Category Archives: Introduction to Electrics 5 day course

I’ve completed this course now. All through this week I’ve been presented my class notes but I’m not sure that I can face doing that now. Not after a week in Dartford and a journey home through the London rush hour. Besides, my grip on the notes is scantier than I would like.

Instead, I’d like to sum up the week. It has been fucking superb. I’ve had lots of education in my life: O-levels, A-levels, a degree. I’ve worked the streets. I’ve trained in law, completed the bar finals and practised as a barrister. I’m a qualified mediator and a self-taught techie to boot but nothing has been as thorough, as engaging and as downright enjoyable as the week I’ve just gone through. I am not being paid to say this. Our tutor, Alan, was excellent. His mix of dark humour and sensibility as he trespassed over the risks involved in electrocution scenarios was fantastic.

If you ever read this Alan, thank you very much indeed. I was sorry to hear that you won’t be teaching us next week. You were first rate. If I ever become big in the electrical business, I will be headhunting you. Your encylopedic knowledge of the regulations, quick witted explanations and attention to detail combined to inspire me. I came home thinking not only would I just rewire my own house, which had been my original aim, but that I would also become an electrician. If this seems like gushing praise that’s because it is. My regular readers will know that I rarely come on that strong. In fact, if you do read this Alan, this is not a come on. I liked you a lot, an awful lot, but not in that way.

Many many thanks also to Steve, who assisted Alan in the practical workshops. He was endlessly patient with me. I did find the whole business rather complicated and am unsure of my chances of success right now, at the half-way stage in the 2 part course, but that doesn’t detract from the quality of the teaching, only from my own ability.

If you want to read my course notes over the previous four days, go ahead, but remember that they are just what I think I have learnt. Here’s day one, day two part one and day two part two, day three and day four. Today (day five) we learnt how to make and test a final ring circuit, picked up some shortcuts and were generally bombarded with all sorts of other useful tips.

Every day this week, I’ve been posting what I think I’ve learnt on this course. Whether I’ve understood everything is a matter for conjecture. I was getting more confident with each passing day but today a series of mishaps knocked me back. Each related to my inability to screw down the face of a socket outlet. Eventually the screw snapped off inside the threaded section of the back plate. I took this very badly.

Before going on to post anything else today I’d like to point out that I have perhaps been a little hard on Dartford. Yesterday I called it a shit hole. It’s not all that bad. Posting from my phone makes outer hard to sync photos with the relevant text but you can see the delightful river I get to walk up each morning on my way to Able Skills training centre.

Also, the milestone pointing out that you could always escape to London, even if you had to travel by Shanks’ Pony.

The local Conservative Club struck me as oddly out of touch with national standards. The letter “B” in its sign is in a different font from the rest of the sign, suggesting that perhaps it has been damaged and poorly repaired. A sign of excitement past?

An indication of excitement in the future was the fire in a timber yard, just getting underway as I came past on my way home this evening.

Here’s a copy of my notes for today without all the diagrams I drew. It’s late and I can’t face photographing all the diagrams and then posting the fiddly business of matching them up with the relevant text from my phone. If the android wordpress app people could do that, I would donate!

…

Everything can be reversed polarity and all the appliances would still work (except some brands of boilers). However that situation is dangerous because out means that removing the fuse will not make the system dead. Out will kill the power because it breaks the circuit but the conductors are still live. Ze and PFC will find reverse polarity. In this scenario the electricity distributor must treat it as an emergency call out (if the fuse is below the meter).

Buy the board you want, rather than reformatting it. With old boards there might be more circuits than fuses. You want to have at least two spare fuses. NIC recommend 25% extra capacity. That may be going too far. In extreme cases the manufacturers will create a bespoke board for you – at a price!

Breakers go down in size away from the main switch and are numbered away from the main switch too, much like street numbers run away from the nearest sorting office, though the Able Skills house in Priory Road, Dartford bears no relationship to the other numbers in the street. Back to the training. There’s no rule about board numbering. Out is just a convention followed by all the manufacturers.

When permanently joining conductors, use crimps and ratcheted crimpers. Pliers are useless. you can crimp anything in an enclosure but a trunk is not an enclosure. Use a crimp of the same colour and sleeving.

RCDs need quick ready access. Garages and half basements which require external access are not clever places. Power sockets in new builds have to be placed off the floor but they don’t have to be moved on an existing build.

We can’t move the main cut out fuse or the meter. No conductors should ever be in contact with thermal insulation.

Here’s a list of the tools you’ll need to do a board change:

1. Distribution board

2. MCBs or RCBOs

With a split load board divide the circuits for the convenience of the householder so that, for example, if a fault occurs sockets are on one RCD and lights are on another. This means that they have the possibility of still being able to see in the dark!

3. tail kit

- 2m of double insulated blue – 2m of double insulated brown and 2m of double insulated green/yellow

4. 4 way earth block

5. Crimps and ratchet crimpers

6. Green/yellow sleeving in different colours

7. Drill, raw plugs and screws

With stud walls, screw to the wood.

Whilst it is acceptable to use a joining block, trails of the correct length are preferable.

Never put more than one cable into the top of a distribution board’s main switch.

Next up we learnt since acceptable shortcuts. With L-E & N-E 500V is always a safe test. However, with L-N there is a chance of damaging components so do a 250V test (a soft test) first (FLUKE don’t have this test. That won’t damage components and will reveal whether anything is connected. The 500V test is the real test. The pass is at the extreme end of the meter. If anything is in the circuit, the result will be 0Ω. Bear in mind that 0Ω might ask also mean that there had been an insulation fail.

Incidentally, copper is now selling for £5 per kilo.

I refer to my crappy sketch showing line and neutral from a transformer being taken together at the board as an acceptable shortcut for an insulation resistance test. You can’t do this with every circuit and must explain on the test form which circuit and why it wasn’t tested, eg “devices vulnerable to testing could not be connected”

Test L-N. If that is 0Ω then put L & N in one crocodile clip and test with CPC. That should be 0Ω. This reduces the tests to two. If it comes back wrong, separate all the cables and do all three tests.

You could measure the insulation resistance before putting any loads in, with just a pure cable length, eg not connecting the shower just yet. Often you’re looking for a fault someone else has caused. For example, a nail hammered through the cable. Wire the lights after the test.

Heating elements and water are famous for tripping an RCD.

If the RCD trips do an insulation resistance test (that tests for earth faults), ramp tests and actually look for something. You can’t remove the circuit from an RCD!

Trunking is not an enclosure!

A ring final circuit, which is only used in the UK, uses less copper. See page 80 OSG. The relevant tests are more challenging. They break down into three parts. Firstly, you have to prove that it is a ring. Secondly, you have to obtain R1+R2 and, thirdly, you need to check the polarity of the sockets.

32A L + N at 2.5 square mm & CPC can cover a maximum floor area of 100 square metres. See page 362 and 363 of the big red wiring regs book.

At this point in my revision I’m losing the will to blog. I’ve been up until 1am every night doing this. I’

Below is what I think I learnt on the third day of my introduction to electricals course courtesy of Able Skills in Dartford. I must emphasise that I am neither connected in any way with Able Skills nor receiving any form of payment from them. I’m simply undertaking the course and blogging about it simultaneously, which may be foolish since I would like to complete it before they discover my blog. I have been a little critical but mainly, luckily for both them and me, I’ve been praising the disarmingly engaging manner on which Alan and Steve have been teaching us. Their attention to detail has been impressive and their good humour in endless supply. I’ve done all sorts of education (as you can tell from my strapline) but this crew has easily created the best atmosphere of any class I’ve ever been in. As a group of students, we’re about as eclectic as you can get but have gelled nicely, even with yours truly! Classmates help each other out. There is shared auto-didactivism but I’d better stop there before I am accused on hyperpolysyllabicsesquipedalianism. Confess to being unsure of correct spelling of 8th syllable in that word but I don’t think spoils the joke!

Back to what I learnt today… first up we were told that the picture on page 79 of the On Site Guide (which I’m going to call the “OSG” from now on because you can never have enough TLAs) was an acceptable way to perform the CPC CONTINUITY TEST (the picture includes a temporary link between line and the distribution board’s CPC) but it is less safe than disconnecting the circuit’s line and CPC from the distribution board and joining them with a connector block because you might forget to remove the temporary link and cause a fire.

When terminating two conductors together, never twist them together because one of then might well snap when you screw the termination down. Instead push them in side by side and screw down into each of them from above. This is a Part P question.

The order of the tests was finalised today but remained to be fine tuned next week.

Earthing Systems

There are five wiring systems. Three are legal in a public supply and the other two are used in private commercial systems, such as coal mines.

Legend:
T – Earth
N – Neutral
C – Combined
S – Separate

The first system is TN-S. This has two cores, live and neutral. They are protected by an armored shell which is also earth. The wiring diagram for this system is shown on page 15 of the OSG.

All new properties since about 35 years ago are supplied via a different system, the TN-C-S, which is also called protected multiple earthing (PME). This system only has one core, which is live. Its armoured shell is both earth and neutral.

The third system is called TT. There are two cores, live and neutral, but no earth. The earth has to be built manually at the domestic property. This is now rare in England and Wales and is mainly found in rural areas. These cables are always strung up overhead on wooden posts. This saves the trouble of finding them in fields and avoids farmers getting electrocuted.

Maximum Permitted Impedances

Max Ze on TN-S = 0.8Ω
Max Ze on TN-C-S = 0.35Ω
Max Ze on TT = 200Ω

TT is an unstable system. Ze on TT will change according to the climactic condition of the ground. 5-50Ω is the expected figure. TT systems always had RCDs because they are so inefficient. However, that might not be a 30mA RCD. It might be 100mA. That does not comply with the 17th Edition. The OSG recommends using RCBOs with a TT system. You might use a 100mA RCD before a life saving 30mA RCD. That is inconvenient though because a fault can shut down the entire house.

Incidentally, do not trust a £10 stud detector.

A £100 stud detector would be trustworthy.

You need to be able to identify the three different earthing systems.

In Able Skills’ building the Ze measurements are artificially high because there is a second distribution board between the original one and the testing bays.

If the earthing resistance is above the permitted maximum, it must be reported.

If you need to identify the location of underground services (gas and water pipes), use a decent metal detector. With a TT system, the first thing to do is to look for the overhead cable. The second thing to do is to locate the earth conductor.

A split load consumer system or RCBOs can be used with any system. Don’t buy £5 RCBOs from the internet because at that price they are bound to be dodgy. They’ll be type C or D, which we don’t want.

Don’t forget to never pay cash for anything. Set up a trade account at a wholesalers. Persuade them to give you a discount from the start of the account.

Multi-function testers are made by AVO (the megger), FLUKE (ROBIN), DI-LOG, METREL, KEWTECH and INSTROTECH. You might get a used model from a calibrater.

There is no maximum for R1 or R2. The maximum Zs is limited for BSEN 60898 MCB and BSEN 61009 RCBOs (B type). See table 2D on page 103 of the OSG. We will only use the breakers listed on page 49 of the Electricians Guide to the Building Regulations.

6A – 6.18Ω
10A – 3.71Ω
16A – 2.32Ω
20A – 1.85Ω
32A – 1.16Ω
40A – 0.93Ω
45A – 0.82Ω

You can see a pattern above. As the amps doubles, the impedance halves. Zs has to be below the tabulated maximums. You don’t need to remember these figures because they are in the table but they can be
calculated from the first two sets of figures in the table anyway. Just to complicate matters, there is a different set of values for the design maximums. The measured maximums listed above are multiplied by 1.25 {I’m unsure about this multiplier’s accuracy} to give you a safety margin. As domestic installers we just need to know that there are two sets of values, one for design and one for testing.

A verification inspection confirms that an installation complies with the wiring regulations. The regulations are considered, by the Health & Safety Executive, to comply with the Electricity at Work Regulations 1989.

People become blase about the risks we were warned. Later I realised that I had already become blase, when not paying attention properly to my crocodile clips during a live test. Made a mental note to always retain my ultra caution. My risk adversity as my friend Anita would call it.

Live working should be avoided unless it is unreasonable not to do so.

Isolation procedure

1. Check it is ok to isolate the circuit
2. Lock off the circuit and tag it.
3. Check that there is no dangerous voltage present.
4. Check the voltage again.

A single person our a single organisation must be responsible for the design.

Inspection must be more than just visual. Touch and smell can be employed too.

When testing R1 + R2, always record the highest figure from each of the loads in the circuit. This will be the last ceiling rose for example. Always do the continuity tests first. Always put the line probe on last and remove it first to prevent the neutral crocodile clip from waving around the line contact.

Put the highest short circuit current on the test form.

RCD Testing

Page 91/93 of the OSG deals with this. You have to test the 30mA RCD or RCBO, even if it its brand new. Under fault conditions the RCD will take differing times rio detect the fault. The resulting delay in shutting down the power will vary according to where it is in the cycle. Domestic electricity is supplied at 50Hz (50 cycles per second).

Until very recently, seven tests were performed to test an RCD. One tester (an AVO megger) has conflated the first two tests. Doubtless the updated models of the others will follow that lead. The first two tests are duplicitous.
The seven 30mA RCD tests are:

1. 15mA @ 0°
2. 15mA @ 180°

This establishes that the RCD will not trip at faults below the danger threshold. The RCD should either not trip or else trip in approximately two seconds.

3. 30mA @ 0°
4. 30mA @ 180°

The RCD should operate at

5. 150mA @ 0°
6. 150mA @ 180°

The RCD should operate at 40mS (0.04S). This is much faster than a fuse or a circuit breaker. It gets faster as the current goes up.

7. Push the RCD test button – this ensures that the mechanism hasn’t seized up.

An auto RCD test is exactly the same but quicker. only record the highest figure from each pair of results.

Ramp Test

Another test, which isn’t required for the paperwork but is nonetheless a useful fault finding test is the ramp test. Not all multi-function testers can do the ramp test. It ramps up the current to the RCD t

For some reason the wordpress app on my phone would not let me post any more words to Part 1 of my review of the 2nd day of the Introduction to Electricals course run by Able Skills in Dartford, which is a little irritating. I’m sure I haven’t reached a limit imposed by my own installation of wordpress.

Anyway, if you’ve not seen my earlier posts on this topic you really should start at the beginning before moving onto the link above. Only then will this post make sense.

I was just explaining what I’ve learnt about the calibration of multi-function testers. The basic point is that you do not need to return them to the manufacturers for recalibration but they must be calibrated once per year.

The third test is the EARTH FAULT LOOP IMPEDANCE TEST. When a circuit is dead, we call resistance “resistance” bit when it is live, we call it “impedance”. It is still measured in Ohms and I still don’t know if I have an omega symbol to hand on my phone… … Okay, I do now. Call me a perfectionist but I just grabbed one from the net and saved it to my phone. Here it is, in all its glory:

Ω

Z is the symbol for impedance. Zs is the measure of the entire earth fault return path. “s” stands for system. Ze is the external earth fault return path. In other words, Ze measures the potential fault current entering the domestic property: the PCF. That little s and that little e should be below the line that the Z sits on but that is definitely beyond what the phone can cope with, as is the groovy diagram I drew on my note paper. Okay, the diagram I could show but it turns out that I’m not the perfectionist after all. It’s late and I’m not trying to teach this shit, only report what I learnt.

Ze or PFC is tested at the domestic distribution board. Zs is tested at the load. Both Ze and Zs include testing cabling outside the domestic property because they both measure back to the subsystem. Although the brackets are unnecessary, the formula always includes them to show that the bits inside the brackets are dead: Zs = Ze + (R1 + R2).

If the impedance at a distribution board is 0.25Ω, I = V/R = V/Ze = 230V/0.25Ω = 920A. That would mean that the potential fault current (PFC) with a line to earth fault would be 920A.

At the distribution board we measure L-E, which is the prospective earth fault current (PEFC) and the prospective short circuit (PSCC). The highest figure goes on the test form as the PFC.

If Ze is very high, you must inform the distributor. Ze and PFC are inversely proportional. The little plastic circuit breakers used in domestic distribution boards have a fixed maximum PFC, which is written on them. Therefore, the circuit breaker’s figure must be above the PFC. If there’s different PFCs in the circuit breakers, go by the lowest limit. You have no control over Ze and the PFC.

It is illegal to access your electricity distributor’s meter and main fuse. It is also fucking dangerous. In order to work on the distribution board inside a residential address, you need an isolator switch between the meter and the distribution board.You will need to ask the electricity distributor how to shut off the power to the property legally. Any new arrangement might be chargeable!

The rest of the day was spent doing some basic tests and modifying circuits. I thought that the tuition was superb, with a staff to student ratio of 1:5.5 in the electrical bays. No question was too simple. We were given lots of encouragement. So far, I’m very pleased to be on this particular course. (I’m not on any payment from them.)

Quite worn out after the second day of my training course at Able Skills in Dartford. Not sure that I can properly relate all that I learnt today because much of out was practical work and I didn’t feel like saying, “slow down a bit, I’d like to film that”. Today we concentrated on testing.

The tests fell into two categories: the dead tests and the live tests. Dead tests have to be performed first. Then everything was switched on for the live tests. I don’t mind admitting to feeling a slight tingle at the point of turning everything on that was just nervous anticipation rather than my circuit malfunctioning.

The first dead test is the CPC CONTINUITY TEST. Obviously it’s crucial to check the basic safety of the circuit first! CPC, as I reported yesterday, is the new acronym for “earth”. The test checks whether the CPC wire is continuous throughout the circuit. For it to do its job properly, there cannot be any breaks or gaps in it. The only way of proving that it is continuous is to measure resistance on it. To do that, it has to be connected (with a connector block inside the circuit board but without anything being connected to the circuit breakers at the board) to the line wire. To use the technical jargon, R1 (line) and R2 (CPC) are joined. Next the multi-function tester is set to measure resistance. The green and red contacts are put together and the reset button pressed to return the readout to zero. That removes any measure of resistance in the tester cables, which might otherwise skew the result. Then the red contact is put on the switch line (so as to ensure the switch part of the circuit is included in the test) and the green contact is put on the CPC. Both these points of contact were inside my ceiling rose. The readout will give a figure of less than the maximum. When the switch is thrown, the circuit created between R1 and R2 is broken and the readout will go to its maximum. We are using the AVO Megger 1552. Its maximum readout for resistance says “>999.99″. After some initial confusion as to the correct points of contact to test inside each of the roses I made yesterday, I was relieved when the test was successful. The higher of the two figures recorded has to be written on the test form. It does not matter what the figures are so long as they are beneath the maximum because that shows that current is flowing all the way along the CPC wire. In other words, it is continuous! In effect, we use line (R1) as a test cable.

For reasons that we were told would be explained later, the next test we had to be sure to complete would be the third test in the running order. This is the POLARITY TEST. Polarity is tested as part of the CPC CONTINUITY TEST. Polarity is correct when all single pole devices are in the line conductor. I just realised that when I was using the word “wire”, I should have been saying “conductor”. Having insufficient time to reword what I’ve just written, I’ll use the jargon correctly from this point on. Fuses, switches and mini circuit breakers (MCBs) are all single pole devices. They cannot be in the neutral conductor (RN) because if they were, it would be impossible to turn off the loads on the circuit. Lamps would be permanently live.
Cutting back a CPC is bad practice. Leaving it long allows switches to be changed in the future. For example, someone might want to add dimmer switches years later.

A line to CPC fault is called an Earth Fault. A line to neutral fault is called a Short Circuit. These faults are caused by overcurrents.

The conductors entering a residential house in England & Wales are: line, at 230v, and neutral & earth, at 0v.

The fuse will blow under fault conditions. In normal conditions, the resistance of a load will prevent a short circuit across an appliance.

Another type of fault is an overload. This occurs when a householder has connected too high a load across a circuit. For example, a leaky water tank might give rise to the wild notion that out would be a great idea to swiftly dry out the suddenly wet plaster by connecting six 3kW heaters to one ring circuit. 6 x 3,000W = 18,000W/230V = 78A. This won’t harm the conductors in the ring circuit because a fuse or circuit breaker will be protecting it. There is no actual fault in the ring circuit.

50-80mA (50/1000 – 80/1000 A) will cause enough damage to the human heart to be fatal. The heart is just a pump which works by dint of electrical impulses. These impulses are very low. Any interference from another electrical source in that range Is likely to cause serious harm.

Fuses are intended and designed to save a circuit, not a human life. To save life, we use a residual current device (RCD). It checks the current flowing out of the device against the current flowing back. If the current flowing back is different from the outgoing current (it would be lower), then must mean that there is a fault. If that difference reaches 30mA, within a few thousandths of a second an RCD will shut down the power. This saves lives.

The downside of an RCD is that it does not protect against an overcurrent. Therefore an RCD is used in conjunction with a circuit breaker. In other words, there are different safety mechanisms for different jobs. An RCD can and should be tested by pushing its test button. That should throw its switch.

Every domestic circuit which has been worked on in any way must be left with 30mA RCD protection. Unprotected circuits can be left alone so long as no work has been done on them – they were not installed, modified or extended.

The second downside to an RCD is nuisance tripping. Heating elements are famous for this.

An RCBO is a device which comprises an MCB and an RCD. Previously, the argument against RCBOs was cost. Now, with some shopping around, they can be got for about £20 including VAT. They used to be £50 each, a price which soon mounts up when lots of circuits were being worked on.

The second test in the list but the third one we learnt today is the INSULATION RESISTANCE TEST. In this procedure, the multi-function tester injects 500V into the system. This is not dangerous because the current is only 1-2mA, which is non fatal. If you touch this circuit when live by reason of the test, it will hurt and make your arm fly around like Harold Lloyd on top of a big drop but it won’t kill you. The three pairs of conductors each have to be tested: (i) Line and CPC; (ii) Neutral and CPC; & (iii) Line and Neutral. The order in which each pair is tested is irrelevant. The minimum acceptable readout is 1,000,000 Ohms (still no idea if omega symbol exists on my swype keyboard). If the resistance is lower than that, it means that two conductors are in contact with one another. They should not be touching! The test sheet has two columns to deal with this test. The first is marked “Line/Live” – that is for the line and neutral test. The second column is marked “Live/Earth” – that is for two tests: the line & CPC test and the neutral & CPC test. All loads have to be removed for the insulation resistance test. If they are not, the test will give a low reading which is unsatisfactory. Also, some equipment can be damaged by this test, for example anything with a silicon chip in it. The test is only for the cables. Sockets are not loads. Switches have to be closed to test that part of the circuit. Two crocodile clips are used so that there is no risk of touching the connectors (which I previously and perhaps erroneously called the multi-function tester’s ‘contacts’).

These first two tests (the CPC CONTINUITY TEST and the INSULATION RESISTANCE TEST) will identify 99% of faults. If there is no RCD, the insulation resistance test is the only way to test the insulation.

Don’t forget, no matter what you’re told, to thoroughly check everywhere (including outside) for forgotten loads. For example, security lights. Take out all lamps and close all switches.

Incidentally, prior to battery driven testers, the insulation resistance test involved hand cranking to generate 500V!

Second-hand multi-function testers have to go through a calibration test, which costs £50. Never buy a tester without an intact serial number. Apart from meaning that it has been stolen, the serial number is required for calibration. (DI-LOG 9083P is £200 cheaper than the testers used by Able Skills – the AVO Megger 1552, now discontinued. It’s been replaced by three models: the 1710, which is £460 plus vat; the 1720, at £660 plus vat; and the 1730, at £840 plus vat.) Wholesalers have calibration days. The tester must be calibrated once a year. You don’t necessarily have to return it to the manufacturers to get your calibration certificate. There’s even a mobile calibrator – google him to find out when he’s in your area.

I’m doing a course on an introduction to electrical stuff in Dartford this week. Next week, with luck, I’ll be obtaining some minor qualifications in the field. All courtesy of a training company called “Able Skills”. I was rather nervous this morning but our tutor, Alan, was impressively engaging and everything we did today was essentially a recap of bits of my old O-Level in Physics, which I did get a B in.

Shockingly, the first thing I learnt was that a multi-function tester compliant with the 17th Edition of the wiring regulations will cost between £300 & £1,000. To be Part P registered, one has to complete an annual assessment with one of the governing bodies, which costs about £450 a time.

In 2008, our official domestic voltage was “harmonised” with Europe at 230v but in fact nothing has changed and it is still 240v. Bizarrely, all the maths we do uses the official figure. Either way this voltage its easily enough to stop your heart permanently. Voltage is the measure of electrical pressure pushing a current around. Current measured in amps is the measure of flow but it is represented by the letter “I” because “A” was already spoken for. The main switch on a domestic circuit board is 100 amps. Resistance to flow is measured in ohms (using the Greek symbol for Omega which I can’t find on my swype keyboard). Ohm’s Law is the basis of all electrical calculations. It is represented pictorially by a pyramid with V at the top and I & R (I’m using R to mean resistance until I can locate the pesky omega symbol.)

   V
I     R

Therefore, V=IxR, I=V/R and R=V/I. In other words, I and R are inversely proportional. In a domestic scenario V will always be measured at its nominal value: 230v. Thus, if a shower draws 50A, the resistance of the connected load (the shower unit) must be 4.6R. There is a negligible resistance in a circuit itself. The main resistance is in the load. The load’s resistance cannot be altered.

The Power Law is represented by another pyramid with P (power measured in Watts) at the top and I & V below, like this:

     P
I         V

Therefore, P=IxV, I=P/V and V=P/I. The most useful equation is I=P/V. Every domestic appliance is rated in watts.

On a domestic circuit only a fuse, a switch and a load can be connected in series. If the circuit contains two loads, they must be connected in parallel to each other. This decreases resistance in the circuit because it creates more pathways for the current to flow around.

An open circuit is off. A closed circuit is on. These words properly describe the status of a switch. The circuit always goes through a protective device first, then the switch and finally the load.

Prior to 2008, the three wires in a domestic cable were called live, neutral and earth. After 2008 they are called line, neutral and CPC (circuit protective conductor). The first two are both live conductors.

In 2004-2006, the wiring colours changed. Previously they were red, black & green/yellow striped. Now they are brown, blue and green/yellow striped. The old colours can stay in place if they are functioning properly.

R1 is the resistance of the line conductor, R2 the resistance of the CPC and RN the resistance if the neutral.

The wiring regulations are going to be amended at the end of the year with a new book being brought out next year. This means that the book I bought on the recommendation of Able Skills only has a life expectancy of three months. Methinks that they could have warned me.

Wires are measured by their cross-sectional area (csa). This is always marked on the drum they are sold on but not always marked in the cable. You could write it onto a sample of each cable to keep it in your tool box. Resistance increases with a wire’s increased length. Any kink or stretch which reduces csa renders a cable into scrap. A good tip is to knock up a test board out of plywood and practise making circuits on it. When clipping a cable to a wall, always put the mail below the cable to support it.

The only way to test a CPC is continuous is by checking its resistance.

After all that theory, it was time for practice. First up I had to learn how to wire a ceiling rose with a single switch. Next came two roses with two switches, each of which worked its own light. After that, there was the slightly more tricky business of two ceiling roses operated by two switches. I made one of those in the afternoon but before I got to that giddy height, I had to learn the unusual circuit for a two-way and intermediate switch.

I hope I’ve got all this right and not missed anything out.