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MEASUREMENT TECHNIQUES:
SAFETY
In most situations, the voltages being measured for power quality analysis are lethal.
Please adhere to this warning with a "life and death" view, also ensuring all safety precautions are taken to eliminate injury as well as fatality. This point just cannot be stressed enough. Unfortunately there are still too many people being killed through ignorance and stupidity. There is one superb rule
if in doubt, don't.
If you are in anyway unsure of how you are going to connect up voltage and current probes then it is best not to tackle the measurement until safe plans are clearly mapped and in place. Remember to consider both yourself and anyone else working with you.
The First Step is "One Step Back":-
When you open a panel up start by stepping back and draw the circuit out, first in your mind and then on paper. Don't neglect the paper phase. It's so easy to be lazy, but it could be costly. If you have someone with you (you should always have someone with you when doing dangerous work) then talk through what you have planned. You may even trip yourself up before making a disasterous mistake.
Consider the Instruments and their Input Connectors:-
A prime example is the extremely dangerous practice of using standard electronics oscilloscopes for use in power quality. These use BNC connectors that are metal cased. Should such a case come into contact with a live point, there is likely to be many sparks - followed by even more sparks in the form of words! A standard desk-top oscilloscope is not suitable for use on the mains. You ignore this warning at the risk of your life!
There are a number of safety connectors available in both the common 4mm test plugs as well as BNC connectors. These drastically reduce the possibility of bringing an earthed point to a live connection. Please use them.
A word of warning when using hand held rechargable instruments :-
It is so easy to forget that the mains powered charging adapters have breakdown voltages. If the adapter is used simulataneously while measuring a voltage with the "common" input above the breakdown of the charging adapter, a flashover in the adapter will more than likely result in the adapter, instrument, and device being measured suffering damage. If the dissapated energy is high enough there is a possibility of things "blowing up in your face" - literally!
Beware of Colour Codes :-
There are at least three different colour codes used throughout the world, and there are probably many more when considering there may be ones generated for a specific site. Black in some parts of the world is Neutral, and in other parts it is Live. So too for Blue and White. Know the colour code of the site you are working on, especially an old building in a country that has since then adopted a new colour code!
Before connecting up any instrumentation, ensure the voltages that are expected are the ones seen (use a highly insulated DMM or similar). Some instruments are sensitive to polarity and it is not uncommon to find Live and Neutral reversed. This could have dire consequences for the instruments, let alone the readings (especially if you are powering the instrument from the test points!).
Only Use Safe Test Leads :-
When doing measurements many will be concerned with the capabilities of the instruments but cast not a thought towards the leads connecting the instruments to the test points! During this type of work it is imperative that test leads are suitable for the voltages being tested as well as ensuring they are in a suitable (and that means perfect) working condition. My rule is: All test leads used for testing mains and LV (<1kV) are minimum CAT III 1000V rated.
Then, do not neglect or abuse the test leads. Worn or frayed test leads are a potential hazard to the user. Always inspect test leads before connecting the instrument to the test points. A very reliable method is to lay the test leads out on a large area while running each through your hand. Any distortions in the outer protection, be it cut, burnt, frayed, etc., will be quickly felt as it rubs against your fingers.
Fuse Each Test Point :-
I shudder each time I hear of how some or other twit professing to be a power quality investigator coming exceedingly close to burning himself, simply because they omitted to install a fuse in the test lead or connection point. With fused crocodile clips complying to GS38 now being freely avilable from the likes of Silvertronic (www.silvertronic.co.uk), there is absolutely no excuse to have a test lead burn off a busbar during testing.
Stay Alive :-
Safety can be summarised in one simple statement....
"Always have the ultimate aim in mind -
talking about your findings
over a well deserved cup of coffee."
A Word On Calibrations >>
© 04.12.03
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