medium voltage cable insulation rating?
medium voltage cable insulation rating?
(OP)
Does anyone have information on the selection of insulation rating for medium voltage cable. The procedure or calculations needed to select 100% insulation or 133% insulation.






RE: medium voltage cable insulation rating?
Reference NEC Article 310 and IEEE Red Book.
RE: medium voltage cable insulation rating?
100% insulation - ground faults clear within 1 minute
133% insulation - ground faults clear within 60 minutes
173% insulation - ground faults can persist indefinitely
RE: medium voltage cable insulation rating?
http://www.okonite.com/engineering/electrical-formulas.html
RE: medium voltage cable insulation rating?
RE: medium voltage cable insulation rating?
Does anyone know when this change occurred?
RE: medium voltage cable insulation rating?
RE: medium voltage cable insulation rating?
RE: medium voltage cable insulation rating?
RE: medium voltage cable insulation rating?
RE: medium voltage cable insulation rating?
In other words, the line-to-ground fault current (3Io)is equal to or greater than 0.6 times the three phase fault current ("Applied Protective Relaying" by Westinghouse).
However, I too cannot understand the statement in Okonite document as indicated by Modula2 above.
RE: medium voltage cable insulation rating?
RE: medium voltage cable insulation rating?
ANSWER: This statement translated in numeric terms could be indicated as follow:
-1< Xo/X1< -40
This equate to a Coefficient Of Grounding (COG = 110%) per ANSI (Former ASA Std. C84.1-1954).
For our colleagues in the IEC marketplace, the COG=110% represent an Earth Fault Factor (EFF=0.191).
The above electrical parameters indicates that this is an ungrounded (unearthed) system that in the event of phase to ground fault the overvoltage on a sound phase could rise up to 10% above the line-to-line voltage. This is obvious that a 100% rated insulation cable do not met the requirement in term of insulation strength to withstand the overvoltage creating dielectric overstresses on the cables that will induce a premature insulation failure.
Please remember that Cable is rated related with the voltage phase to ground (Vo).
- Wyes grounded system Vo/Vll=0.58.This is applicable only to systems where the normal voltage between the cable conductor and the insulation shielding tape, or metal sheath, will not exceed 58% of the phase to phase voltage.
Ungrounded systems Vo/Vll>0.58 and beside that the system may operate for a limited period of time with one phase accidentally grounded. To protect the cable against this faulted condition, it has arbitrarily been decided to reinforce the insulation thickness increasing the cable rated voltage by 33% resulting that for 133% insulation level Vo/Vll=0.77