210v from 3 phase for lights when switched 0v. Why?

Thread Starter

g60chubb

Joined Sep 13, 2014
18
So here is a head scratcher.
I have a machine running on three phase. L1 L2 L3 N E. (Neutral isn’t used)
So I’ve tapped into L1 and neutral and taken it to a switch. Switch is reading 200v. There is an existing light attached to the machine running on an external single phase supply. My plan is to tap into the machines power so I can get rid of the external power supply for the light. So I did this, I wired the light into the switch. When off I still get a reading of 200v from the input, switch it on and get a reading of 0v. Something is happening to the light that must cancel out the voltage. Maybe an existing circuit that no longer works.
Can’t figure it out. The light is in the depths of the machine so can’t see it.
 

Dodgydave

Joined Jun 22, 2012
8,377
Is the switch in the Live Side!! On a Star formation..

Measure the Voltage with a load connected, to prevent false readings.
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
So assume this is a Star secondary with the star neutral?
What is your phase to phase voltage?
Where is your light connected between if not using Neutral?
Max.
 

Thread Starter

g60chubb

Joined Sep 13, 2014
18
So assume this is a Star secondary with the star neutral?
What is your phase to phase voltage?
Where is your light connected between if not using Neutral?
Max.
Im assuming so. I’m getting 420v between phases. But between L1 and neutral 199v L2 and neutral 260v and L3 and neutral 200v
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
It sound as though either you have a large load unbalance or some discrepancy in the transformer.
Is there a number of other phase to neutral (1ph) circuits?
Max.
 

Thread Starter

g60chubb

Joined Sep 13, 2014
18
It sound as though either you have a large load unbalance or some discrepancy in the transformer.
Is there a number of other phase to neutral (1ph) circuits?
Max.
Just been looking and can’t find any three to single phase set ups. It’s an old building. 2 machines are running lights off a separate supply and would of definitely been integral from standard so I’m assuming they didn’t work due to the problem in the supply.
 

MaxHeadRoom

Joined Jul 18, 2013
19,014
This wouldn't be a 3ph delta with winding tap for a neutral possibly?
This was often done in the past.
Max.
 

Thread Starter

g60chubb

Joined Sep 13, 2014
18
Sounds like it. Just been reading up on it. I’ve only worked on three phase star systems so haven’t come across it before. This is an old mining building in Cornwall so who knows how old it is.
 

Thread Starter

g60chubb

Joined Sep 13, 2014
18
Sounds like it. Just been reading up on it. I’ve only worked on three phase star systems so haven’t come across it before. This is an old mining building in Cornwall so who knows how old it is.
 

gerty

Joined Aug 30, 2007
1,250
Since the neutral isn't being used by the machine... are you sure it's connected at the source end. Your phase to neutral voltages are all over the place and when you connect a load the voltage goes away, to me it sounds like you are reading a induced voltage...
 

LesJones

Joined Jan 8, 2017
2,317
Measure the voltage between earth and the terminal that you think is neutral first with the switch open and then closed. Post the readings.
Les.
 
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