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.