Meter Safety
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Safe meter usage
Using an electrical meter safely and efficiently is perhaps the most
valuable skill an electronics technician can master, both for the sake of their
own personal safety and for proficiency at their trade. It can be daunting at
first to use a meter, knowing that you are connecting it to live circuits which
may harbor life-threatening levels of voltage and current. This concern is not
unfounded, and it is always best to proceed cautiously when using meters.
Carelessness more than any other factor is what causes experienced technicians
to have electrical accidents.
The most common piece of electrical test equipment is a meter called the multimeter.
Multimeters are so named because they have the ability to measure a multiple of
variables: voltage, current, resistance, and often many others, some of which
cannot be explained here due to their complexity. In the hands of a trained
technician, the multimeter is both an efficient work tool and a safety device.
In the hands of someone ignorant and/or careless, however, the multimeter may
become a source of danger when connected to a "live" circuit.
There are many different brands of multimeters, with multiple models made by
each manufacturer sporting different sets of features. The multimeter shown here
in the following illustrations is a "generic" design, not specific to
any manufacturer, but general enough to teach the basic principles of use:
You will notice that the display of this meter is of the "digital"
type: showing numerical values using four digits in a manner similar to a
digital clock. The rotary selector switch (now set in the Off position)
has five different measurement positions it can be set in: two "V"
settings, two "A" settings, and one setting in the middle with a
funny-looking "horseshoe" symbol on it representing
"resistance." The "horseshoe" symbol is the Greek letter
"Omega" (Ω), which is the common symbol for the electrical unit
of ohms.
Of the two "V" settings and two "A" settings, you will
notice that each pair is divided into unique markers with either a pair of
horizontal lines (one solid, one dashed), or a dashed line with a squiggly curve
over it. The parallel lines represent "DC" while the squiggly curve
represents "AC." The "V" of course stands for
"voltage" while the "A" stands for "amperage"
(current). The meter uses different techniques, internally, to measure DC than
it uses to measure AC, and so it requires the user to select which type of
voltage (V) or current (A) is to be measured. Although we haven't discussed
alternating current (AC) in any technical detail, this distinction in meter
settings is an important one to bear in mind.
There are three different sockets on the multimeter face into which we can
plug our test leads. Test leads are nothing more than specially-prepared
wires used to connect the meter to the circuit under test. The wires are coated
in a color-coded (either black or red) flexible insulation to prevent the user's
hands from contacting the bare conductors, and the tips of the probes are sharp,
stiff pieces of wire:
The black test lead always plugs into the black socket on the
multimeter: the one marked "COM" for "common." The red test
lead plugs into either the red socket marked for voltage and resistance, or the
red socket marked for current, depending on which quantity you intend to measure
with the multimeter.
To see how this works, let's look at a couple of examples showing the meter
in use. First, we'll set up the meter to measure DC voltage from a battery:
Note that the two test leads are plugged into the appropriate sockets on the
meter for voltage, and the selector switch has been set for DC "V".
Now, we'll take a look at an example of using the multimeter to measure AC
voltage from a household electrical power receptacle (wall socket):
The only difference in the setup of the meter is the placement of the
selector switch: it is now turned to AC "V". Since we're still
measuring voltage, the test leads will remain plugged in the same sockets. In
both of these examples, it is imperative that you not let the probe tips
come in contact with one another while they are both in contact with their
respective points on the circuit. If this happens, a short-circuit will be
formed, creating a spark and perhaps even a ball of flame if the voltage source
is capable of supplying enough current! The following image illustrates the
potential for hazard:
This is just one of the ways that a meter can become a source of hazard if
used improperly.
Voltage measurement is perhaps the most common function a multimeter is used
for. It is certainly the primary measurement taken for safety purposes (part of
the lock-out/tag-out procedure), and it should be well understood by the
operator of the meter. Being that voltage is always relative between two points,
the meter must be firmly connected to two points in a circuit before it
will provide a reliable measurement. That usually means both probes must be
grasped by the user's hands and held against the proper contact points of a
voltage source or circuit while measuring.
Because a hand-to-hand shock current path is the most dangerous, holding the
meter probes on two points in a high-voltage circuit in this manner is always a potential
hazard. If the protective insulation on the probes is worn or cracked, it is
possible for the user's fingers to come into contact with the probe conductors
during the time of test, causing a bad shock to occur. If it is possible to use
only one hand to grasp the probes, that is a safer option. Sometimes it is
possible to "latch" one probe tip onto the circuit test point so that
it can be let go of and the other probe set in place, using only one hand.
Special probe tip accessories such as spring clips can be attached to help
facilitate this.
Remember that meter test leads are part of the whole equipment package, and
that they should be treated with the same care and respect that the meter itself
is. If you need a special accessory for your test leads, such as a spring clip
or other special probe tip, consult the product catalog of the meter
manufacturer or other test equipment manufacturer. Do not try to be
creative and make your own test probes, as you may end up placing yourself in
danger the next time you use them on a live circuit.
Also, it must be remembered that digital multimeters usually do a good job
of discriminating between AC and DC measurements, as they are set for one or the
other when checking for voltage or current. As we have seen earlier, both AC and
DC voltages and currents can be deadly, so when using a multimeter as a safety
check device you should always check for the presence of both AC and DC, even if
you're not expecting to find both! Also, when checking for the presence of
hazardous voltage, you should be sure to check all pairs of points in
question.
For example, suppose that you opened up an electrical wiring cabinet to find
three large conductors supplying AC power to a load. The circuit breaker feeding
these wires (supposedly) has been shut off, locked, and tagged. You
double-checked the absence of power by pressing the Start button for the
load. Nothing happened, so now you move on to the third phase of your safety
check: the meter test for voltage.
First, you check your meter on a known source of voltage to see that it's
working properly. Any nearby power receptacle should provide a convenient source
of AC voltage for a test. You do so and find that the meter indicates as it
should. Next, you need to check for voltage among these three wires in the
cabinet. But voltage is measured between two points, so where do you
check?
The answer is to check between all combinations of those three points. As
you can see, the points are labeled "A", "B", and
"C" in the illustration, so you would need to take your multimeter
(set in the voltmeter mode) and check between points A & B, B & C, and A
& C. If you find voltage between any of those pairs, the circuit is not in a
Zero Energy State. But wait! Remember that a multimeter will not register DC
voltage when it's in the AC voltage mode and visa-versa, so you need to check
those three pairs of points in each mode for a total of six voltage
checks in order to be complete!
However, even with all that checking, we still haven't covered all
possibilities yet. Remember that hazardous voltage can appear between a single
wire and ground (in this case, the metal frame of the cabinet would be a good
ground reference point) in a power system. So, to be perfectly safe, we not only
have to check between A & B, B & C, and A & C (in both AC and DC
modes), but we also have to check between A & ground, B & ground, and C
& ground (in both AC and DC modes)! This makes for a grand total of twelve
voltage checks for this seemingly simple scenario of only three wires. Then, of
course, after we've completed all these checks, we need to take our multimeter
and re-test it against a known source of voltage such as a power receptacle to
ensure that it's still in good working order.
Using a multimeter to check for resistance is a much simpler task. The test
leads will be kept plugged in the same sockets as for the voltage checks, but
the selector switch will need to be turned until it points to the
"horseshoe" resistance symbol. Touching the probes across the device
whose resistance is to be measured, the meter should properly display the
resistance in ohms:
One very important thing to remember about measuring resistance is that it
must only be done on de-energized components! When the meter is in
"resistance" mode, it uses a small internal battery to generate a tiny
current through the component to be measured. By sensing how difficult it is to
move this current through the component, the resistance of that component can be
determined and displayed. If there is any additional source of voltage in the
meter-lead-component-lead-meter loop to either aid or oppose the
resistance-measuring current produced by the meter, faulty readings will result.
In a worse-case situation, the meter may even be damaged by the external
voltage.
The "resistance" mode of a multimeter is very useful in
determining wire continuity as well as making precise measurements of
resistance. When there is a good, solid connection between the probe tips
(simulated by touching them together), the meter shows almost zero Ω. If
the test leads had no resistance in them, it would read exactly zero:
If the leads are not in contact with each other, or touching opposite ends
of a broken wire, the meter will indicate infinite resistance (usually by
displaying dashed lines or the abbreviation "O.L." which stands for
"open loop"):
By far the most hazardous and complex application of the multimeter is in
the measurement of current. The reason for this is quite simple: in order for
the meter to measure current, the current to be measured must be forced to go through
the meter. This means that the meter must be made part of the current path of
the circuit rather than just be connected off to the side somewhere as is the
case when measuring voltage. In order to make the meter part of the current path
of the circuit, the original circuit must be "broken" and the meter
connected across the two points of the open break. To set the meter up for this,
the selector switch must point to either AC or DC "A" and the red test
lead must be plugged in the red socket marked "A". The following
illustration shows a meter all ready to measure current and a circuit to be
tested:
Now, the circuit is broken in preparation for the meter to be connected:
The next step is to insert the meter in-line with the circuit by connecting
the two probe tips to the broken ends of the circuit, the black probe to the
negative (-) terminal of the 9-volt battery and the red probe to the loose wire
end leading to the lamp:
This example shows a very safe circuit to work with. 9 volts hardly
constitutes a shock hazard, and so there is little to fear in breaking this
circuit open (bare handed, no less!) and connecting the meter in-line with the
flow of electrons. However, with higher power circuits, this could be a
hazardous endeavor indeed. Even if the circuit voltage was low, the normal
current could be high enough that am injurious spark would result the moment the
last meter probe connection was established.
Another potential hazard of using a multimeter in its current-measuring
("ammeter") mode is failure to properly put it back into a
voltage-measuring configuration before measuring voltage with it. The reasons
for this are specific to ammeter design and operation. When measuring circuit
current by placing the meter directly in the path of current, it is best to have
the meter offer little or no resistance against the flow of electrons.
Otherwise, any additional resistance offered by the meter would impede the
electron flow and alter the circuit's operation. Thus, the multimeter is
designed to have practically zero ohms of resistance between the test probe tips
when the red probe has been plugged into the red "A"
(current-measuring) socket. In the voltage-measuring mode (red lead plugged into
the red "V" socket), there are many mega-ohms of resistance between
the test probe tips, because voltmeters are designed to have close to infinite
resistance (so that they don't draw any appreciable current from the
circuit under test).
When switching a multimeter from current- to voltage-measuring mode, it's
easy to spin the selector switch from the "A" to the "V"
position and forget to correspondingly switch the position of the red test lead
plug from "A" to "V". The result -- if the meter is then
connected across a source of substantial voltage -- will be a short-circuit
through the meter!
To help prevent this, most multimeters have a warning feature by which they
beep if ever there's a lead plugged in the "A" socket and the selector
switch is set to "V". As convenient as features like these are,
though, they are still no substitute for clear thinking and caution when using a
multimeter.
All good-quality multimeters contain fuses inside that are engineered to
"blow" in the even of excessive current through them, such as in the
case illustrated in the last image. Like all overcurrent protection devices,
these fuses are primarily designed to protect the equipment (in this
case, the meter itself) from excessive damage, and only secondarily to protect
the user from harm. A multimeter can be used to check its own current fuse by
setting the selector switch to the resistance position and creating a connection
between the two red sockets like this:
A good fuse will indicate very little resistance while a blown fuse will
always show "O.L." (or whatever indication that model of multimeter
uses to indicate no continuity). The actual number of ohms displayed for a good
fuse is of little consequence, so long as it's an arbitrarily low figure.
So now that we've seen how to use a multimeter to measure voltage,
resistance, and current, what more is there to know? Plenty! The value and
capabilities of this versatile test instrument will become more evident as you
gain skill and familiarity using it. There is no substitute for regular practice
with complex instruments such as these, so feel free to experiment on safe,
battery-powered circuits.
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REVIEW:
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A meter capable of checking for voltage, current, and resistance is
called a multimeter,
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As voltage is always relative between two points, a voltage-measuring
meter ("voltmeter") must be connected to two points in a circuit
in order to obtain a good reading. Be careful not to touch the bare probe
tips together while measuring voltage, as this will create a short-circuit!
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Remember to always check for both AC and DC voltage when using a
multimeter to check for the presence of hazardous voltage on a circuit. Make
sure you check for voltage between all pair-combinations of conductors,
including between the individual conductors and ground!
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When in the voltage-measuring ("voltmeter") mode, multimeters
have very high resistance between their leads.
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Never try to read resistance or continuity with a multimeter on a
circuit that is energized. At best, the resistance readings you obtain from
the meter will be inaccurate, and at worst the meter may be damaged and you
may be injured.
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Current measuring meters ("ammeters") are always connected in
a circuit so the electrons have to flow through the meter.
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When in the current-measuring ("ammeter") mode, multimeters
have practically no resistance between their leads. This is intended to
allow electrons to flow through the meter with the least possible
difficulty. If this were not the case, the meter would add extra resistance
in the circuit, thereby affecting the current.
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