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Neutral Isolation. A Safety Necessity or a Practical Nightmare?

AMK

How should regulation 537.2.1.7 be interpreted and applied? This regulation, which I believe has been deleted but is still being followed by some, requires the neutral to be isolated with a linked switch or removable link when carrying out isolation. However, this can cause a lot of trouble and inconvenience, especially for 24/7 departments that need continuous power. The Lead Engineer argues that both supplies, essential and non-essential, need to be isolated to work on the Essential Line side isolator, where the neutral is linked between the line and load. But is this really necessary? Regulation 531.2.2 allows the neutral to remain connected if it can be reliably assumed to be at earth potential. Can this be verified prior to work commencing by a simple calculation of Un-E = neutral current x Zn (TN-S) or a voltage test between neutral and earth? Some suggest that these methods are sufficient and practical, and that isolating the neutral is unnecessary and impractical. Others assert that these methods are not reliable and safe, and that isolating the neutral is essential for safety reasons, even in a well balanced 3 phase system having minimal neutral current and a low Zn. What is your view?

Parents

  • • The disconnection of a fixed neutral conductor on TN C S and TN C system is unnecessary and could introduce more significant levels of risk and harm.

    • Disconnection and temporary opening of a Fixed neutral link can result in a situation where the link is not replaced, which can lead to serious con. sequences.

    Always prove dead before commencing working 

    Phase to phases

    Phase to neutral 

    Neutral to Earth 

    • In situations where neutrals are shared the disconnection of the link/conductor can lead to significant voltage instability on dependant connected circuits. in TN-C-S and TN-C. 

  • in reply to Aryadutt

    The disconnection of a fixed neutral conductor on TN C S and TN C system is unnecessary

    Perhaps not always true - say you had local generation as a switched alternative to the grid - the change-over switch would need to switch (and isolate) N to comply with 444.4.7. (Wouldn't be needed in a pure TN-C system of course but the ESQCR prohibits PEN conductors in consumer's installations.)

    That then begs the question, as far as the downstream installation is concerned, how reliable is the N connection with Earth? Moving contacts are generally less reliable than solid joints, and not all disconnection devices guarantee not to close the L contacts if the N has failed to close, or prevent the N from opening if the L contacts have welded closed. If you consider that the N downstream of the change-over switch isn't reliably held at close to Earth potential, can you justify working on such a conductor without isolation?

       - Andy.

  • in reply to AJJewsbury

    Hi Andy, 

    You are right about the generator connection to the system you shall use a 4-pole change-over switch. 

    My comment was mostly above the question about the confusion of neutral isolation. With three poles or single pole switch.

    Many thanks 

    Arya

Reply
  • in reply to AJJewsbury

    Hi Andy, 

    You are right about the generator connection to the system you shall use a 4-pole change-over switch. 

    My comment was mostly above the question about the confusion of neutral isolation. With three poles or single pole switch.

    Many thanks 

    Arya

Children
  • in reply to Aryadutt

    I refer melord to G99…. where there even delta wound generators that depend on the supply neutral and protection systems….

    Russ

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