11.2 Prove–Test–Prove Safe Isolation

Benefits of Working on Isolated Equipment

Dead working—performing PEW on equipment that has been safely isolated—is the default preference for good reason:

• Removes electrical energy as the primary shock and arc source at the work point
• Allows continuity, insulation, and construction work without relying solely on PPE and insulated tools to manage live risk
• Supports clear LOTO and tagging so others do not re-energise the circuit under you
• Aligns with the hierarchy of controls: eliminate the hazard where practicable before relying on administrative or PPE controls

Isolation is not “flipping a switch and hoping.” Competent isolation is a process: identify sources, isolate, secure against re-energisation, prove dead, then work—and restore only under a controlled return-to-service sequence.

The PROVE–TEST–PROVE Rule

Voltage indicators and multimeters fail. Leads break. Ranges get left on the wrong setting. Batteries die. The industry rule that catches a failed instrument is PROVE–TEST–PROVE:

If step 1 fails, do not use the instrument. If step 2 shows voltage, the circuit is not isolated—find the remaining source. If step 3 fails, re-test the circuit with a proven instrument; you cannot trust a “dead” reading from a tester that no longer indicates live correctly.

Practical discipline

• Use a suitable voltage tester or meter rated for the system and category of work.
• Test phase-to-phase / active-to-active, active-to-neutral, and active-to-earth as appropriate so a single open neutral or missing earth reference does not fool you.
• Do not skip the second prove because “it always works.” The second prove is what detects a tester that died during the dead test.

Test Before Touch

Test before touch means: before any conductive contact with conductors, terminals, or exposed conductive parts that might be live, verify absence of voltage by the prove–test–prove process (or equivalent competent method).

Do not treat the following as proof of isolation:

• Switch handle position alone
• A colleague’s verbal assurance without your verification
• A faded tag with no locked isolation point
• “It was off yesterday”
• Circuit labels that may be wrong after alterations

On the exam, answers that skip testing because “the main switch was down” are usually wrong. The main switch may feed only part of the installation; alternative supplies may still be present; the wrong circuit may have been operated.

Identify All Sources of Supply

Modern installations often have more than one energy source. Isolation of the “main switch” alone can leave the work point live or capable of becoming live.

Sources and backfeed paths to actively hunt for include:

Identify → isolate each source → secure → prove dead at the point of work. If you cannot identify the supply architecture, stop and obtain drawings, labels, and site knowledge before opening enclosures.

PV and inverter awareness (high level)

Even when the AC isolator is open, PV modules can still produce hazardous DC in light. Isolation procedures for renewable systems must address both AC and DC sides per the installation design and applicable standards. Exam stems often reward “consider alternative and renewable sources,” not “main switch only.”

Written Isolation Procedures for Complex Sites

Simple single-board domestic isolation may be managed by a competent person with clear LOTO and prove–test–prove. Complex sites—hospitals, industrial plants, multi-board commercial buildings, sites with generators/UPS/PV—need written isolation procedures that define:

• Scope of work and equipment boundaries
• All isolation points and sequence of isolation
• Locks, tags, and who holds keys
• Test points and method (prove–test–prove)
• Residual energy (capacitors, batteries, mechanical)
• Communication with operators and permit-to-work interfaces where used
• Restoration sequence and who authorises re-energisation

Written procedures reduce reliance on memory under time pressure and make supervision of trainees auditable. For EWRB competence culture, treating complex isolation as “in my head only” is a red flag.

Exam habits

1. Always sequence identify sources → isolate → lock/tag → prove–test–prove → work.
2. Reject answers that treat switch position as sufficient without testing.
3. Call out UPS, generator, and PV/backfeed whenever the stem is multi-source or “unexpectedly live.”

Study note

Pair this section with LOTO (11.3) and live-work controls (11.4). Isolation without securing against re-energisation is incomplete; prove–test–prove without identifying all sources is incomplete.