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Idea for a new tester...........

whjohnson

………….which helps work out where a buried wiring fault is? 

Having watched a youtube video

 

 where the electrician finds a short in a cable by measurement and calculation rather than stripping out the whole infrastructure I have been thinking that something like the process employed in the video could be turned into a test meter but I have no idea how to go about achieving it. 

Basically, what I had in mind was something the size of a multimeter with a display which would give a read-out in centimeters. The scale could be made seletable for whole metres too.

You program in the tabulated resistance value in miliohms for each cable size given in the OSG /  regs book onto some kind of chip. 

you make this a selectable range option with a dial on the meter, say 2.5mm/1.5mm for example. 

The range would go from 0.75mm to 10.00mm for the common cable sizes, so 1.00mm/1.00mm, 1.5 mm/1.00mm, 2.5mm/1.5mm, 4.00mm/1.5mm, 6.00mm/2.5mm, 10.00mm/6.mm. 

These are the common twin and earth sizes but an option could be included for cables with cores of the same cross sectional area such armoured or round flex e.g. 2.5mm/2.5mm etc. 

Selecting this would set the internal chip to the correct miliohms range value for that size cable. 

You then measure the ends of the cable - one measurement taken from each end (fault somewhere in the middle) and each measurement result would be displayed in centimeters. 

It would then be a simple case of getting the tape measure out and marking the wall or floor with the values given in centimeters by the test meter. 

Why make something like this? Well, it would speed up fault finding on site by not having to disturb vast amounts of building materials, floor coverings, cosmetic finishes etc. You would only need to focus upon a small area for remedial repairs and it would automate the calculation process to give a distance measurement rather than an electrical one. 

Achievable? If so, how? Being something of an old buffer who is a contacts and relays man rather than 0's & 1' I know nothing about silicon chips and programming etc. 

Going by gut instinct and not mkt research here, but if such an item were available I think I'd buy one. 

Do you think the idea has legs?

Your thoughts? 
 

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  • 0

    Actually, the TDR doesn't care about the cable impedance, because it measures this continuously across the trace. It does need to have a good idea of the cable “velocity factor” but this is easily found with a known length of similar cable. It will find any kind of fault where the cable impedance changes suddenly, and often can see joints or whatever because the impedance changes. They are often used on underground cables by the DNO as one can see all the joins and branches, very handy looking for problems. They are not incredibly expensive for fairly simple instruments, prices increase with range, accuracy, etc. as usual. They work by generating a very fast risetime pulse, usually less than 100ps, and recording the voltage across the test terminals as the pulse is reflected from changes in cable impedance. 100ps gives a resolution of about an inch, which should be good enough for electrics. Ones for high-frequency electronic tests may use pulses with a risetime of 1ps or less and are expensive!

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  • 0

    Actually, the TDR doesn't care about the cable impedance, because it measures this continuously across the trace. It does need to have a good idea of the cable “velocity factor” but this is easily found with a known length of similar cable. It will find any kind of fault where the cable impedance changes suddenly, and often can see joints or whatever because the impedance changes. They are often used on underground cables by the DNO as one can see all the joins and branches, very handy looking for problems. They are not incredibly expensive for fairly simple instruments, prices increase with range, accuracy, etc. as usual. They work by generating a very fast risetime pulse, usually less than 100ps, and recording the voltage across the test terminals as the pulse is reflected from changes in cable impedance. 100ps gives a resolution of about an inch, which should be good enough for electrics. Ones for high-frequency electronic tests may use pulses with a risetime of 1ps or less and are expensive!

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