Outcome 2.3 identify appropriate electrical instruments for the measurement and calculation of different electrical values

1. 
08/2 – Understand standard units of
measurement used in electrical installation,
maintenance and design work.
Outcome 2.3 – Identify appropriate
electrical instruments for the measurement
and calculation of different electrical
values.
Unit 08 Principles of electrical science

2. 
2.1 - Identify and use internationally
recognised (SI) units of measurement
for general variables.
2.2 - Identify and determine values of
basic SI units which apply specifically
to electrical variables.
Last session

3. 
2.3 - Identify appropriate electrical
instruments for the measurement and
calculation of different electrical
values.
Voltmeter, ammeter, wattmeter,
ohmmeter.
This session

4. 
Digital Meters
Digital meters take an input signal and converts it to a series of pulses
Digital signals are either on or off
Advantages of digital meters
• Strong
• No moving parts
• Accurate
• Easy to read
Disadvantages of digital meters
• Batteries need replacing regularly
• Not easy to tell if it is the meter or the circuit that is at fault
• Readings may vary

5. 
Analogue Meters
Analogue signals are variable
which mean they are changing all
the time.
Moving Iron Meter (repulsion type)
A moving-iron meter is based on the principle that when two
magnetic fields oppose each other they deflect or try to
move in opposite directions

6. 
Moving Iron Meters
There are two types of moving iron meters repulsion and attraction which both work in a
similar way
This type of meter is less commonly used now
Advantages
• Cheap
• Fairly robust
• Can be used on a.c and d.c.
Disadvantages
• Uneven scale
• Can be affected by heat
• Can be affected by stray magnetic fields
• Only accurate at the lower end of the scale

7. 
Moving Coil Meter
A coil is wound on an
aluminium frame. It is
supported via a spindle
resting in jewelled
bearings which enable it
to move freely.

8. 
Moving Coil Meter
This meter is much easier to read because the scale is linear,
meaning the accuracy is constant
Advantages
• Even scale
• Accurate
• Unaffected by stray magnetic fields
Disadvantages
• Fragile
• Expensive
• Only measures d.c.

9. 
Voltmeter
• On site for general purposes we use a
multimeter to test for voltage.
• A specific voltage testing device is called a
voltmeter.
• Voltmeter’s measure the difference in
potential between to point on a circuit.
• This test instrument must be connected in
parallel across the load.
• Voltmeters have very high internal resistance
to ensure accurate readings.

10. 
Voltmeters
Voltmeters measure Voltage
Voltmeters have to be connected across the load (In
parallel)
U
Load
U
The voltmeter is connected in parallel with the load.
The same voltage is dropped across the load and the
voltmeter.

11. 
Altering the Range of the Voltmeter
The problem with connecting a voltmeter into a circuit is that the
current is supposed to flow through the load, what happens is
that it splits up and some current passes through the voltmeter.
The voltmeter needs to have a resistance that is very high, so
reducing the current to a negligible amount.
To alter the range of the voltmeter we must connect a resistor in
series with the meter, this is called a multiplier.
Voltmeters

12. 
Ammeter
• On site for general purposes we use
a multimeter to test for current.
• A specific current testing device is
called a ammeter.
• ammeter’s measure the current that
pass through them (current flow).
• This test instrument must be
connected in series with the circuit.
• Ammeter’s have very low internal
resistance to ensure false readings
aren’t given.

13. 
Ammeters
Ammeters measure current
In a circuit it is critically important that the ammeter is placed in the
correct position. (IN SERIES)
To understand why think back to resistors.
In a series circuit the current is constant throughout the circuit.
The ammeter is connected in series with the load and we will assume
that the instrument has a resistance and so a volt drop when the
current flows.

14. 
Increasing the Range of the Meter
If we want to increase the range of the instrument.
Which is sometimes necessary as the current can destroy the
sensitive coil in the meter. We have to place a resistor in
parallel with the meter. This resistor has a very low value and is
called a shunt.
We have in effect two resistors in parallel connected to a resistor
(the load) that is in series with them
Ammeters

15. 
Wattmeter
• On site for general purposes we use
a multimeter to test for power.
• A specific power testing device is
called a wattmeter.
• wattmeter’s measure the current
and voltage that pass through them
and display (apparent) power in
watts.

16. 
Wattmeter
Current coil
(SERIES)
Potential coil
(PARALLEL)
AMMETER
(SERIES)
VOLTMETER
(PARALLEL)
WATTMETER
In DC circuits you can simply measure voltage and
current and using P = I x V and calculate power.
In AC circuits using this method you only calculate
APPARENT POWER. This value will be incorrect unless we
have unity power factor (1).
This is due to factors within a system such as
inductance and capacitance, these factors make he
current lead or lag the voltage. We use P = I x V x cos ø
In circuits with any of these two factors we must use a
wattmeter to measure power.

17. 
Wattmeter
Three meter method
Current coil
Potential coil
Meter two
Meter three
Meter one
Neutral

18. 
Meter one
Meter two
Wattmeter
Two meter method

19. 
Ohmmeter
• Ohmmeter’s measure impedance
(Z) also known as resistance.
• On site for general purposes we use
a multimeter to test for impedance.
• A specific impedance testing device
is called a ohmmeter.
• Impedance is defined as the total
opposition to the flow of alternating
current at a specific frequency.
• Quite often these types of meters
are referred to as LRC metres as the
quantify readings from resistance,
inductance and capacitance.

20. 
Ohmmeter
• You must connect the tester
at each end of the resistive
load or circuit.
• Test leads must be zeroed out
before the test or tested
initially and deducted from
the result total.

21. 
2.3 Identify appropriate electrical
instruments for the measurement and
calculation of different electrical
values.
Voltmeter, ammeter, wattmeter,
ohmmeter.
Consolidation

22. 
Outcome 2 formative assessment
Next session