The legal definition on this was set out in England by Lord Justice Asquith in Edwards v National Coal Board  who said:
“‘Reasonably practicable’ is a narrower term than ‘physically possible’ and seems to me to imply that a computation must be made by the owner, in which the quantum of risk is placed on one scale and the sacrifice involved in the measures necessary for averting the risk (whether in money, time or trouble) is placed in the other; and that if it be shown that there is a gross disproportion between them — the risk being insignificant in relation to the sacrifice — the defendants discharge the onus on them. Moreover, this computation falls to be made by the owner at a point of time anterior to the accident.”
This English decision has since been confirmed by the Australian High Court1.
1Slivak v Lurgi (Australia) Pty Ltd (2001) 205 CLR 304 cited in Bluff & Johnstone (2004) The relationship between Reasonably Practicable and Risk Management (WP 27 ANU National Research Centre for OHS Regulation).
Source: NOPSEMA Guidance Note: ALARP, N-04300-GN0166, Australia, Revision 4, December 2012. Regulatory Guidance. Regulatory Guidance
“Reasonably practicable” means practicable unless the person on whom a duty is placed can show that there is a gross disproportion between the benefit of the duty and the cost, in time, trouble and money, of the measures to secure the duty.
Source: Nova Scotia Offshore Petroleum Occupational Health & Safety Requirements, Canada-Nova Scotia Offshore Petroleum Board, Canada, December 2000. Regulations