Standards for electrical safety

The NFPA (National Fire Protection Association) developed NFPA 79 as a counterpart to EN 60204-1 for the US market. This standard describes the requirements of electrical equipment on machinery.

Below are some of the most commonly used protective measures:

The risk of operating personnel coming into contact with live parts is generally avoided through an enclosure that provides adequate protection. Protection indices for enclosures have been defined as part of the work going into international standardisation. The degree of protection depends on the hazards and the type of work environment.

• Electrical leads are isolated. Circuit diagrams are designed to prevent hazards resulting from indirect contact.
• Machine actuators and reclosers on protective relays shall be positioned so that they can be operated by production personnel without risk of coming into contact with live parts.
• Reclosers shall be positioned so that the actuating device is outside the enclosure containing the live parts.

Of course, these rules are much harder to keep where maintenance staff are concerned. The machine manufacturer must make every possible effort to consider the electrical safety aspect with regard to any anticipated repair work on the machine.

Machines need to be protected from overcurrent, i.e. currents above the rated current. This includes all overcurrents in an intact electrical circuit and all faulty short circuit currents due to insignificant impedance between points at different voltage. Overcurrent protection is guaranteed through fuses or circuit breakers.

The various machine parts are linked earth-side to each other and to the general earthing terminal. Earthing generally occurs via a protective conductor incorporated into the machine's electrical connection cable.

With high power ratings the earth conductor may be positioned in the immediate vicinity of the supply cable, but it may not be incorporated within it.

These protective measures are met if the conductor cannot be replaced by the machine frame. They are not to be applied if the electrical work equipment has double insulation or reinforced insulation.