Safe isolation: lock off, tag out, prove dead

It's 3am. The infeed conveyor to the sorter has stopped with a drive fault. Two of your team are on it. One goes to the panel, finds the isolator marked "CV-04", switches it off and hangs a lock on it. The other is already reaching for the motor terminal box.

Nobody's tested anything. The label says CV-04. It's always been CV-04.

Then someone mentions that CV-04 was rewired in the summer shutdown. And nobody's sure the label was changed.

Everyone on site knows the words: lock off, tag out, prove dead. What goes wrong is the order, and the step that gets skipped because the label looks right and the line's been down twenty minutes.

Start with what the law means by isolation

The Electricity at Work Regulations define isolation as disconnecting and separating equipment from every source of electrical energy "in such a way that this disconnection and separation is secure" (regulation 12). Once it's dead, you must take precautions to stop it becoming charged again while the work goes on (regulation 13).

PUWER says the same for every kind of energy. Kit must have suitable means to isolate it "from all its sources of energy" (PUWER regulation 19). The code of practice adds that isolation means a break in the supply "in a secure manner" so that accidental reconnection isn't possible (L22, paragraph 196).

And one line every fitter should know by heart. Interlocked guards and e-stops "are not suitable as a means of isolation" (L22, paragraph 195).

The order that works

HSE's guide HSG85 sets out a sequence for dead working. Here it is in shop-floor terms.

Identify it. Make sure you've got the right circuit or machine. HSG85 says you should never assume labelling is correct and start work without first proving the kit dead (HSG85, paragraph 47). CV-04 is exactly why.

Disconnect it. Cut it off from every source. If it can store charge, such as capacitors in a drive, make sure the charge has been safely discharged (HSG85, paragraph 48). On a VSD that means waiting out the discharge time on the drive label before you open it.

Lock it. Lock the isolator off, preferably with a safety lock. That means a padlock no other keys will open, or one with a unique combination (HSG85, paragraph 49). If you've pulled a fuse, take it away or lock the enclosure (HSG85, paragraph 50). A plug that's been pulled needs a lockout too.

One lock each. Where several people are working, use a multi-lock hasp or lock-out box. Everyone fits their own lock and keeps their keys on them (HSG85, paragraph 51). The code of practice for PUWER says the same, and that keys should not be swapped between people (L22, paragraph 197).

Tag it. Put a notice at the point of isolation so others know work is going on. HSG85 suggests space on the notice for the name of the person responsible and the date (HSG85, paragraph 52).

Prove it dead. Check the parts you'll touch really are dead. That holds "even if the isolation has been achieved automatically through an interlocking system". On three-phase kit, prove every supply conductor dead (HSG85, paragraph 53).

Prove the tester too

The tester matters. For low voltage, HSG85 points to proper voltage detectors, such as two-pole testers or test lamps. Multimeters aren't recommended for proving dead, because they can be set to the wrong function. Nor are non-contact "volt sticks" (HSG85, paragraph 54).

Then test the tester, before and after use, for example on a proving unit (HSG85, paragraph 55). Prove, test, prove. If the tester died in your hand, a "dead" reading means nothing.

Electricity is only one source

The conveyor might be dead and still dangerous. PUWER covers all energy, and the code of practice asks for a way to release stored energy where possible (L22, paragraph 199). Walk the machine and think about:

• air in the receiver and the lines downstream of the dump valve
• pressure held in a hydraulic accumulator
• a tensioned belt, a loaded spring, a raised lift table
• product on an incline that will run back when the brake's released
• steam or hot water in a jacket

Valves need extra thought. The code of practice warns that valves deteriorate and their effectiveness "often cannot be judged visually" (L22, paragraph 202). Some drains and vents need locking open, not shut (L22, paragraph 196). Our guide to the first ten minutes of a hydraulic failure covers stored pressure in more detail.

Here's a check we like, as experience rather than rule. Once it's locked and proved, try to start it. Press the start button, with everyone stood clear, and confirm nothing moves. It catches the second supply nobody knew about.

Live work is the exception

Live work is only allowed when it's unreasonable for the kit to be dead, reasonable to work on it live, and suitable precautions are in place (regulation 14). "The trailers are due" doesn't pass. Our post on the Electricity at Work Regulations goes through the duties.

The cost of the wrong person on the job

Regulation 16 says nobody should do work that needs technical knowledge to prevent danger unless they have it, or are properly supervised (regulation 16).

At 3am that matters. A mechanical fitter can lock off an isolator. Proving dead inside a drive cabinet, with stored charge and a second supply for the brake, is a different skill. If the only person on shift isn't confident with it, the honest options are to wait or to get someone in. Waiting has a cost per hour. Getting it wrong has a cost nobody can put a number on. Our post on the 30-minute rule helps you decide when to make the call.

And when the job is bigger, or two teams are involved, isolation sits inside a permit to work.

Our view

HSE's isolation guide notes that where incidents happen, the root causes "often include human failures" (HSG253, foreword). In our experience that usually means the steps were known and one got skipped, because the label looked right or the job looked quick. So make the steps physical. Locks and hasps at every panel. A proving unit in every tool bag. A tag with a name on it. If the kit isn't there at 3am, the step won't happen.

What to do on Monday

1. Check every engineer on every shift has their own lock, a two-pole tester and a proving unit. Replace any multimeters being used for proving dead.
2. Pick your three most complex machines. Write a one-page isolation sheet for each, listing every energy source and how to isolate or release it.
3. Check the labels on any isolator touched in the last shutdown against the drawings.
4. Ask your shift leads who on their shift is competent to isolate and prove dead inside a drive cabinet. Plan for the shifts where the answer is nobody.

* HSE guidance and regulation text correct as of 5 September 2026. Check HSG85 on HSE's website and the Electricity at Work Regulations on legislation.gov.uk for the latest. HSG85 paragraph numbers refer to the third edition (2013). L22 paragraph numbers refer to the fourth edition (2014, amended 2018).