Isolated Ground Receptacle. Often used in premises wiring intended for audio and/or computer systems. Usually orange, or denoted with an orange triangle (when of NEMA 5-15/20 type).
http://www.iaei.org/magazine/2006/0...-circuits-got-clean-grounds-or-dirty-grounds/
For more, see http://ecmweb.com/content/basics-isolated-grounding-receptacles .
http://www.iaei.org/magazine/2006/0...-circuits-got-clean-grounds-or-dirty-grounds/
For more, see http://ecmweb.com/content/basics-isolated-grounding-receptacles .