Equipment Over 1000 Volts, Nominal

Article 490 collects the general requirements for equipment operating over 1000 volts — the medium- and high-voltage switchgear, metal-enclosed equipment, circuit breakers, fuses and enclosures found in substations and large facility services. At these voltages electricity behaves differently: it can jump air gaps, so clearances, insulation and guarding all scale up.

The article specifies construction and marking of high-voltage equipment, working space and clearances greater than for low voltage, guarding of live parts, and the warning signs that keep unqualified people away. It's a general framework that other high-voltage articles and standards build on, and it's strictly the domain of qualified workers.

When it applies

Article 490 is the general playbook for equipment operating over 1000 volts, medium-voltage switchgear, enclosures, and the like. It sets the working clearances, the guarding and locking of high-voltage compartments, and the warning signage, all scaled up because the arc-flash and shock energy at these voltages are in a different league.

Common mistakes

• Applying low-voltage working-space numbers to medium-voltage gear, the clearances here are larger for a reason.
• Underestimating the need for proper interlocks and grounding before anyone opens a compartment.
• Skimping on qualified-person requirements and PPE, above 1000 volts nothing is casual.

In the field

I do not let anyone near open medium-voltage gear who is not qualified for it, and we ground the conductors before touching them, every time. At these levels the mistakes are not shocks you shake off, they are the kind you do not walk away from, so the discipline is absolute.