[ALiEN17]

u odnosu da znam interpretirati točno vrijednosti koje ispitujem uređajem Metrel do sada sam to radio 'površnije' na način da kada je bio rezultat ispitivanja i mjerenja OK, onda je i meni bilo ok... sada me više zanima, htio bi ući u dubinu stvari malo više, hvala na pojašnjenjima,... opet zdravo seljački, može li mi netko ovo objasniti, ako nije prekomplicirano :

What is a Z-line and Z-loop test? Are you talking about the options on a test device such as a mft?

If so, Z line measures the impedance Z of the lines R1 + Rn for phase to neutral, and R1 + R1 for phase to phase (depends on your connection)

Z loop measures the impedance of the earth loop - R1 + R2 (line and earth connection Or is the OP asking why such tests are done?

If so, the Z-loop test is to prove the fault current in the event of phase-earth short is sufficiently high to operate the over-current protective device (OCPD) fast enough to have an acceptably low risk of electric shock, or conductor overheating. In a typical UK case (230V, TN system) the "fast enough" is 0.4s for most circuits, 5s for some feeders, etc.

The Z-line test is done as well so the higher of the two prospective fault currents can be recorded, and compared with the ratings of equipment to establish that in the event of a short the OCPD and/or switch-gear can safely interrupt it. As said by pc above, there are two basic reasons, firstly to ensure the impedance is low enough that sufficient current flows in the event of a fault and therefore will operate the protection within the required disconnection times.

Secondly, to determine how much current could flow as a maximum, to ensure that the fault level is not higher than the equipment can withstand or open where appropriate.

There are two basic possibilities, a fault involving earth, and a fault between phases or phase and neutral therefore both have to be checked --- ts basically as it sounds, Z line meausures the impedance between the two line conductors (Live and neutral on single phase, or Live and Live on 3 Phase) and the tester will give you the Prospective Short Circuit Current value also, Z Loop measures the impedance between line and earth, where the tester will also display Prospective Eart Fault Current. A measurement of 48 ohms, in this case indicates it should be a TT system hence the big difference between the two readings.

If it were a TN-C-S the Z line and Z loop readings should be very similar (Because Neutral and earth are the same conductor from the cutout back to the suppliers transformer) however, TN-S systems, the two readings could vary slightly because of the seperate earthing conductor.

Remember, doing the Z loop test you are measuring the resistance down the earth conductor, back to the suppliers transformer, up the windings and back down the live conductor, to the supply cutout and back to your tester.
Z Line is exactly the same but you're measureing down the neutral conductor, up the winding and back down the live. (Or two live conductors on a 3 phase supply)

Hope this makes sence.