6. The system circuitry shall be designed and/or located to limit RF power at each EED (produced by range and/or vehicle transmitter) to a level at least 20 dB below the pin-to-pin DC no-fire power of the EED.
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Electromagnetic environment evaluation should either be by analysis or electromagnetic compatibility (EMC) testing. RF power density levels for facilities are available from the 30 SW and 45 SW for VAFB and CCAFS, respectively, and the KSC Electromagnetics Lab.
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13.4.2. High Voltage Exploding Bridgewire Circuits
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13.4.2.1. All solid rocket motor ignition circuits for payloads using exploding bridgewire (EBW) systems shall include a manual arming and safing plug in addition to an EBW-firing unit (EBW-FU).
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13.4.2.2. An EBW-FU shall be required on all other EBW systems. A manual arming and safing plug may also be required depending on the degree of hazard as determined by the PSWG and Range Safety.
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