The Ultimate Guide to Manufactured Home Testing
Electrical Safety testing of Modular Homes is a complex business, but we’ve set out to help you simplify this application with our Ultimate Guide to Manufactured Home Testing. The controversy of AC vs. DC Dielectric Withstand testing of modular and manufactured homes has been an issue for years, due-in-part to a misunderstanding of the test itself.
Common questions asked by Modular Manufacturers:
1. What part of the circuit do I need to test? / What is the proper test voltage?
2. What happens to the Manufactured Home during a test?
3. Do I test with AC or DC voltage?
4. How can I test the entire manufactured home, with all of its appliances still connected, and not damage any appliances?
This article addresses these questions and other aspects of AC vs. DC Hipot testing.
What part of the circuit do I need to test? / What is the proper test voltage?
The U.S. Department of Housing and Urban Development, Manufactured Housing and Standards Division issued a memorandum in June of 2000 to All Primary Inspection Agencies and All State Administrative Agencies regarding dielectric strength testing of modular and manufactured homes as specified in standard # 24 CRF 3280.810(a).
The memorandum states the requirements of the Dielectric Withstand test:
"The wiring of each manufactured home is to be subjected to a dielectric strength test between live parts and the manufactured home’s ground and between neutral and the manufactured home’s ground. The test is either to be performed between 900 to 1079 volts for one minute or between 1080 to 1250 volts for one second. The test is conducted on each manufactured home to stress the insulation to determine if it is capable of resisting transient power line surges to which the wiring may become subjected, identify any low resistance areas of the insulating material and any dielectric breakdown paths of failures that may exist."
Please note that the test is not to be conducted from phase to phase, or between the hot and neutral conductors as this could result in damaging appliances that may be connected. The standard does not specify whether AC or DC should be used to perform the test, but the voltage ranges indicated in the standards have historically been accepted as AC values.
AC voltages are normally specified as root-mean-square (RMS) values. The RMS voltage is the effective value of a varying or alternating voltage; this value will produce the same power loss as a continuous voltage when applied to a pure resistive load. The peak value of a sine wave is actually 1.414 times greater than the RMS voltage (Figure 1). To perform an equivalent DC test, the test potential would be 1.414 times the RMS value to achieve the equivalent peak voltage. Using the RMS values specified in 24 CRF 3280.810(a) the DC equivalents would be 1273 to1526 volts for a one-minute test or 1527 to 1768 volts for a one second test.
Figure 1: RMS Voltages vs. Peak Voltages
Common Parameters for AC and DC Hipot Testing Modular Manufactured Homes
DC Hipot Test Parameters
| *Test Voltage | **Ramp Time | *Dwell Time | **Trip Current |
| 1273 -1526 volts DC | ≥ 2 – 5 s | 60 seconds | ≥ 1 – 3 mA |
| 1527 – 1768 volts DC | ≥ 2 – 5 s | 1 second | ≥ 1 – 3 mA |
AC Hipot Test Parameters
| *Test Voltage | **Ramp Time | *Dwell Time | **Trip Current |
| 900 – 1079 volts ACrms | ≥ 100 ms | 60 seconds | varies with load |
| 1080 – 1250 volts ACrms | ≥ 100 ms | 1 second | varies with load |
* Values according to H.U.D. specification # 24 CRF 3280.810(a)
** SCI recommended values
What happens to the Manufactured Home during a test?
During a Hipot test, voltage is applied across the insulating material between current carrying conductors and ground. This in fact is very similar to the characteristics of a capacitor, which is simply two conductors separated by an insulating or dielectric material. The circuitry that needs to be tested within a manufactured home can be very capacitive in nature because of the size and surface area of the circuits. For this reason, the majority of the leakage current seen during an AC Hipot test can be a direct result of the capacitive reactance of the circuit and not the insulation resistance. This capacitive reactance is low and can range from less than 50k up to 200k ohms in comparison to the insulation resistance of the home, which is typically greater than 1 megohm. A capacitor will allow AC current to flow through it while it blocks the flow of DC current once it charges up to the applied potential. Current flows through a capacitor when there is a change in the applied potential or charge. When performing an AC test the charge is changing at a rate of 60 cycles per second. This is the reason that AC current cannot charge the capacitive component of the device under test (DUT). This results in much higher leakage currents when performing an AC Hipot test than is observed when performing a DC Hipot test.
Do I test with AC or DC voltage?
AC Hipot testing is often performed in multiple steps. This typically includes separating circuits and disconnecting appliances because some Hipot testers do not have the output capacity to test the entire home. This lack of capacity is usually exhibited by the inability of the tester to reach the specified test voltage without going into failure mode. The operator must then determine if the failure condition is the result of excessive leakage current trip setting or if a breakdown of the insulation caused the failure. Many older testers lack current meters in their design. This makes it difficult for the operator to determine the nature of the failure condition. Circuit breakers must be shut off to isolate circuits and then each circuit must be retested separately.
When performing a DC Hipot test, once the applied voltage charges up the capacitance of the DUT, the true leakage current observed during a DC test is usually very small. The main advantage of DC testing over AC is that manufacturers can use smaller, lighter, more portable instruments and are able to test the complete home with the appliances, GFCI’s, smoke detectors and fluorescent fixtures connected. This allows the manufacturer to test the entire modular home with a single connection as a single test. For these reasons DC Hipot testing of manufactured and modular homes is quickly becoming the preferred method of testing.
The only drawback to testing with DC current is that you typically must ramp up the voltage to keep from having a false failure shutting down the Hipot. This is due to the high initial total current, which includes charging current that flows until the circuit capacitance is fully charged, dielectric absorption current and the steady state leakage current. See Figure 2.
Figure 2: Direct Current vs. Time
The dielectric absorption current is the current that flows into a capacitor following its initial charge due to a gradual penetration of electrical stress into the dielectric. It is also the current that flows out of a capacitor following its initial discharge. This current represents a reversible stored energy that can be released after the applied voltage is removed. Setting the ramp time for a two to three second ramp should solve the problem of false failures by allowing enough time for the DUT to charge slowly and therefore keep the total current below the trip threshold. The trip level of the DC tester can be set to a lower level as compared to the trip level when performing an AC test. The leakage current observed when doing DC testing is a true indication of how good the insulation is because reactive current does not mask true steady state leakage current. The trip level of the Hipot is typically adjusted around 1 milliamp when performing the test with DC, as compared to trip levels greater than 20 milliamps when performing the test using an AC tester.
This brings up the issue of safety. Underwriters Laboratories (UL) and the American National Standards Institute (ANSI) ran tests in the 1960’s to determine the human body’s response to different levels of electrical current. The tests were conducted using a 120 volt 60 hertz source. They determined that on average, 0.5mA of current is the perception level and can produce a startle reaction. Higher current levels in the range of 5 to 10 mA will cause paralysis of the extremities, effectively producing the inability to let go. Currents in the range of 20 to 40 mA between the extremities cause the muscles to contract painfully, making breathing difficult, and leading to asphyxiation. Current levels in the 40 to 70 mA range lasting for one second or longer cause ventricular fibrillation. Currents greater than 70 mA cause severe electrical burns and can cause cardiac arrest.
Modular Home Testing Comparison Advantages and Disadvantages of AC and DC HipotTesting
Testing Advantages
| AC Testing Advantages | DC Testing Advantages |
| Full Test Voltage Can Be Applied Instantly With an AC Hipot tester, the full test voltage can be applied instantly without the need to ramp the voltage. |
No Need to Disconnect Fixtures, Loads and Appliances As long as the test voltage is ramped up slowly, the breaker box can be tested with all of the loads connected, simplifying the testing process and shortening the total test time. |
| No Need to Discharge the DUT The breaker panel doesn’t need to be discharged for an extended period of time. This is very helpful if you are using a DC Hipot tester that does not have an automatic discharge feature. |
Lower Output Current Capacity The output current capacity of a DC tester can be much lower than that of an AC tester. This lowers the cost of the Hipot tester and is less dangerous for test operators to use. |
| Tests DUT in Both Polarities An AC tester performs a Hipot test in both the positive and negative polarities, providing an accurate waveform similar to what the breaker panel sees on its input under normal conditions. |
Accurate Leakage Current Measurements A DC tester provides a more accurate reading of the leakage current by reading only real current. |
Testing Disadvantages
| AC Testing Disadvantages | DC Testing Disadvantages |
| Inaccurate Current Measurements An AC Hipot tester measures total current. Reading total current doesn’t provide an accurate leakage measurement -- it makes your leakage current readings much higher than they are. |
Mandatory Discharge of the DUT Since the DC tester charges up the breaker panel to the test voltage, the DUT needs to be discharged before the test is finished. Failing to do this will result in an energized panel. This is especially important if your tester does not have an automatic discharge feature. |
| Requires Higher Output Current Capability An AC tester requires a higher current output capability than a DC tester. This is due to the current it must supply to both the resistance of the insulation and the capacitance of the cabling and fixturing of the home. Higher output current capability is usually more expensive, and can be more dangerous to testoperators if a shock occurs. |
Tests DUT in One Polarity The DC test only stresses the breaker panel in one polarity, thought by some to be an inaccurate representation of the voltage input the DUT would see under normal conditions. |
Based on the information above, performing a Hipot test using a DC tester that has a trip level set for 1 mA would limit the shock hazard the operator could be exposed to as opposed to conducting a test with AC where the trip level could be set in excess of 20 mA. It is important to note that the DC Hipot will shut down at a much lower current level limiting the exposure from the Hipot. However, this doesn’t mean that the operator will only receive a 1 mA shock. The Device Under Test (DUT), after being fully charged, can act like a capacitor and discharge its stored energy into a load, such as a person who accidentally may come into contact with the circuit. This stored energy or charge on the DUT can be much greater than the energy available from the Hipot. Stored energy is calculated by using the formula VÇC/2. For example, the capacitance value for a filter capacitor in a DC Hipot tester might be 0.01 microfarads. This means that at 1530 volts the capacitor would deliver a charge of 11.7 milijoules of energy. A DUT could have a distributed capacitance of 0.05 microfarads, which at 1530 volts will deliver 56.2 millijoules of energy, five times the energy available from the Hipot.
How can I test the entire manufactured home, with all of its appliances still connected, and not damage any appliances?
Another area of concern for manufacturers is potential damage to appliances within the manufactured home during a Hipot test. Keep in mind that appliances within the manufactured home have already been Hipot tested to comply with their product safety listing. If performed correctly, the test stresses the insulation between what are normally current carrying conductors and the dead metal or ground on the appliance. This is ensured by placing the power switch on the appliance in the on position, connecting the return of the Hipot to the dead metal on the appliance and applying high voltage to the hot and neutral conductors. Performed in this way the test applies an equal potential to both sides of the components within the circuit. This same method should be followed when testing the complete manufactured home. High voltage should be applied to both phases and to the neutral (the neutral must be isolated from ground) and the return should be connected to ground. All switches and circuit breakers should be in the on position (Figure 3). Connecting the Hipot to the manufactured home in this manor will allow testing of the complete home with the appliances connected.
Figure 3: Hipot Tests of Manufactured and Modular Homes
Summary
There has been a trend towards DC Dielectric Withstand testing of modular and manufactured homes. There are advantages and disadvantages to both AC and DC testing which are listed in the chart below. DC testing seems to offer more significant advantages. DC Instruments are more portable and allow the complete home to be tested in one step. The Hipot testers themselves are safer since the current trip setting can be set to a much lower level, thus reducing shock hazard. More importantly the test results obtained from DC leakage current readings give a true indication of the quality of the insulation system. Since the leakage current observed is due to the insulation resistance, and not the distributed capacitance of the manufactured home as in AC testing. While AC testing has been the norm, advanced technologies such as microprocessor control and electronic ramp and dwell timers have made DC testing a more viable and effective means of testing entire modular and manufactured homes.