https://trepo.tuni.fi/bitstream/10024/144323/2/Raja-AhoJussi.pdf

5

I have a 240V+-10V 2 Wire Single Phase (UK) supply with TT earth bonding (earth rod).

Normal case:

Live-neutral: around 240v

Live-earth: around 240v

Earth-neutral: around 0v

With neutral and live swapped:

Live-neutral: around 240v

Live-earth: around 0v

Earth-neutral: around 240v

http://www.ste.lanera.com/strayvoltage/strayvolt/sect2/file32g.html

Neutral-to-Earth Voltage
The voltage measured between the neutral conductor (or a ground lead connected to the neutral conductor) and a remote point on the earth is the highest gradient voltage possible in most cases. Neutral-to-earth voltage may be measured either on the supply circuit (also known as the utility or primary neutral-to-earth voltage) or on the utilization circuit (also known as the customer or secondary neutral-to-earth voltage).
The neutral-to-earth voltage is a special voltage gradient measurement in the earth.

Neutral-to-earth voltage is not stray voltage. This is an important difference. Stray voltage is a voltage gradient measured between two points that a cow may contact at the same time (two nearby steps in a stairway). Neutral-to-earth voltage, on the other hand, is a gradient measured over a distance too great for a cow to reach. Neutral-to-earth voltage is the maximum gradient. Measuring neutral-to-earth voltage typically involves locating a reference point on the earth far from the area of concern and outside of the electrical system’s influence. Usually a cow cannot contact the neutral conductor, or some object connected to the neutral, and a remote point on the earth at the same time and is therefore not exposed to neutral-to-earth voltage.
The neutral-to-earth voltage changes slightly from location to location on the neutral system. The resistance of the wires and connectors allows for these slight and regular changes in voltage along the neutral conductor.
Locating the reference ground rod can be a difficult task. If not done properly, considerable variability and errors in measurement may result.

Some portion of the neutral-to-earth voltage appears on all metallic structures that normally are bonded to the farmstead neutral system (i.e., waterlines, stanchion pipes, etc.). Many of these structures are the grounding electrodes that allow current to flow from the neutral to the earth or vice versa

Minimum would be 0.0V. Maximum could be almost anything (up to Uo). The classic problem with TT systems is that during an earth fault, all the fault current returns via the source electrode, multiplied by the soil resistance, can give a very substantial potential difference between true Earth and the transformer’s star point (i.e. supply N) – the exact figure depending on the resistance of the consumer’s electrode (and any parallel paths – e.g. bonding to extraneous-conductive-parts) as much as the source electrode itself. If the fault isn’t cleared promptly (e.g. due to a faulty RCD or just because it’s on the distribution network) that N can reman at a hazardous voltage for a long time. Hence all isolation devices need to open N as well as L on TT systems.

There are no recommendations of N-PE voltage from safety standards.

However, there are recommendations for EMC (although not from standards) – which have, in some cases, been as low as 5 to 10 V.

There is a solution to this … although not without cost … which is to use transformers to permit the filters to perform as they would in a TN system. This is recommended in the standards for VSDs, and also in BS EN 60204-1. The transformer secondary neutral is earthed, thus forming TN-S circuits in the TT installation.

On a single phase system, 11v is probably on the borderline of being acceptable. In contrast, by the time it reaches 20-50 you almost certainly have an unrealised fault somewhere. The thinking is as follows – the voltage drop in the LN loop may be right at the limit, as high as ~ 10% of supply (20 odd V) Half of this is lost on the live on the way out, and the other half in the neutral wire on the way back. TN-S would be the same, for the same reason. However this is neutral to true earth voltage – there may be an additional offset between true earth (an electrode far away from the near field of your local one and carrying negligible current) and your TT installation electrodes – if there are significant currents running into the electrodes then the metal work of the whole installation may also be a few volts off terra-firma earth voltage. It may be worth installing a test electrode, or even a garden fork or un-insulated screwdriver in the ground a few m from the building and seeing how far off the building earth voltage that is.

Be aware in a TT system can go a bit wild if your user electrodes are lower resistance than the ones at the substation. – if this happens, when the fault to earth comes on, the voltage is divided between the series connection of your electrodes, and the substations transformers electrode(s).
Normally we assume the substation neutral voltage rises a few volts, and most of the 230V or whatever, is dropped in the earth around your your local one, but it can be the other way about – so that the phase you have shorted to ground is nearer true earth if your earth is good enough, then the the whole transformer, neutral and everyone else’s supply bounces up to near 230V for the neutral and upto about 400V for the other phases. So while you have the test electrode in the lawn or flower beds, also check the phase voltage/voltages. you may not have a fault, but your neighbours just might.

Mike.

Just to be clear, there is no limit in BS 7671 on touch-voltage to the general mass of Earth in this regard … only in terms of selection of protective devices and combined resistance of protective conductor and earth electrode alongside this.

Which are explicitly co-ordinated so that a touch voltage of >50V cannot persist – reg. 411.5.3 (ii).

The touch voltage can of course exceed 50V for very short durations, but that’s the same with supplementary bonding where the fault current exceeds Ia.

Which are explicitly co-ordinated so that a touch voltage of >50V cannot persist – reg. 411.5.3 (ii).

Not disputing that, although only true in some circumstances, such as RCDs in TT systems … it’s unlikely that a high touch-voltage will persist in other cases for faults in the installation, ignoring PE faults and leakage currents.

The reason for pointing this out, is that there’s a common misunderstanding that BS 7671 actually serves to limit touch-voltage values (although as you correctly point out, for the most part, it’s aims to limit the time those touch-voltages may persist).