Sencore POWERITE PR57 User manual

Servicing Hot Chassis and Performing the Leakage Test with Your
PR57 “POWERITE”®
The PR57 “POWERITE” provides a very
fast and accurate check of AC leakage
between the AC line and the exposed
metal on an AC-operated device. This
Tech Tip explains this patented leakage
test, why and when it should be
performed, and some extra applications
where it could come in handy.
What is the Leakage Test?
The leakage test assures that the TV set
or other electronic device being returned
to the customer does not have any
exposed metal parts that could give the
customer an electrical shock. Virtually all
service literature lists a safety leakage
test similar to the one shown in figure 1.
This safety check applies to all consumer
units that are connected to the AC line,
but it is most important when the device
does not have an isolation transformer.
The potential of a shock hazard increases
dramatically when the set has a hot
chassis.
Why Don’t More Servicers Perform
the Leakage Test?
Simply stated, many technicians to not
perform the safety leakage test, because
the test is complicated. It requires a good
ground, a resistor/ capacitor combination
“made up” or located among the shop
parts, and the test setup takes time.
Time is something most technicians don’t
have.
Why Should You Perform the
Leakage Test?
Even though you may find leakage on
only one chassis out of a hundred, that’s
Leakage Current Cold Check
1. Unplug the AC cord and connect a jumper between the two prongs on the plug.
2. Turn on the receiver’s power switch.
3. Measure the resistance value, with an ohmmeter, between the jumpered AC
plug and each exposed metallic cabinet part on the receiver, such as
screwheads, connectors, control shafts, etc. When the exposed metallic part
has a return path to the chassis, the reading should be between 240 kΩand
5.2 MΩ.
When the exposed metal does not have a return path to the chassis, the reading
must be infinite.
Leakage Current Hot Check
1. Plug the AC cord directly into the AC outlet. Do not use an isolation transformer
for this check.
2. Connect a 1.5 kΩ, 10 watt resistor, in parallel with a 0.15 µF capacitor, between
each exposed metallic part on the set and a good earth ground such as a water
pipe.
3. Use an AC Voltmeter, with 1000 ohms/volt or more sensitivity, to measure the
potential across the resistor.
4. Check each exposed metallic part, and measure the voltage at each point.
5. Reverse the AC plug in the AC outlet and repeat each of the above
measurements.
6. The potential at any point should not exceed 0.75 volts RMS. A leakage current
tester (Simpson Model 229 or equivalent) may be used to make the hot checks.
Leakage current must not exceed ½ milliamps. In case a measurement is out-
side of the limits specified, there is a possibility of a shock hazard, and the
receiver should be repaired and rechecked before it is returned to the customer.
Fig. 1: Manufacturers include the leakage test in service literature, because
it helps them meet their legal obligation to deliver safe products to the
consumer.

the very reason you need to safety test
every chassis that leaves your shop. It
only takes one leaky chassis to put you
out of commission so you are unable to
work. It only takes one to damage your
expensive test instruments and put them
out of service for a week or longer. Or, it
only takes one zapped customer to bring
a lawsuit against you and your shop. It
only takes one before you realize that it
costs you more not to make the leakage
test on every chassis that goes out the
door.
Most technicians add on an extra charge
to the customer’s bill for performing the
leakage test. They find that most
customers don’t mind paying an extra five
dollars if it means safety for themselves
and their family.
What Causes Leakage?
Basically, any path that will place the
customer into either diret or indirect
contact with the AC line is dangerous.
There are many different ways in which
this can happen. The following is a list of
some of the most common causes of
leakage:
Shorted Antenna Bypass Capacitors:
You may see a lot of shorted antenna
bypass capacitors after a thunderstorm.
The capacitors are in series between the
antenna terminals and the chassis to
isolate the hot chassis from the antenna.
When one of these capacitors is shorted,
the customer notices no difference in the
operation of their receiver because the
RF signal passes through the shorted
capacitor just as well as through the good
capacitor. The problem, however, is the
antenna terminals have raw AC on them.
Improperly Installed Tuner:
Occasionally, replacement tuners are
installed improperly or with the wrong
kind. Isolation capacitors must be
installed properly to avoid the AC line
being tied to exposed metal parts. Also,
insulated shafts on tuners should be
replaced with a similar kind to avoid a
shock hazard on the tuning knob.
Conductive Knobs: If the TV being
serviced has control shafts that are
connected directly to a hot chassis, the
knobs on these shafts must be insulating-
type knobs. Many times, however, knobs
that have been replaced are metal or
chrome plated which may conduct current
resulting in a shock hazard.
Defective Isolation Transformers:
Older TV receivers often have guilt-in
isolation transformers that are supposed
to isolate the metal chassis from the AC
line. But, these older transformers
occasionally develop leakage between
the primary and secondary windings,
causing a hot chassis. This is especially
dangerous on the older sets with metal
cases or with metal pans covering the
bottom of the chassis. All of the exposed
metal becomes a shock hazard.
AC Bypass Capacitors: These
capacitors connect between the AC line
and the chassis in most electronic
devices that use isolation transformers.
The capacitors bypass RF interference,
picked up by the metal chassis, to the AC
line for shielding purposes. Older
capacitors were often the wax-coated
paper type which often develop leakage
as the paper dieletric absorbs moisture.
In some cases, a capacitor shorts
completely, causing a direct connection
between the AC line and the chassis.
These capacitors, like antenna bypass
capacitors, are commonly damaged
during thunderstorms.
Bent Rabbit Ears: Many receivers have
rabbit-ear antennas designed to slide
down inside the back of the case.
Generally, there is a plastic tube inside
the case to isolate the metal antenna
rods from the chassis. But the rods often
get bent. When the bent rods are pushed
down to the “nested” position, they touch
the hot chassis. Not only is the exposed
end hot, the wire coming out the back of
the chassis to connect to the antenna
terminals also carries the full AC line
voltage.
Improper Installlation of Parts:
Forgetting to replace an insulating piece
of “fish paper” under a component.
Foreign Objects Touching the AC Line:
Allowing a piece of wire or solder to fall
between one side of the AC line and the
chassis.
A Broken Safety Ground: The broken
gound may allow a shaft or control to
“float”.
Using Long Metal Screws: Screws that
are too long and go all the way through
the plastic mounting tabs and touch the
metal chassis.
Adding An Earphone: Accessory jacks
on an set that does not have an isolation
transformer – imagine the shock hazard
when connectd directly to the listener’s
head!
Fig. 2: To make a safety leakage test with your PR57, simply plug the
chassis into the POWERITE and touch the probe to all exposed metal
surfaces.

Foreign Objects: Coins, hairpins, or
other metal objects which have fallen
inside the set.
Connecting an External Speaker:
Wiring accessories to a set that do not
have an isolation transformer.
The list could go on and on. The key
thing is that there is a shock hazard any
time a piece of metal comes in contact
with a hot chassis.
The PR57 Leakage Test: It’s Fast and
Simple
The PR57 simplifies the safety leakage
test, because all circuits are internally
referenced to the isolated output of the
PR57. First, the PR57 allows the leakage
test to be made while the unit under test
is still connected to the isolated output.
Other procedures require a direct
connection to the AC line. The test should
be done with the set plugged into the
PR57. This reduces the chance of getting
a shock while performing the test and
also means that you do not have to
remember to move the AC line cord to a
non-isolated outlet.
The PR57 leakage test does not require
earth ground reference because all of the
current paths are referenced back to the
secondary of the isolation transformer.
This allows you to make the test
anywhere, including in a home that is not
wired with grounded outlets.
To perform the Leakage Test with the
PR57:
1. Plug the PR57 into a properly
grounded three wire AC outlet.
2. Push the AC VOLTS OUTPUT
pushbutton and adjust the VOLTS
control for a reading of 117 volts on
the PR57 meter. This establishes a
117 volt reference for uniform test
results.
3. Press the HI SIDE leakage button
and plug the Safety Leakage Probe
into the PROBE jack on the front
panel of the PR57.
4. Touch the probe tip to every user-
accessible piece of exposed metal,
including screw heads, antennas,
antenna terminals, knobs, all control
shafts with knobs removed, handles
or anything that even appears to be
metallic.
5. Read the leakage on the µA
LEAKAGE meter scale of the PR57.
To read the true leakage current,
push the button on the probe while
probing the test point.
6. Push the LO SIDE LEAKAGE push-
button and repeat the leakage test to
the same points tested in step 4.
The Safety Leakage Probe has a switch
which places a current-limiting resistor in
series with the test circuit. The resistor is
bypassed when the button on the probe
is depressed.
The resistor limits the amount of current
to prevent the meter from pegging when a
test point with high leakage is contacted.
This condition occurs when the point
being tested is connected directly to
either side of the AC line.
Any test point that reads close to full
scale (800 µA) with the button in the “out”
position is connected directly to one side
of the AC line. If, for example, the meter
reads full scale when the LO SIDE button
is depressed, there is a direct connection
to the common side of the AC line. This is
the side of the polarized line cord plug
with the larger connector.
Leakage readings which are less than full
scale indicate that there is a leakage path
(but not a dead short) to the point being
tested. Simply press the button on the
Safety Leakage Probe to read the actual
leakage current present.
Severe burns, not
fatal unless vital
organs burned.
Heart stops during
shock, may restart if
current is removed
before death occurs.
Heart Fibrillation,
in 1.4 seconds;
usually fatal.
Breathing stops;
often fatal.
Highest
Danger
Zone
Cannot let go.
Current may increase
to fatal level.
Painful sensation.
Mild sensation.
Imperceivable
10A..
9…
8…
7…
6…
5…
4…
3…
2…
1A…
900mA..
800……
700……
600……
500……
400……
300……
200……
100mA…
90……
80……
70……
60……
50……
40……
30……
20……
10mA
9…
8…
7…
6…
5…
4…
3…
2…
1mA
900µA
800….
700….
600….
500
400
300
200
100µA
UL Limit for
consumer
products
UL Limit for
Hospital
Equipment not
connected
directly to body.
Fig. 3: The pushbutton of the Safety Leakage Probe is depressed when
the actual value of leakage current is to be read on the PR57’s meter.
Fig. 4: The effects of electrical
shock on a human being.

How Much Leakage is Bad?
The chart, Figure 4, shows the effect of
electrical shock on the human body. You
may be surprised to learn that the most
likely range of fatal current is between 30
and 250 mA. This is the area where the
breathing and heart are most severely
affected. The chart is based on a person
weighing 150 pounds. Smaller people,
especially children, are much more
susceptible to shocks at lower current
levels.
There should be no leakage at all, but
this is not practical in real terms.
Components, such as RF decoupling
capacitors and antenna matching
transformers, allow some current to flow.
Underwriter Laboratories has established
guidelines for safe leakage currents. The
maximum UL allowable limit (since 1972)
is 500 microamps. Consumer electronic
devices manufactured before 1972 were
allowed 750 microamps. The leakage test
on the PR57 is calibrated directly in
microamps to compare with the
appropriate standard.
Measuring Leakage at Lower Levels
Some equipment must be kept at a
leakage current which is considerable
less than consumer products. Equipment
such as clinic and hospital equipment
must have less than 100 or even 10
microamps.
This requires more resolution than
provided by the PR57 meter. Simply
place an AC current meter (such as the
Sencore DVM37 or DVM56A) in series
with the Safety Leakage Probe. This
allows you to measure the leakage within
0.1 microamps with digital accuracy.
Checking the Calibration of the
Leakage Test
The current-limiting resistor in the Safety
Leakage Probe is also a calibration
resistor. The calibration of the PR57
leakage test can be tested at any time so
you will know that the readings obtained
during the test are correct.
To test the calibration of the PR57
leakage scale:
1. Plug the PR57 into a properly
grounded three-wire AC outlet.
2. Depress the AC VOLTS OUTPUT
button and turn the PR57 “ON”.
Adjust the AC VOLTS control for a
reading of 117 volts on the PR57
meter.
3. Plug the leakage probe into the
Probe jack and depress the HI SIDE
LEAKAGE pushbutton. Place the
probe tip into the small blade
opening of the ISOLATED OUTPUT
socket in the PR57. The meter
should read 780 microamps without
the button pushed on the Safety
Leakage Probe.
Testing Leakage on Three-Wire Units
The PR57 leakage test is designed to
operate exactly the same on a unit with a
two-wire AC line cord or a 3-wire
(grounded) AC line cord. The ground
connector on the ISOLATED OUTPUT
jack is connected to earth ground through
the power cord of the PR57. This assures
safe operation of the unit under test as its
chassis is maintained at earth potential.
This ground path does not affect the
safety leakage test. The leakage between
the internal circuits and the chassis on
these three-wire units is read exactly the
same as two-wire units. You simply touch
the Safety Leakage Probe to each
metallic cabinet part and read the
leakage directly on the PR57 meter. Do
not defeat the third-wire ground on either
the device under test or the PR57.
Testing for Line Cord and Extension
Cord Leakage
An increasingly common cause of
electrical fires is leakage between the
conductors of line cords and extension
cords. Leakage of this type may
eventually short and/or cause sparks
which may ignite a fire.
The instrument to use to test for leakage
in line cords or extension cords is your Z-
Meter. Measure the leakage between the
two conductors on a two wire cord, or the
leakage between the three conductors on
a three wire cord. In either case, the
leakage should be absolutely zero (with
the cord unplugged from the wall or
device that it supplies power to). If any
leakage is detected, the cord is defective
and should be replaced.
Powerite is a registered trademark of Sencore, Inc. Form #4550 Printed in U.S.A.
Fig. 5: The built-in calibration resistor in the Safety Leakage Probe allows a quick check
of the leakage scale calibration by inserting the Safety Leakage Probe into the Isolated
Output and checking for a full scale leakage reading.