Strange Earth Voltage Reading

Question

Ok, my first question here. In my spare time, I run a small PA company. On Saturday, I had a concert at a sports center where I´d never been before. Power for the event came from a standard 5-pin, 16A outlet. After connecting to my distribution unit, I got the multimeter out and checked the voltages. I found:

Line -> Neutral = 230v (on all 3 phases).

Neutral -> Earth = 0v;

Line -> Earth = 0v.

The last reading obviously threw me. Over the years, I´ve seen all sorts of 'floating' earth to phase voltages, but never 0v.

So I checked the voltage at the outlet on the wall in case my cable or distro was the problem, but found the same thing. I then checked with the guy in charge of the place, but he wasn´t much help. There was no other outlet in the hall, nor would he let me into their mains 'cubby hole' - at least not without their electrician present, etc, etc. All he said was that they have regular events there and have never had any problem with the power.

So I set up, made sure that the mics, etc. were safe and did the gig.

So my question is simply, what circumstances would produce a 0v Line to Earth reading?

Answer 1

Unless there is a wiring fault (missing earth connection), my best guess is that someone was being very nice to you and giving you an isolated power source.

This is common in recording studios - there is (are) isolation transformers that completely float the output Mains supply voltage.

One specific instance that I was involved with was a live recording gig (Procol Harum) where the Hammond organ has an arcing switch internally. The arc was polluting the entire power system and the sound was showing up coming from the guitar amp.

Moving the Hammond organ power lead to its own isolated supply eliminated the audible problem and the concert (and recording) continued. Organ was repaired the next day.

The common scenario I used to be called to was live events where lighting dimmer noise was getting into the audio system. The cure was to install an isolation transformer for the entire audio system - either single phase (if the system was small) or all 3 phases. The transformers I use are Topaz Ultra-Isolator units- 5 KVA each and used either singly or as a triplet for 3-phase.

The rules say that you are supposed to earth the Neutral connection that comes from the output of the isolation transformer but we had some pretty understanding Electrical Inspectors who knew us well. They would allow us to NOT put in that jumper after we showed them the difference in noise level with and without the Earth-Neutral bond. But that was decided on an individual case-by-case basis.