The Stack Archive

Protecting data centres from arc flashes

Mon 12 Dec 2016

Arc Flash

fireworks-ltd-sponsoredArc flash can happen at both high and low voltages – with potentially deadly consequences – meaning data centres must take care to protect their staff, says Jeremy Gadd, Managing Director at GSE Systems Ltd…

Data is central to our lives these days, whether in the form of family photos on the home computer or a spreadsheet on the office server. Losing this information, or being temporarily unable to access it, can be stressful and potentially expensive.

Yet a potential electrical problem called arc flash incidents – in which electricity is discharged through the air like a tiny bolt of lightning – has the potential to play havoc with the data centres that store much of our information. More importantly, arc flash poses a huge potential risk to data centre staff – as it can cause explosions that can maim and kill.

A recent survey by the Ponemon Institute and Emerson Network Power in the US said that the average cost of an unplanned data outage has risen by 38% since 2010, to nearly US$750,000. The figure was derived from a survey of 63 data centres in the USA.

Workers face a higher risk of injury from an arc fault than from an electric shock, and injuries can be severe

The severity of incidents ranged from a drop in reputation through to “damage to mission critical data”. The study concluded that a range of factors – such as the increased use of smart devices – had increased the amount of data flow, making it more imperative to protect data centres from the potential risks from arc flashing.

Electrical equipment used in data centres that are susceptible to arc flash hazards include switchboards, UPS systems, power distribution units, panel boards, busways and remote power panels.

Low voltage incidents 

Arc flash is usually thought to require high voltages, but this is not correct: more than half of all electrical incidents associated with low-voltage (LV) switchgear assemblies involve arc faults.

Workers face a higher risk of injury from an arc fault than from an electric shock, and injuries can be severe: burns, from the high temperatures generated; lung damage, from inhaling vaporised metal droplets; and damage to both hearing and sight. There have also been a number of fatalities.

Most arc flash incidents happen while equipment is being operated, and are generally down to equipment failure. Some of the most common underlying causes are; poor electrical contact; insulation failure; inadvertent contact with energised equipment; and ageing – or poorly maintained – electrical systems. It is impossible to banish the risk of arc faults completely. However, assessing the potential magnitude of these hazards can help to provide information that will allow decisions to be made that could limit damage and prevent injury in the future.

Low voltage analysis

GSE, which offers advice and guidance on how to mitigate the risk of arc flash, analysed more than 6,000 distribution and switchboards at low voltage at a variety of commercial and industrial sites. The results were alarming: it found that almost a quarter presented risks of NFPA 70E Category 3 or higher, while 6.5 percent were in the “Dangerous” category – where personal protective equipment (PPE) alone did not provide adequate protection.

There is a common misconception that system impedances such as transformers can reduce arc flash hazards on LV systems. However, even a small 1MVA distribution transformer can produce a short circuit fault current on the secondary side, of the order of 20-30kA – a substantial value when considering arc flash hazards.

One reason behind the prevalence of ‘low voltage’ incidents is that, when operating and maintaining such systems, protection procedures and training are less stringent – and operations are carried out more frequently. LV installations can often be accessed and operated by people with minimal training, who are less familiar with arc flash than those who operate high voltage equipment. This means that people working on LV equipment could be at greater risk than those who regularly operate high voltage equipment.

Managing risk

Calculating arc flash incident energy requires specialist knowledge. The potential severity of an incident depends on the configuration of the entire electrical system, along with an understanding of system fault currents and protection clearance times. Specialist software is usually required to make these calculations. In addition, a risk assessment – which takes factors such as the condition of the equipment into account – is also a key process.

The challenge is to reduce potential arc flash energy without sacrificing selectivity and network integrity

A proper approach to managing arc flash hazards has three related elements: assessing the extent of the hazard; designing and implementing mitigation measures (reducing the hazard) and managing residual risk (reducing the risk).

Assessing the hazard

Calculating the potential severity of an arc flash requires knowledge of system fault currents and protection clearance times. Specialist software is normally used to analyse fault levels and the time taken to clear these faults,arc incident level calculations are performed in accordance with IEEE 1584. This stage requires special knowledge, and many organisations choose to employ external specialists.

Implement mitigation measures

Because arc flash current is always less than short circuit current, an efficient protection assessment should be performed to ensure that correct settings for overcurrent protection devices, such as relays and circuit breakers are chosen. The challenge is to reduce potential arc flash energy without sacrificing selectivity and network integrity, so the portion of a power system that is disconnected in response to an event is minimised.

Manage residual risk

Once mitigation strategies are in place, an equipment operator must understand the residual risk, and take steps to manage them. These include clear labelling of equipment showing the incident energy levels and its boundaries; proper training for staff involved in switchgear operation and maintenance; compliance with appropriate operation and maintenance procedures; and use of personal protective equipment (PPE) that is appropriate to the level of residual risk.

An increased arc flash risk is often an unintended consequence of the design and operating characteristics of modern data infrastructure

The last three steps are of particular importance in data centres – where staff may have a high level of expertise in areas other than electrical power systems, and where there can be a cultural reluctance to adopt appropriate PPE.

Large and small

Bluffdale-utah-nsa-data-centerArc flash incidents can affect all types of data facility. The US National Security Agency’s huge data centre in Bluffdale, Utah is a prime example: construction of the centre’s 65-megawatt electrical distribution system proved troublesome, as a series of arc flash incidents during commissioning delayed the project by more than a year.

But arc flash incidents are not restricted to large data centres. An increased arc flash risk is often an unintended consequence of the design and operating characteristics of modern data infrastructure. For example, increasingly powerful servers and higher rack densities are creating greater demand on data centre electrical distribution systems.

At the same time, the quest for greater energy efficiency is leading to new distribution system configurations – with fewer, larger transformers. And, high service level requirements increase the likelihood that operations and maintenance personnel will work on – or close to – energised equipment.

System-wide approach

Effective management of this hazard requires a system-wide approach. It involves detailed studies to identify the magnitude of the hazard at every point in the power distribution network, the correct safety management procedures to reduce those hazards, and ongoing management of the residual risks through appropriate operations and maintenance procedures.

When considering the danger of arc flash incidents in data centres, it’s worth remembering that it is ultimately about the protection of workers rather than protection of data, however protecting workers should ultimately assist in protecting the data, whereas only considering the data, would not protect the workers .

Also see:

Gas vs. High Pressure Water Mist in data centre fire suppression – what are the realities?

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