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Article Date: 19th December 2014

ABB Power - Minimising Risk from Arc Flash

Power Distribution - Power Factor Correction - Critical Power Systems

ABB Engineer

Although arc flash incidents in power distribution systems are rare, they are extremely hazardous when they do occur. In a data centre, they could be caused by human error, poor maintenance or legacy equipment. Awareness of the dangers of arc flash has increased dramatically in recent years following the introduction of new guidelines and standards by international trade and safety bodies such as the Institute of Electrical and Electronic Engineers (IEEE) and the US Occupational Safety and Health Administration (OHSA).

During an arc flash incident, the sudden release of energy causes an intense flash of light and a temperature rise as high as 20,000°C, which itself causes thermal expansion of air that leads to an explosive blast.

Not only does this pose significant risk to operators but it can also damage essential substation equipment and cause expensive service outages and down-time.

The key to minimising the risks posed by arc flash is fast detection and clearing of faults. Arcing that lasts 40 ms (milliseconds) or less typically causes no personal injury or switchgear damage, whereas if left for 500 ms or more, an arc has the potential to result in serious personal injury and major damage to switchgear.

Two of ABB’s products are designed to detect and clear faults quickly and reliably. The REA arc flash relays detect faults rapidly and UFES (Ultra Fast Earthing Switch) enables fast switching to earth. Both can be retrofitted into existing installations, improving safety standards and enabling operators to reduce the need for personal protective equipment.

REA arc flash protection relays
The intense light emitted by an arc is the key to the ultra-fast detection and tripping time of ABB’s REA 10 arc flash relays. A long unclad fibre optic light sensor will pick up the light from an arc flash and transmit it to the relay at the speed of light.

In normal operation, both bright light and overcurrent need to be in evidence simultaneously to trip the circuit and the system can be installed into new substations or retrofitted into existing sites.

At up to 60 metres in length, the sensor can cover the same protection zone at a much lower cost than is possible with individual lens sensors, while overcoming any shadowing issues. Because the sensor can be configured in a loop, it can provide regular self-checking of the sensor’s integrity and generate an alarm if a problem is detected.

Ultra Fast Earthing Switch
The Ultra Fast Earthing Switch (UFES) protects switchgear against arc flash incidents by initiating a 3-phase short-circuit to earth in the event of a fault. An extremely short switching time of less than 1.5 ms and rapid and reliable fault detection ensures that an arc fault will be extinguished almost immediately after it arises.

Rapid operation is achieved by channel- ling the uncontrolled release of energy of the arc through a solid metal three-phase earth connection. It’s the low impedance of this earth connection that encourages the arc current to flow through the UFES unit, triggering the switch to open and stop the current.

UFES is available as a withdrawable assembly to insert into a panel or as a box mounted on top or on the end cover of a panel. Once fitted, UFES will spring into operation when the flash of an arc forming triggers one of several optical sensors. Arc detection takes only around 2.5 ms and together with three-phase current detection will lead UFES’s primary switching elements to channel the current to earth and break the circuit, quenching the arc before it forms.

Medium-voltage circuit breakers are typically too slow to provide protection against exceptionally high peak short- circuit currents. By installing ABB’s IS-limiter, operators can detect and limit a short-circuit current at the first rise. This ensures that the maximum instantaneous current remains well below the peak short-circuit current level. The device is ideal as a link between an ‘unprotected’ and a ‘protected’ switchboard as it can significantly reduce voltage drops for a more reliable power supply.

One advantage is particularly helpful to data centre operators. When operated, the voltage in the parts of the system that are not affected by a short-circuit only drops for a fraction of a millisecond. This means that even sensitive computers
remain protected from drops in the system voltage.

The IS-limiter consists of three independent phases, each made up of an extremely fast switch that can carry a high current but has a low switching capacity and a high rupturing capacity fuse arranged in parallel.

An electronic device inside the IS-limiter monitors the current and at the very first rise of a short-circuit current, it decides whether tripping is necessary. When triggered, a small charge opens the switch on the main conductor within 0.5 ms. The current then continues to flow through the parallel fuse, where it is limited and then finally interrupted as the voltage passes through zero.

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