The Effects of Voltage Sags On Electronic Equipment

by Vince Luciani, Innovolt

Do voltage sags damage my MFPs? While much has been written in technical journals on the causes of voltage sags — momentary reductions or loss of voltage — this article will focus on the effects that voltage sags can have on electronic assets.

When voltage sag occurs, the electronics’ internal power supply takes the biggest blow. Normally, when the piece of equipment is off for a long period of time, internal thermally based current inrush protection has a chance to reset and be at the ready to reduce the current when a large and sudden voltage change occurs. The most obvious sudden voltage change occurs when an electronic asset is turned on. However, there are other times in the life of a piece of electronic equipment when sudden voltage changes occur.

But before we get to that, we have to ask why do sudden voltage changes cause current inrushes in the first place? Well, it is because of the capacitors internal to the piece of equipment. Just about all electronic equipment uses capacitors; short-term energy storage devices that are used to turn the AC, or alternating current voltage, that comes out of the wall outlets into DC, or direct current voltages. When voltage is applied across a capacitor, it wants to charge up like a battery. 

Like drinking water from a fire hose

To visualize a capacitor charging, think of the capacitor as a paper cup and the current as the flow of water filling the paper cup. You want to fill the paper cup but all you have is a fire hose. If you turn on the water flow unregulated, you will destroy the paper cup. In order to fill the cup without destroying it, the water flow must be limited in some way.

The rate of water flow in this example is analogous to current flow within your asset. However in electronic circuits, the “paper cup” is not the only thing that gets damaged with high current flow. Anything in the water-flow path (including the capacitors that are causing the problem in the first place) gets damaged. Back in the electronics world, this means the bridge rectifiers, diodes, and anything else in the current path along with the capacitors themselves have their life reduced when current inrushes occur.

So, we now know that sudden voltage changes to capacitors cause the capacitor to create these large current inrushes when they are discharged and need to charge up. We also know that a very common sudden voltage change occurs when the piece of equipment is turned on. So, what do designers do to combat current inrushes at turn on? A Negative Temperature Coefficient Thermistor (NTC) is existent in just about every piece of electronic equipment to limit or regulate the current flow caused by the capacitors when the sudden voltage change at turn on occurs. Their mere presence is an indication that the electrical engineers designing the piece of equipment were aware of the dangers of current inrush.

An NTC is a temperature-sensitive resistor that, at room temperature, has current regulating characteristics (resistance). After current flows through the NTC, it heats up and its internal resistance reduces to near zero. An NTC does a great job at limiting current inrushes as long as the NTC is cooled off to room temperature. A warm NTC takes about two minutes to cool off and reset to current inrush limiting capabilities.

So, assuming prior to every sudden voltage change the voltage was off for more than two minutes, allowing the internal NTC to cool off, then you are fine. Your NTC is all you need to prevent the current inrush that the original designer knew needed to be limited in order to allow your electronic equipment to live a long and prosperous life.

Constant exposure to voltage sags

There is one catch: turn on is not the only time sudden voltage changes occur. Voltage sags and momentary outages occur hundreds of times a year from the power grid, and possibly thousands of times per month or even per week depending on the condition of the electrical service wiring in your building, causing unabated current inrushes to occur each time.

Will one current inrush event kill a piece of equipment? It depends on the piece of equipment, as not all electronics are created equal, but usually it will not. However, multiple unabated current inrushes over time will reduce the lifespan of internal electronic components such as the bridge rectifier, diodes, and capacitors causing your piece of equipment to either fail or malfunction.

Is your equipment susceptible?

How do we know that these electronic components are susceptible to current inrushes? Take a look at the data sheets for the components.  Looking at the specification “Peak Surge Forward Current” in the data sheet, we see the maximum inrush current the part can handle. For example, say the rating is an impressive 200A. However under the “conditions” section, it’s important to note that this is a “non-repetitive 1-cycle peak value.” What that means is that it can handle a 200A current inrush one time. 

Looking further in the data sheet, we see a graph where the manufacturer specifies the number of times their part can handle a current surge (aka, current inrush) of different magnitudes. From the graph, we see that the bridge rectifier — the same one that is in the copy machine talked about earlier — can withstand a single 200A current inrush and as the current inrush magnitude reduces, it can withstand more cycles or current inrush events. 

According to the manufacturer of the bridge rectifier, it can handle about 80 current inrushes at that magnitude.

Now we must ask the question: How many voltage sags or momentary interruptions are my electronics likely to be exposed to during their lifetime? Well, to answer that, we have to talk about where voltage sags come from. The two sources are either from issues on the power grid itself, or from the wiring network inside your structure. 

There are many authorities on power quality for the North American Power Grid. IEEE (made up of academia and independent engineers), EPRI (a joint power company research organization), and NEETRAC (a joint research organization of universities and power companies) are just a few of the many groups actively researching power distribution and power disturbances. While published numbers vary, they all agree that on average our electronic equipment is exposed to hundreds of grid-based voltage sags per year.

The number of grid-based voltage sags, however, pales in comparison to the potential number of building-based voltage sags our equipment is exposed to. If the electrical loads are not balanced (i.e., not distributing current “hogs” properly in the system), one piece of equipment can cause sags that adversely affect other pieces of equipment.

The summary

  •  Unprotected MFPs are going to suffer from poor performance and increased serviced calls
  •  Voltage sags are caused both on the power grid (outside your building) as well as by factors inside the building.
  •  Voltage sags cause electronics to draw abnormally large amounts of current when the voltage recovers.
  •  The device installed to protect from current inrushes does not have adequate time to reset in reaction to a voltage sag or momentary interruption.
  •  These large current draws, or current inrushes, deteriorate components inside your electronic assets.
  •  When internal components fail, your asset fails, stopping revenue generation.

On the Web: Innovolt 

This article originally appeared in the July 2014 issue of The Imaging Channel.