When working with electricity, very often the first measurement needs to answer, “Is voltage present or not?”
This question is a critical part of electrical testing. OSHA and the NFPA 70E®: Standard for Electrical Safety in the Workplace® both direct workers to de-energize all live parts to which an employee may be exposed, unless live conditions are required for troubleshooting.
Pocket-sized, non-contact voltage testers
To help you answer this question, Fluke now offers a family of non-contact voltage testers, the VoltAlert™ AC series non-contact voltage detectors. This little tool is perfect for keeping in a top pocket where it can be easily seen and pulled out for immediate use. If it lights up when placed near a conductor, there is voltage present. Electricians; maintenance, service, and safety personnel; and homeowners can quickly test for energized circuits in the workplace or at home.
Fluke offers a family of non-contact voltage detectors, each fit for a slightly different purpose, but all built to provide quick go/no-go answers.
The 1AC-II VoltAlert voltage detector includes VoltBeat™ technology. When voltage is detected, the tip glows red and the beeper sounds. The 1AC-II also includes an on-off switch for maximum battery life.
The LVD2 Volt Light voltage detector is includes a built-in flashlight, ideal for working in cramped, dimly lit areas. The LVD2 also offers dual sensitivity, turning on a blue light when the probe is 1 inch to 5 inches (2.54 centimeters to 12.7 centimeters) from the source, changing to red when the LVD2 has located the voltage source.
Fluke’s newest electrical test tool is the 2AC VoltAlert voltage detector. The 2AC is “always-on.” This feature lets the user perform immediate voltage checks without having to turn the unit on, allowing faster, safer voltage testing. Always-on is made possible by the innovative Battery Check button; even though the unit is always-on, a quick touch of the battery check* button will confirm that the unit is powered up and ready to test.
The Fluke 2AC tester detects energized circuits and defective grounds, making it an ideal first-line go/no-go tester for an electrician on the factory floor as well as do-it-yourselfers around the house. The tip of the pocket-sized tester will glow red when within close proximity of an outlet, terminal strip, or power cord where voltage is present.
Additional benefits of the Fluke 2AC VoltAlert™ Tester include:
Voltage detection from 90 to 1000 V ac, suitable for a wide range of residential, commercial and industrial needs.
Category IV 1000 V overvoltage rated product for best-in-class user protection.
Integrated clip design, optimized for pocket storage.
The new Fluke 2ac VoltAlert™ voltage detector provides a fast test to determine if ac voltage is present on a conductor.
Voltage detection as part of your electrical test
Testing for the presence, or absence, of voltage is only one step toward fully, and safely, taking your measurements. But the proper user of a non-contact voltage detector can help answer this very basic first electrical test question.
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Historically,
analog null detectors or null meters have been used in electrical metrology to measure small
voltage differences between two points
or to detect a zero current condition where the voltages at two points are the
same. These might include comparing
and measuring the voltage difference between a standard and a device
under test such as in the comparison of of two primary level standards or between
a primary standard and a secondary
calibration standard by interfacing with the instruments directly or
with the aid of a voltage divider. The small differences
in voltage that are measured by
a null detector allow voltages to be
adjusted on standards so that there is effectively
no voltage difference between
two points or two instruments. The ability to
zero voltage, or a null condition, is essential for electrical metrology.
When
precision digital multimeters (DMMs) became available with resolution and
sensitivities comparable to null detectors, they were quickly adopted, and null
detectors were set aside. However, depending on the measurement circuitry
characteristics and the unique characteristics of the DMM, significant
measurement errors can be created. This application note discusses what to
consider when using a DMM as a null detector.
This is
especially important as improvements in new DMMs lessen some of the effects of
these issues. However, in all cases they need to be understood and
appropriately considered.
Since
the 1960’s, commercial voltage and ratio calibration systems have been
available to calibrate dc voltage from very low
levels–on the order of
millivolts–to relatively high values up to
1 kilovolt. A critical component of these systems was the analog high
impedance voltmeter/null meter. These instruments were designed with extremely
high input impedance (10 to 100 MΩ), excellent sensitivity (0.1 µv per
division) and high isolation (on the order of 1012 Ω).
One series instrument, the Fluke 845 Series of High Impedance Voltmeter Null Detectors, was designed so that source loading through leak- age was virtually eliminated regardless of power line, chassis ground, or guard connections. Input voltages were applied through an input divider and filter circuit to a photo-chopper-stabilized amplifier. The input filter minimized the effects of source noise, and the photo-chopper-stabilized amplifier reduced the input current to a few picoamps. The 845AB could be battery operated, so that it was isolated from line power, had an analog input for nulling operations, provided a good measurement response time (5 seconds on the 1 µv range) and in general was easy to use.
Fluke 8588A
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High
precision digital multimeters started to emerge for metrology use in the mid to
late 1970’s. They became the
preferred measurement tool for dc and low frequency ac electrical metrology. As
a bonus, they were easy to use, multi-functional, and easy to automate. In certain applications they easily replaced
the null detectors, yet their dc input amplifiers did not have the advanced design needed to fully match the null detector
characteristics. In some critical respects they had serious shortcomings. One
key area was the input bias current of the meter.
Bias current related errors
One of
the non-ideal aspects of operational amplifiers such as those used in the input of a DMM
is input bias current. This is a
condition where current flows from the amplifier through the input terminals of
the DMM. DMMs use CMOS amplifiers, which minimize
this effect, but a
current still exists on the
order of picoamps. In voltage measurements where the voltage source is being
measured, there is some source impedance in series with the voltage source. A small bias current through a small source impedance creates a negligible offset voltage. While
this offset is in series with the voltage being measured, it has virtually no
detrimental impact because it constitutes just fractions of nanovolts.
However,
with certain measurement
configurations where there is a sizeable source resistance from the voltage
being measured (in the tens of k
ohms), this resulting offset voltage
due to the bias current could cause offset errors of several microvolts. This offset is directly in series with the
measured voltage and is a serious error that must be dealt with if a DMM was to
be used for critical measurements in such applications. Such an error could easily exceed the voltage measurement being attempted.
However,
remember that this bias current characteristic was not a factor when using a
null detector, so it was never something to be dealt with. Eliminating such
errors requires extra measurement steps to compensate and remove these bias
current offsets when using a DMM.
Use of voltage dividers
Electrical
metrology uses primary level voltage standards for establishing traceability
and working level voltage standards to calibrate working standard voltage
sources. Because the voltage standards today are commonly at a 10-volt level, and the working
standard voltage sources range from millivolt ranges up to a kilovolt range,
voltage dividers are used to
simplify intercomparing the sources to the 10-volt voltage standards. For example, a 100-volt source is divided
by 10 to create a 10-volt level for
comparing to the 10-volt working standard. Another example is calibrating a
precision voltmeter at a non-decade voltage,
such as 1.9 volts.
To
create a precise voltage for such a test, a 10-volt voltage standard would be
scaled by 0.19 ratio to create a 1.9-volt level for calibrating the meter.
From these examples, ratios must both be decade values; that is, multiples of 10 (1:10, 1:100, 10:1 and100:1),
as well as other variable ratios – such as from .999999 to 0.000001. There are
many such dividers in common use.
There
are fixed ratio standards, such as the
Fluke 752A Reference Divider, and
variable ratio standards, such as the Fluke 720A
Kelvin Varley Divider. These are regularly used to divide one voltage to match the level of a second voltage
when doing calibrations across
the voltages mentioned above.
The metrologist must confirm or adjust the dividers to insure they have the proper ratios for the intended calibration. These dividers must be balanced before use. Such balancing requires a null detector style of measurement to adjust for and confirm a proper ratio.
Fluke 8588A
Divider balancing errors due to DMM bias currents
The
most commonly used fixed or variable
ratio standards are resistive dividers. When the dividers are balanced there is
always a resis- tance between the two balancing points. This resistance is usually on the order of 25 kOhms to 40 kOhms. So, when you have maximum bias currents of 50 pA, the divider
can generate undesired offset voltages of several microvolts. The metrology
measurements using such divid- ers often
measure voltage levels of zero volts to fractions of microvolts; therefore,
these undesired bias current offsets have a serious effect.
Fortunately,
techniques for eliminating these errors have been developed and are presented
in white papers by Fluke metrologists. Updated ver- sions of those original
papers are included on the Fluke Calibration website so you can refer to them
for complete details about using a precision DMM in place of a null detector.
Using the Fluke Calibration 8588A in place of analog null detector
for self-calibration of the Fluke 720A.
Using digital multimeters in place of analog null detectors for
metrological applications
Recommended solutions to bias current offset
voltages
The best way to correct
this
bias current issue is to properly select the DMM used for
balancing and null detector measurements. Different DMMs have different designs and vary in their bias cur- rent
situations.
Use a newer designed DMM: Since the 1970’s the precision DMM designs have amplifier circuitry with up to 50 pA of bias currents. The newest Fluke Calibration Reference Multime- ters (the 8588A and 8558A) have significantly less bias current, at 20 pA maximum. This bias current is also adjusted to be effectively zero pA when they are originally tested. This virtu- ally removes the bias current offset problem. Also, because the bias current changes very little over time, it remains near this level for an extended period into the future.
Determine the offset and correct for it: In the initial balancing process, at the starting point where the DMM is connected to the divider and configured for the test, but before any external voltage source is to be applied, short the voltage input terminals of the divider and observe any offsets on the DMM. Any voltages measured will be the result of bias currents through the divider’s source impedance. If any offset is indicated, observe if the offset is relatively noiseless, and it is stable over time. If it is both stable and not excessively noisy, then it can be mathematically removed from the balancing measurements. The observed offset can be mathematically removed by doing a measurement offset correction on the DMM.
Reject DMMS with unstable bias currents: A precision DMM can work very well for general metrology measurements. However, a small minority of these instruments have excessively noisy or unstable bias currents. These will not be able to be used to balance dividers.
Expect many Fluke Calibration DMMs to have acceptable bias currents: Most Fluke Calibration Reference DMMs are adjusted for minimum bias current when they are manu- factured. As a result, you will see bias currents at a fraction of their maximum specification. So, with bias currents that show to be 5 pA or less no measurable offsets will show up in the offset test mentioned above.
Periodically test for bias currents: It should be noted that a DMM’s bias current may slowly change over time. Regularly test the DMM for its bias current characteristics, or if it is not frequently used for balancing, then test before use, to see if the bias current is still acceptable.
Fluke 8588A
Conclusion
The Fluke Calibration
8588A Reference Multimeter and 8558A Digital Multimeter are suitable
replacements for analog null detectors, as was the previous generation 8508A
Reference Multimeter. Appropriate corrections must be made for current
emanating from the meter input terminals. Before any other digital multimeter
is used as a null detector, the input bias current and other current sources
must be considered to determine the significance of error generation
to the circuit.
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Fluke
endeavors to build the world’s most accurate, rugged and reliable test
equipment. Our products are calibrated
to be traceable to the International System of Units (SI) through
intrinsic standards or National Metrology Institutes. However, all electronic components and mechanical devices exhibit
drift over time. To ensure that your Fluke product always operates to published specifications,
you must have it recalibrated regularly.
When Fluke develops product
specifications, the design engineers take into account a variety of uncertainty
influences: for example, traceability to the SI; short term stability;
stability due to environmental variation; long term stability; and other
sources of uncertainty based on the product design. The uncertainty due to long
term stability must be defined by a time interval. Fluke defines one or
more-time intervals in the published specifications for each product. The most
common time interval is one year.
Fluke product specifications
are designed so that more than 95 percent of the population for a given model will meet all
specifications at the end of its published interval. This is assured through product design and
is tested by methods such as statistical analysis of reliability and
accelerated life cycle testing.
When customers purchase test
equipment, they need to select an appropriate interval for recalibration. The
recalibration interval may be shorter or longer than the time interval
published in the manufacturer’s specifications because of factors such as
frequency of use or harshness of operating environment. This is why the
calibration quality standard ISO/IEC 17025 states that the calibration
laboratory may not recommend any calibration interval except where it has been
agreed with the customer.
Fluke’s products often
perform within their published
specifications for significantly longer than the stated time interval. Whether
end customers select a recalibration interval based on the published
specification or by any other means, they should evaluate the recalibration
data for their test instruments to ensure that the selected intervals meet
their requirements for quality and
reliability. The publications “NCSL International
RP-1, Establishment and Adjustment of
Calibration intervals” and “ILAC G24: 2007 Guidelines for the determination
of calibration intervals of measuring instruments”
are excellent documents for customers to establish and adjust
calibration intervals based on their usage and quality requirements.
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