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

The breakers are double pole.

The issue is with a lighting radial circuit.

This is a new electrical installation and the electricians are about to finish.

The lights that are connected to the circuit power up fine.

One light is not yet connected to the switched live.

With one CB on I probed (using a multimeter, I aware this might not be the best way to test) between Neutral and earth and there was 230 Volts. I probed at a fixture (neutral to earth) and again, 230 Volts

  • What is an acceptable voltage between Live/Line and Earth?
  • What is an acceptable voltage between Neutral and Earth?
  • Should there be voltage between live or neutral to earth?

Is voltage between neutral and earth normal or could there be a fault?

This video states that there should be 0-0.5V between earth and neutral

This post on Quora states that

A rule-of-thumb used by many in the industry is that Neutral to ground voltage of 2V or less at the receptacle is okay, while a few volts or more indicates overloading; 5V is seen as the upper limit.

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  • In the US, neutral and earth are bonded in the main panel so voltage should be close to zero. You seem to be in the UK, but 230v from neutral to earth seems wrong.
    – JPhi1618
    Jun 11, 2019 at 15:41
  • If an appliance is plugged into that outlet, will it power up? Or does it not work? Jun 11, 2019 at 15:44
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    In the US, 2-wire 240V would be two hots and NO neutral. So where are you? Can you provide pictures of the breaker and the fixture wiring? Jun 11, 2019 at 15:50
  • I've edited the question to provide additional information. Jun 11, 2019 at 15:59
  • 3
    Possible duplicate of Acceptable ground to neutral voltage? Jun 11, 2019 at 16:33

3 Answers 3

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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

So maybe you have a setup were neutral and live got swapped. That's not a safe setup. But to be sure, please check the difference in voltage for all 3 cases.

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  • You're right, the line and neutral was swapped! Jun 12, 2019 at 15:32
  • I'm curious, how do I interpret "around 0v"? Does 3v fall within this range? Jan 23 at 10:00
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It turns out that the supply polarity was incorrect.

The Line & Neutral had been swapped causing an incorrect reading.

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I have a 240V+-10V 2 Wire Single Phase (UK) supply

Note that standard UK supply spec nowadays is 230V +10% -6% (yes the tolerance thresholds are asymmetric). This gives an acceptable voltage range of 216.2V - 253V.

What is an acceptable voltage between Live/Line and Earth?

The voltage between Line and Earth should be similar to the voltage between Line and Neutral.

What is an acceptable voltage between Neutral and Earth?

The voltage between neutral and earth should be small.

I've never seen an official maximum tabulated but based on what I know about UK wiring and doing some quick sums I would say.

  • Single digit volts - fine.
  • 10V-30V - suspicious, but if the system is heavily loaded it could just be the result of volt drop in cables.
  • More than 30V - something is wrong. Either you have a serious fault somewhere, what you thought was the neutral is not actually a neutral, or there is a large ground potential difference between your location and the transformer (which likely indicates a fault in someone elses installation).

A rule-of-thumb used by many in the industry is that Neutral to ground voltage of 2V or less at the receptacle is okay, while a few volts or more indicates overloading; 5V is seen as the upper limit.

That sounds like it was written by an American. Modern american electrical systems always use what we Europeans would call "TN-C-S" while we brits use a mixture of TN-C-S, TN-S and TT.

We brits also tend to feed our houses with single ended supplies, whereas American buildings usually use split phase or three phase supplies with only the final distribtion circuits being single ended.

We also use a higher voltage from Live to Neutral.

Put all these things together and you are likely to encounter higher Earth-Neutral voltages in the UK than you would in the USA.

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