ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Shout Box
Recent Posts
VDE 0100 to introduce AFCIs
by sparky. 01/20/18 05:09 PM
MRI LED lights dimmer control replacement - wow!
by Potseal. 01/19/18 08:52 PM
Video: Inventor of the GFCI self-testing shocks
by Bill Addiss. 01/17/18 11:11 PM
FPE in Germany
by HotLine1. 01/17/18 07:07 PM
Fujifilm Recalls Power Adapter Wall Plugs
by Admin. 01/16/18 07:04 PM
New in the Gallery:
Housebilding DIY wiring
SE cable question
Who's Online Now
0 registered members (), 8 guests, and 12 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Leakage Capacitance #129193
09/29/04 11:09 AM
09/29/04 11:09 AM
E
Electra  Offline OP
Member
Joined: Jul 2002
Posts: 38
Portland, Oregon, USA
Soares Book on Grounding mentions that an ungrounded system is capacitively coupled to ground and the existance of leakage capacitance. I understand what capacitance means, but can anyone explain to me what the actual physical implications there are on the system due to this leakage capacitance. Also, why is this not such of an issue on a grounded system????

Thanks,

Laura J

Tools for Electricians:
Re: Leakage Capacitance #129194
09/30/04 09:06 AM
09/30/04 09:06 AM
J
JBD  Offline
Member
Joined: Jul 2001
Posts: 599
WI, USA
Trying to keep it short and simple, so ask for more details if you want.

All AC systems have a natural capacitive coupling to ground. In a balanced system the associated leakage current cancels simialr to the way the balanced phase currents cancel. But if one of the phase conductors is grounded then there is no canceling of the charging current.

In a low resistance grounded wye system the overcurrent protective devices should operate when a phase goes to ground preventing further problems. However in high-resistance grounded wye systems the OCPD will not open causing the charging currents form the other two phases to increase by a factor of 3 (this is caused by the change of the phase angles and the increased voltage to ground).

The charging current in a high resistance ground system is minimal so even a 3 times increase is not usually a safety hazard. The primary reason we are concerned at all has to do with the settings and operation of protective realys and other ground monitoring equipment.

Re: Leakage Capacitance #129195
09/30/04 10:25 AM
09/30/04 10:25 AM
B
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
 
To effectively damp phase-to-ground overvoltage in an ungrounded system (with complete equipment grounding and bonding in place) the capacitance between phases and ground must be damped by a resistor connected from neutral-to-ground.

“Charging current” for 480V systems is roughly 1 Ampere/1,000kVA of system capacity. [In the case of delta systems, the neutral has to be derived from zig-zag or wye-primary—delta-secondary transformers.]

Here is my favorite story about the very serious problems that can occur with low-voltage ungrounded distribution.




[This message has been edited by Bjarney (edited 09-30-2004).]


Featured:

2017 Master Electrician Exam Preparation Combos
2017 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
MarkC10
MarkC10
CA, Inland Empire
Posts: 43
Joined: September 2013
Show All Member Profiles 
Top Posters(30 Days)
Admin 20
sparky 16
Potseal 15
Popular Topics(Views)
243,572 Are you busy
180,369 Re: Forum
170,846 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.6.1
(Release build 20180101)
Page Time: 0.017s Queries: 14 (0.004s) Memory: 0.9503 MB (Peak: 1.0755 MB) Zlib enabled. Server Time: 2018-01-21 02:39:07 UTC