testing prob

[underfloor]

i'm using a fluke 1652 to test r1 r2 on rising mains unless the run is very long or the cable size is small the majority of readings are 0.00 is this usual or am i daft and doing something wrong

 

[pevvers]

Make sure your leads are nulled properly, but you will find that the massive CSA of rising busbars won't actually give you a huge R1+R2 reading.

 

[Mark_Burgess]

Try checking the resistance of your leads, don't null them, take your reading and subtract the lead value.
Worth a try.

 

[rocker]

Um, I've had a few beers but I'm fairly sure I'm right in saying a long run or small cable CSA will give you a HIGH reading, not a low one. If you are getting a low reading and its a very short run on thick cable then that's probably fine (some testers wont measure less than 0.03 ohms-ish)

 

[petegibbons]

You should be getting some sort of reading.

If you look in Guidance Note 3 (I am not sure what page) there is a table in there telling you the resistance you should get for each size cable.

If you post an example of a cable you have tested, size & type of conductor and length of cable, I will look and see what reading you should of got.

 

[pevvers]

Um, I've had a few beers but I'm fairly sure I'm right in saying a long run or small cable CSA will give you a HIGH reading, not a low one. If you are getting a low reading and its a very short run on thick cable then that's probably fine (some testers wont measure less than 0.03 ohms-ish)

I think that's what he was saying, unless it's a long run or small cable the meter is reading very low. With a rising main, the CSA of the copper bars are pretty big so therefore the reading may be 0.01 ohms or lower if it's only going through a few floors...

To the OP, have you checked your meter against a known resistance value? You can get the resistance values for different CSA's of cable from the OSG, so why not cut off a length, calculate what the reading should be and then measure and see if they tally up? Or if you've got a little more money, get some resistors or a check box just for your own peace of mind. I reckon it's just because you've got big bars of not a great length...

If you post an example of a cable you have tested, size & type of conductor and length of cable, I will look and see what reading you should of got.

As it's a rising main, he would have to look in the tech specs of the manufacturer for the type of busbar.

Examples that I have just found from the MEM Eaton range are for a 250A rising main.

R1 (milliohm/m) - 0.669
PE (milliohm/m) - 0.397.

So a 5 metre length, not including joints, would be - (0.000669+0.000397)x5 = 0.00533 ohms. And I don't know many instruments, unless you use specialist equipment, that will measure that low accurately.

Even 50M of 250A bar, a very high building, would only come up as 0.0533 ohms, so I can quite happily say with the type of test equipment we all use, maybe incorrectly nulled, would quite easily read 0.00 when the magnitude of the readings are so small relatively speaking.

EDIT: I do apologise, I have just seen underfloor post in the chat box at the start of the forum, and he states there that it is rising mains cable, and not busbars.

The theory still stands though if they're big old cables that the resistance will be extremely low, so I don't think I've wasted my time on this occasion!!