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The abbreviation PAT stands for portable appliance testing (Association for Electrical, Electronic and Information Technologies). This association has set the task of uniting research, the definition of standards, testing and certification as well as application consulting for the above-mentioned technical areas under one roof. Currently, there are about 3,500 PAT norms and standards. Many of these regulations pursue the goal of ensuring that persons and property are not endangered by the electro technical devices or systems. In order to avoid malfunctions, injuries and fires, the specified safety requirements must be observed and controlled. In order to be able to carry out the specified tests and measurements, suitable PAT Testers are required as measuring instruments.
The basic PAT standards for safety testing of electrical systems, devices and machines include:
A separate PAT standard regulates the requirements for the devices that are to be used PAT test equipment for testing the protective measures of electrical equipment. 
This is part 16 of PAT 0413 (also DIN EN 61577-16 or IEC 61557).
PAT 0413-16 specifies the requirements for the measuring and test equipment used to check the protective measures of the electrical equipment according to IEC 62638 or medical electrical equipment according to IEC 62353. The specifications of the standard are intended to ensure that the measurement results are comparable, that the user of the test equipment is not endangered and that the equipment to be tested is subjected to only minimal stress during the test.
For the initial and repeat tests of electrical installations according to DIN PAT 0100-600 and DIN PAT 0105-100, measuring and testing devices are required for various tests and measurements. A device that is suitable for the prescribed tests or measurements is called a PAT test device. The designation PAT testing device or PAT Tester can be used just as much for a rotary field tester, which offers no further control functions, as for an installation tester, with which the complete scope of tests and measurements can be covered.
The tests on electrical installations include:
In order to ensure the safety of the electrical installations, the following values should also be measured with the suitable PAT testing device: - the current intensity for overcurrent protection devices
The results of the tests and measurements are recorded in a test and measurement report for the electrical system. Some high-quality PAT Testers offer direct support in the test procedure and in the creation of the protocols by means of special menu functions and evaluation software. The measurement data can be transferred directly to the protocols created with the evaluation software and supplemented with additional information. As a rule, such a PAT Tester is equipped metrologically in such a way that a large part of the checks on single-phase or three-phase connected electrical systems can be carried out and documented with it.
Not only electrical installations, but also the devices connected to them should meet high safety standards and function safely so that neither persons nor material assets are damaged. PAT standards 0701 and 0702 specify the procedure and the individual tests for the control of the electrical devices.
These include:
The units that do not have the required characteristics may not be used any longer. They must be singled out and repaired or disposed of. The following measurements and checks are carried out with the PAT Tester:
Insulation resistance
The insulation resistance measurements are part of the protective measures against electric shock. The usual measuring voltage is 500 volts DC. For circuits with low voltage, it may be reduced to 250 volts or 100 volts. The PAT test device used must allow the test voltage to be changed. The safety limit values 0.25 MΩ, 1 MΩ and 2 MΩ specified in PAT 0701 and PAT 0702 mean that no dangerous flow is possible at this condition. However, reliable insulation must be at values in the ranges 5 MΩ, 10 MΩ, 20 MΩ and higher. Insulation resistances below 5 MΩ indicate insulation defects whose causes must be clarified. The following is measured in each case:
- the resistance of the basic insulation between touchable, conductive parts and the active current-carrying cores
- the resistance of the basic insulation between components with safety extra-low voltage and conductive touchable elements
- the resistance of the insulation between the protective conductor and active current-carrying cores
- the resistance of the protective insulation between elements with safety extra-low voltage and parts with mains voltage
The safety pre-test
In the case of the PAT Tester with a function for safety pre-testing, the outer, neutral and protective conductors are connected to the tester and checked. If the PAT Tester then issues an optical or acoustic warning, no test voltage may be applied because there is a serious insulation fault.
The protective conductor current / substitute leakage current
The protective conductor current may only be measured after the insulation resistance has been successfully checked. After switching on the load to be tested, the current intensity at its protective conductor is measured. 
The value must not exceed the specified values. Alternatively, the substitute leakage current can be measured at the switched-off consumer. In this case, the PAT Tester applies a test current to the outer and neutral conductors and measures whether this causes the current flow in the protective conductor.
The touch current
For simple electrical appliances without a protective conductor, the PAT Tester measures whether the touchable metal parts of the appliance carry current.
The protective conductor resistance
Among the appliances operated with the mains voltage, the protective conductor is one of the most important safety devices. All electrically conductive components of the appliance that can be touched must be connected to the protective conductor, which should reliably discharge the current. When measuring the protective conductor resistance, the PAT Tester applies a test current to the protective conductor. The test is intended to prove that the protective conductor is not interrupted and has such a low resistance that any fault currents that occur are discharged safely.
Specially equipped PAT Testers are often used for the regular complete testing of electrical equipment in companies. These not only simplify the tests by means of the stored test procedures, they also facilitate the inventory and documentation of the tested operating resources by means of the stored databases and stitch coding systems.