Mains Electricity Flashcards
What is the UK mains supply?
230V
What is direct current?
When the current travels in only one direction only
If you plug a DC supply into an oscilloscope you get a straight line.
Examples are batteries and cells
What is alternating current?
An alternating current repeatedly reverses its direction. It flows in one way, then the opposite way in successive cycles.
What is the frequency of mains electricity?
50 Hz
What is the time base dial?
The time base dial controls how many milliseconds each division represents on the horizontal axis
1 ms=0.01
What is the gain dial?( oscilloscope)
The gain dial controls how many volts each cm division represents on the vertical axis.
How do you work out frequency?
F= 1/ T(time period)
How do you work out the time period?
Time base setting * horizontal distance
What if the time period?
The time it takes to complete one cycle
What is the brown live wire?
The brown live wire in a mains supply alternates between high +ve and -ve voltage. The live wire alternates between +325v and -325v.
What is the job of the blue neutral wire?
This is always at 0 V. Electricity normally flows in and out neutral wires only.
What does the green and yellow earth wire do?
The green and yellow earth wire is for protecting the wiring and for safety- it works together with the fuse to prevent shocks and fires. It is attached to the metal casing of the appliances and carries the electricity to earth( away from you), should something go wrong and the live and neutral wires touch the metal case.
Why is copper used for wires?
It is good electrical conductor and bends easily(metal).
Why is the cable grip and insulator made of rubber?
Good insulator, therefore it prevents anyone touching a cable to receive a shock.
Why should your cable be a relevant thickness?
Thicker cables have less resistance, so carry more current, having a relevant thickness cable stops the heating effect.
What are 2 core cables?
Cables that only carry live and neutral wires. They are used for appliances which have a plastic case.
What doubly insulated mean?
Appliance which have a plastic case with no metal parts showing The wires are insulated. These do not need an earth wire connection
How do fuses and earth wires work together to protect you you?
If a fault develops in which the live wire touches the metal case, then because the case is earthed, too great a current flows in through the live wire,through the case and down the earth wire
This surge in the current melts the fuse when the amount of current is greater than the fuse rating. This cuts off the live supply and breaks the circuit
This isolates the whole applicable making it hard to get an electrical shock. It also prevents fires.
What are circuit breakers?
Circuit breakers are an automatic device for stopping the flow of current in an electric circuit as a safety measure.
How do circuit beakers work?
When the circuit breakers detect a surge in current in a circuit, they break by opening of a switch( electromagnet is in series with the live wire. If the current in the live wire is too large, the magnetic field of an electromagnet pulls the switch contacts apart).
What are the advantages and disadvantages of circuit breakers?
- They can be reset, so they are more convenient than fuses which need to be replaced
- They are more expensive to buy than fused
- They can’t be used in every circuit
What is the RCCB?
Residual current circuit breaker
How does an RCCB work?
Detects a difference in current between live and neutral wires.
What are the advantages and disadvantages of the RCCB?
RCCB or mor sensitive than CB and can detect small current changes( which might not be detected by a fuse).
These small changes can be fatal.
Disadvantages is the same as CB but it’s more expensive.
