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