Safe work on MV switchgear requires a controlled isolation process performed by authorized, competent persons under the applicable national rules and site safety system. Device position indication alone is not proof that a circuit is de-energized.

Why this topic matters

Wrong-source isolation, backfeed from VTs or embedded generation, stored energy, induced voltage and incorrect earthing points can leave hazardous voltage present even when a breaker is open. The safety boundary must be verified from the actual network.

Scope and engineering boundary

This article explains engineering principles and document requirements; it is not a switching instruction and does not authorize any person to operate equipment. IEC 61936-1, applicable national implementation, operating rules and manufacturer instructions govern the site procedure.

Core engineering principles

Identify every possible source

The isolation plan should include normal, alternative, auxiliary, VT, generator, capacitor and induced sources. SLDs and labels must correspond to the as-built installation.

Secure against reconnection

Open or isolated devices must be controlled through the approved lockout, key, tag or access system. Interlocks support safety but do not replace organizational control.

Verify absence of voltage correctly

A suitable, rated and verified detector or test system is used at the defined points, with its correct operation established before and after the test as required by the procedure.

Earth and short-circuit at approved points

Earthing devices must have adequate making and withstand capability and be applied in the safe sequence. The earth location should protect the work zone from reconnection and induced or transferred voltage.

Application workflow

  1. Step 1: Prepare an approved switching and work plan from the current as-built network.
  2. Step 2: Identify and isolate every credible source, including backfeed and auxiliary sources.
  3. Step 3: Secure isolation devices and control access under the site safety system.
  4. Step 4: Verify absence of voltage at the specified points using suitable equipment and procedure.
  5. Step 5: Apply fixed or portable earths and short-circuits at approved locations as required.
  6. Step 6: Establish the work zone, protect against adjacent live parts and issue the work authorization.

Practical engineering example

Opening a feeder breaker does not isolate a cable if the circuit can be energized from the remote end. Both ends, VT connections and any embedded generation must be considered before proving dead and applying earths.

Common mistakes

  • Relying only on a mimic or position lamp.
  • Ignoring VT secondary backfeed or remote-end supply.
  • Treating an interlock as the lockout system.
  • Applying portable earths without a rated connection point and approved sequence.

Design and review checklist

  • Authorized procedure and current SLD are available.
  • All normal and alternative sources are identified.
  • Isolation is secured against reconnection.
  • Absence of voltage is verified by the approved method.
  • Earthing protects the complete work zone.

Standards basis and official sources

Engineering note: Confirm the contracted edition, amendments, corrigenda, national adoption, project specification and manufacturer instructions before applying a requirement. This article explains engineering use and does not reproduce or replace the standard.

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