Managing the risks of high fault current

Mark Pocock :

Hi. I'm Mark from the Electrical Safety Office. Let's talk about high fault current or prospective short circuit current. Simply put, a fault current is the current that flows when a short circuit occurs.

There have been several recent incidents where workers were injured from high current arc flash while working around energised parts. Every day electrical workers are placing themselves at risk if they do not understand the potential hazard of high fault currents while working on or around energised electrical equipment.

To get an idea of how powerful and deadly high fault currents can be, let's think about your average welder. Using only 140 amps, the temperature of an electric arc it produces is about 3,500 degrees Celsius, and it melts metal. Imagine the heat generated by 20,000 or 30,000 amps of potentially uncontrolled current, which you could find at certain electrical installations. These fault currents can produce an arc flash with temperatures up to 20,000 degrees Celsius. This is hotter than the surface of the sun.

In installations with large switchboards or where the supply transformer is close, the potential for high fault current is very real. For example, a 500KVA transformer with a five percent impedance would have a fault level of 14,000 amps. This is why it is very important to consider the environment and the nature of the work you're about to do. A switchboard with a high fault current requires the highest levels of control, because the risks are very high. Working near energised parts in these circumstances can be just as dangerous as performing actual live work, and appropriate control measures must be implemented. This includes testing and visual inspections.

Before you start working, determine the potential short circuit current at the point within the installation where the work will be done, and put the controls identified during the risk assessment into place. A quick visual indicator of the potential fault current can be as simple as identifying the physical location of the supply transformer. Remember the highest level of risk control is to eliminate the hazard altogether by isolating and de-energising the equipment.

High fault currents are not just limited to large main switchboards. They can also occur within other locations of the installation, such as smaller switchboards, electrical supply pillars or large electrical equipment.

The level of prospective short circuit current will vary due to several factors, such as the electricity supply transformer fault current, the circuit length and the cable size. But won't a circuit breaker or fuse stop fault current? In a high current arc flash, the protection may not be enough to interrupt the fault current, which can continue to arc across the protected device even after it is operated. This arc can self-sustain long enough to cause an incredible amount of damage to equipment and severe burns to anyone in the vicinity.

An arc flash can be caused by unsafe work practices and procedures, foreign materials or a breakdown of busbar insulation, electrical equipment failure such as a switch, circuit breaker or loose cables, contacting energised equipment when you're using uninsulated tools, using test equipment not designed or rated for the job.

Electrical work must not be carried out on electrical equipment while energised simply because it's merely convenient for the electrical equipment to stay energised while the work is being performed. Remember it is illegal to perform electrical work on energised electrical equipment unless it is necessary in the interests of health and safety that the electrical work is carried out while the equipment is energised – for example, it may be necessary for lifesaving equipment to remain energised and operating while the electrical work is carried out on the equipment – or it is necessary that the electrical equipment to be worked on is energised in order for the work to be carried out properly, or it is necessary for the purposes of testing by a competent person to determine whether or not the electrical equipment is energised, or there is no reasonable alternative means of carrying out the work.

You can also examine the fault current limit of the installed local and upstream protective devices. If you have any doubts regarding your safety or that of the installation, confirm with the manufacturer that sufficient protection exists. If your investigation reveals inadequate protection, advise the owner both verbally and in writing of the hazardous situation.

More information about electrical work on energised equipment, including mandatory controls, can be found at electricalsafety.qld.gov.au.

Always remember the safest way is to de-energise the electrical equipment prior to commencing any work.

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