HIPOT Testing for isolation

What is HIPOT Testing (Dielectric Strength Test):

• Hipot Test is short name of high potential (high voltage) Teat and It also known as Dielectric Withstand Test. A hipot test checks for “good isolation.” Hipot test makes surety of no current will flow from one point to another point. Hipot test is the opposite of a continuity test.
• Continuity Test checks surety of current flows easily from one point to another point while Hipot Test checks surety of current would not flow from one point to another point (and turn up the voltage really high just to make sure no current will flow).

Importance of HIPOT Testing:

• The hipot test is a nondestructive test that determines the adequacy of electrical insulation for the normally occurring over voltage transient. This is a high-voltage test that is applied to all devices for a specific time in order to ensure that the insulation is not marginal.
• Hipot tests are helpful in finding nicked or crushed insulation, stray wire strands or braided shielding, conductive or corrosive contaminants around the conductors, terminal spacing problems, and tolerance errors in cables. Inadequate creepage and clearance distances introduced during the manufacturing process.
• HIPOT test is applied after tests such as fault condition, humidity, and vibration to determine whether any degradation has taken place.
• The production-line hipot test, however, is a test of the manufacturing process to determine whether the construction of a production unit is about the same as the construction of the unit that was subjected to type testing. Some of the process failures that can be detected by a production-line hipot test include, for example, a transformer wound in such a way that creepage and clearance have been reduced. Such a failure could result from a new operator in the winding department. Other examples include identifying a pinhole defect in insulation or finding an enlarged solder footprint.
• As per IEC 60950, The Basic test Voltage for  Hipot test is the 2X (Operating Voltage) + 1000 V
• The reason for using 1000 V as part of the basic formula is that the insulation in any product can be subjected to normal day-to-day transient over voltages. Experiments and research have shown that these over voltages can be as high as 1000 V.

Test method for HIPOT Test:

• Hipot testers usually connect one side of the supply to safety ground (Earth ground). The other side of the supply is connected to the conductor being tested. With the supply connected like this there are two places a given conductor can be connected: high voltage or ground.
• When you have more than two contacts to be hipot tested you connect one contact to high voltage and connect all other contacts to ground. Testing a contact in this fashion makes sure it is isolated from all other contacts.
• If the insulation between the two is adequate, then the application of a large voltage difference between the two conductors separated by the insulator would result in the flow of a very small current. Although this small current is acceptable, no breakdown of either the air insulation or the solid insulation should take place.
• Therefore, the current of interest is the current that is the result of a partial discharge or breakdown, rather than the current due to capacitive coupling.

Time Duration for HIPOT Test:

• The test duration must be in accordance with the safety standard being used.
• The test time for most standards, including products covered under IEC 60950, is 1 minute.
• A typical rule of thumb is 110 to 120% of 2U + 1000 V for 1–2 seconds.

 Current Setting for HIPOT Test:

• Most modern hipot testers allow the user to set the current limit. However, if the actual leakage current of the product is known, then the hipot test current can be predicted.
• The best way to identify the trip level is to test some product samples and establish an average hipot current. Once this has been achieved, then the leakage current trip level should be set to a slightly higher value than the average figure.
• Another method of establishing the current trip level would be to use the following mathematical formula:  E(Hipot) / E(Leakage) = I(Hipot) / 2XI(Leakage)
• The hipot tester current trip level should be set high enough to avoid nuisance failure related to leakage current and, at the same time, low enough not to overlook a true breakdown in insulation.

Test Voltage for HIPOT Test:

• The majority of safety standards allow the use of either ac or dc voltage for a hipot test.
• When using ac test voltage, the insulation in question is being stressed most when the voltage is at its peak, i.e., either at the positive or negative peak of the sine wave.
• Therefore, if we use dc test voltage, we ensure that the dc test voltage is under root 2 (or 1.414) times the ac test voltage, so the value of the dc voltage is equal to the ac voltage peaks.
•  For example, for a 1500-V-ac voltage, the equivalent dc voltage to produce the same amount of stress on the insulation would be 1500 x 1.414 or 2121 V dc.

Advantage / Disadvantage of use DC Voltage for Hipot Test:

• One of the advantages of using a dc test voltage is that the leakage current trip can be set to a much lower value than that of an ac test voltage. This would allow a manufacturer to filter those products that have marginal insulation, which would have been passed by an ac tester.
• when using a dc hipot tester, the capacitors in the circuit could be highly charged and, therefore, a safe-discharge device or setup is needed. However, it is a good practice to always ensure that a product is discharged, regardless of the test voltage or its nature, before it is handled.
• It applies the voltage gradually. By monitoring the current flow as voltages increase, an operator can detect a potential insulation breakdown before it occurs. A minor disadvantage of the dc hipot tester is that because dc test voltages are more difficult to generate, the cost of a dc tester may be slightly higher than that of an ac tester.
• The main advantage of the dc test is DC Voltage does not produce harmful discharge as readily occur in AC.
  It can be applied at higher levels without risk or injuring good insulation. This higher potential can literally “sweep-out” far more local defects.
• The simple series circuit path of a local defect is more easily carbonized or reduced in resistance by the dc leakage current than by ac, and the lower the fault path resistance becomes, the more the leakage current increased, thus producing a “snow balling” effect which leads to the small visible dielectric puncture usually observed. Since the dc is free of capacitive division, it is more effective in picking out mechanical damage as well as inclusions or areas in the dielectric which have lower resistance.

Advantage / Disadvantage of use AC Voltage for Hipot Test:

• One of the advantages of an ac hipot test is that it can check both voltage polarities, whereas a dc test charges the insulation in only one polarity. This may become a concern for products that actually use ac voltage for their normal operation. The test setup and procedures are identical for both ac and dc hipot tests.
• A minor disadvantage of the ac hipot tester is that if the circuit under test has large values of Y capacitors, then, depending on the current trip setting of the hipot tester, the ac tester could indicate a failure. Most safety standards allow the user to disconnect the Y capacitors prior to testing or, alternatively, to use a dc hipot tester. The dc hipot tester would not indicate the failure of a unit even with high Y capacitors because the Y capacitors see the voltage but don’t pass any current.

 Step for HIPOT Testing:

• Only electrically qualified workers may perform this testing.
• Open circuit breakers or switches to isolate the circuit or Cable that will be hi-pot tested.
• Confirm that all equipment or Cable that is not to be tested is isolated from the circuit under test.
• The limited approach boundary for this hi-pot procedure at 1000 volts is 5 ft. (1.53m) so place barriers around the terminations of cables and equipment under test to prevent unqualified persons from crossing this boundary.
• Connect the ground lead of the HIPOT Tester to a suitable building ground or grounding electrode conductor. Attach the high voltage lead to one of the isolated circuit phase conductors.
• Switch on the HIPOT Tester. Set the meter to 1000 Volts or pre decide DC Voltage. Push the “Test” button on the meter and after one minute observe the resistance reading.  Record the reading for reference.
• At the end of the one minute test, switch the HIPOT Tester from the high potential test mode to the voltage measuring mode to confirm that the circuit phase conductor and voltage of HIPOT Tester are now reading zero volts.
• Repeat this test procedure for all circuit phase conductors testing each phase to ground and each phase to each phase.
• When testing is completed disconnect the HIPOT Tester from the circuits under test and confirm that the circuits are clear to be re-connected and re-energized.
• To PASS the unit or Cable under Test must be exposed to a minimum Stress of pre decide Voltage for 1 minute without any Indication of Breakdown. For Equipments with total area less than 0.1 m2, the insulation resistance shall not be less than 400 MΩ. For Equipment with total area larger than 0.1 m2 the measured insulation resistance times the area of the module shall not be less than 40 MΩ⋅m2.

Safety precautions during HIPOT Test:

• During a HIPOT Test, There may be at some risk so to minimize risk of injury from electrical shock make sure HIPOT equipment follows these guidelines:

1. The total charge you can receive in a shock should not exceed 45 uC.
2. The total hipot energy should not exceed 350 mJ.
3. The total current should not exceed 5 mA peak (3.5 mA rms)
4. The fault current should not stay on longer than 10 mS.
5. If the tester doesn’t meet these requirements then make sure it has a safety interlock system that guarantees you cannot contact the cable while it is being hipot tested.

• For Cable:

1. Verify the correct operation of the safety circuits in the equipment every time you calibrate it.
2. Don’t touch the cable during hipot testing.
3. Allow the hipot testing to complete before removing the cable.
4. Wear insulating gloves.
5. Don’t allow children to use the equipment.
6. If you have any electronic implants then don’t use the equipment.