Arc flash relays meet code compliance

Revised NEC 240.87 requirements make mitigation easier to achieve.

06/04/2015


Arc flash relays use either point or fiber-optic light sensors to detect an arc and send a trip signal to the circuit breaker in <1 ms. Courtesy: Littelfuse

Besides wanting to comply with state regulations, managers knew that insurance providers expect equipment to meet minimum design standards, which the NEC provides. They discovered that the code change makes it easier to protect workers and equipment from arc flash hazards.

In 2014, the NEC was revised in multiple areas. Changes to Paragraph 240.87 have made compliance simpler and less expensive. One easy-to-apply device can simplify meeting the new requirements while providing important additional safeguards for people and equipment.

The previous version of the Code

Paragraph 240.87 of the 2011 edition of the NEC, which was titled Non-instantaneous Trip, said that whenever using a circuit breakerwith a rating of 1,200 A or higher (or one that could be adjusted to 1,200 A or higher) that did not have an instantaneous trip function, one of the following was required:

  1. Zone-selective interlocking
  2. Differential relaying
  3. An energy-reducing maintenance switch with local status indicator.

Zone-selective interlocking addresses a shortcoming of selective coordination. It involves interconnecting downstream and upstream circuit breakers: If a short circuit or ground fault occurs on a branch circuit, the breaker feeding it will trip instantaneously, and simultaneously send a signal to the breaker just upstream, telling it to use its time-delay function instead of tripping instantaneously.

This is important because, otherwise, a downstream short or ground fault could draw enough current to cause both breakers to trip instantaneously, killing power to branch circuits other than the one with the fault.

An unfortunate drawback is that an arc flash may not immediately draw enough current to trip either breaker, resulting in personal injury and damage or destruction of equipment.

Differential relaying uses current transformers at the inputs and outputs of the electrical equipment being protected (zones). When a fault occurs, the zone in which the input and output currents do not match is the location of the fault, and the appropriate breaker is tripped. This is complicated and expensive and takes up a fair amount of space.

An energy-reducing maintenance switch manually sets the current pickup lower and the time delay faster, to trip the breaker feeding a panel as fast as possible while someone is working on it. If there is an arc flash, the breaker should trip instantaneously and limit the energy delivered. This reduces the level of PPE required for the panel. The switch must be activated manually before beginning the maintenance activity to provide protection, and deactivated afterward to prevent future nuisance tripping or miscoordination.

The current version of the Code

The 2014 edition of the NEC changed the name of paragraph 240.87 to Arc Energy Reduction and focused more on personnel protection. As explained in the “2014 NEC Handbook,” the change to this section removes the mention of instantaneous tripsetting from the criteria for applying this requirement. 

Instead, it is now required where the highest continuous current trip setting in a circuit breaker is rated or can be adjusted to 1,200 A or higher. It allows for two new methods that can make the job of compliance a good deal easier:

  • An energy-reducing active arc flash mitigation system
  • An approved equivalent means.

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