MIL-DTL-55668 Cord, Electrical; Audio, Subminiature \(Retractile And Straight\)This specification covers subminiature audio helical retractile cords and straight cords with either stranded wire or tinsel wire conductors, for accessories associated in for the use with microphone, telephone, switchboard, and associated communication equipment. These cords are intended for operations at audio frequencies and temperatures from -55 Degrees C to +85 Degrees C with a high degree of flexibility. Each retractile cord is designed for extension in normal service up to 6 times its nominal retracted helical length.

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MIL-DTL-55668D

4.7.5 Shield coverage. Shield coverage shall be calculated as follows:

NPd

K

100

sin a

where:

K = Percent coverage

N = number of tinsel strands per carrier

P = picks per inch of cable length

d = diameter of tinsel strand in inches

a = angle of serving with axis of conductor

Tan a  2 D  d  *

where:

D = diameter under served shield, in inches

C = number of carriers

4.7.6 Testing of jacket material. Properties of the jacket material shall be specified in table II, except as

indicated herein.

4.7.6.1 Ozone resistance test. The specimen shall be a completed cord or helical section thereof. The

specimen shall be air-oven aged at 701 C for a period of 1681 hours in an unstressed condition and

allowed to cool to room temperature prior to placement in the ozone chamber. A 2 inch (50.8 mm)

(minimum) section of the helical section shall be stretched 5:1 and maintained in this elongated condition

while in the ozone chamber and during examination for cracks at room temperature.

4.7.7 Cord retractility. Prior to meeting any extensile and retractile requirements, the completed cord

shall be conditioned by extending the coiled portion to five times its retracted length and allowing it to

retract freely for a total of 6 cycles. After this conditioning, the cord shall be placed on a horizontal

surface and the coils examined for contiguity (see table IV).

4.7.7.1 Breaking load. The test specimen shall be a straight cord or a retractile cord, as applicable. The

straight cord shall be approximately 1 foot long; the retractile cord (or section thereof) shall contain at

least five helical turns and shall be approximately 1 foot long in extended length including a straight

section. For retractile cords with less than five turns, the entire cord shall be used. The breaking load

shall be determined on a power driven tensile testing machine. The test shall be made at the rate of 12

inches (305 millimeters) per minute. The conductors of the cord may be connected in series to an

indicating or signaling device or automatic stop, or both. The arrangement shall be such that an

indication will be given when there is a disruption of electrical continuity due to the breaking of the

conductors of the cord under test. The minimum load shall be as specified in table IV.

4.7.7.2 Cord extension force. The finished cord, or helical portion thereof, in a horizontal position shall

be fastened at one end, with all coils contiguous. The length of the helical portion shall be measured and

a tensile force applied to the free end of the cord, sufficient to extend the helical portion seven times its

retracted length. The maximum force required for this extension shall be as specified in table IV.