Last Updated: October 20, 2026
Safe isolation is the practice of disconnecting electrical equipment from its power supply and preventing accidental reconnection until work is complete. It’s a fundamental control measure that protects workers from electric shock, arc flash, and electrocution.
The safe isolation procedure is not optional. It’s a legal requirement under the Health and Safety at Work etc. Act 1974 and is reinforced by guidance like HSG47 (Avoiding Danger from Live Electrical Work). When done correctly, it removes the risk entirely. When shortcuts are taken, people die.
Why this matters: electrical hazards are invisible. You cannot see electricity flowing through a cable, and you cannot feel a live conductor until it’s too late. The only way to be certain work is safe is to follow a systematic process that identifies, isolates, locks off, and proves the circuit is dead before any work begins.
At SJB Electrical Training, we’ve trained hundreds of maintenance teams, contractors, and industrial workers on the correct 10 step safe isolation procedure. The difference between teams that follow it rigorously and teams that cut corners is stark. One group goes home. The other doesn’t.
A proper safe isolation procedure follows a logical sequence. Each step builds on the last. Skip one, and the entire process fails.
Step 1: Identify the item of equipment to be worked on.
Know exactly what you’re isolating. Not the general area, the specific piece of equipment. Is it a motor, a contactor, a distribution board, or a cable? Get this wrong, and you might isolate the wrong circuit entirely.
Ask yourself: What am I actually working on? Where is it? What does it look like? If you’re uncertain, stop and get clarification from someone who knows.
Step 2: Identify the correct point of isolation for that equipment.
Every piece of electrical equipment has an isolation point. This is usually a switch, isolator, or circuit breaker that disconnects it from the live supply. Find the correct one. Look at the equipment label, the circuit diagram, or ask a competent person.
Common isolation points include:
Step 3: Locate the isolation point physically.
Now go find it. Trace the circuit. Follow the cable. Look at the switchboard. Make sure you’ve identified the right switch before you touch anything. Take a photo if it helps you remember where it is.
Step 4: Switch off the supply.
Open the isolator or circuit breaker. Move the switch to the OFF position. Do not just turn off a light switch or unplug something, use the proper isolation point for the equipment.
Step 5: Lock off and tag out the isolation point.
This is critical. After switching off, physically lock the isolator in the OFF position using a padlock. Attach a warning tag or caution notice that states:
This prevents someone else from accidentally switching the equipment back on while you’re working on it. Lockout/tagout is not bureaucracy, it saves lives.
Step 6: Verify the isolation is effective.
Check that the equipment has actually stopped. If it’s a motor, make sure it’s no longer spinning. If it’s a light, confirm it’s off. If it’s a pump, check that flow has stopped. Do not assume the isolation worked, verify it with your senses.
Step 7: Select an appropriate voltage indicator.
You need a tool to prove the circuit is dead. A voltage indicator (also called a voltage tester or test instrument) detects the presence of electrical voltage. Choose one appropriate to the voltage you’re testing:
Step 8: Prove the voltage indicator before use.
Before you test the circuit, test your tester. Use a proving unit, a device that generates a known voltage. Switch on your voltage indicator and touch the proving unit. The indicator should show a reading. If it doesn’t, the tester is faulty. Do not use a faulty tester.
This step proves your test instrument is working correctly. It’s not optional.
Step 9: Test for the absence of voltage at the point of isolation.
Now test the isolated circuit. Touch the test leads of your voltage indicator to the terminals or conductors at the isolation point. The indicator should show zero voltage. No reading. No lights. No sound. Dead.
If it shows voltage, stop immediately. Do not proceed. The isolation has failed. Investigate why before continuing.
Step 10: Prove the voltage indicator after use.
After testing the circuit, test your proving unit again. The indicator should show a reading again. This confirms your tester was working both before and after you tested the circuit. If it doesn’t show a reading now, you cannot trust the test you just performed.
This is the final verification step. It closes the loop.
A checklist keeps teams consistent. Use this framework to build your own, tailored to your site and equipment:
| Step | Action | Check | Sign |
|---|---|---|---|
| 1 | Identify equipment | Equipment name and location confirmed | |
| 2 | Identify isolation point | Circuit diagram checked; correct switch located | |
| 3 | Locate isolation point | Physically found and accessible | |
| 4 | Switch off | Isolator moved to OFF position | |
| 5 | Lock off and tag out | Padlock applied; tag attached with details | |
| 6 | Verify isolation effective | Equipment stopped; no movement or output | |
| 7 | Select voltage indicator | Tool chosen for voltage range | |
| 8 | Prove indicator before use | Proving unit tested; indicator shows reading | |
| 9 | Test for dead | Voltage indicator shows zero at isolation point | |
| 10 | Prove indicator after use | Proving unit tested again; indicator shows reading |
Print this. Laminate it. Use it on every job. Consistency prevents mistakes.
The “prove dead” procedure is steps 8, 9, and 10 combined. It’s the moment of truth, when you confirm the circuit is actually safe to work on.
Here’s how it works in practice:
Before testing the circuit:
Testing the circuit:
After testing:
If the circuit still shows voltage after isolation, stop work. Investigate:
Do not proceed until you’ve found and resolved the problem.
Your voltage indicator is only as good as your confidence in it. A faulty tester gives false reassurance, the most dangerous situation on site.
Voltage indicator types:
Analogue testers (needle gauge) are durable but require practice to read accurately. Digital testers (LED or LCD display) are clearer but depend on batteries. Specialist testers for three-phase supplies detect all three phases simultaneously.
Proving units:
A proving unit is a portable device that generates a safe test voltage. It allows you to verify your tester works without needing access to a live circuit. Most proving units generate 110V or 230V AC.
When selecting a proving unit:
Best practice:
Always carry both a voltage indicator and a proving unit on site. Test the indicator on the proving unit before work, test the circuit, then test the proving unit again. This three-point verification is your assurance that the test was valid.
Mistake 1: Isolating at the wrong point.
Teams sometimes isolate a general circuit breaker instead of the specific isolation point for the equipment. Result: other equipment on the same circuit loses power.
Always use the isolation point designed for that specific equipment.
Mistake 2: Skipping the proving steps.
Some workers test the circuit directly without proving the voltage indicator first. If the tester is faulty, they’ll get a false negative (the circuit shows dead when it’s actually live).
Never skip step 8 or step 10.
Mistake 3: Forgetting alternative supplies.
Equipment might have a backup power source: a generator, a UPS, or a secondary circuit.
Before isolating, ask: Is there any other way this equipment could be powered?
Mistake 4: Assuming the circuit is dead because the main switch is off.
The main switch might be faulty. The circuit might be miswired. There might be stored energy (capacitors, batteries). Never assume, always test.
When to stop work:
A delay is always better than an accident.
Safe isolation is not complicated, but it demands discipline. Every step exists because someone died without it. Follow the procedure exactly. Lock off. Tag out. Prove dead. Verify your tester before and after.
At SJB Electrical Training, we work with teams across industrial sites, utilities, healthcare facilities, and construction to embed this procedure into daily practice.
Proving a voltage indicator before and after use confirms the test instrument is working correctly. A faulty indicator could fail to detect live voltage, putting you at serious risk. Prove before use to establish the device functions; prove after to ensure it detected voltage properly during your test. This two-stage verification is a core part of the prove dead procedure and is required under HSG47 guidance.
Stop work immediately. Do not proceed without positively identifying the correct isolation point. Contact a competent person or the equipment owner to establish which circuit or supply controls the item of equipment. Working on the wrong circuit risks leaving live parts exposed and creates a serious hazard. If uncertainty persists, treat the entire circuit as live until you have confirmed safe isolation.
Use a voltage indicator to test for absence of voltage across all live conductors and between live and earth. Test at the point of isolation, then test again at the item of equipment you will work on. Both tests must show no voltage. This is the prove dead procedure. If any test shows voltage, the circuit is still live and you must not proceed. Always prove your voltage indicator before and after testing using a proving unit.
Before re-energising, confirm all work is complete and safe. Remove all tools, test instruments, and temporary barriers. Check that locking devices and warning tags are removed only by an authorised person. Verify no one is working on the circuit. Inspect the item of equipment for damage or foreign objects. Only then restore the supply. Document the re-energisation in your site records to maintain a clear audit trail of safe working.
