Every product is subjected to a suite of tests. What are the purposes of these tests? Often, we just perform the tests as prescribed in a standard, and with whatever conditions are specified by the certification house we are currently dealing with. I have found that it is worthwhile to consider not what the standard or certification house requests, but rather what is the “thing” that is being tested, and what is its relevance to the safety of the product.
Let’s look at a few of the popular and universal tests that are commonly applied to products.
This test is to measure the input current and input power as a function of input voltage. The product is adjusted or stimulated to consume maximum current or power. Note that the test has no pass/fail criteria as do most of the other tests. The input current and input power for specified input voltages are recorded.
What do we use the test data for? Some standards imply the purpose of the test is related to proper sizing and loading of the supply to which the product is connected. Indeed, this is true for permanently connected equipment where the building wiring is specifically installed for the equipment. For plug-and-socket connected equipment, the building wiring is already installed; the issue is whether the building wiring has sufficient capacity to carry the additional load imposed by the product.
However, what is the safety issue? Whether permanently installed or plug-and-socket connected, the building wiring up to the point of product connection, is required by building codes to be adequately protected by circuit breakers or fuses. No matter what load is connected to building permanent wiring for either permanently connected products or plug-and-socket connected products, the installation remains safe.
The usual use of the test data is to evaluate the product rating markings. However, such data is not related to the safety of the product. If the rating markings are incorrect, there is no safety issue. The worst that can happen is nuisance tripping of building overcurrent devices. This, in itself is not a hazard, although remedies to nuisance tripping may result in hazardous situations.
The major safety issue for which we use input test data is to determine the adequacy of the current rating of the various primary circuit components. To prevent overheating, the current ratings of various primary components must be equal to or greater than the primary current. Components that must be considered include the power plug current rating, the power cord wire ampacity rating, the appliance coupler current rating, the fuseholder current rating, the power switch current rating, internal wire ampacity rating, internal connector rating, etc.
Another safety issue related to the input test is the temperature of various insulating materials within the product and the temperature of heated accessible parts on the product. As a general rule, maximum heating occurs when the product consumes maximum power. Thus, the “normal temperature” test should be conducted at the input voltage for maximum power. However, the power difference as a function of input voltage is usually a low percentage of total power. Unless internal temperatures are very close to their ratings, the actual input voltage at which the temperature test is conducted is not usually significant.
Some certification houses assert that maximum temperature of some devices within products is not related to maximum input power; in such cases, only the certification house can specify the input voltage at which temperatures should be determined. Other certification houses specify the input voltage at which the temperature test is to be conducted regardless of power.
The purposes of the input test are:
- Determine whether the rating markings are acceptable.
- Determine whether the primary components are suitably rated.
- Determine the input voltage at which the temperature test should be conducted.
LEAKAGE CURRENT TEST
For grounded products, this test is to measure the current in the protective grounding conductor. For
two-wire products, the test is to measure the current between accessible conductive parts and ground. In some cases, leakage current is measured following humidity treatment. Why should humidity affect leakage current?
This test has pass/fail criteria which are specified in the standard to which the product is evaluated. The measured value is recorded and compared with the standard. Often, the purpose of the test is purported to be that of determining whether an electric shock is possible in the event of an open ground, or from accessible conductive parts of a two-wire product.
To identify the purpose of this test, let’s look at what one would do to address a problem of excessive leakage current. Or, putting the question another way, what does one do in the design of a product to control or minimize leakage current (ignoring EMI suppression capacitors)?
To control leakage current, we must first know the source of the leakage current. Since there are no electrical components connected to the ground circuit (or to accessible conductive parts), where does the current come from? The current comes from the stray capacitance between the primary circuit and the ground circuit (or to accessible conductive parts). The dielectric of this stray capacitance is the insulation between the primary circuit and the ground circuit (or accessible conductive parts). Therefore, to control leakage current, one must minimize the stray capacitance of the primary circuit. This is done by increasing the distance between the two plates of the capacitor (increasing the distance between the primary circuit conductors and grounded or accessible parts).
Some insulations may be hygroscopic (i.e., may absorb moisture). The presence of moisture within an insulator will alter the overall dielectric constant, thus increasing the value of capacitance. If the value of capacitance increases, so will the value of leakage current. Therefore, some standards specify humidity treatment prior to the measurement of leakage current.
The purpose of the leakage current test is:
- Determine whether the insulation from the primary circuit to grounded or accessible parts is adequate to prevent electric shock.
DIELECTRIC WITHSTAND (HI-POT) TEST
This test applies a relatively high voltage between the primary circuits and the protective grounding conductor. For two-wire products, the high voltage is applied between the primary circuits and accessible conductive parts (or foil wrapped around accessible non-conductive parts). In some cases, the test follows humidity treatment. Why should humidity affect this test?
This test has pass/fail criteria which are specified in the standard to which the product is evaluated. Note that this is not a measurement in that no value of any parameter is recorded.
What is the safety purpose of this test? To answer this question, we need to identify what part fails when the product fails the test and we need to identify the consequences of that part failure. Since we are applying a voltage between the primary circuits and the grounding circuit (or accessible conductive parts), the part we are testing is insulation. The insulation between any point of the primary circuit and the grounding circuit is either solid or air, or both solid and air in series.
In the event of a hi-pot failure, there is a failure of either the solid insulation or the air insulation. If the failure is solid insulation, then a conducting path is impressed upon the surface or through the solid insulation, and the insulation is destroyed catastrophically, becoming a resistor of indeterminate value. The resistance may be sufficiently low value to allow an electric shock to occur.
If the failure is air insulation, then a conducting path exists for the duration of the test. When the high voltage is turned off, the system returns to normal because air is a renewable insulation. A shock could exist for the duration of a primary circuit overvoltage.
So, the failure of the primary-circuit-to-ground insulation could result in an electric shock. But, why test with a voltage often more than 10 times the rated input voltage?
Inductors have the property of storing energy in magnetic fields. Usually, energy in magnetic fields is converted to some other energy form such as the kinetic energy of a rotating shaft (of an electric motor). Occasionally, magnetic energy is released as a high-voltage impulse into the power distribution system. Such releases are normal (e.g. – during the starting process of an electric motor).
Because high-voltage impulses are impressed upon the power line, all insulations on a power distribution system (including product internal insulations) must have sufficient electric strength to withstand not only the normal system operating voltage, but also the normal system overvoltages. Consequently, product mains-to-ground insulations must be tested with a high voltage to confirm that the insulations will not break down when subjected to high-voltage impulses, which normally occur on power distribution systems.
For type-testing, there is merit in converting this test from a pass/fail test to a measurement of the breakdown voltage of the weakest insulation in the product. This is done by increasing the voltage until breakdown occurs, recording the voltage, and examining the unit to identify the failed insulation. This tells you the margin between the required electric strength and the actual electric strength. It also tells you what the weakest insulation is. This is valuable information in the event of a failure of the production line hi-pot test.
Some authorities now advocate that the weakest insulation should be a specific air insulation especially installed in the product, where the breakdown voltage of that air insulation is less than that of the weakest solid insulation. This construction has the advantage of protecting the solid insulation from catastrophic breakdown in the event of ANY overvoltage. The breakdown voltage of the air insulation can be set at any convenient value. However, safety standards authorities and certification house authorities commonly do not permit breakdown of either air or solid insulation at any value less than that specified in the standards.
The purposes of the dielectric withstand (hi-pot) test are:
- Determine whether the insulation from the primary circuit to grounded or accessible parts has sufficient electric strength to withstand
the worst-case overvoltage which could occur in service.
- Determine the insulation with the least value of electric strength.
This test is to measure the normal operating temperatures of various components and materials. (For the moment, we will ignore the fact that some standards specify measurement of temperatures under fault conditions.) The measured temperatures are compared with maximum temperatures specified in the standard.
Why do we measure temperatures? What is the safety consequence of a component or material exceeding the temperature specified in the standard? How do we choose what components and materials to measure? Why does the standard specify some components and materials and not other components and materials?
Probably the most obvious reason to measure temperatures is to prove that accessible parts are not hot enough to cause a burn injury. But what is the purpose of measuring internal product temperatures?
All components and materials will fail as a function of temperature. Products commonly use metals for conductors and for structure. For metals, the temperature for failure of either the conductor function or the structural function is sufficiently high that it can be ignored.
However, products also commonly use thermoplastic for insulation and for structure. For thermoplastics, the temperature for softening can be of the same order as the normal temperature for power dissipating components such as power resistors and power semiconductors. If the structural function of a thermoplastic is weakened, so, too, may be its insulating function. Failure of an insulator may result in electric shock or electrically caused fire.
Therefore, we need to measure temperatures of thermoplastic insulations and thermoplastic structural parts (assuming the failure of the structural parts will result in a hazard — which usually will be the case). Examples of thermoplastic insulations are wire insulations, connector bodies, transformer bobbins (including EMI filter coil forms), and sheet insulations. Other materials may exhibit chemical change as a function of temperature. If such materials are used as insulators, then we must ascertain that the material operating temperature is less than that at which the chemical change occurs. (The chemical change may also alter the material’s insulating characteristics.) An example of a material which incurs a chemical change as a result of being subject to a high temperature is the epoxy of a glass-epoxy circuit board.
Some components, when heated, can evolve a gas. If the component is sealed, the pressure due to the evolved gas can cause a catastrophic rupture of the container. Some containers will release such pressure in the form of an explosion, while others will release the pressure gradually. An explosion could result in an injury. Examples of sealed components which can evolve a gas when heated include electrolytic capacitors and sealed batteries. Today, most electrolytic capacitors incorporate pressure relief mechanism which prevent explosion. Nevertheless, we still measure and control the temperatures of electrolytic capacitors and batteries.
Often, rather than measure the temperature of the material, we measure the temperature of the heating device, such as a transistor or diode. In this case, we get a worst-case measurement, where the insulation associated with that component can never achieve the temperature of the heating device. Such a measurement accounts for misrouting of wires in case they should bear against the heating device.
The purpose of the temperature test is:
- Determine whether materials are subject to a temperature at which they are likely to fail, where such failure would result in a hazardous condition.
Obviously, we could continue this discussion to cover a large number of tests. But, I believe these four tests are sufficient to illustrate the point. Too often, we just test the product, and record the data. I believe it is useful, for each test, to consider the consequences of failure of that test, and what one would do to the equipment to make it pass the test. This exercise forces one to consider what is being tested, and how it fits into the “big picture,” the overall set of components that make the product safe.
Richard Nute is a product safety consultant engaged in safety design, safety manufacturing, safety certification, safety standards, and forensic investigations.