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With the soaring cost of oil, manufacturers and consumers are switching to alternative energy sources. These sources include solar, wind, renewable resources such as wood and biomass, and fuel cells. This article will discuss various standards covering alternative energy sources, with the primary focus being on electrical safety and other electrical testing.

One organization dedicated to renewable energy is the American Council on Renewable Energy, or ACORE. ACORE works to bring all forms of renewable energy into the mainstream of America’s economy and lifestyle. In addition, the National Renewable Energy Laboratory (www.nrel.gov), operated by the U.S. Department of Energy, is our country’s primary research and development laboratory for renewable energy.

Part 1—Solid Fuel
Wood-Fueled Furnaces
Wood stoves have been used for years for primary and supplementary heating in residential buildings. Today there are standalone furnaces capable of heating spaces up to 11,000 sq. ft.

There are three different types of furnaces. The central furnace is a self-contained product that delivers heated air through ducts or heated water to the residence. The heated air is circulated by a fan (i.e., forced hot air) or natural convection (gravity feed).

The combination furnace uses a combination of solid-fuel such as wood with fuel-oil. These furnaces can burn both fuels separately or simultaneously. They can have a common combustion chamber or separate combustion chambers.

The third type of furnace is the supplementary furnace. The supplementary furnace is designed to connect into a common hot air system by means of a plenum, use of a heat exchanger, or directly connected into a boiler.

These three types of furnaces are covered by a common safety standard, UL 391. The Canadian standard is CAN/CSA B366.1-M91. Most stoves sold in North America meet both UL 391 and CSA B355.1-M91.

UL 391 4th Ed., Solid-Fuel and Combination Fuel Central and Supplementary Furnaces, was last updated in October 2006. This standard applies to manually fueled and solid-fuel furnaces, which are intended to burn fuels such as wood, coal, and other biomass fuels.

The 4th edition has the following three changes from the previous edition:

Requirements for electrical convince receptacles;
Corrections to bonding conductor tests;
Miscellaneous references to NFPA 211, ANSI MC96.1, UL 61058 and wire size terminology.

Electrical Safety Testing for Wood-Fueled Furnaces
Electrical safety testing called out in UL 391 includes Dielectric Withstand testing, Bonding Joint testing and Bonding Conductor testing. Manufacturing and production testing requires that Dielectric Withstand testing be performed on every furnace.

Dielectric Withstand testing requires hipot testing at 60Hz for one minute between live parts (mains) and dead metal parts, and between live parts of high voltage and live parts of low voltage. The test voltage between live parts and dead metal parts is 1000VAC plus twice the rated voltage, which is typically 1250VAC for North America. The test voltage may be reduced to 1000VAC for motors less than ½ hp (373 Watts) and input voltage no more than 250VAC. No breakdown shall occur during the test time. Breakdown is defined as a rapid and uncontrolled current flow. Arcing is acceptable as long as the arcing stops when the voltage is reduced.

Low voltage circuits are tested at 500VAC, 60Hz for one minute between low voltage parts of opposite polarity, and between low voltage parts and dead metal. Low voltage circuits are defined as having a maximum potential of 30VAC, 42.4V peak, or 42.4VDC.

The hipot tester used should have a 500VA or higher transformer, or less than 500VA if the high voltage potential can be maintained during the test. Most digital hipot testers have load regulation and meet the latter requirement.

Production testing for dielectric withstand is to be performed on every furnace. The requirements are the same as outlined above, with the following exceptions:

If the furnace uses a low voltage circuit, the circuit shall be connected to chassis or dead metal parts during hipot testing between mains and dead metal parts.
The test time can be reduced from 60 seconds to one second if the test voltage is increased by 120%.

Figure 1: Production Test Dielectric Withstand

The equipment for Dielectric Withstand testing shall have a visual indication that high voltage is being applied, visual and/or audible indication of failure, and a manual reset switch in the event of failure.

Bonding Joint testing requires the application of 30A between any single bonding joint that uses friction or spring action. The voltage drop across the joint cannot exceed 10mV initially, and 15mV after the part has been attached and removed 10 times. This type of test can be performed with a ground bond tester as described in the section on grounding conductor tests. There are no requirements for production testing.

Grounding Conductor testing, or Ground Bond testing, verifies the integrity of the ground connection on the furnace. The test requires application of twice the rating of the branch circuit over-current protection device required to electrically protect the equipment. The test time varies depending upon the test current. The conductor shall not open during the test.

This type of test is performed using a ground bond tester, which applies a constant current for a specified time with a pass or fail indication. It is very similar to a hipot tester. It is important to review CSA requirements, specifically CSA 22.2 No. 0.4, Bonding of Electrical Equipment, for production and compliance tests for bonding conductors.

Figure 2: Grounding Conductor Test

Pellet Stoves
Pellet stoves have become a popular alternative for supplementary residential heat. A pellet stove is an appliance that burns compressed wood or biomass pellets to create a source of heat for residential rooms or for entire buildings. By slowly feeding fuel from a hopper into a burn-pot area, pellet stoves create a constant flame that requires little to no physical adjustments. Pellets are made from compacted sawdust, wood chips, bark, agricultural crop waste, waste paper, and other organic materials. Some models can also burn nutshells, corn kernels, sunflower seeds, cherry pits and small wood chips.

Pellet stoves are more convenient to operate and have much higher combustion and heating efficiencies than ordinary wood stoves or fireplaces. As a consequence, they produce very little air pollution. In fact, pellet stoves are the cleanest of solid fuel-burning residential heating appliances, with combustion efficiencies of 78%–85%.

The heating industry has considerably shifted toward biomass stoves and heating devices based on efficient combustible and renewable resources. The first pellet stoves were developed in the late 1970s and early 1980s and, with the prices now soaring over $4.00/gallon for fuel oil, pellet stoves are a popular alternative. Companies such as Maine Energy Systems hope to convert 10% of households in the state of Maine to wood pellet heat during the 2008-2009 heating season.

Pellet stoves do require electricity to operate and, under normal usage, they consume about 100 kilowatt-hours (kWh) or about $9 worth of electricity per month. As they operate from electricity, they are required to meet the UL 1482 safety standard.

Electrical Safety Testing for Pellet Stoves
UL 1482 5th Ed., Solid-Fuel Type Room Heaters, was updated in February 1996. This standard covers room heaters that have a freestanding fire chamber, and which are radiant or circulating, including freestanding wood stoves, fireplace inserts, and pellet stoves. The Canadian standard covering these products is ULC 5627.

Electrical safety testing called out in UL 1482 includes Dielectric Withstand testing, Bonding Joint testing, and Bonding Conductor testing. Manufacturing and production tests require Dielectric Withstand testing be performed on every stove.

Dielectric Withstand testing outlined in the standard requires hipot testing at 60Hz for one minute between live parts (mains) and dead metal parts, and between live parts of high voltage and live parts of low voltage. The test voltage between live parts and dead metal parts is 1000VAC plus twice the rated voltage, which is typically 1250VAC for North America. The test voltage maybe reduced to 1000VAC for motors of less than ½ hp (373 Watts), and input voltage of not more than 250VAC. No breakdown shall occur during the test time. Breakdown is defined as a rapid and uncontrolled current flow. Arcing is acceptable as long as the arcing stops when the voltage is reduced.

The hipot tester used should have a 500VA or higher transformer, or less than 500VA if the high voltage potential can be maintained during the test. Most digital hipot testers have load regulation and meet the latter requirement.

Production testing for dielectric withstand is to be performed on every furnace. The requirements are the same as outlined above with the following exceptions. If the furnace uses a low voltage circuit, the circuit shall be connected to chassis or dead metal parts during hipot testing between mains and dead metal parts. The test time can be reduced from 60 seconds to one second if the test voltage is increased by 120%.

Figure 3: Production Test Dielectric Withstand

The equipment for Dielectric Withstand testing shall have a visual indication that high voltage is being applied, visual and/or audible indication of failure, and a manual reset switch in the event of failure.

Leakage Current testing measures the current available from exposed conductive surfaces and ground. This test is commonly referred to as earth line leakage for products with a three-prong power cord, and enclosure leakage for products with a two-prong power cord.

All products that use an AC line power source have some associated leakage current when the device is turned on and operating. This leakage current normally flows from the AC line source through the ground path in the product and back to earth ground through the ground blade on the power cord.

On products without a ground blade (or when the ground on the product has a fault), a potential can develop on metal surfaces of the product. If an individual then comes in contact with the exposed metal surface, he or she then becomes the ground path for the product. Under this condition a certain amount of leakage current flows through the individual exposed to the metal surface. If the leakage current is extremely low (typically less than 0.5mA), the individual should not notice that they are the path for the current flow. At levels higher than this, the individual can experience a startled reaction or worse.

During Leakage Current testing, the product is powered up and the leakage current flowing though the ground wire and/or any exposed metal on the product is measured using a multimeter or dedicated leakage tester. The standard requires a 1500Ω resistor and 0.015uF capacitor in the current measurement circuit to simulate the impedance of the human body. This provides a more realistic idea as to how much current would flow through a typical individual. The multimeter or leakage tester needs to have a frequency response from DC to 100 kHz and measurement error of not more than 5%.

The Leakage Current testing discussed here is different from the measurement of leakage current during a hipot test. During a hipot test, high voltage (generally greater than 1000V) is applied between the hot and neutral lines and the ground of the device under test (DUT), and the leakage current is then measured. In Leakage Current testing, the product is powered on and operated via standard line voltage (e.g., 120VAC), and the leakage current is then measured using a circuit to simulate the impedance of the human body.

The configuration in Figure 4 is called out in the standard. The switches shown on the mains side are to switch between normal and reverse mains conditions.

Figure 4: Leakage Current Test

The maximum leakage current is 0.75mA for a blower rated 250VAC or less.

Bonding for Grounding testing, or Ground Bond testing, verifies the integrity of the ground connection on the stove. All exposed or accessible dead metal parts shall be connected to protective ground. The test requires application of twice the rating of the branch circuit over-current protection device required to electrically protect the equipment for a period of two minutes. The conductor shall not open during the test.

There is no requirement for this test in production, but there is a requirement for Ground Continuity testing. In addition, CSA 22.2 No. 0.4, Bonding of Electrical Equipment, should be reviewed for additional grounding requirements and tests. The Ground Continuity test requires that all appliances be tested for electrical connection between ground and the appliance. This test can be performed with an ohmmeter, battery/buzzer combination, or similar items. Most hipot testers incorporate a Ground Continuity test that meets these requirements.

Part 2—Solar Power
Solar power is once again back in the spotlight as a viable source of alternative energy. Solar power is based upon the photovoltaic cell. The term photovoltaic refers to any device which produces free electrons when exposed to light. The photovoltaic cell is the smallest discrete element in a photovoltaic module that performs the conversion of light into electrical energy to produce a DC current and voltage.

Crystalline photovoltaic cells are manufactured using two different techniques, either crystal growing or casting. The growing technique creates a silicon wafer by pulling it from molten silicon. The casting process uses a reusable graphite mold to produce blocks of silicon. The silicon blocks are then cut into wafers.

A photovoltaic cell typically produces 0.5V. To increase the voltage, 36 cells are typically assembled into a photovoltaic module to provide enough voltage to charge a 12V battery. A photovoltaic module also includes ancillary parts, such as interconnections, the terminals intended to generate DC power from sunlight. A photovoltaic module is often referred to as a solar panel.

Electrical Safety Testing for Photovoltaic Cells and Panels
There are several standards that cover various aspects of photovoltaic module testing, including the following:

UL 1703, Flat-Plate Photovoltaic Modules and Panels
IEC 61730, Photovoltaic (PV) module safety qualification
IEC 61215, Crystalline silicon terrestrial photovoltaic (PV) modules - Design qualification and type approval
IEC 61646, Thin-film terrestrial photovoltaic (PV) modules – Design.

UL 1703 and IEC 61730 are the basic standards, covering requirements for safety and construction of photovoltaic modules to address prevention of electrical shock and fire hazards. These two base standards are to be used in conjunction with IEC 61215 and IEC 61646. IEC 61215 is specific to crystalline silicon modules, and IEC 61646 to thin-film modules. UL 1730 will soon be superseded by UL IEC 61730.

IEC 61730-1 -2 1st Ed. – 2004, and UL 1703 3rd Ed. - 2002
Electrical safety tests called out in IEC 61730 and UL 1703 include Dielectric Withstand, Bonding Path Resistance, and Insulation Resistance. Manufacturing and production tests require Dielectric Withstand and Ground Continuity testing be performed on every module.

Bonding Path Resistance testing outlined in the standard requires the resistance between the ground terminal or lead and any accessible conductive part be less than 0.1 ohms. The resistance is to be measured with a test current of 2 times the fuse rating.

Ground Continuity production testing requires only that there is an electrical connection between the ground terminal and all accessible conductive parts. Note that, if CSA certification is required, CSA 22.2 No. 0.4 – 2004, Bonding of Electrical Equipment should be reviewed. When a production continuity test is required by the standard to verify electrical connection, this requires a 10A Ground Continuity test.

Hipot (Dielectric Withstand) testing is performed with test conditions of (1000V + 2 x max voltage) DC. The voltage shall be ramped up from 0V to required value within five seconds and held for one minute, with leakage current not exceeding 50uA. The reason for ramping up the voltage is to prevent false failures due to capacitive in-rush current. For modules that have a system voltage of less than 30VDC, the test voltage is 500VDC. The voltage is to be applied between all current-carrying parts and all accessible parts. The tests are to be performed on samples after humidity, temperature, water spray and corrosive atmosphere tests have been performed.

Production testing for Dielectric Withstand is required under the same conditions as above; test time can be reduced to one second if voltage is increased by 120%. A module or panel with a system voltage rating of 30V or less is not required to be tested.

Wet Insulation Resistance testing is carried out by immersing the panel into a solution for two minutes. The insulation resistance is then measured between the shorted output terminals of the module and the solution. This test is intended to verify that the solar panel or the solar cell array has insulation high enough to reduce the possibility of fire and electrocution hazards, even when the module is wet.

Insulation Resistance testing is performed using a test voltage of 500 VDC. The minimum resistance is 400 Mohms for a module having an area of 0.1m2 or less. Larger modules are required to have the measured resistance times the area of the module greater than 40 Mohms*m².

IEC 61215 2nd Ed. - 2005
The specific standard for crystalline silicon modules is IEC 61215. This standard requires the safety tests similar to the base standards.

Insulation Resistance Test or Withstand Voltage Test
Withstand Voltage testing is carried out by shorting a positive terminal and a negative terminal of the solar panel or a solar panel array, and then applying a predetermined voltage between the live electrical section and the outer housing (which refers to the bottom surface reinforcement member, the frame, and ground terminals). This test is intended to verify that the product (the solar panel and the solar panel array) is free from any dielectric breakdown attributed to aging of insulation. As for a test method and test apparatus for Withstand Voltage testing, it is important to apply a direct current across test terminals of an object to be tested.

Withstand Voltage testing is performed with test conditions of (1000V + 2 x max voltage) DC. The voltage shall be increased from 0V to the required value within five seconds and held for one minute with leakage current not exceeding 50uA. Production testing for Dielectric Withstand is required under the same conditions as above, or test time can be reduced to one second if voltage is increased by 120%.

Wet Leakage Current Test or Insulation Resistance Test
Insulation Resistance testing is carried out by shorting a positive terminal and a negative terminal of a solar panel or a solar panel array, and then by applying a predetermined voltage between the live electrical section and the outer housing (which refers to the bottom surface reinforcement member, the frame, and ground terminals). This test verifies that the solar panel or the solar cell array has insulation high enough to reduce the possibility of fire and electrocution hazards.

There are no other particular requirements on the test method and test apparatus. Preferably, the test apparatus includes the function of discharging a charge accumulated during the resistance test of the insulation resistance having a capacitive component. The insulation resistance test is performed using test conditions of an application voltage of 500 V for an application time of one minute, with a minimum limit typically 400 Mohms or (40 Mohms*m²)/Area(m²)

IEC 61646 2nd Ed. - 2008
The specific standard for thin-film terrestrial photovoltaic modules is IEC 61646. This standard requires the same safety tests to the base standards of IEC 61730 and UL 1703.

Inverters
Devices such as inverters, converters, charge controllers and interconnection equipment that convert DC to AC also have to have Hipot and Ground Impedance testing. UL 1741 and IEEE 1547 cover inverters and other equipment.

For products that employ power connectors, those connectors are normally investigated using UL 498, Attachment Plugs and Receptacles, and/or UL 1977, Component Connectors Used for Data, Signal and Power Equipment Applications. Note that electrical connectors must be certified for outdoor use in wet locations with exposure to sunlight (i.e., UV exposure resistant). If a customer should choose to create a listed power connector for use with photovoltaic modules and panels, also note that these devices must be designed robustly enough to withstand use as a DC circuit interrupt device under overload conditions, as outlined in UL 498 and UL 1977.

UL 1741, Inverters, Converters, Controllers and Interconnection System Equipment for use with Distributed Energy System, covers a variety of devices used in conversion of electricity from a variety of sources, including fuel cells, photovoltaic, and wind energy systems, to another form of electricity for both grid and non-grid connected systems. This standard is to be used in conjunction with and to supplement IEEE 1547, Interconnecting Distributed Energy Systems with a Power System. IEEE 1547 covers general requirements, such as voltage regulation, synchronization, integration of grounding, monitoring and isolation, to manufacturing and commissioning tests.

UL 1741 1st Ed. – 1999 Testing
Dielectric Withstand testing is required both for compliance and production testing. The dielectric withstand is applied between input and output wiring to accessible dead metal, and between input and output wiring to accessible low voltage and limited energy parts including terminals.

Test voltages for Dielectric Withstand testing are based upon rating voltage. Products with a 250V or less rating are tested at 1000VAC for 60 seconds or 1200VAC for one second. The equivalent DC can be used if the product can be damaged by AC potential. Products rated over 250V are tested at 1000VAC + 2 x rated voltage for 60 seconds, 1000VAC + 2.8 x rated voltage for one second, 1400VDC + 2.8 x rated voltage for 60 seconds, or 1700VDC + 3.4 x rated voltage for one second.

The equipment for Dielectric Withstand testing shall have a visual indication that high voltage is being applied, visual and/or audible indication of failure, and manual reset switch in the event of failure. If the tester has a transformer less than 500VA, there must be a voltmeter in the output. Testers with a transformer of 500VA or greater shall have a voltmeter, indicating voltage selector or visible marking indicating output voltage.

Ground Impedance testing is required in compliance testing. The equipment ground to any other metal part that is required to be grounded shall not exceed 0.1ohms. The impedance is measured at 25A and a frequency of 60Hz. The open circuit compliance voltage shall not exceed 6V. n

Robert M. Brown is the vice-president of technical operations at QuadTech Inc., and can be reached at rbrown@quadtech.com.

References

UL1703 3rd Ed. – 2002
CSA 22.2 No. 0.4 – 2004
UL 1741
UL 391 4th Ed
UL 1482 5th Ed
www.mainewoodfurnaces
www.solarbuzz.com
www.furtonics.co.uk
www.harmonstoves.com
www.evergreen.solar.com