Electric to log cabin

[diycooper]

Thanks for reply, I do not think I am fully capable of carrying out design work but think I have the knowledge to start and ask questions. I think I am capable of running cable and sockets and know ohms law. I also wouldnt do anything without being told fully that it is ok to do so.

I have read all that site you gave also.

 

[diycooper]

cooper - please at least be honest with yourself, if not us.

Do you really think that you are capable, genuinely capable, of carrying out electrical design work?

And if you're not, do you really think that you should try to muddle through on the back of asking questions here as and when you happen to realise there's something you don't know?

You've shown, by the questions you've asked and the mistaken assumptions you had been working to, that you really don't know enough to be attempting to do this design.

The project you plan to do may not be "rocket science", but I can assure you that it involves knowing far more than you think it does.

There's nothing wrong with asking questions here - they can be a useful part of a learning process, but they are not a substitute for proper structured studying. The key term there is "learning process" - you cannot learn all the things you need to know just by asking questions here. It isn't structured enough - it won't provide you with a way to progress where each step builds on what you learned before.

You can't carry out a job of this magnitude by asking whatever random questions happen to occur to you. You've already shown that you have some dodgy misconceptions - what if you get something wrong because you have no idea your knowledge is wrong? What if you miss something because you simply have no idea it even exists, and just don't realise you don't know it?

• //www.diynot.com/wiki/electrics
  
  
• //www.diynot.com/wiki/electrics:books
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/electricity.html
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/domesticinstallations.html
  
  
• http://www.tlc-direct.co.uk/Book/1.1.htm[/list]I strongly suggest you get stuck into the last link right away - it won't give you design ideas, and unfortunately it doesn't refer to the current edition of the Wiring Regulations, but it's free, and will still give you a good grounding which you can augment with more up to date publications. And it might help you realise the scope of what you need to learn.

• 
  
  To be honest yes I do now as I have had a spark out to do work today and mentioned what I thought and he said yes he would do it exactly the same way !

 

[Taylortwocities]

OK, so you have a little knowledge

And we know what the maxim is about that!

So, can you reply to my earlier, basic, question?

Also, what is your supply type?

 

[diycooper]

OK, so you have a little knowledge

And we know what the maxim is about that!

So, can you reply to my earlier, basic, question?

Also, what is your supply type?

240 volts . 100 amp fuse .

 

[Taylortwocities]

OK, so you have a little knowledge

And we know what the maxim is about that!

So, can you reply to my earlier, basic, question?

Also, what is your supply type?

240 volts . 100 amp fuse .

No, no, no

TN-S, TT, TNC-S something else maybe???

You see, its more than Ohm's Law

 

[JohnW2]

Also, what is your supply type?

240 volts . 100 amp fuse .

No, no, no ... TN-S, TT, TNC-S something else maybe??? ... You see, its more than Ohm's Law

It's certainly a lot more than Ohm's Law, but I think you're probably being a bit unfair by playing this jargon game. To the best of my knowledge, "supply type" does not have a unique definition, and is certainly not necessarily synonymous with "Type of earthing system", which I reckon makes it a fairly 'bad question' (unless one is prepared to accept a whole range of answers)! I would have said that, depending on context, things like "overhead","3-phase", "local generator", "priority", "isolated", or countless other things, could all be valid answers to your question.

I'm not disagreeing about the OP's limited knowledge, but I don't think you were really demonstrating/testing it in a very fair way!

Kind Regards, John

 

[diycooper]

OK, so you have a little knowledge

And we know what the maxim is about that!

So, can you reply to my earlier, basic, question?

Also, what is your supply type?

240 volts . 100 amp fuse .

No, no, no

TN-S, TT, TNC-S something else maybe???


You see, its more than Ohm's Law

If I am honest I do not know ! , But the spark said he would do it same way and as the cable is already in place from MCB, does it matter as the art is already in place ?

 

[ericmark]

When I left school in 1970's most houses has fuse protection and we did not have anywhere near as mush electrical stuff in our house. My dad was proud his house had 5 sockets (built 1952) where his father-in-laws had just 2.

Today however we have at least 5 sockets per room often many more and we use MCB's to protect not fuses.

Although the MCB is in the main better than the fuse it has a fixed point at which the magnetic part of the trip will stop working where the fuse was a sliding scale. So slightly out with a fuse means it takes slightly longer than it should to trip. Slightly out with a MCB however means the magnetic part will not work so it takes a lot longer to trip.

We as electricians measure what is called the loop impedance which is similar to resistance but for AC. And we select the MCB not only for the run current but also so that if there is a short circuit it will trip within a cycle. The trips have a multiplier letter in front of the number B = x5, C = x10, D = x20. So a B10 MCB will trip in less than a second at 50A but below that it will take a lot longer.

So this is where Ohm's law comes in 230v and 50A need a resistance less than 4.6 ohms now some of this resistance will be on the incomer around 0.35 ohms so you can have around 4.25 ohms. Jump to a B32 and then you drop to 1.44 - 0.35 = 1.09 ohms. Now to measure 1 ohm you need at least 200ma flowing and this can't be done with a multi-meter.

So the main problem with DIY is lack of test gear. It takes seconds to plug in a loop impedance meter and see if the resistance is low enough but if you have not got one then the calculations take some time.

 

[diycooper]

When I left school in 1970's most houses has fuse protection and we did not have anywhere near as mush electrical stuff in our house. My dad was proud his house had 5 sockets (built 1952) where his father-in-laws had just 2.

Today however we have at least 5 sockets per room often many more and we use MCB's to protect not fuses.

Although the MCB is in the main better than the fuse it has a fixed point at which the magnetic part of the trip will stop working where the fuse was a sliding scale. So slightly out with a fuse means it takes slightly longer than it should to trip. Slightly out with a MCB however means the magnetic part will not work so it takes a lot longer to trip.

We as electricians measure what is called the loop impedance which is similar to resistance but for AC. And we select the MCB not only for the run current but also so that if there is a short circuit it will trip within a cycle. The trips have a multiplier letter in front of the number B = x5, C = x10, D = x20. So a B10 MCB will trip in less than a second at 50A but below that it will take a lot longer.

So this is where Ohm's law comes in 230v and 50A need a resistance less than 4.6 ohms now some of this resistance will be on the incomer around 0.35 ohms so you can have around 4.25 ohms. Jump to a B32 and then you drop to 1.44 - 0.35 = 1.09 ohms. Now to measure 1 ohm you need at least 200ma flowing and this can't be done with a multi-meter.

So the main problem with DIY is lack of test gear. It takes seconds to plug in a loop impedance meter and see if the resistance is low enough but if you have not got one then the calculations take some time.

Right ok. So are you saying I ma better getting the sparky to do it even though he is going to do it exactly the same way ?

 

[plugwash]

Now to measure 1 ohm you need at least 200ma flowing

BS.

and this can't be done with a multi-meter.

Depends on the multimeter.

I just hooked a fluke 75 multimeter up to a resistance box and steped from 0 to 1 ohm in 0.1 ohm steps and it remained within 0.1 ohm of the selected value.

So for most domestic circuits that aren't right on the edge of non-compliance a reasonable (not bottom of the barrel but far from top of the line) multimeter should be sufficient.

For the heck of it I then zerod out the test leads on an agilent 34405A multimeter and hooked it up to the same resistance box and it remained within 0.03 ohms of the selected value (and I suspect the inaccuraries were in the resistance box not the multimeter).

 

[JohnW2]

Now to measure 1 ohm you need at least 200ma flowing

BS.

I agree. There's absolutely no reason why, per se, one needs 'high' currents to measure resistances of that order - after all, just 1mA through 1Ω would result in a 1mV voltage across that resistance, which is quite measurable. It could be that if there are joints within the path whose resistance is being measured, the effective resistance of less-than-perfect joints may change a little according to what current is being passed through them, but that's a rather different matter.

Kind Regards, John

 

[ban-all-sheds]

To the best of my knowledge, "supply type" does not have a unique definition, and is certainly not necessarily synonymous with "Type of earthing system",

To the best of my knowledge when used in the context it is here it absolutely does include the earthing system, and minimally contains only that.

You may add the number of phases, whether it's overhead, what its capacity is etc, but you may not omit the earthing system.

 

[JohnW2]

To the best of my knowledge, "supply type" does not have a unique definition, and is certainly not necessarily synonymous with "Type of earthing system",

To the best of my knowledge when used in the context it is here it absolutely does include the earthing system, and minimally contains only that. You may add the number of phases, whether it's overhead, what its capacity is etc, but you may not omit the earthing system.

If 'supply type' were formally defined somewhere to have that meaning, then I would have to agree with you. However, I've never seen such a definition, have you? BS7671 does not seem to use the phrase at all that I can find - about the closest it gets seems to be 'Supply Arrangement', but it does talk of 'Types of system earthing".

Anyway, this has become a bit moot. The OP has now indicated that he wouldn't have known the answer if he had been asked about the "earthing system" of his supply/installation - thereby, I suppose, confirming TTC's point.

Kind Regards, John

 

[diycooper]

Also the sparky has said he would use 2.5 cable on a 32 amp MCB. Earlier on in the topic I got told this was wrong. Could someone please answer this then I have 5 properties 3 new builds and 2 ex council. I had the ex council ones rewired about 5 years ago and the sockets are on a 32 amp MCB. I have had electrical checks on these properties and all passed. Can someone advise me why 32 amp is wrong because that means 2 different sparks have wired my houses wrong and the sparks who passed are also wrong ?

 

[ban-all-sheds]

Also the sparky has said he would use 2.5 cable on a 32 amp MCB. Earlier on in the topic I got told this was wrong.

It is for a radial.

Could someone please answer this then I have 5 properties 3 new builds and 2 ex council. I had the ex council ones rewired about 5 years ago and the sockets are on a 32 amp MCB.

They'll be rings.

You see, its more than Ohm's Law

Anyway - I thought you said you'd read all of this: http://www.tlc-direct.co.uk/Book/1.1.htm