One subject that comes up many times is the question of testers, and whether or not it is necessary to fork out hundreds of pounds on test equipment, when a simple 'plug-in' tester seems to do it all?
I answered this question the other day by stating that you cannot get to Australia by bicycle - which got some guys thinking. Some thought you could, some thought you could get most of the way but would need a boat at the end, some thought you would need a boat (or plane) at the beginning, in which case you might as well go all the way by plane!
Which is exactly how it is with testers. If you want to test properly, and have the full range of tests available to you, a proper multifunction tester is required, which will set you back £700-£1000 or more. Various models exist from a number of companies, and my particular preference is either the Megger 1730 series or the Kewtech KT65.
If you want to test TN-S or TN-C-S single phase installations then there is a good economical solution, for example the Kewtech KT63, which will give you small change out of £360. It does not provide Earth Electrode testing or phase rotation, but covers all other standard tests.
What has been suggested is that much of the important electrical testing can be done with a simple to use, plug in socket tester. There are quite a few on the market, one of the 'better' ones states that it can adequately test earth fault loop impedance, and RCD's, and check wiring faults and polarity, a bold boast and at about £60 quite a bargain if it can do all that successfully.
I looked into the specification for this unit, as one of my students had bought one as an affordable testing solution, and was keen to see how suitable it was.
The tester in question gave an audible and visual display using a mixture of warbler and L.E.D.'s. From the specification it gave a 'PASS' signal when the measured earth fault loop impedance was within 1.8 Ohms. Now that obviously is showing that there is an earth of sorts, and that the wiring is correctly connected, but I disagree that this should be taken as a satisfactory 'PASS' !
Taking a measured value of 1.8 Ohms, and applying the (Rule of Thumb) 0.8 factor for the increase of temperature under fault conditions (conductor temperature increase to max. 70 degrees C), this would suggest that an overall value of earth fault loop impedance during earth fault conditions could be 2.25 Ohms. This would equate to a fault current of 102A and an operating time of 35 seconds for a type 32A type B mcb!
Not really the level of safety I would be looking for, when you consider the conductive casing of a Class I piece of equipment could remain live for 35 seconds, with someone dangling on the end of it!