MEASURING INSTRUMENTS

Meggar:  Meggar is a portable insulation tester used for measuring resistance of any insulation in the order of mega-ohm in which an ohmmeter and a d.c. generator are incorporated in single unit.

1. Minimum 10000 ohms resistance can be measured by meggar insulation tester.

2.It can not be used in live line.

3. Since a meggar is driven through gearing and a centrifugally controlled slipping clutch then the generator speed and output voltage remains more or less constant even when the handle speed is variable, therefore there will be no appreciable variation of reading in slow speed and average speed rotation of the meggar.

4. In an ammeter or a voltmeter when the pointer moves the controlling or restoring torque is produced by two spiral hair springs wound in the opposite direction. One end of each spring is attached to the spindle of the pointer while other end is attached to some fixed point. When the instrument is not in use, the two springs are in their natural position without any tension or compression with the zero position of the pointer. But in case of a meggar the deflecting torque and the restraining torque of the moving element is directly proportional to the supply voltage. As the restraining or controlling torque is proportional to the supply voltage obtained from the generator the use of control springs is eliminated. So the pointer floats over the scale and does not point to zero except when the instrument is in operation.

5. An extra terminal known as ‘Guard’ terminal in addition to the usual ‘Line’ and ‘Earth’ terminals is provided in the high voltage meggar to guard the instrument against the effect of surface leakage surrounding the line terminal within the tester itself or the connection of line terminal on the testing object. The guard terminal connected to the negative terminal of the generator. If there is any leakage current, it will be trapped by the guard ring and returned directly to the negative terminal of the generator without passing through the current (deflecting) coil of the instrument and thus eliminating erroneous indication.

6. Maximum 50 ohms resistance can be measured by an earth testing meggar.

Ohmmeter: An ohmmeter is a moderately accurate direct reading instrument used for the measurement of the resistance of a circuit connected across their terminals. It consists of a moving coil milli-ammeter, a dry cell, a fixed resistance and a variable resistance, all mounted within the instrument.

Types of Ohmmeters used in practice:

(i) Measurement of resistance

(ii) Point to point continuity test of circuits.

(iii) End to end continuity test of a long cable.

DC voltage is applied with insulation testing meggar because with a.c. voltage the capacitance effect will give wrong results.

With DC voltage the contact resistance between the buried electrode and the soil will be slightly higher than with AC due to the effect of electrolysis set up by the passage of current causing a film of gas to be liberated round the electrode and also to set up a back e.m.f. To avoid such effect to back e.m.f. due to electrolysis effect AC voltage is applied with meggar earth tester.

Frequency meter: A frequency meter is a direct reading instrument which indicate the supply frequency of a circuit directly. It has two terminals. The frequency meter is connected across the supply line just like a voltmeter.

Classification of frequency meters: Three main classes of frequency meters -

(i)         The vibrating read type instruments depending for their operation upon the phenomenon of mechanical resonance.

(ii)        The deflectional electrodynamics type instruments employing electrical resonance.

(ii)        The deflectional moving iron type instruments whose action depends upon the variation of impedance of an inductive circuit with frequency.

Moving iron type frequency meter is commonly used.

Power factor meter: (i) Dynamo-meter type which may be either of electro dynamic or of ferro dynamic form and (ii) Moving iron type. Both types are available either for single phase or for three phase load.

There are two sets of coils, namely

(i)         Fixed coils also known as current coils and

(ii)        Moving coils also known as pressure coils.

There are (i) two fixed coils connected in series with the load and carry the current in the circuit under test and (ii) two moving coils fixed rigidly at an angle at 90⁰ apart are connected across the supply lines through a resistance and an inductance respectively.

(a) For balanced load there are (i) two moving coils fixed with their planes 120⁰ apart and connected through resistance across two different phases of the supply circuit, (ii) two fixed coils connected in series with the third phase carrying the current in the line.

(b) For unbalanced load there are (i) three fixed coils being displaced 120⁰ apart from each other and connected in series in the three lines (ii) three star connected moving coils being displaced 120⁰ apart from each other and connected through non inductive resistances to the three phases.

(i) In single phase power factor meter  there are 4(four) terminals, (ii) in three phase three wire power factor meter there are 8(eight) termi9nals for balanced load and 9(nine) terminals for unbalanced load. (iii) In three phase four wire power factor meter there are 9(nine) or 10(ten) terminals.

Potentiometer: Potentiometer is an instrument used for accurate measurement of a low range unknown e.m.f. or potential difference up to 1.5 volt by balancing it against a known potential difference produced by a known current passing through a resistance.

Clip - on meter: It is also known as tong tester is a portable hand operated C.T. based ammeter which has a laminated core arranged in such a way that the core can be opened out at a hinged section by pressing a trigger-like projection to encircle the current carrying conductor. During operation the current carrying conductor acts as a single turn primary whereas the secondary is connected across a standard ammeter conveniently mounted on the handle itself. It is used to measure the line current flowing through a core of a cable without disconnecting the cable leads. There is no electrical connection between cable line and measuring instrument.

Phase sequence indicator: Phase sequence indicator is nothing but a small poly phase motor having a stator consisting of three coils and a metal disc rotor having an arrow marked on it which determines the order of phase rotation in a three phase system. The rotation in the direction of arrow indicates the correct phase sequence i.e. R -> y -> B. The phase sequence can be corrected by interchanging any o the two leads.

 Animated flasher: When a flasher is used for sequential operation to give appearance of movement in luminous sign by using quick succession of gradually varying drawings then it is said to be animated flasher.

High voltage animation flasher: For more complicated control if it is required for spelling and animation effects in one sub-circuit flasher is usually connected on the HT side since this enables parts of a sign fed from one transformer to be operated at different times. Such flasher connected on the HT side is called as the high voltage animation flasher.

Low voltage animation flasher: Where the whole sign is required to flash on and off at predetermined intervals the flasher unit is usually connected on the LT mains side. Several small transformers up to a total capacity of 1000 volt-amps may be operated by one switch. When a big animated luminous sign requires power of more than 1000 volt-amps then it is divided into several sections each having a separate transformer of capacity not more than 250 volt-amps and lights up fully in each sequential operation. In that case flashers area introduced in the LT mains of transformers of different sections. Such flashers connected on the LT side are called as low voltage animation flashers.

Photo electric control: It can be used to make the sign automatically switched on when the level of illumination falls below a predetermined value and switched off again when the illumination is restored.