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ITIC Plotting of All Power Quality Events

Brief History

Back in 1977 a group formed with the mission of developing a curve to show the ability of computer equipment to ride through voltage events.  This group then was called the Computer & Business Equipment Manufacturer’s Association (CBEMA).  This group published a curve as part of IEEE Std 446.  Fast forward to 1996 the curve became the Information Technology Industry Council (ITIC).  In that year they made some tweaks and published the curve still being used today.  This curve is a characteristic curve meant to represent typical loads and how sensitive the loads are to voltage events.  Some loads have been found to be have longer ride through times and some loads have been found to be less tolerant.

CBEMA Curve and ITIC Plot Examples

How to Read the Curve

Looking first at the y-axis you will see the % of nominal voltage, and the x-axis represents time typically shown using a logarithmic scale.  The idea is as long as the voltage is contained within the boundaries the load will be maintained.  The examples above show events plotted both in and out of the boundaries.  Equipment and system designers key in on the zero voltage limit which is 20 milliseconds in the current ITIC curve seen on the right. 

LayerZero Products & The ITIC Curve

From a product perspective the ITIC curve can be analyzed in a few different ways.  First from a ‘Performance’ perspective like a static switch that will complete transfer within the limits of the ITIC curve.  Secondly, the products can monitor and report out power quality events plotted on the ITIC curve.


A static transfer switch (STS) simply has two inputs and one output.  The STS will break down each incoming source into 128 samples per cycle (7.68 kHz Sampling).  The purpose of the switch is to determine if the preferred source is acceptable or not.  The customer defines what is acceptable and the STS will determine to stay on the preferred source or transfer to the alternative source.  The transfer time to get over to the other source is going to happen well within the limits of the ITIC curve.  This time will vary based on the nominal rating of the STS voltage and the type of loads downstream. 

Examples of ITIC Plots in Static Transfer Switches

Scenario One is the voltage of the STS matches the voltage seen by the loads (i.e. 208 VAC).  With this scenario there are not any magnetic loads downstream of the STS.  The STS will sense and transfer in less than a ¼ cycle (4.17 milliseconds).  This is depicted by the curve below on the left. 

When the STS is switching at 480 volts there will typically be downstream PDU’s with transformers.  Sometimes in critical electrical systems the phase relation ship of the two sources will drift apart.  In this case doing a ¼ cycle transfer with out of phase sources will cause the transformer to draw large inrush currents.  These inrush currents could potentially trip upstream overcurrent devices and take down large blocks of critical load.  For these transfers LayerZero deploys a transfer algorithm called ‘Dynamic Phase Compensation’.  The inrush is practically reduces to  nothing at all and the transfer time is lengthened to around a ½ cycle (8.33 milliseconds).  This is still well within the ITIC curve.

Monitoring & Reporting

Regardless if it is a STS, a PDU, or RPP LayerZero can deploy an embedded metering system and constantly monitor the voltage.  If an event occurs like the one seen below, the user will be able to download the event from the equipment.  In the past the customer would need to find someone on staff or elsewhere to read and interpret a waveform.  Not everyone is well versed in doing that task.  However, everyone can clearly see if an event falls outside the ITIC curve. 



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