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Thursday, 4 September 2014

When is a PAT tester not a PAT tester?

On our various Portable Appliance Testing courses I sometimes get asked whether one could use a £20 multimeter for PAT testing. I usually go through the reasons why this is not safe. Browsing on the web, I have come across what looks like a multimeter with a mains socket on the front that describes itself as a 'simplified version of a standard PAT tester'.



This is a very dangerous trend as it will lead customers to believe that they are actually carrying out a safety check that is meaningful. This blog sets out the reasons why using this instrument could PASS appliances that are dangerous.

A Portable Appliance Tester does 2 basic tests. One is the Earth Continuity Test and the other is the Insulation Resistance test.


EARTH CONTINUITY

This test is to check that the Earth Bonding is sound and can protect the user in case of a fault. The specification is 0.1 ohm. What this means in practice is that if a fault current of 10 amps should flow, the voltage on the user touchable metal parts of appliances is less than 1V. Even when the specification is relaxed for appliances with long leads, it rarely goes above 0.5 ohm which still only represents a voltage of 5V under fault conditions.

The equipment being marketed as a 'perfect compact instrument for regularly checking the safety of electrical appliances' states that it passes appliances when the resistance is less than 10 ohms. Faulty appliances with Earth Continuity of 8 ohms will be passed by this tester. However under fault current conditions the user may be exposed to voltages upto 80V which is dangerous and can be lethal in certain circumstances.

INSULATION RESISTANCE

This test is to check the quality of the insulation protecting the user. On Class I appliances this is specified as being more than 1 Mohm and on Class II appliances this is specified as being more than 2 Mohm. More importantly the voltage used for testing should be 500V in order to suitably bring out any faults. For surge protected appliances this can be relaxed to 250V. There are some testers that use a fixed 350V for testing, but increase the PASS limit (to typically 2 Mohm and 4Mohm) to compensate for this.

Most multimeters, while perfectly capable of testing resistance, do so with a low voltage. The specification of one instrument claiming to be a PAT tester, states that it uses a voltage of 9V which is way too low to properly stress the insulation and highlight any faults. It also has a PASS limit of 1Mohm, which while being acceptable for Class I appliances is not high enough for Class II appliances. 

Multimeter versus Portable Appliance Tester

The notes above highlight why a multimeter cannot be used to carry out Portable Appliance Testing. The danger of marketing an instrument with multimeter type measurement capability as 'capable of checking the safety of electrical appliances' is that many customers will be driven by price and will not understand that the tests that they are carrying out will PASS appliances that have faults that are possibly lethal to the user. 

In summary:






 


2 comments:

  1. This would have been a much better article if it covered the various multimeters that are capable of conducting the PA Tests.

    This article gives the impression that no multimeters is capable of conducting 500Vdc insulation tests: many can selectively go from 100 to 1000V.

    In theory, if someone actually knows what they are doing, they could use a decent multimeter to conduct a PA Test, they just wouldn't have the convenience of plugging in the appliance, hitting the test button and being told pass or fail.

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  2. Not come across a Multi Meter where you can set the voltage at which resistance is measured. Most Digital Multi Meters (Not to be confused with a Megger) test resistance using a very low (<1 V) voltage.

    To measure the Earth Continuity resistance, one needs a meter capable of reading down to milli ohms. One is lucky of a multi-meter can read down accurately down to 1 ohm.

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