Differential Ground Fault Protection

[Dave Loucks]

Differential ground sensing is useful in several situations:

• Multiple, grounded sources on 3-phase, 4-wire system switched with 3-pole devices (neutral is not switched)
• Current flow can reverse (this makes using ZSI more difficult to apply)

The following narrated screencast discusses how a differential ground system can be applied to isolate ground faults to within electrical switching systems (switchboards, switchgear) or even buildings.  This design does not require current transformer (CT) wiring to extend outside the zone of protection.

Narrated Screencast Differential GF Sensing: http://pps2.com/v/1/dgfs.php

The simulations shown in this model are based on LTSPICE.  More information on LTSPICE is available from http://www.linear.com/designtools/software/.  I've created another screencast that provides some basic information on how to use LTSPICE to run the simulations shown for this differential ground fault simulation.

Narrated Screencast LTSPICE: http://pps2.com/v/1/lts.php

Download LTSPICE simulation files: https://app.box.com/s/wib5s1biu901mhlungqc1fs611zt5iay/url]

Special Note:
Occasionally a different kind of differential protection will be necessary.  Refer to the following Eaton application notes for more information:

• [url=http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/wp027003en.pdf]Ground fault isolation with loads fed fromseparately derived grounded sources[/url]
• [url="http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/wp027004en.pdf"]Grounding methods in missioncritical facilities[/url]