Electrical Safety
Electrical Isolation and Permit to Work: Step-by-Step Safe Isolation Procedure
Safe isolation is the process of making electrical equipment dead, proving it is dead, and securing it against re-energisation before any work begins.
Every electrical fatality caused by working on live equipment is preventable with a correctly followed safe isolation procedure.
Safe Isolation
Permit to Work
Lockout Tagout
Prove Dead
IEC 60204-1
Hello everyone, today we are going to learn about electrical isolation and permit to work. We will understand the full safe isolation procedure step by step, the roles involved in issuing and receiving a permit to work, the types of electrical permit to work, and the common mistakes that cause accidents during electrical maintenance.
Working on electrical equipment without a valid permit to work is illegal under most national safety regulations. It causes fatalities every year.
The safe isolation procedure is the sequence of actions that keep maintenance engineers alive.
Electrical Isolation and Permit to Work: 7-Step Safe Isolation Procedure
1
Identify the equipment to be isolated
Identify every circuit and energy source feeding the equipment. Check single-line diagrams and distribution board schedules. For complex plant, prepare a written isolation schedule listing every switch and isolator to be opened.
2
Obtain the permit to work
The permit issuer issues the permit to work after verifying the isolation plan. The permit states the equipment, isolation points, hazards, and work scope. The performing authority signs to accept the permit.
3
Isolate all energy sources
Open every switch, circuit breaker, and isolator from Step 1. Open all three phases on three-phase systems. Check for alternative supply paths, capacitor banks, and UPS feeds. Also isolate the motor control circuit for motor-driven equipment.
4
Apply locks and danger tags
Fit a personal lock to every isolation point from Step 3. Each person fits their own lock. Only the lock owner may remove it. Attach a danger tag with the holder's name and date.
5
Prove the equipment is dead (verify absence of voltage)
Use an approved voltage indicator to verify absence of voltage. First prove the indicator works on a known live source. Test all phases, neutral, and earth at the work point. Then prove the indicator still works.
6
Discharge stored energy and apply earthing where required
Discharge capacitors and allow drives to fully discharge before touching terminals. For medium and high voltage equipment, apply earthing clamps to all conductors at the point of work.
7
Carry out the work, then restore in reverse order
Work may now begin. When complete, remove all tools, refit all covers, remove earthing clamps, and remove personal locks in reverse order before the permit issuer cancels the permit.
4 Types of Electrical Permit to Work
Electrical PTW
Issued for work on electrical equipment that has been de-energised and isolated. Covers switching, termination, testing, and maintenance of LV and MV electrical systems.
Hot Work PTW
Issued for work that generates heat, sparks, or flame near energised equipment. Required alongside the electrical PTW when welding or grinding near live cable routes.
Confined Space PTW
Issued for entry into cable tunnels, underground chambers, or any enclosed space where electrical work is performed. Requires additional hazard assessment for oxygen deficiency alongside the electrical isolation.
Live Working PTW
Issued when work must be carried out on live conductors. Requires written justification that isolation is not practicable and approved live working tools and arc flash PPE rated to the system voltage.
Roles in a Permit to Work System
Permit Issuer (Authorised Person)
The competent electrical person who prepares and issues the permit. Responsible for verifying that the isolation is complete, the permits are correctly filled, and the hazards are correctly identified. The only person who can cancel the permit and authorise re-energisation.
Performing Authority (Competent Person)
The person who accepts the permit and carries out or supervises the work. Responsible for fitting their personal lock, verifying the equipment is dead before touching it, and removing their lock when work is complete.
Issuing Authority (Site Manager)
The management representative who controls the permit system. Approves the scope of work, ensures adequate resources, and maintains the permit register. Does not need to be an electrical specialist but must understand the permit system.
Isolation Team Member
Any additional person carrying out work under the permit. Each team member fits their own personal lock to every isolation point. No team member may start work until their own lock is in place and they have personally verified absence of voltage.
Common Safe Isolation Mistakes That Cause Accidents
| Mistake | Why It Is Dangerous | Correct Practice |
|---|---|---|
| Isolating the wrong circuit | Equipment remains live at the point of work despite the isolation. | Verify identification by testing for absence of voltage at the point of work, not at the isolator. |
| Using a single-phase test lamp as the only voltage check | A single-lamp test only checks one phase. The other phases may still be live. | Use an approved voltage indicator (AVI) rated for the system voltage and test all phases, neutral, and earth. |
| No personal lock on the isolator | Another person can restore the supply while work is in progress without knowing work is ongoing. | Every person working on the equipment fits their own personal safety lock. No exceptions. |
| Not discharging capacitors or VFD DC bus | Capacitors in motor drives, power factor correction banks, and UPS systems store lethal charge after isolation. | Wait for the drive display to go blank (typically 5 to 10 minutes). Measure DC bus voltage before touching terminals. |
| Restoring supply without checking work is complete | Tools left inside equipment, open cable ends, or removed earth clamps cause flashover or shock when supply is restored. | The permit issuer must confirm with all performing authorities that work is complete before cancelling the permit and restoring supply. |
Critical Rule: Never remove another person's personal lock.
Only the lock owner may remove their own personal lock. If a lock owner has left site, the procedure for removing an absent person's lock requires written authorisation from senior management, a physical check that the person is not on site, and a full inspection of the equipment before re-energisation. There are no shortcuts to this process.
Only the lock owner may remove their own personal lock. If a lock owner has left site, the procedure for removing an absent person's lock requires written authorisation from senior management, a physical check that the person is not on site, and a full inspection of the equipment before re-energisation. There are no shortcuts to this process.
Tip: Prove, Test, Prove the three-step voltage verification sequence.
Before verifying absence of voltage at the point of work: (1) Prove the voltage indicator is working on a known live source. (2) Test all conductors at the point of work for absence of voltage. (3) Prove the voltage indicator is still working on a known live source after the test. If the indicator fails at step 3, the test at step 2 is invalid and must be repeated with a working instrument. This three-step sequence is required by BS GS38 (UK Health and Safety Executive guidance) and equivalent standards in other countries.
Before verifying absence of voltage at the point of work: (1) Prove the voltage indicator is working on a known live source. (2) Test all conductors at the point of work for absence of voltage. (3) Prove the voltage indicator is still working on a known live source after the test. If the indicator fails at step 3, the test at step 2 is invalid and must be repeated with a working instrument. This three-step sequence is required by BS GS38 (UK Health and Safety Executive guidance) and equivalent standards in other countries.
Watch: Safe Isolation Procedure Explained
Safe Isolation and Permit to Work Questions Engineers Ask
What is a permit to work in electrical maintenance?
A permit to work authorises specific work on electrical equipment. It records the isolation points, hazards, and work scope. Work must not begin until the permit issuer and performing authority sign it.
What is the safe isolation procedure?
Safe isolation has seven steps: identify the equipment, obtain a permit, isolate all energy sources, apply personal locks, prove absence of voltage, discharge stored energy, then work. Restoration is in reverse order.
What is the Prove Test Prove sequence?
Prove Test Prove means: prove your voltage indicator works on a known live source, test the isolated conductors for absence of voltage, then prove the indicator still works after the test.
Why does each person need their own lock?
Each person fits their own lock so the supply cannot be restored while any one is working. If one person removes their lock, the supply stays off because others' locks remain.
What standard covers electrical safe isolation in the UK?
The Electricity at Work Regulations 1989 is the primary legislation. HSE Guidance Note GS38 covers voltage indicator selection and safe isolation in detail. IEC 60204-1 covers safe isolation requirements for machinery installations.
Related Articles on This Site
- What Is Arc Flash? Causes and Prevention
- NFPA 70E Electrical Safety Explained
- Electrical Safety PPE Selection Guide
- Hot Work Permit Explained
- MCB vs MCCB vs ELCB vs RCCB
External References
- GS38: Electrical Test Equipment for Use on Low Voltage Electrical Systems | HSE UK
- HSG85: Electricity at Work Safe Working Practices | HSE UK
What We Learn Today
- The safe isolation procedure has 7 steps: identify, obtain a permit to work, isolate all energy sources, apply personal locks and tags, prove absence of voltage using the Prove Test Prove sequence, discharge stored energy, then work and restore in reverse order. Skipping any step creates a risk of electrocution.
- Every person working on isolated equipment must fit their own personal safety lock to every isolation point. Only the lock owner may remove it. No other person, regardless of seniority, may remove another person's lock.
- Verify absence of voltage using an approved voltage indicator rated for the system voltage. Test all phases, neutral, and earth at the point of work, not at the isolator. Capacitors, VFD DC buses, and UPS systems store lethal charge after isolation and must be discharged before touching any terminal.
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