Wednesday, April 09, 2008

The 17th Edition IEE wiring regs BS7671:2008

I went to a superb training day at my institute to get all tooled up for the 17th Edition of the IEE wiring regulations - BS7671:2008, standards fans! These updated regs replace the venerable 16th Editions (4th revision!) in July and although they will only apply to new designs and aren't retrospective for older installations I thought it was important to start getting up to speed. A lot of the changes are due to the IEE / IET's involvement with the European standards body CENELEC.
When I started at the Beeb the 15th Edition was still in force and some of the things that came in 1991 with the 16th were universally mistrusted by engineers at the time. I remember reading a couple of days before a senior engineer job interview (internal) that they had relaxed the regulation on how close bays powered by different phases could be. That question came up in the interview and with a but of quick thinking I stretched out my arms and replied about this far! - the engineer questioning me saw I understood the principle and I got the tick! (In case you're wondering - you don't want to be able to put one hand on one phase of live and our other hand on a different live phase!).
Anyway - as mentioned it was a very interesting day at Savoy House (how do they afford to keep such a prestigious building?!). I have a scan of the 1st Edition from 1882 here - it runs to four pages! I've also stuck the various presentations on my server here as they are well worth looking at if your interested in electrical safety and standards.

Anyway, some of the significant changes are;

  • Terminology - There are several re-definitions. The phrases direct and indirect contact are replaced with basic and fault protection. These cover the protective measures you design-in for normal operation and for fault conditions. Section 4 of Darrell Locke's presentation covers this well.
    There are also changes to the 'zones' found in bathrooms (and elsewhere).

  • Expanded use of RCDs - They pretty much insist that RCDs need to be incorporated into all domestic circuits unless there is a good reason not to. They have this concept of experienced and instructed persons and if you're not one of those your an ordinary person who who only be exposed to RCD-protected circuits. Apparently this aspect was fought by the banks who worry about computers being wrongly taken down by residual current detectors. Whilst chatting over lunch every engineer I spoke to thought this was a good addition and if it makes us a bit more innovative in how we lay out supplies all the better.
  • Recognising the problem of harmonics - This is something that is a perennial problem when every piece of equipment has a switch-mode supply (like a typical datacomms or television machine room). I've often hooked an oscilloscope across the mains in the various facilities where I've worked and what you see is rarely a sine wave! These reverse-leaked harmonics cause RCDs to misbehave and mean that you don't get predictable heating effects in linear (i.e. non-inductive) loads. In inductive loads the acronym CIVIL tells us that the voltage leads the current;

  • New specs for allowable voltage drops - previously voltage drop from supply point (consumer unit) to the furthest point was allowed to be 5% - this is now set at 3% for lighting circuits. Apparently this was a French specification.

  • New special location definitions - in the case of a TV outside broadcast vehicle an NIC recognised inspector now has to check every time the supply is re-connected - that may be problematic! I'm sure there has to be a way around this. Either that or every SIS-Link/BBC/Sky engineer will become an inspector!

  • Phase sequences - Incredibly up until now the regs don't have anything to say about maintaining the sequence of the three phases. You don't want suckers to become blowers!

  • New earth loop impedance (0.3 ohms) and minimum insulation impedance (0.5M ohms)
The regs aren't legally binding but if you are in court under EAWR (electricity at work regulations) the man in the curly wig will look on you favourably if you took them seriously and showed you abided by them.
Best phrases heard today; A corpse changes everything and (in relation to a current-carrying conductor) Is is as hot as a cup of tea?!

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