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UL 1971 27.1.3(a) requires that listed strobes used for fire alarm signaling to hearing-impaired occupants flash at between 1 and 2 flashes per second:

a) Signaling lights shall produce a candela output in effective intensity in accordance with Tables 27.1 – 27.3 and Figures 27.1 – 27.3. The flash rate shall not be less than 1 hertz or greater than 2 hertz over the rated operating voltage range.

However, in practice, UL 1971 listed strobes, such as the System Sensor L-Series, are produced with a specified flash rate of 1 flash per second. This raises a question: is there a measurement tolerance on the 27.1.3(a) flash rate requirement, or does the listing process for these strobes require the flash rate to be strictly between 1 and 2 Hz over the operating temperature range, time (when the strobe is free running), and any unit-to-unit variation that may be present?

ThreePhaseEel
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  • The original thinking would have been to allow such a wide tolerance that any old timer could meet it with room to spare. Nowadays they are probably all MCU controlled and precision timed. You could in theory raise a lawsuit against the ones running at 0.999Hz for non-compliance, but... –  Oct 18 '20 at 19:00

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Don’t have easy access to that standard but NFPA 72 is supposed to incorporate the UL standard and it says:

18.5.3 Light, Color, and Pulse Characteristics. 18.5.3.1 The flash rate shall not exceed two flashes per sec- ond (2 Hz) nor be less than one flash every second (1 Hz) throughout the listed voltage range of the appliance.

So no tolerance is, strictly speaking, allowed for, so I suppose a prudent designer should aim to be within the tolerance range over temperature and voltage and all other variations. Of course if the nominal rate is 1.01 or 1.001Hz, the data sheet for the alarm will likely state 1Hz, so without measuring the actual rate it is not possible to tell what they have done.

Spehro Pefhany
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