It is widely recognised the higher the voltage of an electrical power system, the  greater the risk for people working on or near energised conductors and equipment. Although the electric shock hazard increases with voltage, another hazard known as electrical flashover or arc flash, can actually be much worse at lower voltages. Electrocution is the first thing that usually comes to mind when someone is killed or injured by electricity; however, this is not the only hazard that exists. An electrical arc flash can be devastating causing severe burn injury and even death. In part two of this article, Mike Frain and Jim Phillips explain what can be learnt from American calculation methods of calculating risk management for live proximity working

Part one of this article can be viewed in the Features section of this site or in Electrical Review August 2009

Another key component of the AFCS is to produce warning labels that can be placed on electrical equipment such as switchgear, motor control centres, panelboards, switchboards and similar equipment. In addition to providing a warning, these labels list the calculated incident energy, PPE requirements and AFPB as well as information about the electric shock hazard. Many commercially available computer programs have integrated the arc flash calculations and warning label production into their software to make the AFCS a natural extension of a short circuit and coordination study. Arc flash calculations can also be performed manually and a free calculation guide with worksheets and examples can be downloaded at www.brainfiller.com.

Since there are many variables and scenarios that, if not properly addressed, can lead to incorrect results and ultimately jeopardize a worker's safety, the arc flash calculation study should only be performed by properly trained and experienced individuals.

Although much research has already been conducted in the area of arc flash, much is yet to be learned. There is presently a joint collaborative effort underway between the NFPA and IEEE that will take arc flash research to the next level. This multi-year effort has a budget of over $US6m and will attempt to answer questions such as how to calculate DC arc flash, blast pressure, arc sustainability and much more.

THE SHORT CIRCUIT PARADOX
The magnitude of incident energy available during an arc flash is directly dependant on the short circuit current flowing through the air gap and the time it takes an upstream protective device to clear the fault. In general, the greater the short circuit current the greater the incident energy, however this is not always the case.

It is a commonly held belief the greater the available short circuit current is at a given location, the more damage can occur. When it comes to evaluating a protective device's interrupting and withstand capability, this is a true statement. However, in the case of arc flash, it is quite possibl a lower short circuit current can cause the upstream protective device to take longer to operate and actually increase the overall incident energy exposure.

The time current graph in Figure 1 (please open the PDF at the bottom of this article) can be used to illustrate this paradox. The horizontal axis of the logarithmic graph represents current in amps and the vertical axis represents time in seconds. The time current curve, also known as the tripping characteristic defines the relationship between current and tripping time. Time current curves will typically have an inverse characteristic meaning time and current are inversely proportional to each other. The greater the current the less time it takes the device to operate, and the lower the current the longer it takes to operate. Many protective devices will have an instantaneous trip function defined by a vertical band as shown on the graph. If the current exceeds this value, it will trip in just a few cycles. However, if the current is less than the instantaneous value, the device will trip with some time delay.

This graph illustrates that if the short circuit current flowing to the arc flash is 12,000 amps, the protective device will trip instantaneously resulting in incident energy of 2.4 cal/cm2. If the current drops to 6,000 amps, the device will no longer trip instantaneously, but instead will time out to 6 seconds. Even though the short circuit current is less in this case, because of the increase in the protective device's tripping time, the overall incident energy increases to over 100 cal/cm2. This paradox shows how important it is that an arc flash calculation study includes many operating scenarios to evaluate the effect that the short circuit current will have on the device clearing time and ultimately on the incident energy.

DO ARC FLASH CALCULATION STUDIES MATTER?
At this point in time it is unlikely a UK Health Safety Executive (HSE) Electrical Inspector will ask to see an arc flash calculation study based on IEEE 1584 calculations after an electrical burn accident whilst doing live work. Instead, the inspector would likely ask for an account of what the worker was doing there in the first place. If it can be shown that it was unreasonable in all the circumstances for the conductor to be dead and it was reasonable in all the circumstances for that person to be at work on or near that conductor while it is live, then the third part of regulation 14 from the EAWR 1989 will come into question. In other words, were suitable precautions (including where necessary, the provision of personal protective equipment) taken to prevent injury. In addition to competent staff, the precautions should include where appropriate, the use of adequate PPE, insulating barriers or screens, suitable test equipment and leads and accompaniment. Other precautions should include; providing adequate information to the person carrying out the work about the live conductors, equipment, foreseeable hazards and determining the arc flash protection boundary in which only authorised persons wearing appropriate PPE can be admitted.
Working safely in accordance with the requirements of the Electricity at Work Regulations 1989 is about decision making. This includes the decision to work live in the first place through risk assessment. One of the factors that would need to be taken into account in deciding whether live proximity work could proceed is stated in the memorandum of guidance to the EAW Regulations as "the level of risk involved in working live and the effectiveness of the precautions available set against economic need to perform that work". Even testing of electrical systems needs the same degree of care in decision making. There are danger areas to be identified and as can be illustrated below, the testing of low voltage transformer secondary terminals is an area where very high incident energy levels can be present even at 208 volts.

There are many American owned companies with operations in the UK and Europe as well as other international organisations that apply US standards for electrical safety and the arc flash hazard. Mike Frain has worked with American companies to ensure US standards are used to supplement existing UK regulations rather than replace them.

NEXT STEPS
There is no substitute for safe work practices, and the goal should be to work only on de-energised equipment. That said, live work is sometimes necessary and justifiable. Well documented and up to date safe working practices should be made available for people who carry out any work on or near electrical systems. The HSE guidance note HSG85 Electricity at Work - Safe Working Practices is a valuable resource in this respect. It clearly sets out decision-making flowcharts on whether to work live or dead and also provides some guidance on live working procedures. An arc flash calculation study can be instrumental in the performance of a more comprehensive risk assessment by providing documentation of the calculated incident energy as well as the appropriate personal protective equipment and arc flash protection boundaries.