Under normal conditions, an electrical device produces a minimal amount of leakage current due to voltage and internal capacitance, but the insulation can break down, resulting in excessive leakage current that can cause shock or in extreme cases, electrocution.
Insulation resistance testing is carried out by applying a high DC voltage, causing a small current to flow through the insulator surface. Insulation resistance is measured in megohms, so an insulation resistance tester is also known as a megohmmeter or megger.
Also known as dielectric withstand testing or ‘high potential’ or ‘hipot’ testing, the test is a means to qualify a device’s ability to operate safely during rated electrical conditions. A single instrument called a ‘hipot tester’ is often used to perform this test.
If current is less than a specified limit at the required test potential and time duration, the device meets the dielectric withstand requirement. A dielectric withstand test may be done as a factory test on new equipment, or may be done on apparatus in service as a routine maintenance test.
Typical hipot equipment leakage current trip limit settings range between 0.5-20mA and are set by the user. The objective is to choose a current setting that will not cause the tester to falsely trip during voltage application, while at the same time, selecting a value that minimizes possible damage to the device under test should an inadvertent discharge or breakdown occur.
In addition to over-stressing the insulation, the test can also be performed to detect material and workmanship defects, most importantly small gap spacings between current-carrying conductors and earth ground.
When a product is operated under normal conditions, environmental factors such as humidity, dirt, vibration, shock and contaminants can close these small gaps and allow current to flow.
Three types of hipot tests are commonly used, which differ in the amount of voltage applied and the amount or nature of acceptable current flow: dielectric breakdown test, dielectric withstand test, and insulation resistance test.
Turning to specific brands of insulation resistance testers, the Megger MIT515 5kV Insulation Resistance Tester is the entry-level product in Megger’s range that can measure resistance levels up to a range of 10 teraohms (TO).
Built into its own protective carrying case, the MIT515 includes five preset voltage ranges and displays resistance, voltage, leakage current, capacitance, battery status and time constant. It also includes several preconfigured diagnostic testing procedures including polarisation index (PI) and dielectric absorption ratio (DAR) testing.
The Hioki 3455 High Voltage Insulation Resistance Tester is suitable for measuring insulation resistance on high voltage equipment such as transformers, cables and motors. It has five selectable DC voltage ranges of 250V, 500V, 1kV, 2.5kV and 5kV, with the ability to measure insulation resistance to five teraohms.
The Hioki 3455 can also automatically calculate and display polarisation index and dielectric absorption ratio, and features the ability to carry out step voltage testing that by gradually increasing applied voltage to measure effectiveness of insulation in steps of 25V up to 1kV and 100V steps above 1kV.
The 3455 has the ability to store up to 100 data records along with date and time information. The user can then connect the instrument to a PC by USB cable and use Hioki application software to generate reports and carry out other analyses.
The Metrel MA2060 ISOTEST Insulation Tester can be used to test insulation of electrical appliances and instruments, High Voltage cables and transformers, electrical traction systems, motors and other equipment.
This device also boasts an extremely high measurement range and can be used to test insulation resistance up to 500 teraohms, as well as voltage to 600V AC or DC. It also features multiple test voltages: 250V, 500V, 1000V, 2500V and 5000V. The ISOTEST is battery powered and lasts for around 2500 tests per set of batteries.
Martindale Metrohm E3511 5kV Analogue Insulation Tester measures resistance to 100 gigaohms and comes with four selectable test voltages of 500V, 1kV, 2.5kV and 5kV and it can also provide AC and DC voltage measurement to 1000V. Testing results are displayed on a built-in analogue display.
Also included as part of this insulation resistance tester is full over voltage and over current protection for all measurements. It is also powered entirely by rechargeable batteries, allowing the tester to keep working for 16 hours without charging.
All electrical insulation is susceptible to degradation over a time period. At the start of its service life the insulation applied to an electrical system – whether its a common appliance such as a kettle or a high powered device like a transformer – should be strong enough to make leakage current non existent or negligible and ensure that the electrical installation/appliance can be used safely. High voltage insulation testers verify the insulation of an electrical installation to see if action must be taken to prevent dangerous electrical incidents.
For further information, please visit PASS Ltd’s specialist High Voltage Insulation Testers web page.