When a cardiac arrest happens, the chance of survival reduces by 10% for every minute that passes.
A defibrillator administers an electric shock to help re-start a heart.
Having fast access to a defib can be a life-saver. No training is needed to operate one, they can be fully or semi-automatic and will not deliver a shock if it is not needed.
Below is some documentation regarding specification and installation of defibrilators and safes:
Wellmed DefibSafe specification
A guide to AEDs by RCUK and BHF Dec 2013
Ecopad brochure & specification