Lockout Tagout (LOTO): Safe Working Procedure on Switchboards
lockout tagout: Lockout tagout is the procedure that makes re-energisation of an isolated circuit physically impossible while work is in progress; simply switching a breaker off does not count as safe working.
Contents
Most electrical accidents are caused not by a faulty device but by unexpected re-energisation. While one person works inside a board, another can close the breaker, an automatic transfer system can operate, or stored charge on a capacitor bank may not have been released. The lockout tagout procedure exists to close all of these scenarios: the supply is isolated, the isolating point is secured with a physical lock, a tag identifies who is working and why, and the absence of voltage is verified by measurement.
What lockout tagout is and why it is needed
Lockout means securing the isolating device in the open position with a mechanical lock; tagout means stating legibly who fitted that lock and why. Together they turn the procedure from a declaration of intent into a physical barrier. Opening a switch and leaving a note beside it is therefore not enough: without a lock, the possibility of somebody closing it remains. The same logic applies to pneumatic, hydraulic and mechanical energy; even when the work is inside a board, the other energy sources of the driven machine must be isolated too.
The five step safe working sequence
The sequence accepted on site is: stop the load in an orderly way and isolate the circuit; lock the isolating point; fit the tag; release stored energy, which means discharging capacitors and stopping mechanical motion; and finally verify the absence of voltage with a suitable instrument. That last step is the one most often skipped, yet on its own it saves the most lives. The instrument is proved on a known source before and after use to confirm that it is working.
Designing the board for lockability
Whether the procedure can actually be followed starts with the design of the board. Main and outgoing devices that can be locked in the open position, lock holes that stay accessible with the door closed, and separate isolation for every outgoing way all make operation directly easier. In withdrawable architecture, being able to lock a drawer in the isolated position provides an additional layer while work goes on inside the cubicle. In compartmented boards the ability to work on one outgoing way while the others stay energised is the single most valuable feature for combining continuity with safety.
Group work, multiple locks and handover
| Method | What it assures | Where it is used |
|---|---|---|
| Device lock | Securing the device in the open position | Main and outgoing switching devices |
| Multi hole hasp | A separate lock for every person | Work carried out in groups |
| Door or compartment lock | Preventing unauthorised access | Areas open to non technical staff |
| Drawer isolated position lock | Physical isolation within the cubicle | Withdrawable motor control centres |
| Tag | Who is working and why | With every lockout |
When more than one person works on the same circuit, each fits their own lock using a multi hole hasp, and the circuit can only be energised once the last lock has been removed. At shift change the locks are handed over person by person and the handover is recorded. No lock may be removed by anyone other than its owner; for unavoidable situations there must be a defined, written and authorised exception process. Where these rules are not written down, the procedure collapses the first time the schedule gets tight.
Records, training and common mistakes
The mistakes seen most often on site are: isolating only the control circuit and leaving the power circuit live, overlooking a standby supply or transfer system, not waiting for stored energy in capacitors and drive DC links, and skipping the verification of dead state. Most of these come from haste rather than lack of knowledge. A short checklist, an up to date single line diagram kept on the board and regular drills are the three measures that most effectively turn a written procedure into practice.
Key takeaways: lockout tagout
- A lock is essential — Opening a switch and leaving a note is not safe working; the isolating point is secured with a lock.
- Verification — Absence of voltage is confirmed by measurement and the instrument is proved on a known source.
- Stored energy — Capacitor banks and drive DC links are allowed to discharge and mechanical motion is stopped separately.
- Design support — Lockable devices, compartmentation and drawer isolation locks make the procedure workable.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Is switching the breaker off not enough?
It is not. Without a lock the device can be closed by someone else, even accidentally. Automatic transfer systems and standby supplies can also operate on their own. The lock and the tag are the only elements that physically preserve the open position.
How is locking handled when several people work on one circuit?
A multi hole hasp is used and each person fits their own lock. The circuit can only be energised when the last lock owner has removed theirs. At shift change locks are handed over person by person and the transfer is recorded.
Where is stored energy found in a switchboard?
The most common sources are compensation capacitors, the DC link of a variable frequency drive, battery banks of an uninterruptible supply and spring charged mechanisms. Each is discharged or brought to a safe state separately, after which absence of voltage is verified by measurement.
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Devpan designs switchboards with safe working features such as lockable isolating points, compartmentation and drawer isolation locks. Share your operating procedure and maintenance scenario and we will plan the board architecture around them. Related Devpan solution: panel service and maintenance.
