Ghost voltage in cable

nikoldije · Jun 7, 2016

I have a problem with voltage that should not exist.

I send 230 VAC from circuit breaker to auxilary contact of contactor in another cabinet and than bring it back. It goes and returns over 2 wired cable.Distance between cabinets is around 15-20m.

Problem is that i have around 190VAC in second wire even if aux contact is not closed. I disconnect cable from other side and there is still voltage in second wire even if i is not connected to anything

Help please!

emeka · Jun 7, 2016

i really dont understand your question,but having voltage in the second wire even if aux contact is not closed.the aux might be a normally closed contact.

nikoldije · Jun 7, 2016

It is not NC 100%

I send 230VAC from terminal 1 through wire 1 to aux NO contact terminal 3 and i should not have any voltage on terminal 2 where second wire come back from aux NO contact 4.

Problem is that i measure 180VAC on terminal 2. I have that voltage even if i disconect wires from aux NO contact so now there is only cable connected to teminals 1 and 2 and nothing is connected on other side.

I checked the cable there is no short circuit between two wires

JohnCalderwood · Jun 7, 2016

what are these wires connected to ?

Are they connected to a PLC input card?

Can you give us some more information to try and assist?

A small wiring diagram for example?

nikoldije · Jun 7, 2016

http://postimg.org/image/y3ynzfkkr/

I hope this clarifies it a little.

Eather one of aux contacts can turn on the coil but even when they are open i get the 180VAC.

rdrast · Jun 7, 2016

What reference are you measuring to? Are you measuring directly across the coil?

At first glance, it sounds like you are measuring to another floating neutral.

John.S · Jun 7, 2016

Hi,

Question: with what instrument are you measuring that voltage ?
If it is with an electronic mutimeter, you can pick up induction voltages from nearby cables, because of the high impedance of your measuring instrument.
You have to measure with a device that puts some load on the leads you are measuring.
We use measuring device that is called a Duspol.

gr, John.

nikoldije · Jun 7, 2016

Yes i measure with classic multimeter

Reference is neutral conductor from power supply cable

John.S · Jun 7, 2016

Well, put some load over your measuring leads, maybe you have something like a lightbulb of 220v/40W at hand. ( make sure you can not touch the leads in case there is something different going on and the 220V is present at your lightbulb).

Big chance that your ghost voltage is gone now.
The induction voltage give you some reading on your measuring device, but there is no power behind it, so a little load will make that volage drop to zero.

gr. John

redbarron55 · Jun 7, 2016

Induced voltage is very common when wires are run parallel to each other. The voltage may read high due to no load on the circuit.
Often the voltage is high enough to be read as high when fed into a high impedance input card.
In this case an external load may be enough to reduce the voltage to a low enough level to read true.
Otherwise an interposing relay might have to be used.

nikoldije · Jun 7, 2016

Thank You for help

When i put load voltage drops to 0.8V

shooter · Jun 7, 2016

Take your meter and just hold one of the leads, for sure there is voltage on it.
The drawing is different from what you tell us, if a wire is not connected to anything the voltage is unknown.

Boorabee Parker · Jun 7, 2016

John.S said:
Well, put some load over your measuring leads, maybe you have something like a lightbulb of 220v/40W at hand. ( make sure you can not touch the leads in case there is something different going on and the 220V is present at your lightbulb).

Big chance that your ghost voltage is gone now.
The induction voltage give you some reading on your measuring device, but there is no power behind it, so a little load will make that volage drop to zero.

gr. John

This is why I have a set of test lamps in my kit. DMM will give you all sought of readings when inductance or bad neutral are in the system

Ghost voltage in cable

nikoldije

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