ANSI/IEEE Device Numbers. Protective relays are commonly referred to by standard device numbers. For example, a time overcurrent relay is designated a 51 device, while an instantaneous overcurrent is a 50 device. Multifunction relays have combinations of device numbers. A 27/59 device, for example, is a combination under/over voltage relay.
At 0Â° Celsius the speed of sound in U.S. textbooks is 331.3 m/s. At 20Â° Celsius the speed of sound then is 343.21 m/s, rounded 343 m/s. At 0Â° Celsius, the speed of sound in German textbooks is 331.5 m/s. At 20Â° Celsius the speed of sound then is 343.42 m/s, rounded 343 m/s.
For sound waves in air, the speed of sound is 343 m/s (at room temperature and atmospheric pressure). The wavelengths of sound frequencies audible to the human ear (20 Hzâ20 kHz) are thus between approximately 17 m and 17 mm, respectively.