New standards cut back system failure risks.

In May 2004, Underwriters Laboratories (UL) revised UL 1971, regulating how operating currents are to be measured and how voltage ranges are to be listed and published.

Historically, fire-alarm system designers used 24-volt current draw for computing voltage drops on audible and visible (AV) circuits. However, voltage drops based on 24-volt current draw only provide an illusion of lower current draw, placing the reliability of the circuit in question.

UL recognized confusion within the industry because AV appliance circuit calculations were being performed with the wrong current draw. They saw the potential for circuit failures.

UL Max to Denote Current Ratings

Seldom is the voltage at AV devices exactly 24 volts. This is due to the natural voltage drop in the circuit. Depending where the device is located on the circuit, the voltage could drop to its lowest operating voltage.

This means that because device currents vary with applied voltage, the only way to assure the power supply will provide enough current to the entire circuit is to measure current for each device at its highest value.

Based on this logic, UL now requires that current-rating published in installation manuals are to symbolize the maximum current draw (UL Max) over the listed voltage range. These true current ratings will:

1. Reflect device power usage more accurately.
2. Ensure compatibility between devices and outputs on the control panel, regardless of manufacturer.
3. Present comparable current-draw data amongst manufacturers.

Incorrect Data is Still Printed Today

Despite UL’s efforts, some manufacturers are still printing 24-volt current draws on their data sheets. Current draws based on 24 volts can confuse industry professionals who don’t realize that these specifications shouldn’t be used for circuit calculations.

To eliminate confusion throughout the industry, System Sensor is advertising a white paper that gives details and examples of the UL 1971 revision.

You can download a free copy of the white paper, and you can sign up for one of System Sensor’s Fire Protection seminars to learn about the latest fire-detection technologies and UL requirements, by visiting System Sensor’s website atwww.systemsensor.com.

Tags: A/V Notification, Smoke detection, Speakers and Strobes, UL

Posted in Design and Installation, UL, Winter 2005