Understanding the How and Why of Electrical Product Safety Testing
Knowing why to perform required electrical safety tests is as important
as knowing how to perform them.
so many electrical product safety standards currently in use and many
civil and legal actions pending in various courts around the world,
electrical safety testing is more critical than ever to ensure that
all products are safe before they reach the end-user. Fortunately, the
majority of manufacturers are fully aware of the hazards associated
with electrical equipment and the ramifications of noncompliance with
relevant safety standards or test house agreements.
Electrical safety tests can be roughly divided into two areas: those
tests carried out during the approvals process by test houses (known
as type tests) and those carried out at the end of each production line
by the manufacturer (known as routine production tests).
For type tests, a product is subjected to tests and evaluations in
accordance with a specific product safety standard. For production tests,
a manufacturer can select a few tests, ensuring that each product is
subjected to those tests in accordance with its own procedures. Most
manufacturers, in particular information technology (IT) equipment manufacturers,
choose four primary product safety tests to be routine at the end of
the production line. These include dielectric, insulation resistance,
ground continuity, and leakage current.
These tests are designed to ensure that the user does not get electrocuted
or otherwise hurt by operating a piece of equipment that has hazardous
voltages or high fault current as a result of electrical fault. This
article looks at the fundamentals behind each test and analyzes the
reasoning behind each test, as well as discussing appropriate limits
The dielectric strength test, also known as dielectric withstand test
or hipot test, is probably the best known and most often performed production-line
safety test. It is part of virtually every standard, which indicates
its importance. The hipot test is a nondestructive test that determines
the adequacy of electrical insulation for the normally occurring overvoltage
transient. This is a high-voltage test that is applied to all devices
for a specific time in order to ensure that the insulation is not marginal.
Another reason for conducting the hipot test is that it also detects
possible defects such as inadequate creepage and clearance distances
introduced during the manufacturing process.
During type testing, the hipot test is applied after tests such as
fault condition, humidity, and vibration to determine whether any degradation
has taken place. The production-line hipot test, however, is a test
of the manufacturing process to determine whether the construction of
a production unit is about the same as the construction of the unit
that was subjected to type testing. Some of the process failures that
can be detected by a production-line hipot test include, for example,
a transformer wound in such a way that creepage and clearance have been
reduced. Such a failure could result from a new operator in the winding
department. Other examples include identifying a pinhole defect in insulation
or finding an enlarged solder footprint.
Most safety standards use the 2U + 1000 V formula as the basis for
testing basic insulation, where U is the operating rms voltage. Although
this formula is a guideline, each standardin particular IEC 60950refers
the user to a specific table in the standard showing the exact test
voltage based on the working voltage measurements.1
The reason for using 1000 V as part of the basic formula is that the
insulation in any product can be subjected to normal day-to-day transient
overvoltages. Experiments and research have shown that these overvoltages
can be as high as 1000 V.
Test method. Normally the high voltage is applied between the
two parts across the insulation being tested, such as the primary circuit
and metal enclosure of the equipment under test (EUT). If the insulation
between the two is adequate, then the application of a large voltage
difference between the two conductors separated by the insulator would
result in the flow of a very small current. Although this small current
is acceptable, no breakdown of either the air insulation or the solid
insulation should take place. Therefore, the current of interest is
the current that is the result of a partial discharge or breakdown,
rather than the current due to capacitive coupling.
Another example would be to test the insulation between the primary
and secondary circuits of a power supply. Here, all the outputs are
shorted together. The ground probe from the hipot tester is placed in
contact with this cable bunch, and the high-voltage probe is placed
in contact with L and N connectors, which are shorted together (see
Figure 1). The EUT does not run during the hipot test. It must also
be noted that when applying the high voltage during the type test, the
ideal situation would require that not more than half of the prescribed
voltage be applied, and then raised gradually over a period of 10 seconds
to the full value and maintained for 1 minute. Most test equipment,
however, either turns on directly to the full voltage or has an electronically
|Figure 1. Typical hipot test setup.
Test Duration. If the test is part of an agency certification
process, then the test duration must be in accordance with the safety
standard being used. For instance, the test time for most standards,
including products covered under IEC 60950, is 1 minute. However, when
testing products in the production line, it is normally not practical
to hipot test each product for 1 minute, and manufacturers normally
conduct the test to a much shorter time, such as a few seconds, but
with higher voltages. A typical rule of thumb is 110120% of 2U
+ 1000 V for 12 seconds. The duration and procedure should be
in agreement with any test houses concerned. It should be noted that
although the reduced time and increased voltage are approximate, experiments
and the manufacturers' data sheets indicate that each insulating material
has its own specific voltage-time characteristics.
Current Setting. Most modern hipot testers allow the user to set the
current limit. However, if the actual leakage current of the product
is known, then the hipot test current can be predicted. Choosing the
trip level really depends on the product being tested. The best way
to identify the trip level is to test some product samples and establish
an average hipot current. Once this has been achieved, then the leakage
current trip level should be set to a slightly higher value than the
average figure. Another method of establishing the current trip level
would be to use the following mathematical formula:
The reason for the factor of 2 is that the line leakage current provides
current through a single Y capacitor while the hipot test provides current
through capacitors on each line. By solving the equation for I (hipot),
one can predict the hipot test current. Therefore, the hipot tester
current trip level should be set high enough to avoid nuisance failure
related to leakage current and, at the same time, low enough not to
overlook a true breakdown in insulation.
Test Voltage. The majority of safety standards allow the use
of either ac or dc voltage for a hipot test. When using ac test voltage,
the insulation in question is being stressed most when the voltage is
at its peak, i.e., either at the positive or negative peak of the sine
wave. Therefore, if one decides to use dc test voltage, one must ensure
that the dc test voltage is Ö2 (or 1.414)
times the ac test voltage, so the value of the dc voltage is equal to
the ac voltage peaks. For example, for a 1500-V-ac voltage, the equivalent
dc voltage to produce the same amount of stress on the insulation would
be 1500 x 1.414 or 2121 V dc.
One of the advantages of using a dc test voltage is that the leakage
current trip can be set to a much lower value than that of an ac test
voltage. This would allow a manufacturer to filter those products that
have marginal insulation, which would have been passed by an ac tester.
It must be noted that when using a dc hipot tester, the capacitors in
the circuit could be highly charged and, therefore, a safe-discharge
device or setup is needed. However, it is a good practice to always
ensure that a product is discharged, regardless of the test voltage
or its nature, before it is handled.
Another advantage of a dc hipot tester is that it applies the voltage
gradually. By monitoring the current flow as voltages increase, an operator
can detect a potential insulation breakdown before it occurs. A minor
disadvantage of the dc hipot tester is that because dc test voltages
are more difficult to generate, the cost of a dc tester may be slightly
higher than that of an ac tester.
One of the advantages of an ac hipot test is that it can check both
voltage polarities, whereas a dc test charges the insulation in only
one polarity. This may become a concern for products that actually use
ac voltage for their normal operation. The test setup and procedures
are identical for both ac and dc hipot tests.
A minor disadvantage of the ac hipot tester is that if the circuit
under test has large values of Y capacitors, then, depending on the
current trip setting of the hipot tester, the ac tester could indicate
a failure. Most safety standards allow the user to disconnect the Y
capacitors prior to testing or, alternatively, to use a dc hipot tester.
The dc hipot tester would not indicate the failure of a unit even with
high Y capacitors because the Y capacitors see the voltage but don't
pass any current.
The insulation resistance test is also known as a Megger test. Its
objective is to measure the total resistance between any two points
separated by insulation. The test, therefore, determines how effective
the insulation is in resisting the flow of electrical current. The voltage
is typically around 5001000 V dc; hence, the current is very low.
Because the current is so low, this test is useful for checking the
quality of the insulation not only when a product is first manufactured,
but also over time as the product is used.
Test Procedure. The EUT is connected to the measuring instrument,
and the voltage is ramped up from zero to the final value, which in
most cases is 500 V dc. Once the voltage reaches the selected value,
it is kept at that value for a brief period (typically up to 5 seconds)
before the resistance test is measured. The measured value should be
very high (typically in the megohm region). The insulation resistance
test is mandatory in some product safety standards, including IEC 60065
and UL 6500.2,3
Also known as the ground bond continuity test, the ground bond test
must be conducted on all Class I products. The purpose of the test is
to ensure that all accessible conductive parts of the product that could
become live in the event of a single insulation fault are connected
securely to the final earth point of the supply input. In other words,
a ground bond test verifies integrity of the ground path by applying
a high-current, low-voltage source to the ground-path circuit.
Compliance is checked by measuring the resistance of the connection
between the protective earthing terminal or earthing contact and the
parts to be earthed to ensure that resistance does not exceed certain
values when subjected to a high current as specified in various product
safety standards. It is important to bear in mind that from the constructional
and design points of view, the protective earthing conductors should
not contain switches or fuses.
Test Requirements. Most safety standards require the following
parameters for conducting the ground bond test:
The EUT must be subjected to a high ac
or dc current with a low test voltage for a certain period.
The voltage drop between the protective
or earthing contact and the part to be earthed must be
The resistance must be calculated from
the current and the resulting voltage drop using Ohm's law.
The resistance should not exceed certain values as stated in various
safety standards. For example, IEC 60950 requires that the test voltage
not exceed 12 V. The current can be either ac or dc at 1.5 times the
current consumption of the product or 25 A, whichever is greatest. The
test duration must be 1 minute, and the resistance of the connection
between the protective earthing terminal or earthing contact and parts
required to be earthed must not exceed 0.1 W.
This value does not include the resistance of the power cable. Some
standards, such as CAN/CSA-C22.2 No. 60950-00 or UL 60950 with Canadian
deviation, require the test to be conducted at 30 A and for 2 minutes
if the current rating of the circuit under test is 16 A or lower.4,5
Understanding Resistance Values. With the exception of the Canadian
standard, most standards require 25 A for 1 minute. The value of 25
A for 1 minute represents the worst current and the longest operation
time of a mains overcurrent device. The maximum 25 A is approximately
1.5 times the mains circuit breaker value installed for most pluggable
type A cord-connected equipment rated up to 16 A. The Canadian National
Wiring Code requirements are very similar to these in the sense that
they assume that fuses are expected to operate no more than 1 minute
at twice their rated current. Because most mains circuits are protected
with a 1516 A fuse, the fault current would be 30 A for no more
than 2 minutes.
Some standards, including IEC 60950, 3rd ed., have named the leakage
current test "touch current." This refers to the electric current through
a human body or through an animal body when it touches one or more accessible
parts of installation or equipment. There is also another concept known
as "protective conductor current," and this refers to the current that
flows in a protective conductor. A protective conductor current, therefore,
can never be the source of an electric shock because, by definition,
the protective conductor is connected to earth.
If touch current is excessive, an operator could receive an electric
shock, which could result in a serious injury, depending on a person's
body weight. Typically, currents of more than 1.0 mA can cause an electric
shock to an operator. The shock may or may not be serious, depending
on the amount of the current and the body weight.
Like the other tests, the leakage current test is also a very important
safety test, and most safety standards require the test to be conducted
under various conditions such as normal operating condition, switches
open as well as closed, reversed line polarity, and so on. The measured
earth leakage current must not exceed specified limits during any of
the tested conditions. Table I shows some typical limits.
Type of Equipment
Terminal A of Measuring Instrument Connected
Maximum Touch Current mA rms1
Maximum Protective Conductor Current
||Accessible parts and circuits not connected to protective earth
|Movable (other than handheld but including transportable equipment)
|Stationary, pluggable type A
||Equipment main protective earthing terminal (if any)
|All other stationary equipment
not subject to the conditions of 5.1.7
subject to the conditions of 5.1.7
5% of input current
|1 If peak values
of touch current are measured, the maximum values are obtained by
multiplying the rms values by 1.414.
|Table I. Typical earth leakage currents.
One of the biggest contributors to leakage is the capacitance between
ac lines and earth, i.e., the Y capacitors. These Y capacitors are normally
placed in the circuit to control electromagnetic interference. It should
be noted that some standards, such as IEC 60950, allow higher earth
leakage current than 3.5 mA only for Class I stationary equipment that
is either permanently connected equipment or that is pluggable equipment
type B, provided certain conditions are met. These conditions are listed
in Clause 5.1.7 of IEC 60950.
Equipment. When measuring the earth leakage current of IT equipment,
it must be noted that the measuring instruments should satisfy the requirements
of Annex D of IEC 60950, which simulates the worse-case impedance of
the human body. The use of an isolating transformer during the test
is also highly recommended (see Figure 2).
|Figure 2. Typical earth leakage current setup.
Any capacitive leakage in the transformer must then be taken into account.
If for any reason the use of an isolating transformer is not possible,
then the EUT must be mounted on an insulating stand, and appropriate
safety precautions must be taken. Such measures compensate for the possibility
that the body of the EUT may carry a hazardous voltage.
Test Method. Most standardsin particular IEC 60950require
that the EUT be energized for this test. The input voltage applied to
the EUT is typically adjusted to 110% of the highest rated mains voltage
and the highest rated line frequency. As mentioned previously, for safety
reasons, it is highly recommended that the EUT be powered via an isolating
These tests are performed on both Class I and Class II products. For
Class II equipment, the test is made to accessible conductive parts
and to a metal foil with dimensions of approximately 10 x 20 cm, which
is attached to the enclosure of the product. The metal foil simulates
a human hand contact.
This test should also be conducted in all possible combinations such
as normal operating condition, switches open as well as closed, reversed
line polarity, etc. Equipment designed for multiple power sources, only
one of which is required at a time (e.g., for backup), must be tested
with only one source connected. Although most standards do not require
the earth leakage current test to be carried out for 100% of the units
in a production line, some standards, such as those for medical products,
do require a 100% production-line test.
Because any electrical safety test involves some risk of electrical
shock, it is crucial that certain precautions be taken to avoid shock
and injury to operators. Listed below is a sampling of precautions that
can minimize the danger of electrical shocks and ensure all-around safety:
Train operators in the basic theory of electrical circuits and
explain the object of each test.
Review and update all safety test procedures regularly.
Locate the testing area away from walkways and crowded areas on
the shop floor.
Guard the testing area with nonconductive barrier.
Mark the testing area with a clear and visible sign such as "Danger"
or "High Voltage Present."
Mark the testing area with a clear sign indicating "Qualified Personnel
Ensure that all test equipment is properly connected to a reliable
Reconfigure all testers (where possible) with push-button switches
so that operators must use both hands to activate the test equipment
or, alternatively, provide the equipment with a safety interlock that
automatically shuts down the high voltage when a safety switch on
the EUT is opened.
Connect the complete test setup to a palm-type switch that can
shut off the power to the test bench in case of an emergency.
Record Keeping and Identification
CENELEC has released a standard, EN 50116, which essentially defines
the routine electrical safety tests and their procedures to be applied
during or after the manufacturing process of IT equipment certified
or declared as complying with EN 60950.6,7
It is extremely important for manufacturers of electrical products
to ensure that all test results, including routine production tests,
are clearly and adequately documented and kept on file for possible
inspection. Although this may not be required by all agencies, keeping
accurate test records is not only good engineering practice, but could
also be vital in defending a legal action should such a case arise.
Most modern test equipment can produce test results in an electronic
format that can be stored or printed when needed.
It is also necessary for manufacturers to ensure that all test equipment
is calibrated regularly and that such records are kept on file. In fact,
most test houses expect to see a log for daily calibration of the hipot
tester on the production line. This log confirms that the hipot tester
has indeed been applying high voltage to the EUT. It is also vital to
ensure that all electrical safety tests are carried out on units that
have been returned for repair or service.
On the production line, there are generally three product states: not
tested, tested-passed, and tested-failed. The status of any product
must be clearly apparent to ensure that untested products are not shipped
and that only tested-passed products are shipped. For instance, a red
tag attached to the unit can indicate tested-failed. Untested products
are those that have neither a tested-passed nor a tested-failed indicator.
Electrical safety testing at the design and development stages as well
as at the production stage is vital to ensure that all products are
safe before reaching the end-user. The four tests described in this
article are among the most fundamental tests that manufacturers of electrical
and electronic products should conduct routinely. It is important to
use correct test equipment and adequate and accurate test procedures
so that sufficient testing is conducted and operator safety is considered.
The author wishes to thank Rich Nute of Hewlett-Packard for his valuable
1. IEC 60950, 3rd ed., "Safety of Information Technology Equipment,"
International Electrotechnical Commission (IEC), Brussels, 1999.
2. IEC 60065, "Audio, Video and Similar Electronic ApparatusSafety
Requirements," IEC, Brussels, 1998.
3. UL 6500, "Audio/Video and Musical Instrumental Apparatus for
Household, Commercial, and Similar General Use," Underwriters Laboratories
Inc. (UL), Northbrook, IL.
4. CAN/CSA-C22.2 No. 60950-00, "Safety of International Information
Technology Equipment," CSA International, Toronto, ON, Canada.
5. UL 60950, "Safety of Information Technology Equipment," UL,
Northbrook, IL, 2000.
6. EN 50116, "Information Technology EquipmentRoutine Electrical
Safety Testing in Production," European Committee for Electrotechnical
Standardization (CENELEC), Brussels, 1996.
7. EN 60950, 2nd ed., "Safety of Information Technology Equipment,
including Electrical Business Equipment," CENELEC, Brussels, 1992.