Hi There,
We have just installed a new 800kVA transformer on site with a impedance of 4.75%. However when it comes to taking the EL reading at source we are getting 0.12-0.14ohms compared to what we expect of 0.01-0.02 ohms.
Other than checking the cable connections, changing the meter or having faulting TX can anyone give me some ideas why this may be. Has anyone come across this before?
Further to this, you'd not normally measure it in situ at all unless there was evidence of a problem and even then you may be better to look at voltage drops on switching of really big load banks of a decent fraction of the rating.This is in effect all the tester does, but given its size it can only switch a small test current, so in a low impedance circuit, the voltage drop it tries to detect is 10% of SFA and small compared to lots of other voltage changes due to other things like oxidation and contact resistances, so its gives a funny answer, a bit like a fruit machine - you pull the handle get some random digits, and perhaps a lemon 
Mike.
Both these machines are made and have application. The one on the left is used for filling out EICR forms on cheaper rented property I think....
But joking aside, it is important to understand the limits and likely variation from the desired result in both cases. - failure to do so can cause a rapid loss of profits and credibility. Over the range of Zs for which it is designed to be used the basic loop testers are very good, outside that, they are not to be trusted. Testers, know your limits and if in doubt check the maker's catalogue data - usually the uncertainty is so many % plus a few digits in the last place, assuming shiny new leads and a following wind..
Further to this, you'd not normally measure it in situ at all unless there was evidence of a problem and even then you may be better to look at voltage drops on switching of really big load banks of a decent fraction of the rating.This is in effect all the tester does, but given its size it can only switch a small test current, so in a low impedance circuit, the voltage drop it tries to detect is 10% of SFA and small compared to lots of other voltage changes due to other things like oxidation and contact resistances, so its gives a funny answer, a bit like a fruit machine - you pull the handle get some random digits, and perhaps a lemon
Mike.
Both these machines are made and have application. The one on the left is used for filling out EICR forms on cheaper rented property I think....
But joking aside, it is important to understand the limits and likely variation from the desired result in both cases. - failure to do so can cause a rapid loss of profits and credibility. Over the range of Zs for which it is designed to be used the basic loop testers are very good, outside that, they are not to be trusted. Testers, know your limits and if in doubt check the maker's catalogue data - usually the uncertainty is so many % plus a few digits in the last place, assuming shiny new leads and a following wind..
