The 18th Edition is almost upon us...

It's been nearly a decade since the 17th Edition came into force.

Currently at the committee-stage the 18th Edition will come into force at the end of 2018. The request for comments runs through until the end of August and is well worth signing up to the BSI to read the proposal and have a say if you see anything of note.

The most interesting change (for me) is section 8 - energy efficiency. All the changes to sections 4 onwards are better definitions and tighter specs for RCDs and earth leakage etc which are all very important but build on principles that are well established.

I'll post more as I get familiar with the draft, but here is the intro stolen from the IET site;

New section - energy efficiency 

The worldwide need to reduce the consumption of energy means that we have to consider how electrical installations can provide the required level of service and safety for the lowest electrical consumption. The draft proposals enable a client to specify the level of energy efficiency measures applied to an electrical installation. Installations can also be awarded points for energy efficiency performance levels, for example, transformer efficiency. These points can be added together with points for efficiency measures to give an electrical installation an efficiency class, ranging from EIEC0 to EIEC4, depending on the number of points awarded. The new section will cover several energy efficient areas, such as electric vehicles, lighting, metering, cable losses, transformer losses, power-factor correction, and harmonics.

Earth Leakage - in water!

I recently bought an Agilent U1191A clamp-meter. This is a piece of test equipment that can measure current flowing in a conductor without having to break the circuit (how you would if all you had was a digital multimeter). The jaws physically couple around the conductor in question and by induction you can measure the electrical current flowing in the conductor. 

Clamp meters have moved on somewhat since I last had to buy one (a Fluke; sometime in the late nineties). This one is a pretty competent DVM as well as being able to sample and hold min, max and average values across all setting. For most days it could definitely do double duty against my Amprobe 37XR multimeter - EXCEPT the Agilent doesn't have non-contact voltage detection (the Amprobe does!). Anyhow - how do you get four and a half digits of resolution across multiple ranges on a brand-name test set for less than a hundred quid? Engineers today, don't know they're born...!

Today I was called to a customer's site where they have three canal-barges, each with two or three edit rooms on board. In the bilges of each boat there is room for little half-height equipment cabinet where they have the shared-storage chassis, network switches etc. They've been suffering an unusual number of equipment failures (motherboards dieing etc) and since they also seem to have RCDs tripping out as a regular feature my first thought was earth leakage.

Here is a picture of the electrical termination point for each boat - two 32A feeders go into the hull, one for the pumps and one for the mains distribution board. The cables are permanently suspended in the water (and have been for many years!) and the ones I inspected had clearly been submerged for so long there has been lots of water ingress into the rubber jacket of the cables. One felt almost ready to crumble in my hands.

Insulation has both electrical resistance and capacitance – and it conducts current through both paths. Given the high resistance of insulation, very little current should actually leak. But -- if the insulation is old or damaged, the resistance is lower and substantial current may flow. Additionally, longer conductors have a higher capacitance, causing more leakage current. Attaching the clamp meter to the incoming earth bond (pre-the consumer unit) measured a massive 100mA of leakage current. This not only risks the equipment being fed off this supply - there is an imbalance between the live and neutral cores and Class-1 equipment is often upset by this, and power supplies can pass this residual current to the earth-plane on PCBs.

More worryingly you've also compromised the safety action of any RCD (Residual Current Devices) in the feed. 

So - my advice was; replace those 32A feeders with marine-grade power cable as soon as possible.

Three phase electrical supplies and technical earth

I had a customer ask a question yesterday about why single phase in a machine room is considered a good thing and why technical earths need to be unified. Here's what I wrote; it's not a complete chapter & verse but it's a good excuse to put in a link to the episode of the Engineer's Bench that Hugh & I did in 2012.

He's confusing the phase that his various supplies are on with the "best practise" requirement of a unified technical earth. Twenty years ago the 16th Edition of the IEE regs forbade powering bays next to each other with different phases. There is more than 400v of difference between the mains phases and so it was thought that having that much potential difference within reach of each other was more dangerous than should be allowed; hence the practise of powering the machine room off a single phase.

However, the other consideration is that the power company charges you 3 x your most heavily used phase. So, if you're averaging 100A on the red (your machine room maybe?) but only 10A on the blue & yellow phases you'll be paying for 300A of current whilst only using 120A - if you don't "balance the phases" your electricity can (worst case) cost you three time per KW/h than it should. So, what with the improvement in RCDs and the cost issue the 3rd revision of the 16th Edition (and it's carried into the 17th) allows for mixed phase supplies in machine rooms. Looking up section 514 all that is required is labelling;

"6.1 Labels to be Provided The following durable labels are to be securely fixed on or adjacent to equipment installed in final circuits. (i) Unexpected presence of nominal voltage (U or Uo) exceeding 230 V Where the nominal voltage (U or Uo) exceeds 230 V, e.g. 400 V phase-to-phase, and it would not normally be expected to be so high, a warning label stating the maximum voltage present shall be provided where it can be seen before gaining access to live parts. (ii) Nominal voltage exceeding 230 volts (U or Uo) between simultaneously accessible equipment For simultaneously accessible equipment with terminals or other fixed live parts having a nominal voltage (U or Uo) exceeding 230 volts between them, e.g. 400 V phase-to-phase, a warning label shall be provided where it can be seen before gaining access to live parts."

On the subject of a technical earth - it's an entirely different consideration to mains phases but best practice is that you run all your tech feeds - bays and edit desks etc, back to the same earth bus-bar in the MCR. This is then run to the incoming feed supply. It's not good to try and tie it to the domestic "cooking" earth somewhere upstream of the incoming supply. The attached PDF is what we spec to customers' electricians.  It’s worth pointing out that getting the earth’s wrong is not unsafe; you can have volts of difference between two earths and they still both work as effective safety earths, but a hundred mV of earth differential between and edit suite and MCR will be a problem – the HD/SDi signal is only a volt big after all and zero level analogue audio is 775mV.

I did a podcast on the subject; http://youtu.be/rL1ZiciXRBg

Requirements for Electrical Supply for Systems Integration projects

If we are obliged to power equipment sited in a machine room and/or edit suites we ask that the customer’s project manager and electrician read and sign these notes to ensure a proper configuration for the mains supply. The difference between an optimal arrangement of mains power and one that merely satisfies the requirements of safety legislation will be the difference between a smooth-running facility and one that is bedeviled by hum on signals and corrupt data streams. Attention to detail initially will save money and result in a robust system.

1. Circuit breakers - Our requirement is that the customer’s electrician provides a separate spur connection for each bay and all feeds are provided via a D-rated 16A MCB. We recommend the area is protected by an Earth Leakage Breaker. For the edit rooms an MCB-protected 16A mains feed terminated in a Commando connector is required. Since most equipment used in modern television production represents inductive loads C-rates breakers found in domestic and office premises will results in unnecessary supply interruptions.
2. It is important that the customer’s electrician runs the earths for the edit rooms back to the same earth bus-bar as the mains feeds to the bays thus creating a technical supply for all production/editing equipment.
3. The practice of tying the domestic ‘cooking’ earth to the technical earth should be avoided as a quick and cheap way of unifying the earths between the edit suites and machine room. Although this satisfies the requirement of a safety-earth it means that the technical earth is now united with the dirty earth.
4. Be aware that new projects that start after June 2008 have to conform to 17th Edition of the IEE regs (BS7671:2008). These notes are meant as additions to legal requirements and should be included in Root6’s Scope Of Works submission.
5. Testing- we will regard demarcation of responsibility for the machine room cabinets and edit suite desks as being at the 16A Commando connector – we will provide a standard set of tests results (earth continuity, Insulation, run-current, leakage, and flash-test) from that point for every circuit. We ask that the electrical contractor provided us with a copy of his test results as detailed in IEE.17th.ed and Part-P.

For a class-1 device you need a mains earth!

We had a monitor that was giving people mains shocks off the SDi BNC connectors. I opened it up and there was no earth from the mains inlet to the metal chassis. No then - if you want to have no internal earth you have to have a plastic case and one other layer of insulation between the user and any current-carrying conductors (that can be a layer of air - but the device must be 'double-insulated' AKA 'class-2'). I hope this was a manufacturing fault and not a design oversight!
It's not unusual for equipment (particularly with a resonant or switch-mode power-supply) to have the internal earth float at half-mains (with a very high-impedance to the power-source, no real ability to deliver any current), but that's why you need a safety earth connection if you can touch the chassis-earth (on the BNCs, for example) and not get a little belt!

before - oh dear! - after - much better!

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?!