Tutorial: Mechanic - electrician Topic: Basics of electrical engineering the 2nd. year Measurement of resistance Prepared by: Ing. Jiří Smílek Projekt Anglicky v odborných předmětech, CZ.1.07/1.3.09/ je spolufinancován Evropským sociálním fondem a státním rozpočtem České republiky.

 Definition of resistance: Electrical resistance is a fundamental property of all passive and active elements : appliances circuits Insulation – electrical equipment The basic unit- 1 Ω (ohm ) 1 Ω is the resistance of a conductor which passes through a current of 1 A at the ends of the conductor is 1 V Range of measured resistances in practice small - from to 1 Ω middle - from 1 to 10 6 Ω large - from 10 6 to Ω

 Used method: Direct method - resistance is measured by Ohmmeters showing the value measured of resistance deviation or a figure. measurement is quick and easy deflection apparatus corresponds to the size of an unknown resistance - reverse calibrated voltmeter value R x →∞ corresponds to the mechanical zero on the right end of the scale electrical zero - the left end of the scale deflection of the hands depends on the size of voltage sources - must be zero balance - adjusted by the device

 Used method: The direct method - measuring the evaluative ohmmeter. is based on the magneto electrical ratio system Direct method - measuring bridge method - Wheatstone bridge in a balanced state there does not flow a measuring diagonal current, then

 Used method Comparative method- R X ≐ R V Principle - Distribution of voltage across the resistor in series we know the internal resistance of voltmeter R V Procedure first we measure the voltage source U 1 – R X short-circuit switch then the voltage U 2 – in series with V meter is connected a resistor R X voltage is divided in proportion of resistances R V and R X The method is suitable for resistors approximately the same size as the resistance of V-meter

 Used method Comparative method - for small resistances R X ≪ R V Principle - there is a voltage division across the resistor in series in proportion to their resistances R N - normal resistance (specific resistance of known size) Procedure we measure the voltage source U X we measure the voltage source U N if we measure on the same scale then the following applies :

 Used method: Comparative method - for large resistors R X ≫R A Principle - distribution of currents in parallel resistors in inverse proportion to their resistances Procedure we measure the current resistance R X we measure the current resistance R N if we measure on the same A- meter then the following applies:

 Used method: Indirect method - Ohm's method using Ohm's law, resistance is found by measuring current and voltage drop over the resistor The resistance is calculated from Ohm's law according to the formula a)circuit for measuring small resistances b) circuit for measuring large resistances

 Used method: Indirect method - Wenner method - measuring the voltage drop Used in practise: great importance to work safety – the size of earth resistance depends on the emergence of a potential dangerous contact voltage with the structure of appliances earth resistance is the resistance between the earthing and the ground ground resistance is an earth resistance earthing including resistance of ground leads - entire ground distribution Principle methods the voltage drop occurs only in the vicinity of earth electrodes in the space between the ground electrode is a large cross-section - there is no voltage drop The resistance of earth electrodes is calculated:

 Describe what it is electrical resistance ◦  Write what methods can measure el.resistance ◦ direct –  ◦ indirect –   Describe what is different in the comparison methods for measuring of large and small resistances ◦ Large resistances - ◦ Small resistances -

 Write the measurement procedure for resistance in a V- meter with comparative method for RV ≐ RX ◦  Describe what is the principle of Ohm’s method for measurement of resistance ◦  Explain why when there is a greater distance from the earthing there is no voltage drop, even when going through the earth is an el. current

 Mužík, J. Management ve vzdělávání dospělých. Praha: EUROLEX BOHEMIA, ISBN  Operační program Vzdělávání pro konkurenceschopnost, ESF 2007 –  Dostupné na: konkurenceschopnosthttp:// konkurenceschopnost  Učebnice Elektrická měření – Ing. Pavel Vylegala, SŠE Ostrava, 2006  Učebnice Elektrické měření – SNTL, 1981  Elektrotechnika - Laboratorní cvičení pro bakalářské studium - Doc. Ing. Miloš Hammer, CSc.