The Canadian Electrical Code, Section 36 defines high voltage as any voltage in excess of 750 volts. Rule 36-006 specifies all of the locations and situations where special signage is required to warn persons of high voltage hazards, awareness being extremely important for protection against electric shock. Another important consideration—access to high voltage areas must at all times be confined to people with special qualifications for entering and working in such areas and therefore all such areas must be accurately identified.
Up-to-date knowledge helps keep people away from danger. In this article, we will review the CEC requirements for high voltage warning signs provided in Rule 36-006, where and in which circumstances they are required.
Without exception, a warning sign such as “Danger—High Voltage” or “Danger—Volts” is required at the entrances of electrical rooms, transformer vaults, high voltage substations and other areas that hold high voltage equipment, to warn the general public and otherwise unqualified people of the high voltage hazards, and also to help limit access to people with specialized qualifications.
High voltage warning signs must also be posted on high voltage switchgear, conduit and cables at points of access, on removable covers or other places where one might accidentally make contact with conductors or high voltage components. Warning signs must also be fastened at least every 10 metres on exposed cables and on cable trays.
Isolating switchgear cannot be safely operated under load and therefore will also require a sign to caution the operator not to open or close the switch before the load has been disconnected by other means. An exception is provided when the equipment is interlocked with a load-breaking device such as a circuit-breaker or load-break switch to prevent opening or closing the switch until disconnection is made by some other means.
An exception, signage is not required for a high voltage horn-gap isolating switch on the primary side of a transformer when the switch is key-interlocked with its secondary side circuit-breaker to prevent opening or closing the switch unless the circuit-breaker has been locked open. Opening the secondary side circuit-breaker releases a key that unlocks the horn-gap switch so that it can be operated safely.
Signage is always required next to high voltage fuses, to warn against their removal or installation unless power is switched off to the fuse holders. Rule 36-006 permits an exception for metal-enclosed switchgear because Rule 36-208 requires that the fuse compartment door must be mechanically interlocked with its controlling switch to restrict access to the fuse compartment until the switch is open. This also prevents closing the switch until after the fuse compartment door is locked.
Rule 36-006 requires warning signs at switching points where the possibility of feedback is present. Examples would include automatic or manual switches for transferring power between an electrical utility supply and an emergency generator. This requirement also applies to tie switches and circuit-breakers interconnecting different systems such as cogeneration, parallel generation and other circumstances where the supplies are interconnected.
When an electrical facility is connected to more than one source of power, a permanent single-line diagram must be posted within sight of the high voltage switchgear that identifies all sources of electrical energy and all points of disconnection. Rule 36-214 specifies that where there is the risk of voltage feedback, there must be a means of visible isolation. This would also include preventing voltage feedback from the low voltage system interconnections in double-ended substations. In such cases, draw-out secondary circuit-breakers are often used to provide visible isolation between the high voltage supplies.
Rule 36-006 also makes some very specific requirements for high voltage switchgear signage where it is essential to accurately identify all the components and points of supply. In addition to warning signs, of maximum importance will be an accurate and up-to-date single-line diagram superimposed on a block diagram showing:
- All available means of isolation, switching and interlocks
- All power supplies both under normal and emergency conditions
- Identification of all high voltage equipment in each cubicle with clear references between the equipment and the diagram
The referenced diagram must be displayed where it can be easily seen from the high voltage switchgear. It should be emphasized that such diagrams must always be kept up-to-date. Many serious electrical accidents have resulted in the past from using inaccurate information.
Where high voltage switchgear has removable covers, signs must be provided warning against entry before the power has been disconnected. If an electrical utility is responsible for isolation, this information should also be provided.
As with earlier articles, you should always consult the local electrical inspection authority in each province or territory for a more precise interpretation of any of the above.