Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • Replies 41 replies
  • Subscribers 40 subscribers
  • Views 4452 views
  • Users 0 members are here
Options
You may also be interested in

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

  • I saw an advertisement for the new Wylex "domestic" three phase consumer unit which has a three pole main switch with the neutral wired directly into the neutral bars.

    I thought all domestic installations have to have a switched neutral, I checked BS7671 and found I was wrong. 

  • in reply to Sparkingchip

    The risk of the neutral being not connected when the phases are - which requires an offset contact in the 4 way switch block, is considered worse than the risk of leaving it connected.

    And to be fair to the DNOs in most cases the neutral really is at or very near earth potential of course.

    I have also been shown pictures of an expensive mistake with wring to a 3p and n main switch in  large installation where the neutral was not routed through the offset contact, but one of the phases was. The switch was just fine, but sometimes when it was operated for maintenance  some unrelated random failure would occur. Apparently It took ages to work out what was happening.

    Mike.

Reply
  • in reply to Sparkingchip

    The risk of the neutral being not connected when the phases are - which requires an offset contact in the 4 way switch block, is considered worse than the risk of leaving it connected.

    And to be fair to the DNOs in most cases the neutral really is at or very near earth potential of course.

    I have also been shown pictures of an expensive mistake with wring to a 3p and n main switch in  large installation where the neutral was not routed through the offset contact, but one of the phases was. The switch was just fine, but sometimes when it was operated for maintenance  some unrelated random failure would occur. Apparently It took ages to work out what was happening.

    Mike.

Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines