Dielectric voltage withstand test in thick film heater test

According to CSA,UL and IEC standard, almost all kinds of electrical safety standards require the products to be tested with voltage resistance.It can be seen that voltage test is an important part of electrical safety standards.Dielectric Voltage Withstand Test also known as High Voltage Test, a test that determines whether a device's insulation material and spatial distance meets requirements by applying a voltage higher than its rated value and maintaining it for a certain time.

Why required dielectric voltage withstand test ?

Normally, the voltage waveform in a power system is a sine wave.In the operation of the power system, due to lightning strike, operation failure or improper coordination of electrical equipment parameters, the voltage of some parts of the system suddenly increases, greatly exceeding its rated voltage, which is called over voltage.Over voltage can be divided into two types, one is the over voltage caused by direct lightning strike or lightning induction, called external over voltage.The amplitude of lightning impact current and impact voltage is large ,short and destructive. However, due to the shielding protection of factories or tall buildings,the 3-10kV and below overhead lines in towns and general industrial enterprises, the probability of direct lightning strike is very small, relatively safe.The other is caused by the energy conversion or parameter changes inside the power system, such as fitting the no-load line, cutting off the no-load transformer, and the single-phase arc grounding in the system, which is called the internal over voltage.Internal over voltage is the main basis for determining the normal insulation level of various electrical equipment in the power system.That is to say, the design of the insulation structure of the product should not only consider the rated voltage but also consider the internal over voltage of the product use environment.Dielectric voltage withstand test is to test whether the product insulation structure can withstand the internal over voltage of the power system.

Test point and test voltage value

Test points and test voltage values are determined according to the relevant standards of specific products.The US and Canada have new IEC-based standards in addition to their own North American standards.Use the testing feature of "Motor-Operated Appliances (Household and Commercial)" CAN / CSA-C 22.2 No.68-92 and "Portable Electrical Motor-Operated and Heating Appliances: General Requirements" C22.2 NO. 1335. The standard of 1-93  introduces the characteristics of American and Canadian standards.

CAN/CSA-C22.2 No.68-92

Requirements:

An AC voltage of the appropriate frequency test time shall be applied for up to 1 minute between the charged part of the product and the uncharged conductor that may be grounded. The specific test voltage is as follows:

A：Equipment with a rated voltage of 31~250 V and a test voltage of 1000 V.

B.  Equipment with a b rated voltage of 251~600 V, with a test voltage of 1000 V + twice the rated voltage.

C.   Rated voltage is 31~250 V, no grounding and human touch equipment, test voltage is 2500 V.

D.  For low voltage 30 V or below, the test voltage is 500 V. (The leakage current can be set by itself, with 0.5mA 5mA 10mA of different company / customer requirements, generally less than 20mA)

Dielectric voltage withstand test and breakdown test

The specific voltage value for the test is specified in the standard.The withstand test can only indicate that the insulation structure of the product can withstand the test voltage, but not stand for how high can withstand.If you need to determine the insulation strength, the breakdown test is required.A breakdown test is that the test voltage at which value the dielectric is breakdown.When the electric field strength exceeds a certain limit, the current passing through the medium and the voltage applied to the medium does not conform to Ohm's law, but it increases suddenly.For an electrical product, if its insulation is through, then will loses the operational function.This result is completely different from the withstand test. Withstand test ensure that the product is without defects and can work safely and normally.Therefore, the test voltage of voltage test is proved to be safe, reliable and effective through practice, and should be strictly comply with it.Some factories increase the voltage in order to show the quality of the voltage test, some even double voltage required.It is harmful and useless.Because first, doing this may reduce the qualified rate of the product.Second, although it has passed the test, it may damage a part of the insulation structure and reduce the safety of the product.Third, it may damage some components, so that the quality of the product is reduced, shorted the product life. It is often mentioned that " how much leakage current is considered to withstand pressure breakdown?" In fact, this leakage current is difficult to accurately determine accurately.Because when the high voltage is applied to the product, a stable small leakage current is generated between the insulation.For each product, the leakage current changes.This leakage current will increase rapidly when the insulation is punctured.Therefore, under normal circumstances, as long as the over current relay action current (namely the leakage current) of the voltage test instrument is set to be slightly higher than the leakage current generated by the normal high voltage of the product.Note that the leakage current here is essentially the same as the leakage current test, which is a tiny current generated by the insulation system under the voltage.The difference is that the leakage current in the withstand voltage test is generated at high voltage, while the leakage current in the leak current test is generated at a rated voltage.

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