Transcript Unit 51: Electrical Technology - News

Unit 51: Electrical Technology
The Properties of Insulators
Course Aims
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NDGTA
At the end of this course the learner will
be able to describe…
1. Describe the electrical properties of
insulators: conductivity; resistivity
2. Describe the mechanical properties of
insulators: tensile strength; rigidity
3. Compare and contrast different insulating
materials
4. Describe the insulating properties of liquids
and gases
So what is an
Insulator?
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An insulator is a substance (solid, liquid or
gas) that resists the flow of electrical current
A true insulator is a material that does not
respond to an electric field and completely
resists the flow of electric charge. In practice,
however, perfect insulators do not exist.
Dielectric materials with high dielectric
constants are considered insulators. In
insulating materials valence electrons are
tightly bonded to their atoms.
The Function of
Insulators in Electricity
NDGTA
• The function of an insulators (dielectrics) is to
support or separate electrical conductors without
allowing current through themselves.
• For this reason they can be used as supports
that attach electric power transmission wires to
pylons.
Ceramic used as an
insulator
Insulators
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• Glass, paper, Teflon are all examples of ‘good’ insulators.
These are regarded as having low bulk resistivity
• Remember R = ρl/A
• A much larger class of materials are still "good enough" to
insulate electrical wiring and cables. Examples include
rubber-like polymers and most plastics. Such materials can
serve as practical and safe insulators for low to moderate
voltages (hundreds, or even thousands, of volts).
A copper conductor insulated
by an outer layer of
polyethylene
Insulators
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2-core mineral insulated
(MI) pvc sheathed
3-core copper wire
power cable with
individual colour coded
insulating sheaths
contained within an
outer protective sheath
– double insulation
Co-axial cable
with dielectric
insulator
supporting a
central core
Insulation
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Electrical insulation is the absence of electrical
conduction.
Insulators have a large ‘band gap’.
This occurs because the "valence" band containing the
highest energy electrons is full, and a large energy gap
separates this band from the next band above it.
There is always some voltage (called the breakdown
voltage) that will give the electrons enough energy to
be excited into this band. Once this voltage is
exceeded, the material ceases being an insulator, and
charge will begin to pass through it. However, it is
usually accompanied by physical or chemical changes
that permanently degrade the material's insulating
properties.
Breakdown
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Insulators suffer from the phenomenon of electrical breakdown.
When the electric field applied across an insulating substance exceeds in
any location the threshold breakdown field for that substance, which is
proportional to the band gap energy, the insulator suddenly turns into a
resistor, sometimes with catastrophic results.
During electrical breakdown, any free charge carrier being accelerated by
the strong E-field will have enough velocity to knock electrons from
(ionise) any atom it strikes. These freed electrons and ions are in turn
accelerated and strike other atoms, creating more charge carriers, in a
chain reaction. Rapidly the insulator becomes filled with mobile carriers,
and its resistance drops to a low level.
In air, “corona discharge" is normal current near a high-voltage
conductor; an “arc" is an unusual and undesired current.
Similar breakdown can occur within any insulator, even within the bulk
solid of a material. Even a vacuum can suffer a sort of breakdown, but in
this case the breakdown or vacuum arc involves charges ejected from
the surface of metal electrodes rather than produced by the vacuum itself
Uses
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• Insulators are commonly used as flexible coating
• Since air is an insulator, in principle no other substance
is needed to keep power where it should be. Highvoltage power lines commonly use just air, since a solid
(e.g., plastic) coating is impractical. However, wires
which touch each other will produce cross connections,
short circuits, and fire hazards.
• In coaxial cable the center conductor must be supported
exactly in the middle of the hollow shield in order to
prevent EM wave reflections.
• Finally, wires which expose voltages higher than 50V
can cause human shock and electrocution hazards.
Insulating coatings help to prevent all of these problems.
Uses
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In electronic systems, printed circuit boards (pcb) are made from epoxy
plastic and fibre glass. The nonconductive boards support layers of copper
foil conductors.
In electronic devices, the tiny and delicate active components are
embedded within nonconductive epoxy or phenolic plastics, or within baked
glass or ceramic coatings.
In microelectronic components such as transistors and ICs, the silicon
material is normally a conductor because of doping, but it can easily be
selectively transformed into a good insulator by the application of heat and
oxygen. Oxidized silicon is quartz, i.e. silicon dioxide.
In high voltage systems containing transformers and capacitors, liquid
insulator oil is the typical method used for preventing arcs. The oil replaces
the air in any spaces which must support significant voltage without
electrical breakdown.
Other methods of insulating high voltage systems are ceramic or glass wire
holders, gas, vacuum, and simply placing the wires with a large separation,
using the air as insulation.
Insulating Properties
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• Properties of insulators
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Resistivity
Maximum voltage capability
Operating temperatures
Mechanical strength
• Student Research: Find out information regarding the
above properties for…
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Poly-vinyl-chloride (PVC)
Butyl-rubber
Glass
Paper
Oil
Air