POWER LINE PROBLEMS -- AN INTRODUCTION

When failure strikes a product for which we are responsible, we will spend considerable time and money to find and resolve the problem. Many such failures occur in electronic products because the product design engineer failed to give adequate heed to the two major nemeses of electronic products- overheating and power line disturbances. The subject of this paper is the latter of these, but both are deadly to solid-state devices. Also, power line problems can be the cause of overheating.

There has been a quantum leap in the last few years in digital technology. Microprocessors are everywhere. Soon they will control virtually everything. All electrical and electronic products are being made "smart". They are no longer limited to computers, which everyone expects to be finicky gadgets, but are also found in washing machines, microwave ovens, office copiers, radios, and VCR's to just name a few. Soon every home and business will have a dozen, if not several dozen microprocessors operating within its confines. And most of this equipment will be plugged into the power line. The power line is not only the life blood of these valued products but it is also the demon which will destroy them without adequate safeguards.

In order to discuss this we will need to make some definitions. The following is a list of terms and definitions:

1. Steady-State Voltage - Steady-state rms voltage values are those that stay constant for 10 seconds or longer.

2. Nominal or Normal Voltage - The normal planned voltage for a system. For most equipment this is 110-125VAC.

3. Power Failure - A power failure is a zero voltage condition lasting for more than one cycle (1/60 second) on any one of the three phases being used.

4. Blackout - A total power failure lasting several seconds to hours or more.

5. Brownout - A planned and generally announced city or regional-wide reduction by the utility in the steady-state voltage possible due to heavy electrical consumption.

6. Undervoltage - A continuous reduction in the steady-state voltage below the normal voltage. Similar to brownout but this term is generally used to refer to unplanned reductions or localized problems within the building or circuit.

7. Overvoltage - A continuous exceeding of the normal voltage.

8. Sag - Sags are cycle-to-cycle decreases in the power line voltage on any one of the three phases below the normal value. The lasting time of a sag is its duration in the number of cycles of the 60HZ line frequency that the disturbance is below, and returns to the normal level.

9. Dip - Dips are very short (but visible in incandescent light bulbs) decreases in the normal power line voltage. These are similar to sags but are noticeably faster. A dip is a fast sag.

10. Surge - Surges are cycle-to-cycle increases in the power line rms voltage on any of the three phases above the normal voltage. Lasting time for surges is measured the same way as are sags.

11. Swell - Swells are longer term multi-cycle slow rising surges lasting from a few seconds to several minutes.

12. Impulse - An impulse is a very short term disturbance up or down superimposed on the AC sine wave that typically lasts between 0.5 and 100 microseconds. In-phase going impulses which increase the instantaneous voltage are called spikes. Out-of-phase impulses which decrease the instantaneous voltage are called notches.

13. Spike - An over voltage impulse ranging from 400V to 5600V or more which is superimposed on top of the AC sine wave. These, when over 600V, are potentially very damaging disturbances.

14. Notch - An under voltage impulse similar to a spike but of reverse polarity to the instantaneous value of the AC sine wave so as to take a momentary notch out of the sine wave. Notches are typically too fast to see. These typically last as long as spikes but can be up to several milliseconds. In addition, spikes and notches usually come in pairs or in an oscillating series. For every notch there is usually an immediately following spike due to power line inductances and capacitances.

15. Transient - A transient is any short-term non-normal event on the power line. All power line disturbances are transient by definition.

16. EMI - Broad spectrum electromagnetic noise interference either conducted on the power line directly from the source or radiated to the power line then conducted to the susceptible equipment.

17. RFI - Electromagnetic noise interference in the radio spectrum.

18. TVI - Electromagnetic noise interference in the television spectrum.

19. EMP - A very large and very fast rising electromagnetic pulse caused by catastrophic events such as lightning or nuclear detonation.

20. Harmonic - Harmonics are sinusoidal currents and voltages with frequencies that are integral multiples of the fundamental power line frequency. Harmonics distort the normal sine wave.

21. Normal Or Differential Mode - Events which occur across the normal current carrying wires of the power line-hot wire to neutral wire. Also called the transverse or metallic mode.

22. Common Mode - Events which occur from the current carrying wires, hot and neutral, relative to the safety wire-ground.

23. MOV - Metal Oxide Varistor. A voltage dependent resistor. A low cost but very effective protection device. They can handle large surges and switch in 1 to 5 nanoseconds. It works by absorbing voltage surges and spike impulses. They are most effective when used in groups of three to provide both differential mode and common mode protection.

24. Silicon Avalanche Zener Diodes - A solid-state junction device. These devices are very fast acting but have low energy handling capability. Switching speed is in picoseconds. They work by shunting the surge or spike impulse around the protected circuit.

25. Gas Discharge Tube - A calibrated spark gap in a gas filled chamber. These devices are relatively slow, activating in microseconds but can handle very large surges. They work by shunting the surge or spike impulse around the protected circuit.

26. EMI/RFI Filter - A circuit or device containing series inductive (load-bearing) and parallel capacitive (nonload-bearing) components which provides a low impedance path around the protected circuit for high frequency noise. Filters also attenuate impulses since a Fourier's Analysis of a spike will reveal it is composed of high- frequency waveforms. Filters and surge suppressors when used together thus act synergistically.

27. UPS - An Uninterruptible Power System which provides a steady source of electric energy to a piece of equipment.

28. Continuous UPS - A UPS system for which the load is continually drawing power through the batteries and inverter and never directly from the normal AC power line.

29. Standby UPS - A UPS system which normally connects your equipment to the normal AC power line with the batteries and inverter in standby mode. When the power line is weak or fails it transfers the load to the batteries and inverter without any load malfunction and without any user action. When the power line returns to normal the load is automatically retransferred back to the AC power line.

30. Voltage Surge Suppressor - A fast acting circuit or device containing MOVs, Silicon Avalanche Diodes, Gas Discharge Tubes, or other components which suppresses voltage surges and spikes to a safe level. The energy in the surge or spike is either dissipated as heat in the protection components or is diverted to earth ground by the circuitry.

31. Voltage Regulating Transformer - A continuous acting transformer, usually ferro-resonant, which instantaneously regulates the output voltage to normal levels despite wide swings in the input voltage.

32. Voltage Stabilizer - A switching device which selects appropriate windings of a transformer to maintain normal output voltage levels despite wide swings in the input voltage. Switching occurs after the power is non-normal for a few seconds. This device is commonly confused with a voltage regulating transformer but they differ greatly in response time. Stabilizers are generally too slow to protect solid-state devices.

33. Power Line Conditioner - A PLC is a combination of a voltage regulating transformer with a super isolation transformer which provides smooth, regulated, noise free, AC voltage with no ohmic connection between input and output. A PLC will solve most problems other than complete power failure.

How major a problem are power line disturbances? Because of the numerous failures of otherwise well designed electronic equipment there has been quite a bit of research done on this subject. Much of it is not available to the general public or even to many engineers. Not until, that is, a problem develops and nobody can find out the cause. Then finally someone thinks of -POWER LINE PROBLEMS!

A two year study by IEEE members Martzloff and Hahn completed in 1970 shows that surges and impulse voltage spikes can occur as frequently as twice per hour in a typical residence with peak values of 1500V to 2500V. Spikes in industrial environments are even more frequent and severe. Spikes as high as 5600V were recorded during lightning storms.

A study by IBM in 29 different locations throughout the U.S. showed that an average of 50.7 voltage spikes occur per month which in their study was 39.5% of the total disturbances.

A two year study by Bell Laboratories showed that most locations will experience approximately 25 power line disturbances per year 87% of which they found to be sags below 96 volts. Voltages below 96V can cause the power supply in most equipment to malfunction and thus lead to equipment failure.

Some independent service organizations have reported that power line disturbances are the cause of 70% of the failures of some equipment. I myself have frequently seen equipment improperly operating and damaged by power line disturbances.

So indeed, while studies disagree as to which type of disturbance is the most frequent, they all agree that power line disturbances are a major problem and it is widespread. Once the nemesis of esoteric products such as computers, it now is affecting us all because of the widespread use of microprocessors and other digital devices in everyday products. Power line disturbances have ruined the marketability of products which will not hold up in the field and have likewise destroyed the credibility of the engineers and companies who designed them.

What are the solutions? Well, we begin by becoming aware of the existence of the problem. The physical design solution boils down to those we can put into the product and those which we use externally to keep the disturbances outside the product or minimize their impact.

Internal solutions involve allocating adequate time and money to the low- tech, but very important part of the product--the power supply--in order to make the equipment less susceptible to powerline disturbances. Engineers must select power supplies which have adequate operating margins to perform as required in undervoltage and overvoltage conditions. Great care must be given to the impulse response of the power supply so that fast transients and high frequency noise don't sneak through and disrupt or damage the equipment. Surge suppression devices such as MOVs, Silicon Avalanche Zener Diodes, and Gas Discharge Tubes and noise suppression devices such as EMI/RFI Filters should be used to suppress any disturbances that come up the power line before they get to the DC power supply of the equipment. Lastly, the design engineer must test, test, test, and test some more. All to often people use field applications as their testing ground. It's too late then if there is a serious deficiency in the power supply. Murphy's law will get them sure enough if they don't do adequate testing. Accelerated testing must be done for the anticipated life of the equipment for all of the above listed types of power line disturbances. If an engineer ignores the power line disturbance problem it will come back to haunt him 10,000 times over after the product is in the field.

External solutions are ideal solutions for after-the-fact design problems and for very large disturbance problems which, as was pointed out above, do occur. Sometimes this is the only practical economic solution. For simple brownouts and undervoltage the voltage stabilizer is fine. Protection from sags, dips, notches, brownouts, and undervoltage can be provided with voltage regulating transformers. Total protection from blackouts, brownouts, sags, dips, notches, and total power failures can be achieved with a fast acting emergency power system such as a standby Uninterruptible Power System (UPS). Protection from spikes, surges, and noise can be provided by plug-in combination surge and noise suppressors. These devices, when externally applied, usually perform better than similar devices used internal to the equipment. For no matter how well the design engineer designs his protection within the equipment the disturbance is already in the equipment before the protection can work. Power line disturbances propagate along the wires at some fraction of the speed of light. Take the example of particularly damaging disturbances such as spikes. All protection devices have a finite turn on time. For MOVs, the most commonly used protection device for spikes, this is typically five nanoseconds. The disturbance will propagate approximately four to five feet in this time. Therefore, if there is a sneak path through the power supply, and there usually is, protection devices within the unit will not be fast enough without the use of super fast and very expensive protection schemes. Also, very large disturbances can punch through the internal defenses and then the next stop is the delicate and expensive high-tech components. Poof! There goes a five hundred dollar main logic board. Thus, by making your first line of protection further upstream on the power line at the outlet or in the service panel you get the advantage of using the intervening wire and the typical 6' cordset on the equipment as a propagation delay line giving the suppressors in the external protector time to work fully before the disturbance reaches the equipment. Thus adequate protection can be provided externally with low cost parts. External protection, even with the best internal protection, should always be recommended for use with high-tech equipment as low-cost insurance if for no other reason.

In conclusion, it should be apparent that power line problems are a real world phenomena which we have to deal with if we are to continue to expand our use of high-tech digital equipment. However, solutions as outlined above are available.