What is a "borrowed" or "floating neutral"?

lobsterbob · Oct 5, 2010

Hello

I'm working on a maintenance contract in an office block. The lighting is in a trough type system which uses flouresent tubes - with dimmers it's approx 15 years old I've put together a quote to get this changed as it no longer meets the current lighting levels. But my question is I've been told that there's floating neutrals or they borrowed a neutral(WTF?) not understanding this I had a look and the system has been wired without a neutral conductor - it's been snipped off. It's also wired in series so when one goes out it takes two lights out - coupled with this there's also a permenent live for the emergency lights.
How is this posssible? Am i been thick i thought you needed a neutral to make a circuit ect but this has none it has an earth and phase - they're not using the earth as neutral so...... Can someone please explain this and let my weiry head rest!! Hope I've explained this well enough.

general error · Oct 5, 2010

A floating neutral can occur when equipment connected to two or more phases have their neutral wires connected together but not to the incoming main neutral . If one phase disconnects then the no load voltage on the floating neutral is at the same e as the remaining phase , so caution is advised.

Its possible the lights were wired using 2 or 3 phases to supply alternate lights.
The idea was to reduce strobing in the days when lighting phosphors did not maintain luminescence as well as they do today.

If this is the case there will be at least two live feeds with 415 v between them ,

If power factor correction capacitors are not fitted it,s also possible that the installer found excessively high neutral current through the inductive ballasts and just snipped the wire off to cure the problem.

Jabbajaws · Oct 5, 2010

Hi Lobsterbob, im not sure how this applies in your situation, but l can tell you that in a domestic installation, a 'borrowed' neutral is a neutral that has been taken from a different circuit.

This is a common problem when fitting a consumer unit, in a dwelling where a neutral has been 'taken' to complete a circuit and one of the most regular places it happens is on the stairs, where a two-way lighing circuit exists.

Years ago most domestic dwellings had only one lighting circuit, and it was placed on one fuse. However as the years went by and various things were altered, it was decided that it would be better to split the load into two circuits, so that if one went off, due to operation of the fuse, the other one stayed on.

As there was no neutral return for the upstairs, due to them being separated, it was common practice, to 'borrow' a neutral from another circuit, which caused the RCD to operate when the new consumer units with RCD protection. Hope that helps...

Knobhead · Oct 15, 2010

If you have a floating neutral. So long as the load on each of the two supply phases remain equal the current in the floater remains more or less static.

The voltage = 433 / 2 = 217V where as phase to neutral would be 433 / SQRT(3) = 250V

Problems start when a fitting is disconnected or burns out. Load on that leg is reduced and therefore the voltage rises. The load on the 2 legs must equal each other, so say we have 20 fittings divided in to two groups with a loading of 5A. Disconnect one fitting the loading falls to .95 of 5A = 4.75A. The voltage on the fully loaded leg drops to 206V while the lightly loaded leg rises to 227V. With a few duff fittings the system becomes unstable, one bank will have insufficient voltage to work so that fitting are constantly trying to strike, the other leg is going mental because of the voltage changing.