×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Cable Insulation Hi-Resistance Ground

Cable Insulation Hi-Resistance Ground

Cable Insulation Hi-Resistance Ground

(OP)
The 4.16 KV system has an NGR, 5A continuous. For a ground fault in the zone of service transfomer secondary and 4.16 KV SWGR main breaker, that breaker will open. The transformer primary has only a fuse, which will not open for the small current. The fault will then persist and be alarmed, but will only be cleared eventually by pulling the primary fuse on the pole. The elevated phase to ground voltage on two phases would be 173%. I believe the 100% 5 KV cable is not proper and the next higher available 8 KV insulation level is required, between the service transformer and SWGR. Cable would be 3/c, armoured, jacketted, probably unshielded. Does anyone have similar experience?  

RE: Cable Insulation Hi-Resistance Ground

8 kV cable would be better.  I'd check with cable manufacturer.   

RE: Cable Insulation Hi-Resistance Ground

173% of the Line to neutral voltage = 2.4*1.732 = 4.16 kV! You are still within range if you calculated 173%! However, if you were ungrounded, the line to neutral voltage could be very high (capacitive charging of lines)!
 

RE: Cable Insulation Hi-Resistance Ground


I agree that the L-N voltage would only still be 4.16kV on the two ungrounded phases and the cable would be within spec.

I dont understand however if ungrounded how the capacitive charging would charge this voltage to be higher?

RE: Cable Insulation Hi-Resistance Ground

Ungrounded systems can experience voltages much greater than line-to-line voltage during ground faults due to "repetitive restrike".  Restriking arcs can pump up the voltage to ground by charging the capacitance to ground in the circuit faster than it can be discharged.  High-resistance grounded systems provide just enough resistance to get the time constant down low enough to avoid this.  



 

RE: Cable Insulation Hi-Resistance Ground

dpc,
great explanation! short and precise!star to you!

rockman,
Consider the line insulation as a capacitor. When an intermittent ground strikes, this "capacitor" charges; discharges on the negative cycle through the grounding resistor. If the charge was not drained (high resistance grounding), another "restrike" will pump up the charge; hence the voltage increases. Could be more than the line to line voltage if not interrrupted in time.

RE: Cable Insulation Hi-Resistance Ground

(OP)
Cable spec of 5 KV 100% is a phase to phase nomenclature. It is 2.887 KV phase to ground cable rating. On the continuous L-G fault, we have 4.16 KV L-G on two of the phases, which is 144% of the 2.887 cable rating.

If we require a cable of 4.16 L-G, that means 1.73x4.16 = 7.2 KV phase-phase min. Next avaialble size is 8 KV. A 5 KV 173 % would also work, but I don't see any available.

We can also refer to IEEE 242-2001, Protection and Coordination of Industrial and commercial Power Systems, Chap 9, Conductor Protection, p 286, which lists the insulation level requirements 100%, 133%, 173%, and calls for the 173% rating if the ground fault persists past one hour, 133% for up to one hour, and 100% for up to one minute. Mine could persist much longer than one hour.

RE: Cable Insulation Hi-Resistance Ground

(OP)
This is in addition to my previous post. Consider the situation if we intentionally left the transformer neutral ungrounded, but instead solidly grounded one of the phases. And operated continuously like that, supplying only 3 phase loads. What would the required cable insulation level be then?

I also see, my old SouthWire Power Cable Manual gives the same 100/133/173% insulation requirements as Ieee 242.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources