Cable types & selection Flashcards
True or false:
The location of the proposed cable plays a major role in cable selection.
True.
If an area has a high ambient temperature or may come into contact with mechnical damage/ stress, the appropriate cable needs to be selected.
True or false:
The voltage of the proposed load plays no part in cable selection.
False.
Cables have a capacity, if the cable is too small it may not power the load.
True or false:
The type of building doesn’t matter when it comes to cable selection.
False.
You need to be aware of what you can and can’t actually put in the building.
True or false:
Being aware of environmental stresses (mechanical, environmental etc.) is important when selecting a cable.
True.
If it’s a damp environment the cable needs appropriate protections.
True or false:
All cables have protection against electromagnetic interferance by default.
False.
Some cables have insulation which protects against interferance, but most do not.
True or false:
The accessibility of cables by people or livestock is something to bear in mind when selecting a cable.
True.
You don’t want cables to be fucked with by idiots or eaten by livestock.
What does SWA stand for?
Steel Wire Armored
What are the three main advantages of SWA cable?
- Variety of types & lengths & sizes
- Great mechanical strength
- Armouring can be used as CPC
What are the three main disadvantages of SWA cable?
- Expensive to buy and install
- Large & heavy
- Requires special terminations
What are the three things which add additional support to an SWA cable when clipping it?
- Plastic cable cleat
- Saddle
- Tub-tape
Why is the recommended burying depth of SWA cable 600mm?
600mm is longer than the average spade
What are the two main types of fire-resistant cable?
- MICC (Mineral insulated copper clad)
- FP200
What is the mineral which insulates a MICC cable? Why is this mineral chosen?
Magnesium oxide
Because it has a very high ignition temperature
Name three potential advantages of MICC
- Fireproof
- Mechanical strength
- Waterproof
- Non-ageing
- Small diameter
- High-current capacity
Name three potential advantages of MICC
- Copper cladding can be used as earth continuity
- High corrosion resistance
- High operating temperature
What are the three major downsides of MICC?
- Moisture absorption
- Complicated termination
- Expensive
True or false:
MICC should always be clipped when installed, as movement or vibration can damage the cable.
True.
The copper is brittle.
What is a MIMS cable?
It is a Mineral Insulated Metal Sheathed cable, MICC specifically refers to the copper cladding.
What is the colloqueal name for MIMS cable?
Pyro
What are the two main types of Pyro cable?
Light gauge
Heavy gauge
What do the following codes mean?
4L1.5
2H2.5
- 4 core light gauge 1.5mm
- 2 core heavy gauge 2.5mm
True or false:
MICC & MIMS cables require special terminations.
True.
What two things give FP200 cables and their ilk their fire resistant qualities?
There is a layer of silicon insulation surrounding the conductors, which has a high heat resistance.
AND
There is a thin sheath of metal just underneath the final PVC sheath.
True or false:
The thin metal sheath inside FP200 is very durable.
False.
It’s very fragile and can be damaged when stripping.
What metal is the sheath inside FP200 made from?
Aluminium
What is the main disadvantage of using FP200 cable?
The silicon insulation is very easily damaged, requiring the use of ferrules.
What are the five main advantages of FP200 cable?
- Fire resistant
- Good mechnical strength
- Waterproof
- Easy to terminate
- Cheaper then MICC cables
What is the operating temperature of Butyl rubber cables?
85 degress
Where are you most likely to find Butyl rubber cables?
Inside a boiler room
What are the three types of data cable that you are likely to come across?
What are their respective supported frequencies?
ie. not obselete
Cat 5: freq max 100MHz
Cat 6: freq max 250MHz
Cat 6a: freq max 500MHz
How many pairs of conductors within a Cat 5 cable?
Four pairs of conductors, 8 cables in total.
What is inside a Data cable to shield it from interferance?
A tin-foil like metallic sheath
What is the maximum length that a data cable can be run for?
100 metres
What is the minimum bending radius for a data cable?
At least 4x the overall diameter of the cable
What are the two main types of PVC cable and what are their respective operating temperatures?
Thermoplastic: 70 & 90 °C
Thermosetting: 90°C
What does LSF stand for?
Low smoke and fume
What does LSHF mean?
Low smoke halogen free
What does LSZH stand for?
Low smoke zero halogen
What does LS0H stand for?
Low smoke zero halogen
If you see LSF written on a cable, what does that mean?
Low Smoke and Fume means that if the cable were to catch fire or burn, there would be low amounts of smoke and fumes produced.
Where would you expect to see 6242Y cable?
This is twin and eartch cable, and you would most commonly find this in domestic properties.
True or false:
6242Y cable should always go into conduit, but not trunking.
False.
6242Y (Twin and Earth) should not be used in any containment system.
What is the human meaning for the 6491X?
Single core pvc insulated cable(s).
True or false:
6491X cables should always be in a containment system.
True.
They are very susceptable to mechnical damage.
What is the difference between 3092Y cable and 3093Y cable?
3092Y is a two core flexible cable
3093Y is a three core flexible cable
Complete this sentence:
The larger the cable (csa), the ____ current-carrying capacity
higher
What would happen if you undersize a cable for the desired load?
Why does this happen to the cable?
The cable would overheat and melt, potentially causing a fire.
The reason this happens is because the smaller the csa, the higher the resistance, and the more resistance, the more heat.
What would happen if youoversize a cable for the desired load?
The resultant cable would be more expensive, and potentially be harder to install.
How do you work out the design current (Ib) of a load?
Take the power of the load (watts) and divide it by the supply voltage (usually 230V).
I = P/V
What does Iₙ stand for?
The current rating of the protective device for the circuit
True or false:
The design current rating (Ib) must always be higher than the protective device rating (Iₙ).
False.
Think about it, the rating of the protective device must be higher than the current going to the device, otherwise it would trip all the time.
For eg. if you were to put a 20A fuse on a circuit which had a design current of 24.4A, it would constantly be tripping.
What does Iₜ stand for?
This stands for the current-carrying capacity of the cable itself
What is the equation for working out Iₜ?
What do the symbols in the second half of the equation denote?
Iₜ = Iₙ / Cₐ x Cᵢ x Cg x Cf
The C symbols are correction factors, which are listed in the On-Site Guide.
What are correction factors?
They are environmental factors which may impact the amount of current which can flow through the cable, ie ambient temperature, insulation, grouping, fusing.
True or false:
Heat goes down when current and resistance goes up.
False.
The opposite.
When current and resistance increases, temperature rises.
What does Cₐ stand for?
Ambient temperature (anything above 30C)
What does Cᵢ stand for?
Insulation (is there insulation surrounding the cable?)
What does C₉ stand for?
g
Cable grouping (are there other cables grouped together?)
What does Cբ stand for?
f
Fusing (this only applies if there is a BS3036 fuse)
True or false:
The current carrying capacity (Iₜ) of the cable must always be less than or equal to the current rating of the protective device.
True.
If the current carrying capacity is lower than the protective device, it means that the protective device will not operate unless there is a fault.
If the current-carrying capacity of the cable was higher than the protective device, it would trip all the time in normal operation.
What is the important thing to remember when thinking about voltage drop?
For every metre of cable, you will have a voltage drop.
What is the equation for voltage drop?
Voltage drop = (mV/A/m) x I₆ x L / 1000
Voltage drop = (millivolts per amps per metre) x design current x length / 1000
*b
What is the maximum allowed voltage drop for a power circuit?
5% = 11.5V
What is the maximum allowed voltage drop for a lighting circuit?
3% = 6.9V
