Fuses Thom Perry, QSS Group Inc, CODE 562, NASA/GSFC

 

Background
A fuse is a protective device that breaks the circuit if current exceeds its rated value in order to protect other elements of the circuit from a possibly damaging overload.  Goddard recommends fuses designed and qualified to MIL-PRF-23419 with the additional screening specified herein.  These are ceramic-bodied, cartridge-style fuses with an internal wire that melts to break the circuit during overloads.  The FM04 and FM08 styles have long been established as standard parts for space flight applications.  More recently, subminiature “solid body” FM12 fuses have become available to the specification.  These consist of a thick film resistive element on a thermally insulated substrate that is sealed with a thick film arc suppressive glass.  The thick film vaporizes during current overloads, thus effectively opening the circuit.  With miniaturization in mind, recent Goddard projects are beginning to use SMT fuses utilizing either hollow core, wire-in-air or thick/thin film technologies.  However, given the large variety of SMT configurations available, it is doubtful whether any will emerge in the near future as candidates for preferred listing in the NASA Parts Selection List (NPSL). 

Major Issues
The selection of current ratings for all fuses needs to be determined at specified operating conditions.  The manner in which the fuse is mounted or heat sinked, the ambient temperature and vacuum all influence blow characteristics.  Additional factors to consider are lot-to-lot element resistance variations, degradation as a result of turn ons and turn offs during testing, and the effects of any surge currents and the open circuit voltage on the fuse.  For cartridge style fuses, adherence to derating guidelines provided herein will mitigate the likelihood of nuisance opens while assuring that circuits are adequately protected during current overload conditions.

The FM12 fuses are larger with a higher profile and are more costly than cartridge styles but are still desirable for certain space applications because these parts do not require vacuum derating.  However, the user is cautioned to provide a short-time blow time allowance approximately 30-fold of that experienced with cartridge fuses of the same rating.

Screening and qualification guidelines for SMT fuses were developed using testing experience gleaned from the larger cartridge styles or “solid body” FM12 types, as applicable.  Tables 2 and 3 in Section F3 of EEE-INST-002 have been updated to include the latest testing philosophy for surface mounted fuses.  With more testing experience and some flight heritage, SMT fuses will eventually be broken out into separate testing tables.

Attention is now being paid to the polymer-based “resettable” type fuses that actually function more like a switch.  As the temperature of this part rises to a predetermined level, either as a result of internal heating from a current overload or from an external heat source, the resistance of the part dramatically increases, thus effectively opening the circuit.  As the temperature decreases, the resistance drops back to its original level.  Thus, circuits isolated to protect other circuit elements during overloads can be reactivated.  At least one NASA project is actively involved in developing qualification and screening procedures for flight usage of these parts.