18.104.22.168 Conductor Gap
There are various schools of thought on just how far you have to go in arriving at an exact figure when collecting data on conductor gap. The IEEE 1584 Guide for Performing Arc Flash Hazard Calculations has a number of default gaps for determining the possible arc distance between conductors. The latest version (2018) suggests 152mm for 15kV switchgear and MCCs, 104mm for 5 kV switchgear and MCCs, 32mm for low voltage MCCs and panel boards and 13mm for cable junction boxes. There was a survey some years ago by Jim Phillips on brainfiller.com and he asked if people use the standard default gaps from IEEE 1584 rather than measure them on site. It was almost unanimous that people do use the default gaps. The reasons given were that it was implausible that individuals were actually going to measure the equipment and anyway there are multiple gaps for any given piece of equipment, so which one do you use?
The fact is, that the conductor gap does make a difference otherwise the calculations would not allow for it. I am afraid this is where engineering judgement comes in once again. Looking at any given piece of equipment, you may not know the exact arc gap, but you will have a good idea of what the arc gap is not. In other words, an estimate to see if the default gaps from IEEE 1584 are reasonable and failing that err on the side of a larger arc gap. At lower voltages a larger gap will tend to give a reduced arcing current and the most incident energy per unit time. The tools that have been provided with this guide allow you to play with the conductor gap figure and make up your own mind on a case-by-case basis.
8.3.4 Equipment Condition
It is strongly recommended that there is an assessment of the condition of the equipment during any survey to assess arc flash risk. The following is a good template for assessing this risk which will feature in the final report.
One of the criticisms of US standards in respect of electrical safety and arc flash was that there was a perceived link directly from incident energy levels into the wearing of PPE. So, regardless of what condition the equipment was in, or whatever the task was, if the calculated incident energy level was 40 cal/cm2 then you had to wear a 40 cal/cm2 suit. At one extreme, workers were as a result, switching perfectly safe brand-new switchgear which was often internal arc protected. In 2012, I was asked to peer review a paper titles Risk Management of Electrical Hazards by the author Daniel Roberts, which was presented to the IEEE Electrical Safety Workshop in Florida and introduced the concept of “as low as reasonably practicable” (ALARP) principles based upon internationally accepted OHS Risk Management. The paper was well received as was my paper that I presented to the same conference alongside co-author Jim Phillips on “A European View of Arc Flash Hazards and Electrical Safety”. In the years since then the US consensus standard NFPA 70E Standard for Electrical Safety in the Workplace has developed more of a risk-based approach to the provision of PPE.
Part of the assessment of risk from NFPA 70E has introduced the concept of normal operations and providing that the equipment meets the six conditions below. Normal operation of electric equipment shall be permitted where all of the following conditions are satisfied:
- The equipment is properly installed and commissioned. In other words that the equipment is installed in accordance with applicable industry codes and standards and the manufacturer’s recommendations.
- The equipment is properly maintained. Which means that the equipment has been maintained in accordance with the manufacturer’s recommendations (plus company engineering instructions) and applicable industry codes and standards.
- The equipment is used in accordance with manufacturer’s instructions.
- The equipment doors are closed and secured.
- All equipment covers are in place and secured.
- There is no evidence of impending failure. Which means that there is evidence such as arcing, overheating, loose or bound equipment parts, visible damage, or deterioration.
This gives a template for estimating the likelihood of an occurrence of an arc flash and if all the conditions are met then the likelihood is very low. Should any of the conditions fail then there is a greater likelihood of an arc flash and therefore other measures including use of PPE as a last resort would be necessary.
8.4 Protection Data
When collecting data on protective devices, the vital question must be, which device will operate in the event of an arcing fault. A good example of this is where there is an incoming circuit breaker as part of a factory build switchboard or a distribution switchboard. If there was to be a fault on the load side of this circuit breaker, can it be assumed that this device will clear the fault without being involved in the arcing event? In other words, can the arc propagate to the line side of the circuit breaker? If it can, then the next upstream device must be considered as the means of disconnecting the arcing fault in which case the time to trip will be longer leading to greater severity. A similar course of action must be considered if there are doubts that the incoming circuit breaker is capable of disconnecting the arcing fault because of a lack of maintenance or obsolescence.
The collection of protection data will be aided by some preparation in respect of existing records and also building up a library of protective devices and associated curves. Mainstream manufacturers have improved access to time current characteristic curves, and many provide the means to print the curves and settings. There are also tools available with this guide which will assist in the modelling of curves and settings to help validate data. I have given some description on the relay types as some younger engineers may not be familiar with the history and background on the older technologies.