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AC Surge Protection and Fuse Selection Presented by: Godfrey de la Torre EE 136 Fall 2003 Professor Zhou December 6, 200

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AC Surge Protection and Fuse Selection Presented by: Godfrey de la Torre EE 136 Fall 2003 Professor Zhou December 6, 2003. Introduction. In today’s discussion, we will cover AC Surge Protection and Fuse Selection.

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slide1
AC Surge Protection and Fuse Selection

Presented by:

Godfrey de la Torre

EE 136

Fall 2003

Professor Zhou

December 6, 2003

D. Zhou, Ph.D.

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introduction

Introduction

In today’s discussion, we will cover AC Surge Protection and Fuse Selection.

Many companies implement AC Surge Protection within their products: Belkin, and Newpoint, just to name a few.

The intended market for this application is primarily those of consumer electronics devices like notebook computers, desktop PC’s, televisions, and even audio receivers.

D. Zhou, Ph.D.

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topic 1 why bother with ac surge protection

Topic 1: Why bother with AC Surge Protection?

With many of today’s electronics using sensitive electronic controls, it becomes necessary to protect them from current spikes (surges) caused by lightning.

Lightning causes surges which may destroy sensitive circuitry.

It is not only important to understand why surges occur, but rather to understand that they will occur, and that it is important to guard against them at all costs.

D. Zhou, Ph.D.

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some examples of surge voltage waveforms

Some examples of surge voltage waveforms

The “Ring Wave” Surge Voltage Waveform

According to IEEE Standard 587-1980, the most common voltage surge waveform is shown below:

  • This waveform is referred to as the “ring wave.”
  • This wave may vary from 5kHz to 500kHz, but a typical residential “ring wave” varies anywhere from 30kHz to 100kHz, and has an amplitude as high as 6kV at 200A.

D. Zhou, Ph.D.

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unidirectional surge waveform
Unidirectional surge waveform
  • This waveform is the called the unidirectional wave.
  • This waveform most likely occurs near the “service entrance” of a building, and as a result carries more energy than the ring wave.
  • This wave may carry 6kV and 3kA.

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likely rate of surge occurrences
Likely Rate of Surge Occurrences
  • Some surge protection devices have limited lives, so it becomes imperative to consider the relative rate of surge occurrences.
  • Knowing the relative rate of exposure and the anticipated surge crest gives the designer an idea of the number of surges that may occur within a given area.

D. Zhou, Ph.D.

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location categories
Location Categories
  • The surge stress to be expected depends upon the location of the equipment to be protected.
  • For equipment inside a building, the expected surge stress depends upon the distance between the equipment and the “service entrance.”
  • Category A – equipment located furthest from the service entrance.
  • Category B – equipment near the service entrance
  • Category C – equipment outside the building

D. Zhou, Ph.D.

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categories explained
Categories Explained
  • Category A – the lowest stress category since furthest from service entrance; applies to all outlets(#10 to #14 wire) 30 ft. from Category B, or to those outlets 60ft. from service entrance; voltage stress is 6kV with 200 A max
  • Category B – the highest stress conditions since closest to service entrance; includes bus panels, distribution lines, and lightning systems in commercial buildings; same stress as Cat. A but with 3000 A max
  • Category C – the location outside the building or at the service entrance; stress far greater than 6kV

Note: Since most power supplies are indoors, this report covers only Category A and B designs.

D. Zhou, Ph.D.

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categories explained continued
Categories Explained (continued)…

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available transient suppression devices
Available Transient Suppression Devices
  • Metal Oxide Varistors – at voltages below its turnover voltage, these passive elements exhibit high resistance; at voltages exceeding the turnover point, these elements sink excess current

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available transient suppression devices continued
Available Transient Suppression Devices (continued)
  • Transient Suppression Diodes – essentially two diodes back to back in a shunt configuration; used for its high clamping action to stop transients; its resistance is low in conduction

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available transient suppression devices continued1
Available Transient Suppression Devices (continued)…
  • Gas-Filled Surge Arrestors – handles much larger current than previous devices; effectively shorts to ground when it conducts excess current

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category a transient suppression filters
Category A Transient Suppression Filters

This circuit utilizes metal oxide varistors, transient suppressor diodes, inductors and capacitors. When L1 (a) and L1 (b) conductive excessive current, caps C2 and C3 are charged to a voltage which brings ZD1, ZD2, and ZD3 into conduction. Once these diodes enter conduction, they effectively create a short to ground, thereby protecting the load.

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category b transient suppression filters
Category B Transient Suppression Filters

This design is usually reserved for higher-power applications (when compared to Category A designs), and as such, utilizes gas-discharge tubes, spark gaps, fast-acting fuses, as well as inductors and capacitors. The functionality of this design is similar to that of Category A: when fast transients enter this circuit, varistors V1 through V3, L1, L2, and capacitors C1 through C5 cause conduction in diodes D1 though D3, which short to ground, thereby protecting the load. This design (as well as the Category A design) offers full noise protection as well.

D. Zhou, Ph.D.

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category a transient suppression filter example
Category A Transient Suppression Filter Example
  • Simplorer Schematic

D. Zhou, Ph.D.

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category a results
Category A results
  • Assume varistors are ideal switches that are normally open. These switches short for some turnover voltage VT.
  • For first case, choose VT = 90V. Vsource = 120Vac at 60Hz.

D. Zhou, Ph.D.

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for v source 120v ac at 60hz with v t 90v
For Vsource = 120Vac at 60Hz, with VT = 90V:

The top is the graph of the voltage source. The bottom shows the clamping action of varistors V1 though V3.

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category a transient suppression filter example continued
Category A Transient Suppression Filter Example (continued)
  • Next, try the worst case with Vsource = 6kV at say, 3kHz. Choose VT = 600V, which is a common turnover voltage for varistors.

D. Zhou, Ph.D.

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for v source 6kv ac at 3khz with v t 600v
For Vsource = 6kVac at 3kHz, with VT = 600V:

The voltage source waveform is shown on top, and the resulting clamping action of varistors V1 through V3 is shown at the bottom. Note the second graph “spikes” to 600V, then brings the output to 0 V.

D. Zhou, Ph.D.

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category b transient suppression filter example
Category B Transient Suppression Filter Example

Category B designs are similar to Category A in its topology. In fact, the Category A can be used as a Category B design if sufficiently large devices can be implemented within A. But for brevity’s sake, Billings uses a gas discharge tube, which can be modeled as an ideal switch.

The switch is normally off, but conducts for some terminal voltage. S4 shall be the switch modeling the gas discharge tube.

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category b transient suppression filter schematic
Category B Transient Suppression Filter Schematic

D. Zhou, Ph.D.

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slide22
Now, same procedure for Category B:For Vsource = 6kVac at 10kHz, with VT = 600V, and S4 set to 4000V (to emulate Gas Tube):

Notice similar results as Category A. The output spikes to 4000V, then shorts to ground. The left graph is the input, and the second is the output.

D. Zhou, Ph.D.

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topic 2 fuse selection
Topic 2: Fuse Selection
  • Types of Fuses

There are three main kinds of fuses:

  • Time-Delay Fuses

This fuse is usually the largest among the three, primarily because this fuse provide large amounts of currents for some time without rupturing. These are usually used for devices with large inrush currents, like motors and solenoids.

D. Zhou, Ph.D.

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topic 2 fuse selection1
Topic 2: Fuse Selection
  • Standard Slow Blow Fuse

The fuse at the top is typical of a standard slow-blow fuse. These fuses are low-cost, and are easiest to find among the three classes. These fuses are for standard applications, primarily that of short-circuit protection.

D. Zhou, Ph.D.

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topic 2 fuse selection2
Topic 2: Fuse Selection
  • Very Fast-Acting Fuses

These protect semiconductor devices. These fuses allow the bare minimum energy during overload, and are usually filled with filler (e.g. sand) to quench arc voltage in the event of overload.

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fuse parameters
Fuse Parameters
  • Current Rating – this is the maximum current that the fuse can handle; it must exceed the DC or rms current demanded by the circuit
  • Voltage Rating – this is the fuse’s ability to extinguish the voltage arc that is produced as the fuse melts during fault conditions. Failure to extinguish the arcing voltage may result in explosion.

D. Zhou, Ph.D.

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fuse parameters1
Fuse Parameters
  • Let-through or I2R rating

This parameter identifies the amount of energy that must be dissipated in the fuse before it melts. Having this rating allows categorization of fuses into “fast-blow” or “slow-blow” fuses. Knowing the pre-arcing melting time allows one to find the fault current below.

D. Zhou, Ph.D.

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fuse selection
Fuse Selection
  • For line input fuses, the designer must first study the turn on of the supply and the action of the inrush limiting circuitry at maximum and minimum input voltages and current load.
  • Choose standard or slow-blow fuse that provides sufficient current margin to give reliable operation and satisfy the inrush current requirements.
  • For long fuse life, fuses are chosen to be 150% of Irms,max.
  • The voltage rating must exceed peak supply voltage.

D. Zhou, Ph.D.

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standard fuse example
Standard Fuse Example
  • Here, we consider a standard fuse’s functionality. The fuse shall be modeled as an ideal switch, one that normally conducts, but opens when the current is 10A (i.e. the fuse’s current rating in this case is 10A). The source is an independent current source with 15Arms at 60Hz.

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standard fuse example schematic
Standard Fuse Example Schematic

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standard fuse example schematic results
Standard Fuse Example Schematic Results

The input current waveform is on the left and the output is on the right. Notice how the fuse brings the current to zero.

D. Zhou, Ph.D.

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conclusion
Conclusion
  • AC surge protection and fuses play a key role in load protection.
  • The choice of either Category A or Category B design depends on the distance from the service entrance.
  • Fuses must be selected according to its voltage, current, and I2R rating.

D. Zhou, Ph.D.

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