This is a question we often get asked during the PAT testing courses. On any PAT tester, just press the Class II button. The tester will carry out the Insulation Resistance test and display a PASS - even with nothing plugged into the tester.
Why does this happen? Is the tester faulty? The answer is NO.
To understand this, one needs to understand what the Insulation Resistance test is measuring. It is measuring the insulation between the LIVE/NEUTRAL wires and the Earth (Class I) or user touchable metal parts (Class II) on an appliance.The bigger the insulation measured, the safer the appliance is.
On a Class I appliance we are looking for more than 1 Mega Ohm (ie 1,000,000 ohms) and on a Class II we are looking for more than 2 Mega Ohm (ie 2,000,000 ohm). Now, if we do this test with nothing plugged in, then we are measuring the insulation resistance of air, which is very high - certainly higher than tens of millions of ohms.
This is why, when we do not have an appliance plugged in and do the insulation resistance test, we always get a PASS.
Note - the greater than symbol. Many PAT testers use this symbol, when they measure a resistance that is very high. The symbol for greater than is >. So for example, the PAT tester may show >20 Mohm, meaning that the resitance it is measuring is greater than 20 million ohms.
Why does this happen? Is the tester faulty? The answer is NO.
To understand this, one needs to understand what the Insulation Resistance test is measuring. It is measuring the insulation between the LIVE/NEUTRAL wires and the Earth (Class I) or user touchable metal parts (Class II) on an appliance.The bigger the insulation measured, the safer the appliance is.
On a Class I appliance we are looking for more than 1 Mega Ohm (ie 1,000,000 ohms) and on a Class II we are looking for more than 2 Mega Ohm (ie 2,000,000 ohm). Now, if we do this test with nothing plugged in, then we are measuring the insulation resistance of air, which is very high - certainly higher than tens of millions of ohms.
This is why, when we do not have an appliance plugged in and do the insulation resistance test, we always get a PASS.
Note - the greater than symbol. Many PAT testers use this symbol, when they measure a resistance that is very high. The symbol for greater than is >. So for example, the PAT tester may show >20 Mohm, meaning that the resitance it is measuring is greater than 20 million ohms.
Hi, informative post. Thanks. How do I subscribe to the post alert as mentioned at head of blog?
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