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UL 1449 SPD Testing Terminology Explained

UL 1449 is the Underwriters Laboratories Safety Standard for Surge Protective Devices (SPD). Surges are rapid increases in voltage and current that can occur on AC or DC power circuits that can damage equipment attached to these circuits.   An example of a surge would be a lightning strike on or near exposed power lines. The strike can induce voltages on the power line that can exceed a million volts. These induced voltages rise to their peak amplitudes extremely fast (1 to 2 microseconds), and have relatively short durations lasting up to 1 millisecond (1000 microseconds). For perspective, a full cycle of 60Hz power lasts 16.7 milliseconds. The surge voltage drives high current down the power line. This current, like the surge voltage, increases to peak values in the low (4 to 8) microsecond range and lasting 20 microseconds. SPD’s covered by UL 1449 are devices that quickly sense the voltage increase, turn on, and divert the surge current to ground. While the current is being diverted the voltage drops back to safe operating levels and the SPD resets for the next surge.

With this brief description of what an SPD does, we can move on to the testing that derive the values that appear on the labels and specifications. These tests are performed to ensure the safe operation of the SPD under extreme operating conditions. 

1)   Nominal Voltage: This is the manufacturer declared voltage the SPD is designed to operate within. For an example, an SPD for an outlet in your home would have a nominal voltage rating of 120 Volts

2)   MCOV Maximum Continuous Operating Voltage: The maximum nominal voltage the SPD can withstand without turning on. This value is typically 15 to 20% higher than the nominal voltage. A 15% higher value on a 120 volt nominal circuit would result in a 138 Volt MCOV

3)   VPR Voltage Protection Rating: The VPR is the voltage value where the SPD limits a specific surge. The specific surge used is called a combination wave (voltage and current) surge.  This combination wave surge is 6000 volts with a 1.2 microsecond rise by 50 microsecond duration (1.2 X 50 waveform), and 3000 Amps with an 8 microsecond rise 20 microsecond duration (8 X 20 waveform). This surge is applied to the SPD with it powered by the MCOV. An oscilloscope is used to record and measure the voltage that the SPD limits the 6000 volts to. Three combination wave surges are applied to the SPD sample. The measured limiting voltage values are averaged. The averaged value is then used to select a VPR from a table in the standard. The values in the table are rounded up to the nearest 100V.  For example, if the average is 405 volts, the table shows the rounded up value to be 500V. The resultant VPR of 500V is assigned to the SPD.

4)   In: In represents the value of the nominal (8 X 20 waveform) surge current that an SPD can safely divert 15 times with MCOV applied between each surge. A value of the ( 8 X 20) current is selected by the SPD manufacturer from a table in the 1449 standard. The values are 3000 Amps, 5000 Amps, 10,000 Amps or 20,000 Amps.A 1.2X 50 waveform surge voltage is used to drive the selected current through the SPD. The SPD passes this test if it remains operational after the 15 impulses are applied. To check the operation the VPR test is repeated.

5)   Current Testing:There are 3 different short circuit current tests in UL 1449. 

a)   Short Circuit Current Rating: Must be a value greater than 5000 Amps. The value is declared by the SPD manufacturer and must be equal to or greater than the rating of the system on which it will be installed. For example if the electrical panel in your home has a 10,000 amp fault rating, an SPD installed on it must have a minimum of a 10,000 Amp fault rating
b)   Intermediate Current: test at 100 Amps, 500 Amps, and 1000 Amps
c)   Limited Current: test at 0.5 Amps, 2.5 Amps, 5 Amps, and 10 Amps 


Short circuit current is different than surge current. Surge current flows through an SPD for short (20) microsecond durations. Fault current will flow through the SPD until a disconnect (fuse, circuit breaker or other current limiting device) opens the power circuit. This disconnect could be the main fuse on the utility service or an internal fuse in the SPD. It may take a disconnect (>1000 microseconds) to open and limit fault the current. The fault current will flow through the SPD for much longer period of time compared to a 20 microsecond surge.   

Each of the short circuit tests are performed using twice the nominal voltage that the SPD is designed to operate on. For example, a 120V SPD would have the test performed at 240V. The higher voltage ensures the SPD will turn on and the short circuit current will flow through the SPD. The power source must be capable of delivering the fault current selected. During the application of a current tests the samples will be covered with cheese cloth and tissue paper. 

Short Circuit Current: Three samples are tested by energizing them with twice the nominal voltage and the selected short circuit current (minimum 5000Amps) is induced on the SPD. 
Intermediate Current: Three new samples are tested at each level of 100Amps, 500Amps, and 1000Amps. 
Limited Current: Three new samples are tested at each current level of 0.5Amps, 2.5Amps, 5Amps and 10Amps.

The SPD fails these tests if there is emission of flame, molten metal, glowing or flaming particles through any opening. There can be no charring, glowing, or flaming of tissue paper or cheesecloth, ignition of the enclosure. No openings can be created in the enclosure that allows accessibility to live electrical parts. There shall be no degradation or separation of traces from a printed circuit board.  


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