Barnschop's Life

3 phase Cage Induction Motor go/no go testing

2009-05-15T23:10:00.003Z

How to quickly test a new three phase motor.

Many electricians have never had to determine whether a three phase motor is faulty since the majority of their work is involved with single phase work. The same can also apply to electrical apprentices since this may not have been taught.
There is nothing worse than removing an old motor to replace it with what turns out to be a faulty motor. There are some basic tests that can be carried out to try an minimise the risk of putting faulty three motors into service.

The tests below should be done prior to installation.

Test one is to verify that the shaft is free to turn and that the fan is not catching the cowl. Spinning the shaft should identify siezed bearings by listening for noises in addition to the ease at which the shaft can be turned.

Test two is to verify that all three phases are in balance. More often than not the motor will be already connected in either star or delta configuration and thus terminals UVW should be tested using an ohmmeter. U-V, U-W, and V-W should all give the same value of resistance across them. Where the motor is to be employed with a star/delta starter then the three individual coils can be tested and again should be the same.

Test three involves an insulation test of the phases to earth. Although if configured in either star or delta the three separate windings will be connected together you will see most time served electricians still test each terminal ie. U V and W to earth.

You should notice that the three simple tests outlined above will not in all cases identify faults with a new motor.

Do also investigate any anomalies in the tests above since it might be something that is simple to cure and you will not be looked upon in a good light if you condemn the new motor and increase production losses.

Transducers Part 4 - ultrasonics

2009-05-13T13:47:00.003Z

Hello, is the common shout by many to see if they get an echo coming back from an object or objects. The action of shouting of hello creates vibrations of the air that travel outwards much like the action of a pebble dropped into a pool of water. If the tiny ripples of water created by the pebble hit a solid object then they change direction much like light hitting a mirror. Sound waves produce a similar effect and that is what we call an echo.

Sound is a name of vibrations that we can actually hear, whereas much higher vibrations we perceive as visible light. So there is an whole spectrum between sound we hear and the light we see. The name given to vibrations that are just above most human hearing is called ultrasonic.

No doubt your mind will have already used neural pathways to bring up how bats use ultrasonics to avoid objects and some of you may also be picturing the scan of you before you were born.

The piezoelectric effect is used for the transmitter and receiver. In the simplest form an ultrasonic frequency is sent in short bursts, then the echo received by the same sensor. Electronics are used to measure the time taken for the pulses to return. The time taken being proportional to distance between the sensor and the object reflecting the ultrasonic frequency back.

Ultrasonic transducers are not just used for distance measurments as later blogs will show

Transducer Part 3 - The Piezo-electric Effect

2009-05-13T12:50:00.003Z

Piezo is derived from Greek and means to squeeze. Certain materials that are squeezed or stressed will produce a voltage that is proportional to the level stress applied. Then again your lecturer should have told you all this. All I am doing is drilling through the brain to lay down even more neural interlinks. Although many materials exhibit the piezoelectric effect most common is Quartz crystal.
Another important point is that if a voltage is applied to a piezoelectric effect material it will stress. The effect is therefore reversible and this should be noted since piezoelectric is used in transducers that convert energy into electricity or vice-versa.

If we take a diaphragm and connect to it some piezo material and subject the arrangement to air that is vibrating then electrical voltages are produced proportional to the amplitude of the vibrations. This arrangement you might know better as a microphone. Equally we could with slightly modifications to the design, namely in size, make a device that will vibrate the air in proportion to a varying electrical signal, yes, a loudspeaker.

The air is not the only thing that vibrates since there is also vibration in the earth caused by earthquakes but I doubt many electrical apprentices will get involved with those. Some though will come across devices in industry that measure the vibration of machinery. By processing the vibrations generated by machines with computers it becomes possible to identify failure in components long before total failure occurs.

You are reading this text via a computer that I entered via a computer. Tick tock, tick tock, goes the clock. No I have not lost my reasoning since the piezo electric effect is used both in computers and in certain clocks that you know as quartz clocks. Most objects will have a frequency at which they will vibrate naturally at, this is called the resonant frequency. Quartz crystal is no different, so by differing the dimensions of the crystal you achieve a different resonant frequency.

There are many more applications of the piezoelectric effect used in transducers and sensors so look for blog entries for more later

Transducer Part 2 - The Strain Gauge

2009-05-13T05:35:00.008Z

I have chosen the strain gauge to start with since it uses some of the basic electrical principles that are taught to electrical students. Problem is that for many the theory will have been lost in some obscure part of the brain. Therefore it is time to trigger new neural pathways to this long forgotten collection of brain cells.

Before getting into the theory it was mentioned that a piece of knicker elastic can help understand strain gauges in Part 1 of transducers. With your knicker elastic or even rubber band in both hands, pull at either end. You will notice the the width of knicker elastic becomes thinner the more it is stretched.
No doubt the above demonstration will have triggered the eureka moment for many students of electricity into how the strain gauge works. I say farewell to those for whom the penny as dropped and continue on for the others.

Put these in order of lowest resistance, a 1 metre diameter of copper cable a meter long, 1 metre diameter of copper cable 10 metres long, 1 millimetre diameter of copper cable a meter long, or a 1 millimetre diameter of copper cable 10 metres long.

Did you get:-

metre diameter of copper cable a meter long,
metre diameter of copper cable 10 metres long,
millimetre diameter of copper cable a meter long,
millimetre diameter of copper cable 10 metres long.

So for the same material the resistance is proportional to both it's cross sectional area and length. Those neural pathways are now being formed, I can sense it.

Back to the knicker elastic in which when pulled gets longer and narrower (the csa reduces). Copper does have a very very tiny amount of stretch property like the elastic but so small you are unlikly to notice it, however as with the knicker elastic the more it is stretched the longer in becomes increasing the resistance and the less csa so again increasing the resistance. Once released the resistance returns back to its previous value.

The actual strain gauge that uses copper or another metal will be either a wire looped and bonded onto an insulated material or metal foil again bonded to a non metalic surface.

So hopefully you will now have the gap between your knowledge bridged with neural network links from basic theory of resistance to strain gauges and thus transducers.

Rather than filling up your head with more information about strain gauges at this point I will return later in another blog.

Electrical Transducers Part 1

2009-05-13T04:58:00.005Z

This is again another blog entry to give electrical apprentice the link between the theoretical world taught at technical college and the practical world in which a few will be working in.

So what is a transducer?

Your lecturers will no doubt give you the answer that a transducer is a device that converts one form of energy into that of another. For the electrical apprentice this either means converting from or to electrical energy from another source of energy.

For many electrical apprentices that may later work in industry it is important to understand how transducer devices work, since having this knowledge may help to understand the operation of a particular process or machine. Knowing a process or machine operation is a key to effective diagnosis. Later I intent to, at a later date write a blog on fault diagnosis.

With so many different transducers available I thought I had better split the topic up into many different blogs so that you will not have to search through loads and loads of words on a single page.

Where I can think of an analogy, this will be included so as to assist in the explaination a specific transducer. I always find that electricity, because it cannot be seen is best explained using something more familar that is visible or at least easier to understand.
For example, what happens when you slowly close a tap/faucet on a water pipe? The action of closing the tap causes resistance to the flow of water, thus the flow slows down, eventually stopping. The tap/or faucet can therfeore be likened to the variable resistor found in electrical circuits in that it too offer resistance but this time to the flow of electricity and not water.

In part 2 I am going to be looking at knicker elastic and strain gauges.

Variable Speed Drives or AC Inverters

2009-05-11T06:42:00.004Z

Variable Speed Drives or AC Inverters are a common subject that I get questions asked by apprentices and thus thought I might tell others in my words how these are used in an easy and fairly non technical way.

50HZ (60Hz in the US) is the cycles per second of the mains frequency which most apprentices can tell me. Now we look at a 3 phase squirrel cage induction motor which reveals that it will rotate at a given speed. The fastest motor speed found by the apprentice in the UK will be slightly less than 3000 rpm. So the formula is given N (rpm) = f * 60/P where f is frequency in hertz and P is the pair of poles.

So now asking the apprentice what we can change to alter the speed they are most likely to answer either the pair of poles or the frequency and both are correct. Pole change motors are possible but not very practical and are expensive so we are left with changing frequency.

The black box is now brought into play since it is best at this stage to only give the the very basics. Three phases go into the box and another three come out and then we add a speed control potentiometer. Explained is that the mains three phase frequency is now converted to a different frequency. Now many apprentices at this stage begin to ask what is inside the box since it is a little bit simplistic.

So now the black box is opened so to speak and on the drawing additional boxes are drawn. First there is a rectifier a storage capacitor and a box marked chopper. At the stage of asking about speed control most apprentices will understand how the rectification of AC into DC takes place and also how a capacitor can store electricity. The chopper box internals may or not be understood by some. However the simple explanation is that it chops up the DC to produce AC.

It is at this stage that most will accept the explanation of the basic of how inverters or Variable Speed Drives work?

Basic Three Phase AC Theory Understanding

2009-05-11T05:44:00.003Z

Another subject that often baffles electrical apprentices is in the use of three phase AC. At technical college they are given the theory and little else by the lecturers. Confusion thus remains which can for some inhibit or at least slow down further learning on the subject.

Now at this point I have to mention that most of my experience is in industry and thus these apprentices are likely to be familar with the equipment found there. For those of you reading this that wishing to gain a better understanding I should point out that electrical motors form most of the direct use of three phase AC since lighting is single phase.

I ask the apprentice to demonstrate their understanding of single phase AC by having them draw the sinusodial waveform. I then see if they can draw the three phase sinusodial waveforms which hopefully they can and show that there is a 120 degree displacement between the three. Most have no problem in drawing out the waveform. A question is asked to the apprentice "When one phase is at a maximum in the postive direction what is the value of the other two at that instant." With a little bit of help they point out or can see that this happens

With two magnets it is demonstrated for the purpose of refreshing their knowlegde that two like poles repel each other and that two differing poles attract each other. From this I draw a circle and put on three windings and in the centre is a magnet. I then ask them to look at the waveform and show the direction on the circle that each phase would become postive. I then ask them what happens with the magnet and most indicate that this would turn in the direction that they indicated on the circle.
Have you ever seem the light bulb on someones head glow bright because for most apprentices in my experience it is at this moment that you can almost hear them shout "Eureka".

Silence and now AC

2009-05-11T04:41:00.004Z

How does AC electricity work?

A few apprentices have over time have asked me to explain how AC works, since they comprehend how DC works. They know that you need a flow of electrons around circuit which is sent from the negative terminal of a battery and returns to the positive terminal. They know that these electrons are capable of heating up metal to such an intensity that it produces visible light. Again they also know that the heated metal is found inside light bulbs.

So when asked about AC they know that electrons flow first in one direction and then all goes in reverse and the electrons flow in the opposite direction etc. The problem is that they cannot understand how the circuit is made.

In order to explain how AC works I have then asked them to look at the old fashioned steam train and how the piston moves backwards and forwards and yet creates a movement in one direction. The piston pushes out and turns the wheel, as it pulls back it continues to turn the wheel in the same direction.

So once they understand how a backwards and forward motion can be converted to a movement only in one direction, it is then time to look at railways again. So we now look at a super long train and a big powerful locomotive at one end. The locomotive will only move a few yards in either direction however.

I then point out that it is not the content of the wagons making up the train that is important it is the actual movement backwards and forwards that is important. A secondary reason for using the train is to explain how the relatively slow electron can create electricity at the speed of light. When the locomotive shunts into the trucks/wagons you can hear each collosion as the wagon/truck hits the next and so on. Imagine how long it would take from the first truck/wagon being shunted until the last one gets shunted. Even if the train went from the UK to the USA, it would not take long before a shunt in the UK would be heard in the US.

The apprentice can now translate the locomotive and associated train into electron flow in an electrical circuit and how their backward and forward movement can still produce a linear effect.

17th Edition IEE Wiring Regulations (BS7671:2008)

2008-02-12T02:27:00.000Z

£65 or 10 years in prison, your choice.

The new 17th Edition of the Requirements for Electrical Installations is now available to purchase from your normal sources such as AmazonUK, the price, well that is £65, although you can get secondhand cheaper.

IEE Wiring Regulations 17th Edition : (BS 7671: 2008) (Paperback)

I had to admit to having a smile on my face when reading the reviews at Amazon though. Especially in how it can increase your pulling power. Strange though it may seem, it could make sure that if you read and obey you will have liberty and freedom to chase the girls (or boys). Failure to comply could well result in a manslaughter charge and if found guilty (remembering you have to prove your innocence) could face a lengthy prison sentence. To reiterate, The Electricity At Work Regulations, regulation 29 is a clever bit of law that puts the onus on you to prove you took all reasonable steps. In order prove you took reasonable steps then it would be in your defence that your work complied with that contained in the Red Book or 17th Edition Wiring Regulations BS7671:2008. £65 or 10 years in prison, your choice.

A time to die

2007-11-18T08:38:00.000Z

There is a time for us to die and I have been working on a website called A Time To Die. The aim of the website is to give your advice and hints so that you have everything in place ready for your death. This may be writing a will or making sure you have sufficient funds or life insurance to cover your burial needs.

My friend Chris said that since she is struggling at the moment why should she bother since she would not be here, so a paupers grave would suffice. I took on her point and later discovered that if you have children you can put in your will who you would like your parental responsibility passing to. Although this can be challenged it is there never the less.

Chris went on further to then suggest that she had no plans of dying just yet. To this I had to reply that I had two uncles that were taken early.

What got me started on the website in the first place was that an elderly aunt began to suffer with Alzheimer's disease and Dementia and although she had written a Last Will and Testament, she had not be advised to make out a Power of Attorney. Her estate did not really amount to much but the problems encountered trying to administer her affairs seemed to be insurmountable. Although we managed in the end, being given the POA would have made it far simpler.

If you have had problems similar to those encountered to those mentioned above, why not comment and perhaps I can make use of the information

Remember A Time To Die

Friday Boiler Course

2007-10-14T05:49:00.000Z

Attended a course on steam boilers that did not really cover much more that we knew already.

The sight glass proving method as altered though. For those not in the know, sight glasses are situated on the side of the boiler and tell you how much water is in the boiler. Too little and the boiler can be potentially a bomb just getting ready to explode.

There are three handles or taps on the sight glasses which should all be facing downhill, this gives a quick visual indication that they are set correctly for normal operation. The top is the steam valve, middle is the water valve, and finally the bottom is the drain valve.

New gauge glass procedure is now

steam valve - closed- horizontal
water valve - closed- horizontal
drain valve - open - horizontal

gauge should empty and after initial blast should stop indicating that steam and water valves are holding

Opening and then closing the steam valve will wash the steam line and you shoulkd see shimmers as steam passes through.

Opening and closing the water valve will wash the water line and you should the level rise in the sight/gauge glass.

Close the drain cock, slowly open the water cock, sight glass should fill up, slowly open the steam cock and the water level should slowly drop to match the actual boiler level.

Have a look at the Spirax Sarco website for more on steam boilers

2391 Theory passed

2007-10-14T05:31:00.000Z

Just thought I would let you all know I have passed the theory side of C&G 2391 Inspection and testing. It is now down to the practical and then yet another qualification.
Going on a defribrulator course on Monday so will let you know how that goes.
Been adding some more content to http://www.pullthegirls.co.uk in respect of online dating.
Going to try and get some Autumn photographs too, these will show the colours of Autumn leaves.

An end of Split Consumer Boards??

2007-10-12T13:19:00.001Z

The draft addition of the 17th Edition, BS7671:2008 contains new requirements for bathrooms. All circuits will need to be protected by 30mA RCD so this would mean the lights.

If you look at the logic, what would be the point in having a split consumer unit just for lighting circuits that are not supplying bathrooms or outside lighting?

Welcome any comments on this

Monday, Interview

2007-10-12T13:09:00.000Z

Went for an interview for a tutoring post with respect to the 17th Edition or BS7671:2008. Everyone that knows me thought I would have no difficulty in obtaining the post. I had been on nights and with traffic and getting into the place made me late. Not a good start to the interview, being tired did not help with coming up with suitable answers to the questions either.

On Wednesday I had a feeling that all had not gone well and sure enough I got a phone call saying that I had not been successful. Still even though I cannot tutor people and get paid for it. I can still try and help on here by offering advice on the 17th Edition.

Making money online

2007-10-03T12:31:00.000Z

Making money from the Internet?

The easy way.

Now with no more than a fundamental knowledge of typical word processor usage you too can be earning cash.

To start with you need to sign up for a blog yourself. What is your interests in life, you maybe surprised at how many others have the same interests.

Do not consider Adult Material since this will limit returns or even prevent you from getting accepted.

I can show and assist in helping you to get the most from your blog in terms of revenue. Once you have got your blog set up and a number of postings, leave a comment with the address of your blog. I will leave a link for the next move.

Listen I am doing this to make money, I take no money from you though so do not worry.

How much money is like asking how long is a piece of string. Depends on too many variables to go into here. Suffice to say I will try and get you the most for 6 months.

17th Edition Wiring Regulations

2007-10-01T07:37:00.000Z

In 2008 the 17th Edition of the wiring regulations that are now called BS7671 will come into force.

The final copy is not available yet, but a draft was made avaialble for public comment. Let us look at some alterations between the draft and the present 16th Edition

In the draft it appears that the layout of the regulations follows the International Electrotechnical Commission’s (IEC) numbering system, this means that Part 6 and Part 7 are reversed.

Part 1 additions are protection against voltage disturbances and protection against electromagnetic influences

Part 2 includes some new and changed definitions

Part 4 includes several significantly changes, remember direct contact and indirect contact, these are now basic protection and fault protection respectively. A lot of jiggling with this part as taken place. Supplementry protection with the use of RCD comes under the heading of additional protection.

With the normalised voltage for the UK having changed to 230Volts the earth fault impedances shown in the tables are different.

More to come later

C&G 2391-101 July 2007 Exams

2007-07-27T00:19:00.000Z

I am going to put the questions in my own way but the answers will be the same as those for the exam.

1. What are the three main area to be verified on a new installation?

These would be equipment and material complies with relevant British Standard - all parts are correctly selected and erected - no parts are visibily damaged or defective

2. Ignoring compliance with BS 7671, earthing arrangement and load capacity what else must the designer must ensure when the work is not a new installation?

3. Give three locations that could have to comply to other statutory regulations other than the EAWR.

Approved Document Part P is a statutory document and thus dwellings
Locations that have explosive atmospheres other than Mines and Quarries
Cinemas

4. Give an example of each of the following
Limitation of discharge of energy Electric fence used on farms
Placing out of reach Pickups on electric cranes or overhead wires
Undervoltage Protective Devices Motor controllers

5. (i) What statutory document places a duty on person carrying out inspection and testing Electricity At Work Regulations 1989.
(ii) Which two groups of people does the the inspector have a duty to ensure safety of? Although worded confusingly I think himself and others.

6. What instrument should be used to carry out a continuity test of a circuit protective conductor? Low resistance ohmmeter typically with a digital instrument having a 0.01 ohm resolution
What would be unit of measurement of the above instrument? Ohms
Why would the above instrument and scale be selected? The instrument would be need to be able to measure the low resistance encountered with this kind of test

7. On a TT system which would be the first three live tests to be carried out?
Earth Electrode Resistance
Earth fault loop impedance Ze
Prospective fault current

8. Give correct names for three protective conductors that maybe connected to the MET
Main earthing conductor
Main equipotential bonding conductors
Circuit Protective Conductors

9. A cable have a resistance of 0.183 ohms is 25m in length. What be the resistance if
you had 50m of the same cable 0.366
25m in length but twice the csa 0.0915
25m in length but with half the csa 0.366

10. What are the three reasons for carrying out the ring final circuit test

Continuity of the phase, neutral, protective conductor around the ring. That no interconnections exist. Correct wiring of the ring.

11. Assuming a circuit is safely isolated give three checks to be made prior to carrying out an insulation test.

12. Excluding saunas, bathrooms, and swimming pools, name three other special locations
Construction sites.
Caravan Parks
Agricultral and horticultral sites

13. International Protection rating consists of two numbers, what do the numbers represent
The first letter
The second letter
If an X replaces a number, what would that mean
The first letter signifys protection against ingress of solids eg IP4X 1mm
The second letter signifys protection against ingree of liquids.
Where the X replaces a number it means that the protection as not been checked or that it is irrelevant.

14. When polarity testing as part of initial inspection and testing is being carried out, what are the two stages that shold be carried out. Secondly which instrument would be used?

15. External to an electrical installation what are the component parts of the earth fault impedance for a a TT system.
Even though the question does not specifically request a drawing it does help with the explanation. At the consumer Phase conductor -Main earthing conductor -Earth electrode to General Mass of Earth. At the Distributor, General Mass of earth - Earth Electrode - Main earthing conductor - Neutral on Transformer Secondary.

16. Before an installation is powered up, Zs needs to be obtained. What test value needs to be used. What else is required to be known. Finally write out the formula
Test value is (R1+R), this is CPC + phase conductor resistance)
External earth loop impedance Ze would need to be known
Zs = Ze + (R1+R2)

17. What two methods could be used to ensure the measured value of Zs complies with BS 7671.
What is the maximum accepted value for tabulated values in BS 7671 of 0.35 and 14.1
Where the ambient temperature is known then a compensation factor can be used to determine the maximum value of Zs.
The Rule of thumb is used and is the worst case scenario and reduces values in BS7671 by 75%.
Using rule of thumb
0.35 would be 0.35 * 0.75 = 0.2625 ohms
14.1 would be 14.1 *0.75 = 10.575 ohms

18. A 30mA is required to have three instrument test applied. What are they and what currents would need to be applied.
At 50% of 30mA, 15mA the RCD should not trip
At 100% of 30mA, or 30mA the RCD whould trip in under 200mS
At 500% of 30mA or 150mA the RCD should trip in under 40mS

19. What are the two types of fault current measured at source, which needs to be recorded.
PEFC and PSCC
The largest of the two needs to be recorded

20. Why is prospective fault current important in selection of protective devices?
PFC is important since the protective device would need to be able to break the fault current under short circuit conditions

For questions 21 to 26 I am going to assume you have already have access to the exam paper.

21a Electrical Installation Certificate which would include a schedule of inspection and schedule of test results.
21b The designer
21c Usage and external influences
21d BS 7671, Guidance Note 3, GS38 are all non statutory, Electricity at work regulations is a statutory document
21e Confirm that the installation adheres to the requirements of BS 7671 and that there is no visible damage to the installation
21f Earthing Arrangement - Number and type of live conductors - Nature of Supply parameters - Supply protective device characteristics

22a Although the question is perhaps a little vague I beleive the answer is that since the buildings are attached, the new services would need to be taken to the main earthing terminal

22b

Circuit verified dead using method outlined in GS38
CPC disconnected at main DB and shorted to the phase conductor
At the new DB a low resistance ohmmeter is connected to the phase and disconnected CPC.
The value obtained is (R1+R2)

22c We have 30 metres of 25mm csa cable, given that the milliohm/ metre is 0.727 then (R1+R2) will be 2*25*0.727/1000 which is 0.03635 ohms. Zs is Ze+(R1+R2) from information Ze is 0.1 ohm therefore
Zs= 0.1 + 0.3635 ohms
Zs = 0.4635 ohms

22d The SWA would be glanded at both ends and this would mean that the resistance of the armourings would be in parallel with the CPC resulting a lower value of Zs.

23a You would need the three drawings given in GN3 or similar to explain the three stages of the test since this is a ring final circuit.

The phase and neutral conductor loop values are 40*7.41/1000 using the equation used above, thus giving 0.2964 ohms

The CPC is 40*12.1/1000 or 0.484 ohms

So taking the three loop tests we would PR1 to PR2 (PR) and NR1 to NR2 (NR) as 0.2964 and CPCR1 to CPCR2 (CPCR) being 0.484 ohms

Stage two requires that PR1 is connected to NR2 and PR2 is connected to NR1.

At each socket the value expected is (PR + NR)/4

(0.2964+0.2964)/4 = 0.1482 ohms

Stage three consists of PR1 being connected to CPC2 and PR2 being connected to CPC1

At each socket the value expected this time is (PR + CPCR)/4

(0.2964+0.484)/4 = 0.1951 ohms.

24a Precautions could be removing loads from the circuits such as lamps, being aware that the test voltage could cause damage to some electronic equipment and taking action like linking the phase and neutral conductors together.

24b Factors are the conditions of the individual circuits being tested, since the circuits are in parallel the value of the test will be less than that of the lowest test that would be obtained on individual circuits.

24c The test voltage needs to be 500 Volts. A test result lower than 2 Megohms would require further investigation although the minimum accepted result is 0.5 Megohms.

24d 1/Rt = 1/R1 +1/R2 etc

Since there are 14 circuits with values of minimum 200 Megohms. 1/Rt = 14/200 therfore

Rt = 200/14 Mohms.

= 14.2857 Mohms.

25a and b Circuit 10 is 80 metres using 1.5 mm T&E so CPC will have a csa of 1 mm. The maximum Zs is 4.8 ohms.

The Phase conductor (R1) resistance will be 80*12.1/1000 = 0.968 ohms using values in figure 2.
The Neutral conductor (R2) resistance will be 80*18.1/1000 = 1.448 ohms

Zs = Zdb + (R1+R2) where Zdb is earth loop impedance of new distribution board which from question 22 is 0.4635 ohms .

Zs = 0.4635 + (0.968+1.448)

Zs = 2.8795 ohms

Using the 3/4 rule of thumb maximun Zs value becomes 4.8 *3 /4 = 3.6 ohms

The value of Zs would be acceptable.

(Hard to believe that only 6 marks have been awarded for all that work and why did it require reference to a previous result)

25c A drawing here will assist to explain the procedure

At the furthest point of the circuit with all switches closed connect the earth loop impedance tester to the CPC and then the connect the other lead to the phase conductor, remembering that this is a live test and the risk needs to assessed. The result obtained will be Zs for the circuit. If the circuit was a two or more way arrangement, each way would need to be tested and the highest value would be recorded as the Zs.

26a. The basic drawing can be found in the On-Site Guide. TN-C-S would mean that the neutral and earth would be seperated in the distributors metering at the consumers end. The circuit would also include the main protection device, the protection device for the new DB and also the 20A MCB feeding the air curtain (Just what is an air curtain, not exactly common and therefore likely to create a problem to those taking the exam, IMHO).

26b. PEFC is given by Uo/Ze for the source thus PEFC = 230/.1 = 2.3kA

26c. Since Zdb is higher than Ze it can be assumed that the PEFC will be less than at the source.

26d. For this question we could say refer to question 20 since for me the answer is the same. This time the mark is not 1 but 2 however.

The PFC is important in selection of protective devices since this is the current that the device will need to break in case of a fault of negligible resistance.

These are the answers I have since worked out or found since taking the exam and although many seem to be similar I cannot be certain

I you disagree with the answers please add a comment

What is PART P?

2007-07-26T12:51:00.000Z

The Building Act 1984 in section(1)1 made provisions that regulations maybe made for various purposes. This meant that rather than going to parliment each time additions were needed, the Secretary of State can approve documents. Approved Document P: Electrical safety - 2006 edition is one of these documents, and is commonly referred to as Part P.

Prior to the introduction of Part P, anyone could install electrical equipment into a dwelling. The IEE did have regulations in place in the form of BS 7671, but these were not statutory and were often ignored by some installers (including DIYers). Part P does not limit who can install electrical equipment but does require that certain work will require prior notification to a building control body. There is an exception and that is where the installer is registered with an electrical self-certification scheme.

IT earthing arrangement, where used.

2007-07-24T09:59:00.000Z

Coal mining is one of the places that use an IT earthing arrangement.
Transformers used underground are of a Delta/Star arrangement, namely DY11. The transformer secondary star point is not connected directly to earth but through a current limiting impedance device.
The secondary side goes through a breaker that from memory contains a protection device that would trip if the resistance to earth drops below 20000ohms. Since the transformed supply could be used to supply a number of different parts of the mine called sections. Each section needs to be protected to afford discrimination and this means that insted of 20 k ohms the value is raised to 40 k ohms. Where final circuits are involved then a value of 60K would create a trip of the protective devices. Notice I said all the protective devices.

From memory three resistors are used to create a false neutral which in turn is connected to earth via another resistor. Should an earth fault on any of the circuits fed from the transformer occur then every one of the protective devices would have a voltage drop across the resistor connecting earth to the false star points. This voltage drop is sensed by electronics and causes the protective device to operate.

Prior to the arrangement above a core balance transformer was used. Each phase would go through the core or ring and thus each in turn would set up a magnetic field in the core. Since the current flowing in the three phases would under conditions be balanced, each magnetic field would cancel out the others thus giving no overall net magnetic field.
Under a fault condition there would be an overall magnetic field due to the imbalance and a this would cause a current to flow in a coil wrapped around the core. This current is sensed and would cause the protective device to operate.

C&G 2391-101 July 2007

2007-07-24T08:48:00.000Z

Well the 19 July 2007 finally arrived which for me meant at 18:30 I began the City and Guild's 2391-101 examinination.

The room was full of candidates sitting the exam, of those that took the course with me only one was missing.

Having completed the passed papers with relative ease, I did not feel under pressure, that was until I actually turned the paper over.

The first 20 questions took me one hour to complete. The second 6 questions took 90 minutes to complete. So the two and a half hour exam took that long for me and all the others but for a small minority.

Although the questions were in a similar vane to those I had practised with, the actual wording meant that for me at least, two or more possible answers could have been given.

The scenario for Part B was a Public house extension. Two questions involved an air curtain. (I have since found out that an air curtain is the name for the heating device fitted over many shop doorways.) This led to a little bit of a stall, since I had to try and think what one was.

Question 20 and 26 seemed to both ask the same question and I had to check back through the question paper to verify I had not made a mistake.

Why is PFC prospective fault current important in selection of a protective device? Okay the answer is easy. The protective device needs to be of a sufficient rating to break this current without substaining damage. The other part was easy too, which would need to be recorded the PSCC or PEFC. The answer is the highest since it is this current that determines rating required.

The DB was 20 Way with 14 ways used, the individual circuits tested at >200Mohms. What would the overall test be.

so 1/Rt = 14/R ; 1/Rt= 0.07Mohms

Thus we have 14.286 Mohm.

Perhaps the longest question to answer was on the ring final circuit test showing calculations.

You were given the loop length of I believe 40 Metres. A table gave resistance per metre length of all the csas used in the new installation.
So for 2.5mm this was 7.41 and for 1.5mm 12.1.

So for stage 1 you would have 40*7.41/1000 for phase and neutral conductors. The CPC would be 40*12.1/1000.
So PHr and Nr would both be 0.2964 while the cpc would be 0.484 ohms.

Stage 2 would be (Phr+Nr)/4 ; (0.2964+0.2964)/4 = 0.1482 ohms

while Stage 3 would give (PHr + CPCr)/4 ; (0.2964 + 0.484)/4 =0.1951 ohms

In addition to the calculation you also had to go through the procedure to conduct the 3 stages.

C&G 2391 Inspection and Testing Lighting?

2007-07-17T12:00:00.000Z

Many questions seem to include either a PIR or else discharge lighting. From this I have come to the conclusion that the person setting the question is wanting an insulation test to indicate that the phase and neutral need connecting together to prevent damage.
Discharge lighting generally have a choke to limit the current flow, this creates a reactive component which is reduced by fitting of a capacitor. If you check the rating of capacitors fitted they are normally rated at 250Volts. Testing with a 500Volt insulation resistance meter could damage the capacitor.
PIRs in a circuit contain electronics so should not be tested phase to neutral.

So how would you do an insulation test on a lighting circuit?

Disconnect circuit from supply and verify circuit is dead using approved method outlined in GS38
Check that the testing will not create danger to persons, livestock or property
Link together the phase and neutral
Check your instrument or it's leads are not damaged.
Select 500 Volt.
Securely clip one lead to the earth bar
Prove connection by testing to another earth terminal, this should show 0.0 megohms or short circuit
Clip to the phase/neutral connection and press test, wait for a steady reading and record.
Allow the instrument to remain connected to discharge any stored charge created by the test.
If test is greater than 2 Meg ohms, disconnect leads, reconnect phase and neutrals back into respective places, restore power.
Tests of under 2 Megohms would need further investigation but is not a fail as this is below 0.5 Mohms.

C&G 2391 Inspection and Testing EIC, PIR, MEIWC

2007-07-17T10:54:00.000Z

How I have been revising to pass the exam.

EIC will need to be written as Electrical Installation Certificate
PIR will need to be written as Periodic Inspection Report
MEIWC will need to be written as Minor Electrical Installation Works Certificate

Note that both these need attached to the first two but not the MEIWC.

A Schedule of inspection
A Schedule of test results

You will lose marks if you simply say Schedule of Tests.

To answer which is required, remember the clue is in the question.

A new circuit as been installed? Installed equates to installation, thus could be one of two, but since circuit added, is not a minor undertaking. This leaves us with Electrical Installation Certificate.

A report on the condition is required? There is only one report and this is the Periodic Inspection Report.

Initial Verification, or making sure in the first place, relates to the Electrical Installation Certificate.

Statutory documents are those passed by an Act of Parliment and could be The Health and Safety at Work Act 1974 or more likely the Electricity At Work Regulations 1989.

None statutory documents are likely to be BS 7671, Guidance Note 3, On site guide.

3/4 Rule of thumb Inspection and Testing. 2391 Exam

2007-07-17T10:12:00.000Z

So what is this rule of thumb?

When carrying out testing of an electrical installation the value of the phase and circuit protective conductors resistance is measured together to give (R1+R2).
Ze which could have been either measured, calculated or obtained by enquiry is then added to (R1+R2) to obtain Zs, but you knew that.
For a particular protective device BS 7671 gives tables as to the maximum value of Zs that can achieve disconnection within a specified time.
The problem is that when the value of (R1+R2) is measured the temperature of the conductors are likely to be at ambient temperature. Trouble is that the tables in BS 7671 are not relevant to Zs values measured at ambient temperature.
The resistance of copper is proportional to the temperature, so if the temperature drops the resistance drops.
Rather than having to measure the ambient temperature the 'rule of thumb' method is allowed.

To compensate for the lower temperature the maximum value of the Zs allowed needs to be lowered. 3/4 of the value of Zs is the worst case and thus acceptable to use.

A Zs in the table of say 4 ohms. would result in a corrected value of 4 times 3/4 which is now 3 ohms.

From a C&G 2391 viewpoint you will be given a Zs maximum value for a circuit and a set of Zs values that were calculated or else given (R1+R2) to which you need to add Ze to.

Show the formula you are using so that even if your maths are wrong you will still gets marks awarded.
Corrected Value For Zs = ZsMax X 3/4.

Provided that the calculated value is less than the corrected value then the results are acceptable.

C&G 2391 Inspection and Testing Exam Date Getting Closer

2007-07-15T20:36:00.000Z

The C&G 2391 Inspection and Testing Exam is this coming Thursday. I have been going through the past papers and timing myself and believe I must be missing something with my answers.
Part A which should take a hour to complete is taking me 20 to 25 minutes tops!!
Part B which should take a hour and half is only taking 30 to 45 minutes!!!

Some of the model answers to the past papers are almost the same answers to what I get. The keywords are there but perhaps in a different part of the sentence to those of the model answers. what could I be missing?

I can assume that the C&G are allowing the remaining time for thinking since I would not call myself a super fast writer, especially with a pen and paper.

I have been looking through the adverts for testing instruments to carry out inspection and testing of electrical installations. I wondered about whether is would be wise to get seperate instruments but soon dispelled this notion.

A multifunction instrument is initially cheaper to purchase.
The calibration cost is not that much dearer for a multifunction unit than the cost for a single funtion instrument.
If I lose a function or else a single instrument packs in, I cannot carry out the inspection either way so no advantage either way.
Some manufacturers claim that it is easier to use seperate instruments since there are not so many dials. I think that if someone is competent to carry out inspection and testing then a few dials are not going to be too taxing.
Having worked on some large industrial complexes the last thing I want is 6 seperate boxes to have to carry around, plus my tools.
One set of batteries to replace or carry spares for rather than 3 just in case 50% of the single testers decide to get flat batteries half way through a job.

My conclusion would be a multifunction unit for the reasons given above. Now do I need the extras?

Phase rotation is a must for me having an industrial background, since it can cause damage when large numbers of motors all go in reverse!!!!

I have never had to carry out an Earth Electrode Test but can forsee the day coming when I need to check on an 11kV/400 Volt transformer earth connection.

You ever worked on a construction site or one like i,t with water sludge dirt oil and grease all over the place? If so, then like me you can see the benefit of being able to record the results to download later. Ever dropped your notebook in dirty water and know all your results are lost?

I want an IP67 rated multifunction tester that can be dropped from 3 metres, if only one was made. So will have to settle for a shower resistant one.

C&G 2391 Inspection and Testing Day Questions for Roger 3

2007-07-10T09:01:00.000Z

A few more questions for Roger in respect of C&G 2391 Inspection and Testing

GS38 lists three steps when using an approved test lamp to check that a circuit is dead, list the three steps.
What is the recommended period between calibrations of test instruments.
Who determines the extent and limitations on a PIR?
Name one dead test that could cause injury through electric shock and one live test that could cause injury through electric shock to persons other than the inspector, explain the reasons why?
A system having a TN-S earthing arrangement contains a fixed electric heater that develops an earth fault, sketch the fault path for this arrangement.
What test is required for site applied insulation?
The following are recorded on the schedule of tests, what instruments would have been used to obtain the results? (40mS, 6.23kA, >100M ohms, 0.03 ohms)
Why does the main earthing conductor need to be removed when carrying out a Ze (Zdb) calculation?
What precaution could be employed when carrying out an insulation test between live and earth to prevent damage to sensitive components?
A circuit has a Zs of 500 ohms, show your calculation as to the the RCD to be choosen.
What is the main difference between a type B and type D MCB?
Why when carrying out a polarity check on the incoming supply of a new installation would this not reveal an earth neutral fault?
What disconnection time would be required for a 110 Volt system?
Under what circumstances would a PIR not be required for an industrial installation.
What is the requirement for obstacles used to prevent direct contact.
Give two examples where height is used to protect against direct contact.
What is the difference between conductive metal parts and extraneous metal parts?
Give two examples of enviromental influences that may require an inspector to shorten the period of inspection?
Give five reasons why a PIR may be required on domestic premises.
Where there is likely to be parallel paths present during a PIR, apart from tests what else must be done to check the cpc?
What does the abbreviations PEN, PME, MET mean with respect to BS 7671.
Give three systems that are not covered by BS 7671 but need to be considered during a periodic inspection?