Self tests Flashcards
The AC clamp-on-ammeter operates on the principle of which common device?
A transformer
Identify the fundamental parts of an AC clamp-on-ammeter?
iron core, transformer, coil, meter movement
What care should be taken with respect to the jaws of the clamp-on-ammeter?
keep it clean for accuracy
Name one disadvantage of the old DC clamp on ammeter, commonly called the tong-tester?
Separate insert required for each range
The current in a conductor is 2.2A. If four turns of this conductor are looped through the core of a clamp-on-ammeter, what current on the scale should the pointer indicate?
8.8a (2.2x4) multiply no. of wraps
What is the outstanding advantage of clamp-on-ammeters compared to their in-line counterparts?
do not need to de-energize/disconnect the load
When the hot and return conductors are pooped through the core of the clip-on ammeter, why does it normally read zero?
Net flux is zero due to the cancellation effect
Most AC clamp-on ammeters have a locking device that allows for locking the pointer. What is the reason for this?
To lock in the reading when the scale is not visible, also protects the movement when it’s not in use
Clamp-on ammeters sometimes also incorporate voltmeters. Briefly explain why this is possible?
Clamp-on ammeters use D’Arsonval movement. This movement is also used to measure voltage
How is a shunt connected in relation to the coil of the galvanometer?
In parallel
A shunt is a device that has a very _________ resistance
high
Why are shunt resistors made from materials having a low temperature coefficient?
To ensure small resistance change with temp. change
Full-scale deflection current for the coil in the D’Arsonval mater movement could be as low as?
5 micro amps
Why is it desirable to have the resistance of an ammeter as low as possible?
So that the meter does not change normal operation
What is the advantage of a multi range ammeter?
Convenience, takes more readings than one with a single meter
If the polarity of the leads on a D’Arsonval meter are reversed, the pointer will move _____?
counterclockwise; bent
Name two types of ohmmeters?
Shunt ohmmeter
Series ohmmeter
In the series ohmmeter, the high resistance values are on the ________ side of the scale?
left
In the shunt ohmmeter, the low resistance values are on the _________ side of the scale?
left
Which type of ohmmeter is more suitable for measuring low resistor values?
Shunt ohmmeter
What is the disadvantage of the shunt ohmmeter?
cell discharges every time it turns on which drains its battery
What type of ohmmeter is used in the multimeter?
series ohmmeter
What is the function of the rheostat in the ohmmeter?
to compensate for low battery
What is the purpose of the fixed resistor in the ohmmeter?
It keeps current at a safe maximum
Are the scales of ohmmeters linear or non-linear?
non-linear
State briefly two general methods that allow an ohmmeter to have several ranges
using more than one voltage, using a shunt-type operation
When using a multi range ohmmeter, is it necessary to zero-adjust the meter for each range scale?
Yes
Is the higher voltage battery used for the higher or lower ranges of resistance measurement
higher ranges
To what type of circuit or device must an ohmmeter never be connected?
An energized circuit
What is the reasoning for not connecting an ohmmeter to an energized circuit or source?
It will damage the meter
Give two fundamental uses of the ohmmeter
Testing continuity
Finding resistance of loads
What is the resistance value below which the ohmmeter is considered inaccurate for measuring?
1ohm
What does an infinity reading on the ohmmeter indicate?
an open
what does a zero reading on the ohmmeter indicate?
A short
When testing for continuity, the primary concern is not the circuit _________ value, but to check that the circuit is _________
resistance
correct
If the ohmmeter leads are separated in air, the pointer will read?
infinity
If the leads of the ohmmeter are touching, the pointer will read?
Zero
A megger is mainly used for testing _________ resistance
high resistance
Why is an ohmmeter unsuitable for measuring insulation resistance?
The output voltage is too low
What is the typical megger output voltages?
250, 500, 1000v DC
Function of the clutch in a hand-cranked megger?
To slip if cranked too fast in order to not exceed rated values
List 3 precautions to be observed before you connect the leads of an ohmmeter to a component.
De-energize the circuit
do not hold test lead ends
make sure it has been zeroed
If, when the leads of an ohmmeter are joined, the pointer is still at infinity, what might be the reason?
broken leads
Why does the polarity of the leads of a multimeter reverse when the meter is used to measure resistance?
meter changes from measuring a load to a supply
If the zero-adjust on an ohmmeter fails to bring the pointer to zero when the leads are joined, what is most likely the cause?
battery is too weak
What precaution should you take when measuring circuits that may have capacitors?
discharge capacitors before testing the circuit
Why is it correct to always isolate at least one of the two ends of a component being tested with an ohmmeter?
The reading may be wrong due to parallel circuits
Why should you avoid touching the bare leads of the ohmmeter when checking a component or circuit?
There is a shock hazard or an error in the reading
In a shunt-type ohmmeter, what should you do to the leads while zeroing the meter?
Separate the leads
What is the rule with regard to zeroing when using a multi range ohmmeter on different range settings?
meter must be zeroed when changing from one scale to the next
State three precautions to be observed when working with the megger
- do not connect to an energized circuit
- do not use on low voltage circuits
- be aware charges can be built up by a megger
The watt-hour meter measures units of?
energy
Energy is the product of what two quantities?
power and time
Where is the energy meter in a house typically located?
on the side of house, face pointed out
Identify 5 major components of the energy meter?
current coils, voltage coil, disk, dials, damping magnet
What factors determine the speed of the rotating disk in a kilowatt-hour meter?
current in the current coils (load current) voltage across voltage coil (load voltage)
What is the purpose of the damping magnets in the energy meter?
to create counter-torque on a disk to allow speed adjustments
How can the speed of the aluminum disk be fine-tuned? (adjusted up or down)
varying position of damping magnet
At what height must the energy meter be located?
1.5-1.8meters
which of the three conductors going to the meter is not broken at the meter?
neutral
why does the power supply company place a seal on the meter
to prevent energy theft
