Help me with the simple clear understanding of the following

[ericmark]

Hello guys
Can some one pleas help me with a simple understanding of the following
1) what is continuity testing in an installation

With the earth is shows it will actually work. With ring finals it shows it is a ring. With a radial with no continuity it simply will not work. In real terms we are looking at the reading and using our knowledge gained over the years which tells us what to expect. When I was doing my C&G2391 I failed the installation on the continuity test as the impedance was too high for the distance involved, there was a series of resistors in the test rig to emulate the results expected in a house, I found the resistors and reported them as a serious fire risk, which they were. However the reading was well within spec.

2) insulation resistance testing

In the main cable does not leak, most low insulation resistances are caused by poor cable joints, be it a socket or a junction box. However there are items which will give false readings. Neons, USB sockets, filtered sockets can all cause the reading to be lower than normal. But again walking around the installation first should make one aware of items which may need isolating to get reading required. But aim is to locate sockets in wet walls and the like.

3) polarity testing

In this country we do have single pole switching, polarised sockets, and fuse only in the line of a plug, the latter makes it very important that line and neutral are not swapped even if they are both classed as live.

(I have a fair idea of what they are but look for a more understanding of each)

A) why are they carried out
B) what is the permissible range in testing.

And do you have to work out the diversity factor for every hob or oven even if it falls with in the 32amp reading
Ie an oven at is on a 32 but rated 2.9kw I worked it out and with out using the diversity Mehod it in 32amp use the diversity method and it's even below.
And the hob is 7200w

Lastly any good books out there with simple explanations

Thank you all I know it's a Sunday but if one has a spare time to answer thanks a lot

The range does vary, not got latest version of BS7671 so not going to quote values in case they have changed again. The loop impedance and prospective short circuit current are related with ohms law, many meters will flick between both reading with a flick of the switch, some auto read line - earth with loop impedance and line - neutral with pscc. There are a number of reasons to measure loop impedance.
1) To ensure a MCB will trip with the magnetic part, with with a B type is 3 to 5 times current needed to trip the thermal part of the device. We now also give a 5% extra to be on the safe side. So a B32 will be 32 x 5 + (32 x 5 x 5%) amps, that's 168 amp pscc or 230/168 = 1.37 Ω.
2) To ensure the MCB will not be overloaded if there is a short circuit, often they are rated 10k amps although some RCBO versions can be lower often 7k amps, it is rare to find after the 100A incoming fuse that the pscc is too high, and even if it is we also work with let through current of the fuse, can't remember off hand what a 100A fuse let through amps is, but it is less than 7k.
3) To ensure volt drop is not exceeded, likely this is lower than what is required to trip the MCB, with a B32 MCB with incoming loop impedance of 0.35 Ω likely the limit on a ring final at centre of the ring is 0.97 Ω. However since the incoming impedance can change, and even if over 0.97 Ω in real terms volt drop is unlikely to be exceeded then not that important, since this would be the line - neutral reading which will likely be better than the line - earth reading with a B type the volt drop limit is very close to the magnetic trip limit. One should always read line - earth and line - neutral and go by the worst reading, however in real terms since one is taken in ohms and other in amps we don't really compare.

As to diversity some common sense is needed. With for example an induction hob, within a few minutes of switch on, the mark/space ratio will reduce the average current by quite a large factor, however with a resistive hob the time before the simmerstat starts to switch is a lot longer, also with the induction hob very little energy is wasted but with resistive likely 40% is wasted. If my 40% is correct then a induction hob with a pan of food on it will take 40% less power than the resistive hob with same pan. However the induction hob is likely rated 3.7 kW but the resistive rated at 1.2 kW. So in theory we could allow more diversity with an induction hob than with a resistive hob.

However in a commercial setting food can be rapidly moved on and off the hobs, and although the auto switch off once pan is removed will reduce the power, a cook swapping pans around could cause the hob to give out near the maximum of 1.85 kW there is a maximum time on boost so not full 3.7 kW. So in a commercial setting the cooker could have a higher current draw. But most cookers actually state not for commercial use, so that should not be a problem.

So we look at worst case scenario, what happens if you miss calculate the diversity? The MCB trips. So in real terms no harm done. So my stand alone cooker with everything on could in theory draw over 60 amp, however the manufacturer recommends a B32 MCB, as it would be very hard to draw 60A for more than a couple of minutes before some of the heating units start to cycle.

Having said that there are exceptions, you can get range cookers that only require a 16A supply. You never turn the hot plate off, you just put down the cover and conserve the heat, now these cookers could draw 16A for quite an extended time. So you do need some common sense.

I was told how fitting induction hobs in a commercial kitchen kept the electric bill the same and reduced gas bill. What power was used by the induction hob was matched by the reduction in power used by the air conditioning units. Again a lot depends on the building, and if effective flues can take away the heat which escapes with gas cooking. Gas cooking is very wasteful lucky gas is also very cheap. It is really hard to have a one rule fits all. At some point the designer has to make the executive decision, and stop hiding behind regulations and guides.

 

[ban-all-sheds]

If there were made aware, would they actually care?

They might care if enough students started complaining to C&G, and they were at risk of losing their accreditation. Or if the students started taking legal action to get their fees refunded. Each individual student could kick that process off at very little expense. I imagine the people running the college would be less than impressed if year after year they had dozens and dozens of cases to defend, all making the same complaint.

It is often the way that groups of people, even fairly small ones, grossly underestimate the power that they have to make life a right PITA for organisations if they are prepared to be a PITA themselves.

 

[abwooli]

All I can say is thank to all of u.
Trust me colleges now days a full of shyte!!
Won't believe me if I tell you most of the guys depend on YouTube vids.
Do you guys know this guy on you tube called John ward that's where we get a clearly understanding and full explanation of stuff.

If you really need it badly you have to put in work that's what I say. No harm in asking doesn't make u look stupid it's just you haven't come across.

Thanks guys

 

[abwooli]

They might care if enough students started complaining to C&G, and they were at risk of losing their accreditation. Or if the students started taking legal action to get their fees refunded. Each individual student could kick that process off at very little expense. I imagine the people running the college would be less than impressed if year after year they had dozens and dozens of cases to defend, all making the same complaint.

It is often the way that groups of people, even fairly small ones, grossly underestimate the power that they have to make life a right PITA for organisations if they are prepared to be a PITA themselves.

I attend evening classes cos I wrk during morning
We pay for class from 6pm -9pm part time
But we only get 2hours that's 6pm-8pm then teacher what to go home donot want to miss his train lol

 

[ban-all-sheds]

We pay for class from 6pm -9pm part time
But we only get 2hours that's 6pm-8pm then teacher what to go home donot want to miss his train lol

If that's the case it's hard to see how they would have a leg to stand on if you ask for ⅓ of your money back....

 

[JohnW2]

Just to clarify a few points for the OP ....

There are a number of reasons to measure loop impedance.
1) To ensure a MCB will trip with the magnetic part, with with a B type is 3 to 5 times current needed to trip the thermal part of the device. ...

It's actually 3 to 5 times the 'rating' ("In") of the device. The current needed "to trip the thermal part" is at least 1.13 times that rating.

... We now also give a 5% extra to be on the safe side. So a B32 will be 32 x 5 + (32 x 5 x 5%) amps, that's 168 amp pscc or 230/168 = 1.37 Ω.

I think to say that "5% extra to be on the safe side" may be a little confusing, since it's more important than that. What we now do is effectively reduce the voltage used for the calculation by 5% (i.e. calculate on the basis of 218.5V rather than 230V), thereby almost (but not quite) taking into account the fact that the supply voltage is permitted to fall as low as 216.2V. Before we did this (i.e. when we calculated using 230V, without that 5% 'correction'), a loop impedance that would have been low enough to allow magnetic tripping when the supply voltage was (230V or higher) might not have been low enough if the actual supply voltage was below 230V.

2) To ensure the MCB will not be overloaded if there is a short circuit, often they are rated 10k amps although some RCBO versions can be lower often 7k amps ....

True, but it suspect that it must be very rare to get a loop impedance (L-N or L-E) as low as 0.02Ω or 0.03Ω in a domestic installation.

3) To ensure volt drop is not exceeded, likely this is lower than what is required to trip the MCB, with a B32 MCB with incoming loop impedance of 0.35 Ω likely the limit on a ring final at centre of the ring is 0.97 Ω....

For better or worse, the regulations talk in terms of the voltage drop within the installation - i.e. relative to the origin of the installation, and give guidelines regarding voltage drop relative to the origin. In contrast to the situation in (1) and (2) above, the external part of the loop impedance (e.g. the 0.35Ω you mention) is therefore irrelevant to VD calculations - what matters is the impedance of the circuit within the installation Also, as you have suggested, what matters in terms of VD is the (within-installation part of the) L-N loop impedance (i.e. as used for PSCC determination), rather than of the L-E loop impedance (used for EFLI measurement)

So in theory we could allow more diversity with an induction hob than with a resistive hob.

That's probably true. However, if one wants to stick with the guideline for diversity calculations (as in the OSG), those guidelines do not distinguish between different types of 'cooking appliance', whether in domestic or commercial environments.

Kind Regards, John

 

[ericmark]

I went back to collage to learn more maths, took 'A' level classes, did Maths, Physics and digital Photography if I did three it was just £10 otherwise it would cost around £160 just for one.

The maths was A1, the physics the teacher may have know his subject, but not a clue how to teach it, even the books were simply wrong, it showed a fluorescent tube wiring where they had completely missed out the ballast, if the student had followed the instructions it would either just fail to work or it would go bang, but at least the lecturer, well really a teacher did know his stuff even if he didn't know how to teach it.

As to digital Photography well just don't know how he got away with it, the exam was for A1 8 hours long and A2 12 hours long spread over a few days, to have such a long exam mean it could not be held at the same time as other 'A' level exams, this resulted in something like 6 collage weeks after the exam, he did not turn up for a single class after the exam for A1 when we should have been getting ready for the next year, we had the summer to take pictures for the next year.

Students were really not interested, art was considered an easy way to get points to go to University, the teacher was an ex-newspaper guy, he seemed to know the technical bits of the camera and photoshop, but as far as Art went, he did not really know what he was doing, he said to me, "Who is Turner what type of Photos does he normally take?" when I admitted to position I had placed the camera was to emulate as far as I could what Turner had done when he painted the Abbey.

After getting my 'A' level in digital photography I joined a camera club, for competitions the members are split into beginners, intermediate and advanced, some 4 years on I am still a beginner, having seen others work, I now know there is no way I should have passed as an advanced student and that is what the 'A' in 'A' level means. The course was a complete waste of time, I would have learnt far more simply joining a club. There were students passing with a good grade who admitted they had no idea on how to set the camera they simply left it on auto, there was no written test, you submitted 4 photos and a explanation on how they were processed. Not even the simplest of question asked.

While doing the 'A' levels I considered taking the electrical exams, it seems they included practical, so could not simply sit the exam, needed to do some course work. Remember at this time already had C&G2391 and 2381 as it was then, plus a degree in electrical and electronic engineering, may aim was to return to University and take a level 6, I was only level 5. I also had some 30 odd years experience, but when I did my apprenticeship back in the late 60's early 70's collages issued their own certificates not national ones, so I thought a national recognised certificate would help.

So I sat in on some classes, goodness knows what school the other students had gone to? they did not know Gold, and steel were conductors. Yes it was that bad, the teacher had to really start from the basics, the students really had no idea. When I left school at 16 I knew more than these guys who had left school at 18, clearly raising the school leaving age from 16 to 18 was an error?

With the requirement to start at such a basic level, any adult training can get away with a much trimmed down course. I remembered some 30 years earlier doing courses in the same collage and the level was far higher, although at that time computer training was limited to visiting the main frame room and being able to play a very simple game. Not even a calculator was allowed in exams, log tables and slide rules. The whole idea of sitting an exam at a computer as I did for C&G2382 was as far off as Doctor Who being true.

I can still remember 2πNT/33000 was formula for break horse power. No newtons then, it was poundals, pounds and slugs. Then we went decimal so instead of 32 pounds in a slug we had 980 kilos in a newton how that is called decimal I don't know? 4.2 Joules in a calorie and many more where 10 does not seems to come into it. The metric system was intended to get rid of 33000 in formulas but it didn't.

But learning too much can be a problem, we did some quadric equations as an exercise, most I did quickly, but one needed imaginary numbers, it did not work out with the normal round figures and it took me some time, only to find there was a word missed out of the question, it said find answers to following, it should have said find real answers to the following. It seems students should not have been taught imaginary numbers so test setters had not bothered with the word "Real".

Trying to learn more can some times have wrong results. I remember doing an essay about why the voltage when down on a battery, all about internal resistance etc. Answer expected was "Because it is discharged". Do be careful trying to learn more than the level you are at. Up to and including level 3 there are times when too much knowledge can get you poor results, once you start at level 4 and above, then you are encouraged to think, so with a HNC, or HND or Degree then you do read papers, and gather info form all places. But level 3 is still at a point where "and others" can make the difference between right and wrong when asked what a competent person is. OK gone with amendment 3, but giving a correct answer does not mean it's the right answer with level 3.

 

[davelx]

"we had 980 kilos in a newton how that is called decimal I don't know?"

It's 9.8 Newtons per Kilogramme, not 980 kilos in a Newton. The Newton is the SI unit of FORCE, not weight.

The mass (kGm) of a given object is fixed anywhere in the known Universe, but it's weight (kGw) depends on the force exerted on it by gravity. The unit of force, the Newton is defined as the amount of force needed to accelerate a object of 1Kg MASS at a rate of 1m/s/s. On this planet, the nominal force exerted by gravity on a mass of 1kGm is 9.8 Newtons.

I'm surprised this was not explained to you - I learned it in O level physics. The fact that mass and weight are both measured in kG was a source of confusion then too.

 

[JohnW2]

I'm surprised this was not explained to you - I learned it in O level physics. The fact that mass and weight are both measured in kG was a source of confusion then too.

When I was doing O-level physics, the fact that mass and weight were often both measured in pounds was also a constant source of confusion

Kind Regards, John

 

[jg321]

Do you guys know this guy on you tube called John ward that's where we get a clearly understanding and full explanation of stuff.

He's posted on this thread - @flameport

His videos are excellent, I love them.

 

[ericmark]

When I was doing O-level physics, the fact that mass and weight were often both measured in pounds was also a constant source of confusion

Kind Regards, John

This also confused me, one pound force and one pound weight are both valid units. Acceleration under gravity is 32 feet per second squared so you had 32 poundals in a pound, and 32 pounds in a slug. The slug was imperial equivalent to the Newton but rarely used, in the main it was the poundal and pound, so we had pounds per square inch force, and the torque wrench was either foot/pounds or inch/pounds again pounds as a unit of force. Horse power was also linked back to pounds not slugs. Since the Kilogram replaced the pound the Kilogram should also be a unit of weight or force. 550 pounds x 60 time = 33000 it is pounds not slugs.

As to 'O' level Physics I seem to remember Fletcher's trolley

and it was pounds and poundals that were used, again not slugs.

 

[ban-all-sheds]

Since the Kilogram replaced the pound the Kilogram should also be a unit of weight or force.

No - it should be weight or mass.