- This topic has 1 reply, 2 voices, and was last updated 8 years, 5 months ago by Anonymous.
2013/03/25 at 10:52 am #10975zulfadhliParticipant
- Maximum amount of electrical current a conductor/device can carry before sustaining immediate/progressive deterioration.
- Current rating or current carrying capacity
- RMS Current which a device/conductor can continuously carry while remaining within its temperature rating.
- Insulation temperature rating
Due to heat dissipation of conductor that caused by electrical resistance of the conductor material, insulation temperature rating must be made higher than conductor temperature rating, so insulation will have reliability to protect the conductor.
- Electrical resistance of the conductor materials
All common electrical conductors have some resistance to the flow of electricity. Electric current flowing through them causes voltage drop and power dissipation, which heats conductors. Copper or aluminium can conduct a large amount of current without damage, but long before conductor damage, insulation would, typically, be damaged by the resultant heat.
- Frequency of the current
- Ability to dissipate heat, which depends on conductor geometry and its surroundings; (conductor construction)
- Ambient temperature
Derating of Cable Ampacity
è A reduction in the ampacity of a conductor due to correction factors. Conductors are rated for a specific set of conditions, and when those conditions change, ampacity must be derated.
è If multiple cables (bundled more than 3 cables) are install in closed proximity
è External cooling affecting of the individual cable is diminished.
Circulating Sheath Current in Flat Formation Underground Power Lines
- 3 Phases underground power lines can induce voltages and currents in their recover sheath.
- The sheath induced currents are undesirable and generate power losses and reduce cable ampacity.
- Induced voltages can generate electric shocks to the workers that maintenance power lines (during lives / power off)
- Sheath current also known as circulating current. The value of sheath current is depending on different parameters such as sheath grounding system, the geometry of the cable, the gap between them (spacing).
- Induced current from sheath circulating current can produce power losses in the sheath and decrease cable ampacity.
- Circulating sheath currents generate a magnetic field that adds to the cable magnetic field.
Characteristics of the cable that must take into account:
2013/05/14 at 12:05 pm #13051AnonymousGuest
- Conductor Material
- Conductor Diameter
- Conductor Resistivity
- Number of String of Conductor
- Coefficient of the cable
- Sheath’s Material
- Sheath’s Diameter
- Sheath’s Resistivity
consider the type of earthing…that is very important
like there are normally 3 types of earthing
1.Single point 2.Double point 3.Cross bonding.
each has its merits and demerits..if U wanna know about EHV or HV cable you must have knowledge about earthing also.
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