How to manage arc flashes and maintain networks

30Jun

posted by: John Langley-Davis, Schneider Electric

In recent years, arc flashes have become a hot topic for industry because of the increasing awareness of its dangers, both to personnel and equipment.

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As a society, we increasingly demand an uninterrupted supply of electric power. Prolonged power outages cause loss of business to the power supplier and loss of production to the power consumer.

While safety is paramount in the design of power systems, the reality is that faults occur. The damage caused by faults must be kept to a minimum. The ultimate solution is to selectively isolate the fault as fast as possible, while maintaining the operation of the healthy network parts.

What is an arc flash?

An arc flash is the release of energy through light and heat produced by an electric arc. Sufficient levels of arc flash energy can cause substantial damage both to workers and the equipment in close proximity to the incident. Facility owners will have strategies in place for reducing arc-flash incident levels, but it’s important to understand what is happening inside your systems in order to effectively mitigate the problem.

How do arc protection relays work?

An arc protection relay is a protective device used to maximise personnel safety and minimise the material damage to an installation in the most hazardous power system fault situations. Arc protection systems detect an arc in an installation and measure the fault current.

In an arc flash situation, the arc protection relay immediately trips the related circuit breaker(s) to isolate the fault. An arc protection system operates much faster than conventional protection relays and thus damage caused by an arc short circuit can be minimised.

An arc protection system can principally be implemented in three different ways: as an autonomous master unit system, as a part of a protection relay system or as an intersection between a master unit system and a protection relay system.

Why arc flash protection?

Traditional protection techniques may not provide a fast enough defence against substation faults. Further, high-impedance earth-faults may cause prolonged operation times for earth-fault relays. This in turn can lead to a significant release of arcing energy. This can pose a considerable risk to human life and high-value assets.

To choose the most appropriate means for addressing potential arc-flash incidents, specifying engineers need to understand the safety and reliability goals of a particular site.

Managing risk

Arc flash incident energy builds in direct correlation with the length of time of the incident. For example, double the duration of the arcing fault and the available energy doubles; halve the duration and you cut the energy in half. A number of methods can be employed to reduce the arc flash duration including incorporating Zone-Selective Interlocking Schemes, Arc Flash Maintenance Switches, Low Arc Flash Circuit Breakers, or Arc-Flash Protective Relays. These options can clear faults faster than the over current protective device’s normal operating trip settings, reducing the clearing time and arc flash incident energy released.

Isolating the blast

Containing arc flash energy is critical. In these settings, arc-resistant equipment that protects against blasts may be required. Such products, including medium and low voltage arc resistant switchgear as well as arc resistant low voltage motor control centres, are able to contain the rated values of arc flash incident energy that are not able to be reduced through relays or other methods. Per testing standards, all of the exhaust gases and thermal energies are directed away from the front, sides and rear of the equipment, protecting employees and preventing the proliferation of an arc flash to other systems. This includes preventing the arc flash from:

• Causing doors or covers to open or blow off during the event
• Fragment and eject parts within the protected area
• Allowing the arcing fault to burn through the enclosure
• Allowing cotton indicators spaced about the gear to ignite
• Have any of its grounding connections become ineffective

Over the last several years there has been an increasing amount of learning and awareness around arc flash hazards. To maximise reliability and ensure safety, increased facility performance and arc flash mitigation have to work in tandem.