Guía técnica para una libranza eléctrica industrial segura en media tensión según la NOM-001-SEDE

In my more than 20 years leading industrial maintenance projects in northern Mexico —particularly in the industrial corridors of Torreón, Saltillo, and Piedras Negras—, I've seen it all. If there's one procedure where there's no margin for error, where a single oversight can cost a technician's life and millions in losses due to unscheduled shutdowns, it's the industrial electrical lockout/tagoutelectrical lockout/tagout (libranza). Working with voltages of 13.2 kV, 23 kV, or 34.5 kV demands quasi-military discipline and strict adherence to safety protocols.

Lack of planning, use of inadequate protective equipment, or the omission of a single step in the de-energization protocol are the main causes of catastrophic electrical arcs in substations. A lockout/tagout (libranza) is not simply "flipping the main breaker"; it is a coordinated engineering process involving federal regulations, high-precision physical maneuvers, and millimeter-perfect communication with the Comisión Federal de Electricidad (CFE).

In this technical guide, we will address step-by-step how to execute an electrical lockout/tagout (libranza) in medium voltage ensuring the integrity of your personnel, the protection of your plant's assets, and full compliance with the Secretaría del Trabajo y Previsión Social (STPS) and the Comisión Reguladora de Energía (CRE).

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Table of Contents

• What is an Industrial Electrical Lockout/Tagout (Libranza) and why is it vital in Medium Voltage?
• Regulatory Framework in Mexico: Relationship between NOM-001-SEDE and NOM-029-STPS-2011
• Prerequisites: Updated Single-Line Diagram and Work Plan
• The 5 Golden Rules of Electrical Safety applied to Lockout/Tagout (Libranza)
• Step-by-Step Guide to Executing a Safe Electrical Lockout/Tagout (Libranza)
• Dielectric Personal Protective Equipment (PPE) for Medium Voltage
• Coordination with CFE: Procedure and Protocol for External Lockout/Tagout (Libranza)
• Safe Restoration of Electrical Supply (Normalization)
• Common Mistakes in the Mexican Industry
• Real Case: Lockout/Tagout (Libranza) and Maintenance of a 23 kV Substation in Ramos Arizpe
• Professional Electrical Lockout/Tagout (Libranza) Checklist
• Frequently Asked Questions
• Conclusion
• Need professional help?
• Sources and References

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What is an Industrial Electrical Lockout/Tagout (Libranza) and why is it vital in Medium Voltage?

A industrial electrical lockout/tagout is the set of operational, technical, and administrative maneuvers carried out with the objective of isolating and de-energizing (zero voltage) a circuit, equipment, or an entire substation. This procedure is essential for safely performing preventive maintenance, corrective maintenance, load expansions, or civil works adjacent to energized lines.

In the field of industrial electricityindustrial electrical safety

working with medium voltage (typically defined in Mexico between 1,000 V and 35,000 V) implies risks radically different from those of low voltage. At these voltage magnitudes, air ceases to be a perfect insulator. Improper approach to an energized part can ionize the air and cause an electrical arc without the need for direct physical contact.

Risk difference by voltage level	Parameter	Low Voltage (< 1,000 V)
Medium Voltage (1 kV to 35 kV)	Main Risk	Electric shock from direct contact, burns from short circuit.
Electrical arc (Flashover), extreme thermal burns, pressure wave (Blast).	Safety Distance	Avoid direct contact.
Variable according to voltage level (e.g., 1.5 to 2.5 meters according to NOM-029).	Effect of Current	Ventricular fibrillation, respiratory arrest.
Tissue carbonization, molten metal projection, instant death.	Maneuvers	Generally local, manual, and direct.

Remote, using a maneuvering stick or remote motorized controls. Establishing a medium voltage electrical lockout/tagout (libranza)

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Regulatory Framework in Mexico: Relationship between NOM-001-SEDE and NOM-029-STPS-2011

is not an operational suggestion; it is the only physical and administrative barrier that guarantees maintenance personnel will return home safe and sound at the end of their shift. The legal and regulatory framework governing an in our country is mainly supported by two complementary pillars: the NOM-001-SEDE-2012 (Electrical Installations) and the NOM-029-STPS-2011 (Maintenance of electrical installations in workplaces - Safety conditions).

┌────────────────────────────────────────────────────────┐
│               MARCO NORMATIVO MEXICANO                 │
└───────────────────────────┬────────────────────────────┘
                            │
         ┌──────────────────┴──────────────────┐
         ▼                                     ▼
┌─────────────────────────────────┐   ┌─────────────────────────────────┐
│        NOM-001-SEDE-2012        │   │        NOM-029-STPS-2011        │
│    (Instalaciones Eléctricas)   │   │    (Seguridad en Mantenimiento) │
├─────────────────────────────────┤   ├─────────────────────────────────┤
│ • Diseño y especificaciones.    │   │ • Procedimientos de trabajo.    │
│ • Dimensiones de seguridad.     │   │ • Autorizaciones escritas.      │
│ • Sistemas de puesta a tierra.  │   │ • Uso obligatorio de LOTO.      │
│ • Distancias de aislamiento.    │   │ • Plan de atención a emergencias│
└─────────────────────────────────┘   └─────────────────────────────────┘

Industrial welding nom 001 sede 2012 libranza defines the constructive and design parameters that installations must meet to allow safe maneuvers. For example, it specifies the minimum safety distances around switching equipment, the requirements for grounding systems, and the characteristics of medium voltage enclosures.

For its part, the nom 029 stps 2011 mantenimiento electrico regulates "how" work should be executed. This standard mandatorily requires:

• Having a detailed work plan, authorized in writing by the employer or maintenance manager.
• Implementing an industrial electrical lockout/tagout (LOTO - Lockout/Tagout) system.
• Using measuring equipment with a valid calibration certificate to perform the verification of absence of voltage.
• Training and formally authorizing personnel who will perform the maneuvers.

Both standards complement each other. NOM-001-SEDE ensures that the infrastructure is physically suitable for safe de-energization, while NOM-029-STPS guarantees that the human factor applies the correct operating procedures to prevent accidents.

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Prerequisites: Updated Single-Line Diagram and Work Plan

Before touching any switch or hot stick, planning is the most critical step. A safe lockout cannot be performed based on the memory of the plant's oldest electrician or on ten-year-old blueprints.

The Single-Line Diagram: Your Navigation Map

The single-line diagram is the master engineering document that graphically represents the relationship between the components of the plant's electrical system. According to NOM-001-SEDE, this diagram must be permanently updated and available in the substation.

Before lockout, the engineer in charge must perform a physical "walk-through" to compare the single-line diagram against the reality of the plant. The following must be precisely identified:

1. CFE service entrance points.
2. The exact location of load break switches (SF6, vacuum, or air).
3. Main medium and low voltage circuit breakers.
4. Mechanical or electrical transfers (emergency power plants).
5. Potential energy returns (e.g., interconnected photovoltaic systems or cogeneration).

The Work Plan and Risk Analysis (AST)

The work plan must be a formal document detailing the chronological sequence of each maneuver. It must clearly answer: who operates?, what operates?, with what tool?, in what sequence?, and what is done in case of contingency? This plan must be accompanied by a Job Safety Analysis (AST) that identifies the specific risks of the day (high temperatures, confined spaces, work at heights, etc.) and the corresponding mitigation measures.

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The 5 Golden Rules of Electrical Safety applied to Lockout/Tagout (Libranza)

Any industrial electrician in Mexico and worldwide must know and strictly apply the 5 Golden Rules. These rules represent the standard methodology for preventing electrical risks during de-energized work.

┌─────────────────────────────────────────────────────────────────────────┐
│                        LAS 5 REGLAS DE ORO                              │
├─────────────────────────────────────────────────────────────────────────┤
│  1. Corte visible o efectivo de todas las fuentes de tensión.           │
│  2. Enclavamiento, bloqueo y señalización de los aparatos de corte.     │
│  3. Verificación de la ausencia de tensión.                             │
│  4. Puesta a tierra y en cortocircuito de todas las fuentes posibles.   │
│  5. Delimitación y señalización de la zona de trabajo.                  │
└─────────────────────────────────────────────────────────────────────────┘

Advantages and Disadvantages of Strict Application of the 5 Golden Rules

Ventajas

• Risk reduction to zero: If applied correctly, it is physically impossible for an accident due to electrocution or arc flash to occur in the work area.
• Full legal compliance: Perfect alignment with STPS inspection requirements and international audits (OSHA, NFPA 70E).
• Operational peace of mind: Technical personnel work with the confidence that their life does not depend on someone "not accidentally flipping the switch."
• Process standardization: Allows any qualified technician to understand the status of the installation simply by looking at the locks and physical grounds.

Desventajas / Retos operativos

• Increase in downtime: Implementing the 5 rules can take between 1 and 2 hours before starting the actual maintenance work, which puts pressure on production times.
• Investment in specialized equipment: Requires costly and calibrated tools (contact voltage detectors, certified insulating sticks, high-capacity grounding sets).
• Resistance to change: Personnel accustomed to "quick fixes" often consider the process bureaucratic, requiring constant supervision and cultural change.

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Step-by-Step Guide to Executing a Safe Electrical Lockout/Tagout (Libranza)

Below, we detail the steps for a safe electrical clearance in a typical Mexican industrial medium voltage receiving substation (e.g., 23 kV service connection with air or SF6 gas disconnect switch).

1. Disconnection and Opening of Power Sources

The first step is to perform a visible or effective disconnection of all voltage sources.

Engineer's Tip: Never open an off-load disconnect switch (disconnect blades) if the low voltage main breaker or the medium voltage main breaker are closed. Disconnect blades are not designed to extinguish the electric arc generated when interrupting a load current; opening them under load will cause a violent explosion in the substation.

1. Load shedding: First, open all low voltage derived breakers in the general distribution panels (TGD).
2. Opening the LV main breaker: Open the transformer's low voltage main breaker.
3. Opening the MV breaker: If the substation has a medium voltage breaker (vacuum or SF6), open it.
4. Opening the service disconnect switch: With the system de-energized, use the operating stick (duly tested and clean) to open the disconnect switches or the medium voltage load break switch. Visually confirm that the blades are fully open and physically separated from their contacts.

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2. Lockout/Tagout (LOTO) of Disconnecting Devices

Once the disconnecting devices are open, you must ensure that no one can accidentally or deliberately close them again. This is where the electrical lockout/tagout.

   [ DISPOSITIVO DE CORTE ABIERTO ]
                  │
                  ▼
   [ COLOCACIÓN DE PINZA MULTIPUNTO ]
                  │
                  ▼
   ┌──────────────┴──────────────┐
   ▼                             ▼
[ CANDADO DE SEGURIDAD ]     [ TARJETA DE ADVERTENCIA ]
(Llave única en posesión     ("PELIGRO - NO OPERAR:
 del líder de maniobra)       Personal trabajando")

1. Place a lockout hasp on the operating mechanism of the disconnect switch and the medium and low voltage circuit breakers.
2. Each technician who will work in the substation must place their own red padlock (unique key, not master) on the hasp.
3. Place danger tags that clearly indicate:
   • The name of the person responsible for the operation.
   • The date and time the lockout began.
   • The department and contact phone number.
   • The explicit legend: "DANGER: DO NOT OPERATE - PERSONNEL WORKING ON THE LINE".

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3. Verification of Absence of Voltage with Calibrated Equipment

"Every electrical installation is considered energized until proven otherwise." This is the maxim of electrical safety. Do not rely on illuminated indicators on panels; you must perform the verification of absence of voltage physically.

1. Select the appropriate detector: Use a medium voltage contact or proximity voltage detector, suitable for the voltage range of the installation (e.g., a "Snoopi" type detector or an electronic adjustable range detector from 1 kV to 36 kV).
2. Three-step test (Live Test):
   • Step A: Test the detector on a known voltage source (or use the equipment's self-test button) to ensure it is working correctly.
   • Step B: Measure the voltage on each of the phases on the load side of the open disconnect switch. The detector should not emit acoustic or visual signals.
   • Step C: Retest the detector on the known source or self-test to confirm that the instrument did not fail during the actual measurement.

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4. Grounding and Short-Circuiting of Phases

Even if the line is disconnected, residual voltage may exist due to electromagnetic induction from adjacent lines, capacitive loads from underground power cables (capacitor effect), or atmospheric discharges. Grounding is your physical lifeline.

1. Use certified equipment: Use a set of temporary physical grounds for medium voltage (bronze clamps, highly flexible copper cables with transparent insulation, and an operating stick).
2. Ground connection first: First, connect the ground cable clamp to the physical ground bus of the substation. This rule is unbreakable: the ground connection is always made before bringing the equipment near the phases.
3. Connection to phases: Using the operating stick, connect the clamps one by one to each of the three phases of the de-energized circuit, short-circuiting them together and connecting them to the physical ground of the installation.

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5. Delimitation of the Work Zone and Signaling

Finally, you must physically isolate the area where the work will be carried out to prevent unauthorized personnel from entering the operation or to prevent technicians from inadvertently approaching areas that remain energized (for example, the line side of the CFE disconnect switch).

1. Place physical barriers such as red danger tape, plastic barriers, or insulating screens.
2. Install industrial safety warning signs in accordance with NOM-026-STPS (e.g., "Electrical Hazard", "Authorized Personnel Only").
3. Clearly define the "Safe Work Zone" and the "Danger Zone."

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Dielectric Personal Protective Equipment (PPE) for Medium Voltage

PPE for medium voltage is non-negotiable. It's not about wearing everyday work clothes; equipment specifically designed to withstand the operating voltage level and mitigate the effects of an electric arc (extreme heat, projection of particles) is required.

PPE Specification Table for Medium Voltage Operations (up to 36 kV)

PPE Item	Technical Specification	Main Function
Dielectric Helmet	Class E (according to NOM-115-STPS), resistant to 20,000 V.	Protection against impacts and accidental electrical contact to the head.
Arc Flash Face Shield	ATPV certification according to risk analysis (minimum 12 to 40 cal/cm²).	Facial and eye protection against thermal radiation and metal projection.
Arc Flash Suit	Flame-retardant coverall or hood (Nomex/Kevlar), appropriate protection level (Category 2 or 4).	Prevent third-degree burns from thermal radiation of the electric arc.
Dielectric Gloves	Class 2 (up to 17,000 V use) or Class 3 (up to 26,500 V use) with leather protectors.	Hand insulation during the use of the hot stick or contact with metallic parts.
Dielectric Footwear	With polycarbonate toe cap (non-metallic), insulation resistance according to NOM-113-STPS.	Prevent current flow through the body to the ground (step/touch voltage).
Hot Stick	Telescopic or sectional type, fiberglass, tested under ASTM F711 standard.	Maintain physical safety distance when operating switches or applying grounds.

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Coordination with CFE: Procedure and Protocol for External Lockout/Tagout (Libranza)

When maintenance requires intervening in the overhead-underground transition, the bypass switches on the service pole, or the main transformer itself, it is necessary to request a CFE electrical clearance procedure. CFE is the only entity authorized to operate the public distribution network.

┌────────────────────────────────────────────────────────┐
│             FLUJO DE TRÁMITE DE LIBRANZA CFE           │
└───────────────────────────┬────────────────────────────┘
                            │
                            ▼
 ┌──────────────────────────────────────────────────────┐
 │ 1. Solicitud por escrito a CFE (Mínimo 15 días antes)│
 └───────────────────────────┬────────────────────────────┘
                            │
                            ▼
 ┌──────────────────────────────────────────────────────┐
 │ 2. Pago de derechos por maniobra y desconexión       │
 └───────────────────────────┬────────────────────────────┘
                            │
                            ▼
 ┌──────────────────────────────────────────────────────┐
 │ 3. Definición de ventana de tiempo (Día y Hora)      │
 └───────────────────────────┬────────────────────────────┘
                            │
                            ▼
 ┌──────────────────────────────────────────────────────┐
 │ 4. Arribo del personal de CFE y entrega de la línea  │
 └───────────────────────────┬────────────────────────────┘
                            │
                            ▼
 ┌──────────────────────────────────────────────────────┐
 │ 5. Ejecución del trabajo interno por SINE México     │
 └───────────────────────────┬────────────────────────────┘
                            │
                            ▼
 ┌──────────────────────────────────────────────────────┐
 │ 6. Entrega de trabajos, pruebas y reenergización     │
 └──────────────────────────────────────────────────────┘

Key steps for management with CFE:

1. Submission of Request: A formal letter is submitted to the CFE Distribución offices addressed to the zone superintendent. It must include the service number (RPU), the exact location of the plant, the single-line diagram, the justification for the clearance, and the proposed date and time (usually Sundays in the early morning to minimize impact on the network).
2. Approval and Payment: CFE evaluates technical feasibility (ensuring it does not affect hospitals or critical industries in the area) and issues a quote for "clearance operations." Once paid, the date is formally scheduled.
3. On the Day of the Operation: CFE personnel arrive at the service entrance site. They open the fuse cutouts (CCF) on the transition pole and hand over the "de-energized" installation to the plant manager.
4. Handover and Reception: A handover-reception minute of the clearance is signed. From that moment, the authorized contractor (such as SINE México) can install their physical grounds and begin internal maintenance work.

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Safe Restoration of Electrical Supply (Normalization)

The process of handing over and starting substation operations (normalization) is as critical as de-energization. An error here can cause an immediate short circuit when energizing a system that still has forgotten tools or connected physical grounds.

To normalize, the inverse process of the Golden Rules is followed:

1. Removal of personnel and tools: Ensure all personnel are out of the substation cells and that no tools, cleaning rags, or test equipment are left inside the cabinets.
2. Removal of temporary physical grounds: First, remove the phase clamps using the hot stick. Finally, disconnect the ground bus clamp.
3. Cabinet closure: Secure and close all medium voltage cell doors.
4. Removal of locks and tags (LOTO): Each technician must personally remove their lock from the lockout device.
5. Notification to CFE (if applicable): Inform CFE personnel that work has concluded, that the installation is free of physical grounds and personnel, and that it is safe to re-energize.
6. Closing of disconnectors and circuit breakers:
   • CFE closes the fuse cutouts on the pole.
   • The medium voltage disconnector in the substation is closed (no load).
   • Verify that the phase-to-phase and phase-to-neutral voltages on the secondary side of the transformer are correct.
   • The main low voltage circuit breaker is closed.
   • The branch circuit breakers of the distribution panels are gradually energized to avoid current peaks due to simultaneous motor startup.

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Common Mistakes in the Mexican Industry

Throughout our experience, we have detected recurring operational flaws in various manufacturing plants. Avoiding these errors saves lives and prevents severe fines from the STPS:

• Not updating the single-line diagram: Performing a maneuver relying on plans from 5 years ago. If there was a modification or addition of an unregistered capacitor bank, the risk of energy feedback is extremely high.
• Omitting the voltage detector test (Live Test): Blindly trusting that the detector works because "it's new" or "it was used last week." If the detector's batteries are dead, it will falsely indicate "absence of voltage" on a live 23,000 V line.
• Reversing the connection order of physical grounds: Connecting to the phase first and then looking for ground. If the line has static voltage or induction, the electric arc will jump towards the technician when attempting to connect the ground.
• Leaving LOTO lock keys within everyone's reach: Using locks with master keys or leaving the key attached to the lock "in case it's needed quickly." Lockout is personal and non-transferable.
• Not performing insulation resistance tests (Megger) before re-energizing: Energizing the transformer after maintenance without verifying the insulation. If moisture, dirt, or a forgotten tool remained, the transformer will explode upon receiving voltage.

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Real Case: Lockout/Tagout (Libranza) and Maintenance of a 23 kV Substation in Ramos Arizpe

Project Context

A major auto parts plant located in Ramos Arizpe, Coahuila, required major preventive maintenance for its 2,000 kVA pedestal-type main substation, operating at 23 kV, due to signs of overheating in the medium voltage terminals (fuse holder elbows) detected by infrared thermography. The client could not stop production for more than 8 hours on a Sunday.

+-------------------------------------------------------------------------+
|                    CRONOGRAMA DE TRABAJO (8 HORAS)                      |
+-------------------------------------------------------------------------+
| [06:00 - 07:00] Maniobras de desenergización y LOTO (SINE + CFE)        |
| [07:00 -