Hi,
It's been a long time since I have posted a question. However, have a ' live' situation where I could use the advice of members, experts and sages on the IET forum.
Stated to work in a domestic property single phase supply TT system and when I tested the PFC on the short circuit, the test instrument gives fluctuations in reading from 0.01 ohms to 0.07 ohms
The meter has recently been callibrated and despite this: the thing is jumpy and cannot give consistent readings. I accept that MFT have tolerances and this is a metrel with a resolution of 0.01 with an accuracy rating of -+5digits. So at low ohms it can fluctuate but still be in callibration
I have measured the PFC on the short circuit with a loop tester Martindale which is not callibrated and this is giving a reading of 0.07 ohms.
A consumer unit has been installed which has 6ka breakers and I have checked with the manufacturers Contactum and the board has a conditional rating of 16ka. The supply fuse is Bs1361 type 2.
I have made enquiries with the DNO and they have confirmed that the substation is 50 meters away from the property, it is a 500kva transformers and the supply cable is a 0.1 concentric cable cable of 70mm
This is going to be my assessment job for the NICEIC
The question is this: from the DNO information how do I calculate the pfsc short circuit current
The second question, and this is gleaned from other forums, to achieve greater accuracy in the MFT, it has been suggested to introduce a known resistance. I have made test probes where I have placed a 10watt wirewound resistance of 0.68ohms and used this on the neutral conductor to undertake the test. It has given more consistent readings on the jumpy Mft of 0.06 to 0.08 ohms when I have deducted of the resistor value placed in the test lead
From a gs38 and guidance note 3 basis, would the above practice of placing a known resistor in the test leads be acceptable
I do not intend to undertake the measurement of the PFC with the make shift test probes with the resistor before the inspector, but do feel that in the field it would be method of attaining greater accuracy in testing
Apologies for the length of post but would like some advice.
