Hi all,

What can be causing voltage on neutral line at a light point when the neutral is disconnected from the distribution box?
Below are the observations done.

NOTE 1: Mains voltage is 230VAC
NOTE 2: With neutral connected, everything works well

With neutral disconnected from DB, with filament bulb connected to light point holder, with light switch on, with all other light switches in the same circuit off:
* Voltage from Neutral to Earth -> 230VAC
* Voltage from Live to Earth -> 230VAC

With neutral disconnected from DB, with LED bulb connected to light point holder, with light switch on, with all other light switches in the same circuit off:
* Voltage from Neutral to Earth -> 160VAC
* Voltage from Live to Earth -> 230VAC

With neutral disconnected from DB, with LED bulb connected to light point holder, with light switch off, with another light switch in the same circuit on:
* Voltage from Neutral to Earth -> 183VAC
* Voltage from Live to Earth -> 160VAC

With neutral disconnected from DB, with LED bulb disconnected to light point holder, with light switch on, with all other light switches in the same circuit off:
* Voltage from Neutral to Earth -> 0VAC
* Voltage from Live to Earth -> 230VAC

With neutral disconnected from DB, with LED bulb connected to light point holder, with light switch off, with all other light switches in the same circuit off:
* Voltage from Neutral to Earth -> 0VAC
* Voltage from Live to Earth -> 0VAC

 

Why do you not expect voltage on neutral when one of switches is ON?
The voltage goes through the ON switch, so I would be surprised if you didn't read a voltage for that condition.

 

The voltage you measure from a disconnected neutral to "ground" is dependent on the resistance of your measuring device and the capacitance AND resistance between conductors in all of the connected wiring.
AND GIVEN that a disconnected neutral is defined as an unsafe condition, I ask what is the purpose of this exercise??

 

Hi both, so it is normal to have an LED bulb light up (dim) when neutral is disconnected? Regarding the question why this exercise was done, this actually was not planned Is removed the neutral to re-route and forgot to re-connect. When switching on the mains breaker, it noticed the LED which was on from the switch was working, but dim. Also, UNI-T socket tester UT07A was showing all lights on. Shouldn't this show off-green-off?

 

so it is normal to have an LED bulb light up (dim) when neutral is disconnected?

There likely is enough capacitance from the floating neutral wire to the ground wire (≈20pF/ft for typical wiring cable with probably about 1-2 feet of wire for every square-foot in you house), to light the LED bulb dimly, since it takes only a small current for that.

 

When the neutral is disconnected, there is still a circuit path back through earth to the local distribution transformer grounded secondary.

 

When the neutral at the distribution panel is opened then the two line sides form a voltage divider that is seldom equal, and the smaller of the loads gets the greater of the voltage, Destruction of items often follows.

 

When the neutral at the distribution panel is opened then the two line sides form a voltage divider that is seldom equal, and the smaller of the loads gets the greater of the voltage, Destruction of items often follows.

That's if more than one circuit is turned on at the same time, of course.

 

Thanks all for the feedback. So there is noting "abnormal" with the wiring and with the way things were behaving when neutral was disconnected, correct? Also, I still cannot understand why the UNI-T UT07A socket tester was not showing missing neutral (last one from the picture below)

 

If another circuit fed from the opposite phase had some load connected between that opposite phase and the internal neutral wiring, enough current could flow to light the indicator between the original line side and the neutral. The voltage would be higher, but the display LED driver would not let that show. If the indicators were NEON bulbs it would have been brighter.

Quite a few years ago I was asked to investigate an outlet in a computer room that had caused a new UPS to smoke. ALL THREE indicators were lighted., A voltmeter check revealed 120 volts between the line and the neutral, and 120 volts between the neutral and the ground, AND ALSO 240 volts between the line and ground.
I chased the problem to a 5KVA Sola constant voltage transformer that fed that outlet via a dedicated circuit. An improperly terminated unused end of the center tapped 240 volt secondary had contacted the bare ground wire inside the transformer terminal enclosure. Because the secondary was completely isolated from all other wiring it did not cause any overload.
Proper termination and insulation of the unused secondary solved the problem.

 

At one time in the UK, it was forbidden to connect neutral and GND anywhere in the newly wired residence, the GND & N were only connected at the supply transformer neutral.
The GND path resistance had to be measured & conform to a max value res. from installation back to the transformer using the Earth GND & N.

 

Thanks all for the feedback. So there is noting "abnormal" with the wiring and with the way things were behaving when neutral was disconnected, correct?

If the N & GND is disconnected at the panel, they are still common to each other by way of the dist. transformer grounded neutral point.