Another way of determining the prospective short circuit current at source, or anywhere in the electrical distribution system, is by measurement. Most earth loop impedance testers give us the ability to return a short circuit current value, but extreme care must be taken in the interpretation of results. Such instruments become more inaccurate the closer to the source transformer because of the higher levels of inductance. There are, however, high precision loop testers available that will give an ohmic value of resistance and reactance as well as the balanced three phase short circuit current. Accuracies of 1 milliohm are possible from such an instrument.
That sounds great so why do we not just measure the PSCC everywhere instead of by enquiry and calculation? Well, the answer is that you could, but I would exercise some caution because of the following.
- Live testing is classified as live working insomuch that there will be interactions with live conductors. As we have learnt in this guide, live work must pass the test of reasonableness before being sanctioned.
- For high precision loop testers, the test current is likely to be in the order of three hundred amperes rather than the usual 25 amperes that one would find with standard earth loop impedance testers. Whilst the instruments that I have used are extremely robust and well capable of handling such test currents, they do require another degree of care in application.
- Contributions to PSCC under fault conditions such as from induction motors cannot be picked up using an instrument and may need to be factored in. The IEEE 1584 incident energy level calculator which is appended to this guide does allow for an estimate of motor contribution to be calculated in addition to the source PSCC.
In conclusion, an understanding of the prospective short circuit current, which is available at source, is a particularly important early step. What I mean by the word understanding is, what are the realistic maximum and minimum values likely to be? And what factors are significant to the outcome of the arc flash incident energy calculation? Bearing in mind that the IEEE 1584 calculations have built in confidence factors such as the normal and lower arc current calculation, the exercising of care in determining the source impedance will result in greater credibility and accuracy of the overall risk assessment. I would therefore suggest that instead of making a choice between enquiry, calculation, or measurement, that a combination of the three factors will lead to validation and confidence of the characteristics of the supply sources.
8.3 The Survey
Make friends with the guys at the facility and, if not done at the initial meeting, spare the first hour or so on the first day to carry out a presentation to them. Tell them what you are doing, how you are going about it and exactly why you are doing the work. This is a time to allay some fears that they may have in respect of over engineered and cumbersome PPE and following the process in this guide will lead to sensible management of the arc flash hazard. In this way you can win them over and obtain much more cooperation and hopefully assistance in the information gathering process.
8.3.1 Existing information
It is important to understand that the information that is gathered in the discovery phase will have a bearing on the accuracy of the final calculations. There is often a plethora of information available, some new and some old and often conflicting. Very often circuits have been installed that have not been recorded in the proper way and there will be no information available. There is usually a single line diagram for the high voltage system available at each site and if not, that would signal a major non-conformance. The main thing to understand, is that any information, which is made available from records on site, must always be questioned and validated.
8.3.2 Collecting data.
My approach has been to break down the whole of the electrical distribution system into small chunks and work through circuit by circuit. You will eventually have a valid single line diagram with detailed circuit data, but the raw data can be populated onto a spreadsheet which is split into four separate sections with each row representing one circuit. The first section can be about the panel or equipment detailing the equipment reference, location details, busbar geometry, type and where it is fed from.
The second section is all about the protection for the outgoing circuit with information including circuit reference, make, type, rating, CT ratio, breaking capacity, and settings. The third section relates to the outgoing circuit which will have the cable details such as cable reference, type, size, cores per phase, length and method of installation. The fourth section can pick up the load type and included here will be large motors and particularly induction motors that can contribute to prospective short circuit current. Each row of data for every circuit should be populated in full and any missing data will be easily recognised from the grid. The grid should be large enough to be able to view a whole panel or switchboard onto one single sheet. Keep it simple and try to avoid duplication and in that way, nothing will be missed. Sketch out any circuits that may be more than discrete radial circuits such as bus sectioning and rings.
In addition, take high resolution photographs which are catalogued to the above panel/circuit references. This will serve as an aide memoire in the validation process.
Data collection software is available, usually from the manufacturers of complex modelling software. The programs work well with the associated programme but are not easily interchangeable. See Chapter 13: Complex Software Guide.