The quality of incoming power to a load is important because all electrical and electronic equipment is rated for operation at a specific voltage.
The rated voltage is actually a voltage range. Typically, this range is ±10%. However, with many newer components derated to save energy and operating cost, the operating voltage range is now often between +5% and –10%. This range is used because an overvoltage is generally more damaging than an under voltage.
The amount and duration of voltage change can be classified into commonly used industrial terms. Voltage changes range from total power interruption to very short variations that may last only a few milliseconds (transients).
Power interruptions are classified as momentary, temporary, or sustained power interruption. The different classifications are based on the time power is lost.
A momentary power interruption is a decrease to 0 V on one or more power lines lasting from 0.5 cycles up to 3 sec.
A temporary power interruption is a decrease to 0 V on one or more power lines lasting for more than 3 secs up to 1 min.
A sustained power interruption is a decrease in voltage to 0 V on all power lines for a period of more than 1 min.
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Voltage changes other than power interruptions occur much more often and can be a continued change that causes both short-term and long-term problems. See Figure 1.
Figure 1. Voltage changes, such as voltage sags and swells, under voltage, and overvoltage, are difficult to detect because voltage changes are often intermittent.
A voltage fluctuation is an increase or decrease in the normal line voltage within the range of +5% to −10%.
Voltage fluctuations are commonly caused by over-loaded transformers, unbalanced transformer loading, and/or high impedance caused by long circuit runs, undersized conductors, or poor or loose electrical connections in the system.
A voltage sag is a voltage drop of more than 10% (but not to 0 V) below the normal rated line voltage that lasts from 0.5 cycles up to 1 min.
Voltage sags commonly occur when high-current loads, such as motors, are turned on. When very large motors are turned on, a voltage sag can be followed by a voltage swell as voltage regulators overcompensate during the voltage sag.
A voltmeter can be used to measure the voltage at the power source and at the load to determine if there is a problem with bad connections, too long of a run, or undersized conductors.
Voltmeters with a MIN/MAX recording mode are used to measure the voltage over time and can be used to determine if there is a voltage sag or other voltage fluctuation problem. See Figure 2.
Figure 2. A voltmeter with a MIN/MAX recording mode is used to determine the type of voltage problem.
A voltage swell is a voltage increase of more than 10% above the normal rated line voltage that lasts from 0.5 cycles up to 1 min.
Voltage swells commonly occur when large loads, such as motors, are turned off and the voltage on the power line increases above the normal voltage fluctuation (+10%) for a short period of time.
Voltage swells are not as common as voltage sags. However, voltage swells are more destructive than voltage sags because voltage swells damage electrical equipment in very short periods of time.
Under voltage is a drop-in voltage of more than 10% (but not to 0 V) below the normal rated line voltage for a period of time longer than 1 min.
Under voltage (low voltage) is more common than overvoltage (high voltage) on power lines.
Under voltages are commonly caused by overloaded transformers, undersized conductors, conductor runs that are too long, too many loads on a circuit, or brownouts.
A brownout is a reduction of the voltage level by a power company to conserve power during times of peak usage or excessive loading of the power distribution system.
Overvoltage is an increase of the voltage of more than 10% above the normal rated line voltage for a period of time longer than 1 min.
Overvoltages are caused when loads are near the beginning of a power distribution system or when taps on a transformer are not wired correctly.
A tap is a connection brought out of a winding at a point between its endpoints to allow the voltage or current ratio to be changed. Transformer taps are commonly provided at 2.5% increments.
A transient voltage (voltage spike) is a temporary, undesirable voltage in an electrical circuit.
Transient voltages range from a few volts to several thousand volts and last from a few microseconds up to a few milliseconds.
Two types of transient voltages are oscillatory transient voltages and impulse transient voltages.
An oscillatory transient voltage is a transient voltage commonly caused by turning off high inductive loads and by switching off large utility power factor correction capacitors.
An impulse transient voltage is a transient voltage commonly caused by lightning strikes and when loads with coils (motor starters and motors) are turned off.
Transient Voltage Measurement Procedures
Power analyzer meters are typically used to monitor transient voltages. The size, duration, and time of transient voltages can be displayed at a later time when using power analyzer meters.
When transient voltages are identified as a problem within a facility, surge protection devices (voltage surge suppressors) must be used.
A surge protection device is a device that limits the intensity of voltage surges that occur on the power lines of a power distribution system.
Transient voltage measurements must be taken any time equipment prematurely fails and a transient voltage is suspected. Transient voltage measurements are taken to verify that surge protection devices are working.
A transient voltage measurement is different from a standard voltage measurement because a standard voltage measurement displays the RMS voltage value of a circuit and a transient voltage measurement displays the peak voltage value of a circuit. For example, a standard voltmeter that displays 115 VACS is displaying the RMS voltage measurement of the circuit. The peak voltage of a 115 VAC RMS circuit is about 162 V (RMS voltage multiplied by 1.414 equals peak voltage). See Figure 3.
Figure 3. Transient voltage measurements must be taken any time equipment prematurely fails and transient voltages are suspected.
Transients can damage any electrical or electronic equipment. Transients can also cause problems in electronic circuits without damaging them by producing false signals.
The high, short-duration voltage spikes can also cause errors in digital-based electronic circuits because they operate on low-level digital signals.
Transients should be checked for at the power line input of electronic equipment and transient protection devices (snubbers) should be used to reduce transient problems.
Transient voltage damage can be reduced when electronic devices are plugged into a good surge suppressor with a rating of at least 1200 Joules (J).
Before taking any transient voltage measurements using a power analyzer meter, it is important to ensure that the meter is designed to take measurements on the system being tested.
To test for transient voltages, the following procedure is applied:
- Set the power analyzer meter to transient voltage recording mode.
- Connect the test leads of the power analyzer meter to the meter jacks as required.
- Connect the power analyzer meter voltage test leads to the system being tested. When using a power analyzer meter to record transient voltages, the test leads of the meter must be connected to the powered system before recording starts. The test leads must be connected first because a power analyzer meter will be looking for a voltage higher than the setting of the meter (50%, 100%, 150%, etc.). When the test leads are not connected before the meter starts recording, the meter will record 50% (or whatever the meter is set on) of nothing (0 V).
- Set the transient voltage recording mode to record transient voltages at a set level (50%, 100%, 200%), or above normal voltage. Normal voltage is the voltage applied to the test leads before the meter is set to start recording transients. When a meter is connected to a standard 120 V circuit and is set to record transient voltages greater than 100%, the meter records any voltage over 340 V peak (120 V RMS multiplied by 1.414 equals 170 V peak, and the meter is set at 100%, so 170 V multiplied by 2 equals 340 V peak).
- Allow the meter to record as loads are switched on and off.
- Read and record any findings of recorded transient voltages.
- Correct for system transients by using surge suppressors to filter any transient voltages from the system.
- Retest the system after repairs are completed. 9. Remove the power analyzer meter from the system.