So that you can connect a secondary earth lead from a medical device to the earth socket.
See IEE Wiring Regs BS 7671 Guidance Note 7 Special Locations, there is a section for Medical Locations. The installation should have also been of an isolated type (IT System) with appropriate monitoring.
See
http://www.bender-uk.com/www/bendergmbh/bendergmbh.asp for isolated supply equipment/installations.
Bender UK Ltd
Low Mill Business Park
Ulverston
Cumbria
LA12 9EE
tel 01229 480 123
fax 01229 480 345
Regards,
Mark
