Power Electronics Safety

Safety Rules and Measurements in Power Electronics

Nine rules for safe practice and avoiding electric shocks:

1. Be sure of the condition of the equipment and the dangers it can present before working on it. Many sportsmen are killed by supposedly unloaded guns; many technicians are killed by supposedly “dead” circuits.

2. Never rely on safety devices such as fuses, relays

   and interlock systems to protect you. They may not be working and may fail to protect you when most needed.

3. Never remove the grounding prong of a three-wire plug. This eliminates the grounding feature of the equipment making it a potential shock hazard.

4. Do not work on a cluttered bench. A disorganized mess of connecting leads, components and tools only leads to careless thinking, short circuits, shocks and accidents. Develop systemized and organized work habits.

5. Do not work on wet floors. Your contact resistance to ground is greatly reduced on a wet floor. Work on a rubber mat or an insulated floor.

6. Do not work alone. It is just good sense to have someone around to shut off the power, to give artificial respiration, or to call doctor.

7. Work with one hand behind you or in your pocket. A current between two hands crosses your heart and can be more lethal than a current from hand to foot. A wise technician always works with one hand. Watch your TV serviceman.

8. Never talk to anyone while working. Do not let yourself be distracted. Also, don’t you talk to someone who is working on dangerous equipment. Do not be the cause of an accident.

9. Always move slowly working around electrical circuits. Violent and rapid movements lead to accidental short

   circuits and shocks.

Burns

Accidents involving burns, although usually not fatal, can be painfully serious. The dissipation of electrical energy produces heat.

Four rules for safe practice and avoiding burns.

1. Resistor can get very hot. Watch those five- and ten-watt resistors. They will burn the skin off your fingers. Stay away from them until they cool down.
2. Be on guard for all capacitors which may retain a charge. Not only can you get a dangerous and sometimes fatal sock. You may also get a burn from an electrical discharge. If the rated voltage of electrolytic capacitors exceeded or their polarities reversed they may get very hot and may actually burst.
3. Watch that hot soldering iron or gun. Do not place it on the bench where your arm might accidentally touch it. Never store it away while it is still hot. Some innocent unsuspecting person may pick it up.
4. Hot solder can be particularly uncomfortable in contact with your skin. Wait for soldered joint to cool. When de-soldering joints, do not shake hot solder off; you or your neighbor might get hit in the eye or get it on your clothes or body.

Equipment related injuries

This third class of safety rules applies to all those who work with tools and machinery. It is a major concern of the technician and the safety lessons and found in the correct use of tools.

Five rules for safe practice and avoiding equipment related injuries.

1. Metal corners and sharp edges on chassis and panels can cut and scratch. File them smooth.

2. Improper selection of the tool for the job can result in equipment damage and personal injury.

3. Use proper eye protection when grinding, chipping or working with hot metals which might splatter.

4. Protect your hands, clothes and eyes when working with battery acids, etchants and finishing fluids. They are corrosive!

Measurement with the aid of an oscilloscope in power electronics circuits

In order to observe electrical signals in a circuit, a conventional oscilloscope is normally used. However, to observe signals in a power electronics circuit, it often happens that the conventional oscilloscope cannot be used directly. In order to see why this is so, consider the conventional two-channel oscilloscope shown in figure F1-2.

As you can see, the ground (earth) connector of the line cord is connected to the chassis of the oscilloscope and to the oscilloscope’s circuit common point. As a result of this, the common lead of the voltage probe is connected to the ground. The chassis is connected to the ground as a safety feature to prevent shock due to the chassis assuming a high potential. With this in mind, we can represent the oscilloscope in F1-2 as shown in F1-3.

F1-2: Simplified schematic diagram of a conventional two-channel oscilloscope.

F 1-3: Equivalent diagram of the oscilloscope in F1-2.

Now consider a simple series circuit such as shown in F1-4. in which resistor R₁ is equal to resistor R₂. Obviously, the voltage across resistor R₁ is equal to half the ac source voltage E_s (1/2 E_s).

F 1-4: Observing signals in a simple series circuit with a conventional oscilloscope.

 

In order to observe the voltage across resistor R₁, you might be inclined to connect the oscilloscope to the circuit as shown. However, this connection will modify the circuit because resistor R₂ is now short-circuited to ground by the ground lead of the oscilloscope probe. As a result, the oscilloscope will show a voltage across resistor R, equal to Es, which is incorrect.

To prevent the oscilloscope connection from modifying the circuit and in order to observe the correct voltage across resistor R₁, a voltage isolator would have to be used.

Principle of the voltage isolator and current isolator

The purpose of a voltage isolator is to completely separate the electronic circuit from the instrument that will measure the voltage in the circuit. As a result, the instrument cannot modify the electronic circuit and consequently, the voltage across any circuit element can be measured correctly.

A voltage isolator in its simplest form has two input terminals (1 and 2) and two output terminals (3 and 4) as shown in F1-5(a). The input terminals are connected to a light emitting diode (LED) whose light intensity is proportional to the voltage E₁ which appears across terminals 1 and 2.

A photo-transistor is connected across terminals 3 and 4. This optical pickup produces a voltage E₂ which is directly proportional to the light produced by the LED.

It is obvious that this arrangement will produce a voltage E₂ that is directly proportional to the input voltage E₁. Furthermore, it is clear that the “ground” on the measurement side cannot affect the electronic circuit in any way.

The input terminals are connected to the electronic circuit while the output terminals are connected to the oscilloscope, multimeter or any other measuring instrument.

A current isolator operates the same way, except that the current flows through a resistor of very low resistance connected between the input terminals as shown in F 1-5 (b). The voltage developed across the resistor is proportional to the current, and so the LED emits light in proportion to the input current. It follows that the output voltage E₂ is proportional to the input current I. Again it is clear that the “ground” on the measurement side cannot affect the electronic circuit in any way. In other words, there is perfect isolation between the two.

a) Voltage Isolator

b) Current Isolator

F 1-5: Simple voltage isolator and current isolator.

Actual voltage isolators and current isolators are more complex than these simple figures would lead us to believe, but the basic idea is to obtain the results explained above.

Measurements with analog meters in power electronics circuits

In conventional electrical power technology, analog meters are commonly used to measure various electrical variables. Different types of meter movements allow the measurement of ac and dc variables. Unfortunately, In power electronics, many of the variables of interest are neither smooth dc nor pure sinusoidal ac signals but fall somewhere in-between. Because of this, it is important to know how the different meters work in order to be able to make meaningful measurements of variables in power electronics circuits.

Measuring dc voltages and currents

To measure a dc variable, a dc voltmeter or ammeter may be used. Most dc meters are based on a d’Arsonval movement; the deflection of the needle is proportional t the average value of the current flowing through it. Because of this, these meters can also be used to measure average values. Usually instruments include an internal protection circuit which will limit the value of the input variable to protect the meter movement. This must be kept in mind when using these meters. For example, consider measuring the average value of the current waveform shown in F1-6(a) using a dc ammeter with a current protection circuit which limits the current to 2 A on it 1 A range.

 

a) Typical current in a power electronics circuit.

 

b) Current in a meter with the protection circuit.

F1-6: Current waveforms.

The actual average value of the current can be obtained by calculating the area under the curve in one period and dividing it by the period. For the waveform in F1-6 (a). This gives:

I avg. = 3 (T/3)×(1/T) = 1 A

However, with the ammeter on the 1A range, the protection circuit will limit the input current to 2 A. Therefore, the meter will measure the average value of the waveform shown in F1-6(b) and give a reading of:

I avg. = 2 (T/3) × (1/T) = (2/3) A

The error on the measurement, expressed in percentage, is about 33%. Clearly, a higher range should have been selected on the ammeter to measure the average current. This could have been noticed if the current waveform had been observed on an oscilloscope prior to making the measurement.

Measuring alternating currents and voltages

In general, when measuring an alternating current or voltage, it is the R.M.S (root mean square) or effective value that is of interest. Most R.M.S measuring instruments you will encounter are designed to measure the R.M.S value of a sinusoidal variable; instruments for measuring the R.M.S values of no-sinusoidal variables are available, but they are relatively expensive.

Most ac ammeters are based on a moving iron-vane movement. This type of movement gives the R.M.S value of an alternating signal directly. However, it has a limited frequency range from about 25 to 80 Hz, limiting its usefulness as an accurate measuring instrument (2% accuracy) to 50- and 60-Hz sinusoidal signals. Sinusoidal signals whose frequency ranges between 10 and 25 Hz or between 80 and 150 Hz can also be measured, but the accuracy decreases to about 5%. If one of these ammeters is used to measure a non-sinusoidal signal, the accuracy of the reading affected. The most ac ammeters (could) have a protection circuit that limits the peak value of the signal to prevent damage to the movement.

Most ac voltmeters are based on a d’Arsonval moving-coil movement. The movement measures the average value of half-wave rectified version of the input signal voltage. The meter scales are calibrated to indicate the R.M.S voltage of pure sinusoidal signals, so they should only be used to measure pure sinusoidal voltages. As is the case with the ac ammeters, if one of the ac voltmeters is used to measure a non-sinusoidal signal, the accuracy of the reading is affected. Unlike the ac ammeters mentioned above, the ac voltmeters have a wide frequency response.

To recapitulate, when measuring a signal in a power electronics circuit with an analog voltmeter or analog dc ammeter, it is advisable to first observe the signal to be measured on an oscilloscope in order to get some idea of the magnitude and shape of the signal. This allows the reliability of the meter reading to be verified. In many cases, the voltmeter and ammeter, along with an oscilloscope, can be very useful in determining whether or not a set-up is working correctly.

HANDS-ON EXERCISE

Summery

In the first part of this exercise, you will measure the resistance between two points of you body for dry and moistened skin.

In the second part of this exercise, you will set up in the mobile workstation the equipment required to carry out the exercise.