INTERPRETING THE READINGS:
Understanding N-E voltage
and Ground Current

It was earlier clearly stated that the Neutral-Earth voltage is not common mode noise (see here), however, in the defence of the computer industry this is a relatively important voltage to measure. Although N-E is not in itself a definitive value, it can point to possible Earth problems - but only if it is taken with the pinch of salt that is usually required.

As said, there is this misconception that such a voltage should not exist. It was proven that such a voltage will always exist purely because wiring has impedance. As the physics cannot be negated, 'acceptable levels' are set. Unfortunately those who think they know what they are doing have set this at some ridiculous level of about 1V (those who know they know have this at about 5 volts!).

In unbalanced systems the common-mode voltage is half the Neutral-Live voltage making both the differential and common mode noise relevant (this makes differential mode noise half the concern of common mode noise). In the computer industry there are claims that 10V differential mode noise is the accepted limit. Using the half-voltage rule, this would make the common mode noise equivalent of 5V.

Whatever the level is, the reason this is of concern is to determine whether the whole network is 'moving' or if there is sufficient voltage to cause current to flow from the mains to other parts of the circuitry, usually through the mains filters to then dissipate through signal cables etc. Having no N-E voltage does not mean there is not a problem. This is why really good power quality instruments have both N-G voltage and ground current measurement channels.

Before we can go any further, we need to define what is 'E' in the term 'N-E'. If E is actually Earth, as derived from the point of supply where Neutral and Earth are coupled, then E is really just a second Neutral. This will make N-E nothing more than the voltage that has developed across the two legs (N and E) at the point of measure (probably through the difference in current flowing in Neutral and Earth).

If 'G' is good ol' terra firma, then N-G is going to be the voltage as found between Neutral and the last place Neutral was taken to real ground - this is usually at the point of supply. In this case both the N-G and E-G (protective earth to ground) are significant. If about the same value then it can be assumed the N-G voltage is derived from current flowing in Neutral and/or leakage currents flowing through the real ground.

So how do we tackle the uninformed...

If I were presented with a reading where N-E were of some significant value (somewhere in the order of 3-5V and beyond), then I would immediately find out whether is was 'E' or 'G' that was being referred to. If real ground (water pipe, etc.) then it would prove useful to know what the N-G voltage is. If this is low then chances are this is going to be of little concenquence. This is said with a little reservation as if the voltage is considered high (maybe a limit of 10V) then this could well indicate a bad grounding at the supply point. Things such as cable length and load need to be considered in this statement.

Unless....

There is one thing that must be considered and that is if there is any way currents can flow from the Protective Earth to Ground. If they can, then, regardless of the voltage difference between Earth and Ground, these currents could lead to damage of equipment if the currents flow though the devices. Even structural damage can occur e.g. should the currents flow through a gas pipe!

So, to help you make your mind up as to whether N-E is a worthwhile measurement or not (seeing as we keep darting back and forth between voltage and current!) - 'G' must be established and thus N-G should be Neutral-Ground (as opposed to Earth) and this voltage measured. We revist the rule we've established.

Throughout this book there is one golden rule we keep referring back to; Voltage does not damage, Current does! With N-G this rule is no different. However, there is one factor regarding N-G where voltage can become relevant. If there is voltage then there is a high possibilty of current i.e. that which can destroy.

What we're saying is; N-G is a value that should be measured. It must also be taken in context, and the context is ground current.

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© 17.09.02