The competent person working within the constraints of a PTW should look for some or all of the following precautions to be included to ensure safe working:

• Work must be in accordance with any relevant risk assessment
• Everyone likely to be affected should be informed that work is to be carried out
• Warning signs, barriers, screens, etc. should be posted
• All services and supplies should be positively isolated and padlocked
• All hazardous materials present should be identified and withdrawn if necessary
• Venting the system and allowing any necessary cooling or pressure reduction should be specified
• Appropriate protective clothing and equipment for use during the activity should be specified
• Emergency equipment together with personnel trained in its use should be specified and supplied
• The proper tools for the job should be provided

Any residual mechanical, electrical or pressure energy should be safely released as follows:

• Mechanical – high and low speed rotating elements need to be run down, springs should be released
• Electrical – capacitors should be discharged and batteries should be isolated
• Hydraulic – accumulators and pressurised pipework should be depressurised
• Pneumatic – air from the system needs to be depressurised
• Services – steam, gas or fuel may need to be depressurised, vented, purged or drained

• The primary objective of positive isolation is to provide a safe environment for performing maintenance, repair or replacement operations on facilities or equipment. They physically stop energy flow.
• Examples of positive isolation are –
  • Electrical switch locked in the off/open position
  • Double Block & Bleed
  • Spade & spectacle plates fitted to pipework
• Software cannot be used for positive isolation.

Closure of two block valves in series with an intermediate bleed valve.

Single valve isolation of pressure and gas lines is not normally considered adequate, and should only be considered in low risk situations, such as water lines.