3 Phase query - help!!!

[polo1]

Hope it's ok to post here and on the general forum!!

What would cause me to lose indication of a phase when aplying a load to that particular phase?It only happens when certain circuits are closed/loads applied.

Thanks.
PS It's a 3P&N Sub board with no isolation and I cant source where it's fed from!

 

[Knobhead]

How are you testing, Ph to Ph or Ph to N? On a loaded board, testing Ph to N is useless as you will have back feeds from other loads. You have to test Ph to Ph and note the voltages. It’s no good just saying that phase is live because your test lamp says so, the voltage could be anything!

I’ll bet you a pound to a pinch of $hit there's a blown fuse in the feed to the board!

 

[polo1]

Hi Tony,
thanks for responding. I was only testing with test lamps as all my gear was at a different part of the site. I didn't get the chance to get back to his sub board today, but I'm back tomorow and will have my test gear with me then (and should have a bit moretime to search for the origin!)

I'll let you know what I find.

Cheers.

 

[Engineer54]

How are you testing, Ph to Ph or Ph to N? On a loaded board, testing Ph to N is useless as you will have back feeds from other loads. You have to test Ph to Ph and note the voltages. It’s no good just saying that phase is live because your test lamp says so, the voltage could be anything!

I’ll bet you a pound to a pinch of $hit there's a blown fuse in the feed to the board!

Without any other test Annaliese, I'll second that initial assumption... lol!!

 

[polo1]

How are you testing, Ph to Ph or Ph to N? On a loaded board, testing Ph to N is useless as you will have back feeds from other loads. You have to test Ph to Ph and note the voltages. It’s no good just saying that phase is live because your test lamp says so, the voltage could be anything!

I’ll bet you a pound to a pinch of $hit there's a blown fuse in the feed to the board!

Spot on Tony. Evetually found the source of the supply and two of the three fuses had blown. Now tied the problem down to circuits on one of those two phases, so fault finding Monday.

Cheers.