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Electrical product high-voltage insulation test

2019/10/30

In the CSA, UL, and IEC standards, almost all kinds of electrical safety standards require the product to withstand voltage tests. This shows that withstand voltage test is an important part of the electrical safety standards.
 
 
Dielectric Voltage Withstand Test (also known as High Voltage Test), determines the insulation material and space distance of a device to meet the requirements by applying a voltage higher than its rated value to the device for a certain period of time. This article describes the characteristics of the United States and Canada standard high-voltage insulation test, detailed description of the high-voltage insulation test procedure and its indicators and methods to help Chinese companies to create electrical equipment that truly meet the North American market standards.
 
 
 

Why high-voltage insulation test?

Under normal circumstances, the voltage waveform in the power system is a sine wave. In the operation of the power system due to lightning strikes, operation, faults, or improper matching of electrical equipment parameters, etc., the voltage in some parts of the system suddenly rises and exceeds its rated voltage. This is overvoltage. Overvoltage can be categorized into two categories according to their causes. One is overvoltage due to direct lightning or lightning induction, which is called external overvoltage. Lightning impulse currents and impulse voltages are very large in magnitude, short in duration, and extremely destructive.
 
 
However, due to the overhead lines of 3-10kV and below in towns and general industrial enterprises, due to the shield protection of factory buildings or tall buildings, the probability of direct lightning strikes is small and relatively safe. And the civilian appliances discussed here are not in the above range and will not be further discussed. The other type is caused by internal energy conversion or parameter changes in the power system. For example, when a no-load line is cut, a no-load transformer is cut off, and single-phase arc grounding occurs in the system, which is called internal overvoltage. Internal overvoltage is the main basis for determining the normal insulation level of various electrical equipment in the power system. That is, the design of the insulation structure of the product must not only consider the rated voltage but also take into account the internal overvoltage of the product's use environment. The withstand voltage test is to test whether the product insulation structure can withstand the internal overvoltage of the power system.


Test points and test voltage values

Test points and test voltage values ​​are determined based on the relevant product-specific standards. In addition to the standards of its own North American system, the United States and Canada have new standards based on the IEC. The standard "Motor-Operated Appliances (Household and Commercial)" CAN/CSA-C22.2 No.68-92 and "Portable Electrical Motor-Operated and Heating Appliances: General Requirements" C22.2 NO. 1335.1-93 is used here. To introduce the characteristics of the US and Canadian standard withstand voltage test.

CAN/CSA-C22.2 No.68-92
Requirement: An AC voltage at the appropriate frequency shall be applied for 1 minute between the live part of the product and the non-liveable conductor that may be grounded. The specific test voltage is as follows:

a For equipment with a rated voltage of 31 to 250 V, the test voltage is 1000 V.

b For equipment with a rated voltage of 251 to 600 V, the test voltage is 1000 V + twice the rated voltage.

c The rated voltage is 31~250 V, there is no grounding and can be touched by the human body equipment, the test voltage is 2500 V.
d For low voltage circuits of 30 volts or less, the test voltage is 500 V.
(The leakage current can be set by itself. There is 0.5mA 5mA 10mA. Each company/customer requirement is different, generally less than 20mA)

Double insulated product
 
No.
Test voltage application point
AC insulation strength test voltage (V)
1
Between live parts and non-contactable, non-charged electrical conductors with basic insulation
According to above testing requirement
2
non-contactable, non-charged electrical conductors with basic insulation and accessible electrical conductors
2500
3
Between non-contactable, non-charged electrical conductor with a basic insulation and a foil attached on the surface of an external non-conductive electrical conductor
2500
4
Between reinforced insulated live body and accessible non-charged electrical conductor
4000
5
Between the reinforced insulating live body and the metal foil attached to the surface of the outer non-conductive body
4000
6
Accessible non-conducting electrical conductor (or metal foil attached to the surface of the external non-conductor) Between the metal wrap of the power cord at the entrance of the enclosure (or metal rods of the same diameter as the power cord)
2500
 
C 222 No. 1335.1-93
Voltage application point
Test voltage (V)
Appliance With transformer
Electric appliance with rated power exceeding 0.5 horsepower
Electric appliances and heating appliances with rated power not exceeding 0.5 bhp
1. live parts and accessible parts and different polar lines close to the printed circuit board
--
1000 V+ Double rated power
1000
2. isolated or self-propelled transformer
(a) Secondary voltage < 50 V
(b) The secondary voltage is 51-125 V
500
1000
--
--
 
The difference between DC withstand voltage test and AC power frequency withstand voltage test
There are two types of withstand voltage test: one is the alternating current frequency withstand voltage test, and the other is the DC withstand voltage test.
 
Since the characteristics of the insulation material determine the breakdown mechanism of the AC and DC voltages. Most insulating materials and systems contain a range of different media. When AC test voltage is applied to it, the voltage will be distributed according to the ratio of the material's dielectric constant and size.
The DC voltage only distributes the voltage in proportion to the resistance of the material. In fact, breakdown occurs in the insulation structure, which is often accompanied by electrical breakdown, thermal breakdown, and discharge. It is difficult to separate them. The AC voltage increases the possibility of thermal breakdown compared to the DC voltage. Therefore, we believe that the AC withstand voltage test is more stringent than the DC withstand voltage test.
 
 
In actual operation, when a withstand voltage test is performed, if a DC withstand voltage test is to be used, the test voltage requirement is higher than the test voltage of the AC power frequency. The test voltage of a typical DC voltage test is obtained by multiplying the effective value of the AC test voltage by a constant K. Through comparative tests, we have the following results: For wire and cable products, the constant K is 3; in the aviation industry, the constant K is 1.6 to 1.7; for civil products, CSA is generally 1.414.


Formal test and factory test

Formal testing is used to determine whether the product's insulation structure design meets actual use. It is usually performed immediately after the temperature rise test, the tidal state test, the abnormality test, and some other tests. The test voltage is usually applied to the product for one minute. The factory test is to detect production defects in the production process, not to check if the design of the insulation structure is reasonable.
 
This is usually done before the product is ready to be packaged. Defects in the insulation structure of the product usually have the following conditions: Sharp parts damage the insulation layer of the wire, and the electronic components connected between the wire and the ground or between the primary and the secondary are short-circuited, and the welding of the wire is not good or the welding is off. The creepage distance is reduced, the insulation in the transformer is damaged, and so on. In order to adapt to mass production, the factory test can also increase the test voltage value by 20%, and the test time can be reduced from 1 minute to 1 second.

high-voltage insulation test and breakdown test

The standard will specify the specific voltage value at the time of the test. After a withstand voltage test can only explain the product's insulation structure can withstand the test voltage, But cannot explain how much the product's insulation structure can withstand the high voltage. If the application of insulation materials and the design of electrical equipment require insulation strength measurements, a breakdown test is required. The breakdown test measures the voltage at which a dielectric is broken down. When the electric field strength exceeds a certain limit, the relationship between the current passing through the medium and the voltage applied to the medium does not conform to Ohm's law, but suddenly increases. As shown in Figure 1. At this time, the insulation material is destroyed and the insulation properties are lost.
 
 
For an electrical product, if its insulation breaks down, it loses its operational use. This result is completely different from the withstand voltage test. The withstand voltage test is to pass the test to ensure that the product has no defects and can work safely and normally. Therefore, the test voltage of the withstanding voltage test is proved to be safe, reliable, and effective by practice and should be strictly followed. When we went to the factory for factory inspection, some factories raised the voltage of the withstand voltage test to improve the quality of their products, and some even increased to twice the required level. There is nothing to be gained from doing so. Because the first, doing so may reduce the product pass rate. Second, although the tests have been passed, it is possible to damage a part of the insulation structure and reduce the safety of the product. Third, it is possible to damage some components and reduce the quality of the product and shorten its life.

It is often said that "how much leakage current is considered a withstand voltage breakdown?" In fact, this leakage current is difficult to determine precisely. Because when the high voltage is applied to the product, it will produce a stable and small leakage current in the insulation. Because each product is different in complexity and insulation structure, this leakage current will change. When the insulation is broken down, this leakage current will increase rapidly. As shown in Figure 1. Therefore, in general, just set the overcurrent relay operating current (leakage current) of this withstand voltage test instrument to be slightly higher than the leakage current generated when the product is normally applied with high voltage. Please note that the leakage current in the Leakage Current Test is essentially the same as the very small current generated by the insulation system under voltage. The difference between them is that the leakage current in the withstand voltage test is generated under high voltage, and the leakage current in the leakage current test is generated under the rated voltage.
 
 
 
 
When can be exempt withstand voltage test?
The factory withstand voltage test may consider the exemption when:
l The product's power input is the low voltage (ELV) or extra low voltage (SELV) of the secondary output of other products, such as power supplies or transformers.
l When the finished product uses another power supply as a component, this power supply is CSA certified, with a complete housing and input and output connectors. Since this power supply is CSA certified, it has been subjected to a withstand voltage test, so consideration can be given to eliminating the withstand voltage test of the entire product.
But must pay attention to check if it is damaged before the power supply installing.
l When the signal transformer is used in the same voltage line and it is not used as a safety device, it is considered that the withstand voltage test between the primary and the core can be eliminated.

Requirements of the test equipment
 
For products below 600 V, the U.S. and Canadian standards typically require that the voltage test equipment have a minimum transformer capacity of 500 VA, an output current of 95 mA, and a live-to-ground capacitance of no more than 0.1 mF. The instrument is calibrated at least once a year. Before the formal test, the instrument must be checked. The factory test must check the instrument regularly. This can avoid misjudgment or damage to the product due to the instrument being abnormal.

AC withstand voltage test instruments generally include high voltage test transformers, voltage regulators, voltage measurement systems, and control and protection devices. Figure 2 is a schematic diagram of a manual boost test device for reference.
 
 
 
Test Methods

Here is a typical withstand voltage test method:

1. Check that the mains switch of the withstand voltage tester is in the "OFF" position

2. Unless the instrument is specially designed, all uncharged metal parts must be reliably grounded

3. Connect the wires or terminals of all the power input terminals of the device under test

4. Close all power switches, relays, etc. of the device under test

5. Adjust the test voltage of the Hi-pot tester to zero

6. Connect the high voltage output line (usually red) of the Hi-pot tester to the power input of the device under test.

7. Connect the withstand voltage tester's circuit ground (usually black) to the accessible uncharged metal part of the device under test.

8. Close the main voltage switch of the Hi-pot tester and slowly increase the secondary voltage of the instrument to the required value. Normal boost speed does not exceed 500 V/sec

9. Maintain this test voltage for a specified period of time

10. Slow down the test voltage

11. Disconnect the main voltage switch of the Hi-pot tester. Disconnect the high voltage output line of the voltage tester and disconnect the grounding line of the voltage tester.


The following conditions indicate that the device under test fails the test:

• When the test voltage cannot rise to the specified voltage value or the voltage drops instead.

When the withstand voltage tester shows a warning signal
It should be noted that since there are high voltages dangerous to the human body in the withstand voltage test, special care must be taken when testing. The following points need special attention:

• It must be specified that only trained and authorized personnel can enter the test area to operate the instrument

• Fixed, obvious warning signs must be placed around the test area to prevent other personnel from entering the danger zone

• All personnel, including operators, must be kept away from test equipment and equipment under test when testing

• Never touch the output line when the test instrument is started.
 

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