a little confused - 3 phase theory

[uksel]

here's a bit of as tickler for you (or for me anyway)...

when doing a resitance measurement on a three phase supply, how come i am getting a reading of zero ohms across all three phasses ie. dead shorts? (L1 - L2, L2 - L3, L3 - L1 ALL EQUAL 0 OHMS)

i understand that when measuring the voltage i have a reading of 230v between each phase and earth, and when measuring between any two phases am getting 415, all fine, understood.

but what i dont understand is how there can be a potential difference in voltage for my 415v when a resistance reading is giving me dead shorts, surely this would indicate no voltage difference?

why is there a dead short reading between phases?

 

[Guest123]

Hi.

What size is the armoured and how long is the run??

 

[scotsparky]

Uksel is this a periodic test? if so is it terminated anywhere?

 

[uksel]

it is not a periodic test, was renovating a machine and came across a faulty 3-phase socket, spent about 2 hours trying to find a fault in the machine, turns out it was the dodgy socket. so one thing lead to another, including this thread

 

[Sparky-Tom]

Thats a strange one! When was the instrument last calibrated? Have you tried testing the resistances of known items, such as a 100ohm resistor or the like. See what readings you get?

 

[silva.foxx]

I'm pretty thick and can't visualise where you're testing resistance...

at the socket with the supply off (so back towards the DB)?

at the machine plug (to the machine)?

Please can you elaborate?

 

[stretchyboy]

ditto silva foxx i'm confused too, what test are you actually doing on what and how.
If you are testing live then it's impedance and would never be zero unless your meter is duff
If it's dead test and with continuity then you may be reading the winding of the motor or generator.
we really need some more info........... ;-)

 

[baldsparkies]

Phase to Phase ie R1 of L1 + R1 of L2 resistance of the supplying cable, will most definately contain a resistance reading.
Please tell me you are NOT live testing between phases (400v) thats a big no no you will blow the internal fuse at best or fry your tester,could even put yourself in harms way. Has others have said could you elaborate what and how you have concluded a zero resistance reading between phases.