Earth loop impedance test

[thomo27]

Good Evening

I am currently studying C&G 2330 Level 2 and at a pratical session last week we went through the earth loop impedance test , pls correct me if i'm wrong but from the session if for example you were to do the E.L.I test on the downstairs ring you would take the ZS reading from the furthest socket from the CU to take the worst reading , the test was done by connecting a 3 pinplug from the Megger and inserting the plug into the socket , I believe this gives us ZS , to then work out r1 and r2 I remember the trainer linking the live conductor into the E terminal at the CU then touching a probe onto this point but can't remember where he connected the other probe , pls can you clarify the exact process for taking the r1 and r2 , oce we had the r1 + r2 we then worked out the Ze by subtracting the reading of r1+r2 from Zs, my understanding of the test is that it is a test to find out the time it takes for the fault on the circuit to be returned via E back to the CU to trip the MCB , pls can you confirm or correct me should my understanding be incorrect and also conf the process for taking R1+r2
Kind Rgds

Stephen

 

[DESL]

It is all detailed on pages 69 to 71 of the on site guide.

The supply characteristics need to be measured with the main earth diconnected to avoid parallel paths.

So subtracting (R1+R2) from Zs will not give you true reading for Ze

The reverse can be used for determining Zs = (R1+R2)+Ze

The maximum value of Zs is given in tables 41B1,B2 & D

The lower the value of Zs the faster any cct protection will function so you are thinking along the right lines

e g say on a ring final Zs = 0.5 ohm under short cct conditions a current of 460 A will flow this value can be looked up on the current time graph device to ascertain whether it will operate within the prescribed time.