BS7671 17th my views

[steptoe]

Cables in insulation can overheat if not derated.

I think the best example of this is a 9.5kW shower running happily an a 40A MCB with 6mm T&E,

customer has a grant for extra insulation and 6 months later Im getting a lot of burnt out pull switches in the same street.

had five in the last month, all exactly the same scenario.

 

[unphased]

No this is not how regulations are written. This is a made up, non existent regulation, as such it is an irrelevant example which carries no relevance or input to any topic under discussion. There is absolutely no point any member making up artificial regulations to try and put their point across. The fact that you are using this as an illustration, possibly indicates a major flaw in your views. (For what it is worth, section 511 carries no reference to any BS numbers but does say thing like complying with applicable british standards!)Doc H.

I can see I am totally wasting my time on this forum. To respond in that manner clearly demonstrates that you have no lateral thinking. I wasn't referring to any particular reg the wording is fictitious regs or any particular reg. I was putting over the manner in which it reads to me. I should have used x y or z perhaps. Are you posting as a moderator this time or personal, cuz your far too important aren't you to allow any freedom of wording unless it suits you. section 511 carries no ref to any BS numbers...really...well I never. Its a made up non existent regulation...could have fooled me doc. :Applaud

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Okay I'll say whats on your mind for you, doc. You think I am daft. You think I am 5WW. You think I have a lack of BASIC understanding of electrical knowledge. ohms Law is the basics, doc, not cable grouping. Yes I understood the grouping of cables You are changing the question. I am asking about insulation. There is now a highlighted section on my previous thread that needs to be answered. Theory is all well and good if it is backed up by research. Like I said, the BRE have TESTED cables in insulation and appear to have found no significant change in behaviour compared to cables not in insulation. I will do my best to find the evidence so I can post a link.

 

[a3]

I can see I am totally wasting my time on this forum. To respond in that manner clearly demonstrates that you have no lateral thinking. I wasn't referring to any particular reg the wording is fictitious regs or any particular reg. I was putting over the manner in which it reads to me. I should have used x y or z perhaps. Are you posting as a moderator this time or personal, cuz your far too important aren't you to allow any freedom of wording unless it suits you. section 511 carries no ref to any BS numbers...really...well I never. Its a made up non existent regulation...could have fooled me doc. :Applaud ---------- Post Auto-Merged at 15:41 ---------- Previous post was made at 15:32 ----------

Okay I'll say whats on your mind for you, doc. You think I am daft. You think I am 5WW. You think I have a lack of BASIC understanding of electrical knowledge. ohms Law is the basics, doc, not cable grouping. Yes I understood the grouping of cables You are changing the question. I am asking about insulation. There is now a highlighted section on my previous thread that needs to be answered. Theory is all well and good if it is backed up by research. Like I said, the BRE have TESTED cables in insulation and appear to have found no significant change in behaviour compared to cables not in insulation. I will do my best to find the evidence so I can post a link.

Mr Unphased

Whilst I have not a clue about much that you are talking of, it does appear that you seem to be trying to pick an argument with quite a few on this board, I would certainly not start on the moderator team, they are well respected and do a very good job for no thanks.

I would also suggest that as we have plenty of Student members reading threads here then to make up regulations that do not exist is a little bit silly on an electrical forum board when one would imagine you have BS7671 (green copy) and could have just as easily quoted a regulation and what your opinion of how it should read.

If you really feel you are wasting your time on this forum, then please feel free to go elsewhere, let me know and i will delete your account.

 

[unphased]

The information I am seeking may be BRE Defect Action Sheet 62 so I have requested a copy. Not sure if that is the one though. BRE no longer produce Action Sheets so it will be old.

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Okay. delete my account. I don't feel welcome on here. Seems you are all touchy feely for each other anyway.

Goodbye.

 

[NozSpark]

The information I am seeking may be BRE Defect Action Sheet 62 so I have requested a copy. Not sure if that is the one though. BRE no longer produce Action Sheets so it will be old.---------- Post Auto-Merged at 16:05 ---------- Previous post was made at 16:04 ----------

Okay. delete my account. I don't feel welcome on here. Seems you are all touchy feely for each other anyway.

Goodbye.

Unphased,

I can understand that you feel that the regs should be written in a much simpler way, but TBH they cover most of the installations that we see on a day to day basis, none of which are the same! And I guess that you can also tell that you seem to be alone in your thinking.

Repeating yourself won't change anyones mind, just as others repeating themselves won't change your mind. I think that you have stated your position on this topic as have others, so maybe it's time to let it go?

 

[SPECIAL LOCATION]

The information I am seeking may be BRE Defect Action Sheet 62 so I have requested a copy. Not sure if that is the one though. BRE no longer produce Action Sheets so it will be old.---------- Post Auto-Merged at 16:05 ---------- Previous post was made at 16:04 ----------

Okay. delete my account. I don't feel welcome on here. Seems you are all touchy feely for each other anyway.

Goodbye.

Or possibly just your a bit sulky cuz very few others agree with your suggestions and you can't get your own way??

or that you cannot sensibly debate your point or admit that others ARE entitled to opposing opinions to you....

{especially when there are some rather large holes in your hypothesis}

I think reg 432.67.888888.42 page 56 guidance note 23 torque setting for plastic tie straps is open to missunderstanding...

How can black straps be more durable for shower circuits than Grey straps????

Made up regs and numbers WTF!!!!

headbang

How the heck can a sensible debate be made around made up regs???

bad day explode

and the forum is touchy feely????

Crazy... Crazy!! :shakehead

 

[steptoe]

^^^^

are you seriously suggesting that yopur not allowed to make up regs anymore?

is this an AMD1 thing?

:C

makes things a tad harder for a lot of us now then,

are we still allowed to used a portable corral?

 

[kme]

Woah!

Aren`t YOU the one who stated, before this thread even existed, that you didn`t hold the same views as "most of the other members"? Now you`re whining that we haven`t all collectively changed our minds, because you made up a reg.?

Some of your comments had merit, albeit that the argument behind them was flawed.

If you feel unwanted and unloved, then leave- -its a free country

KME

 

[Sharpend]

Unphased, following a rather brief read of the last page may I suggest a name change to Very Phased. You seem to be getting very wound up by something that you cant change. Perhaps you may wish to study Serenity.

All the best. Sharps

 

[Manator]

I would rather you stayed here Unphased, yes you have strong views and yes some of your argument is lacking substance,but every member is important here.

Discussing anything on an open forum is always going to be open to some personal jibes intended or not, a good debater will stand their ground until the detail of the argument is understood and then adapt to the counter claims or arguments against.

All debates can be carried out in a friendly manner, and you have to remember that whenever you argue so strongly there will always be those who argue against just as strongly.

Some debates I have had here have followed the same vein some I have won and some I have lost, or rather conceded it the nature of good honest debate.

Just stay calm and when it does not go the way you want it to, just do like I do and agree to disagree, give them a smiley and thank them for the time taken to reply.

 

[ProDave]

One of the elements of the regs which I find difficult to agree with. (That's diplomatic enough I think. ; \ ).Cables in insulation can overheat if not derated.

Insulation prevents or inhibits the passage of heat. Under normal use a cable can get luke warm, maybe slightly warm, but hot? For heat to be generated in a cable to a point where it will cause concern the cable has to be overloaded, or under continuous load over a long period. If it isn't then I cannot see how excessive heat, hot enough to cause damage, be it in insulation or not, can cause the need to increase the size of a cable (derate it). If you generate 30A of current in a 1.0mm2 cable it would clearly overheat. When you increase the cable size, more copper, less resistance, current flows nicely, no heat. Taking the same current in a 1.0mm2 cable in insulation the heat generated cannot dissipate so the damage is catastrophic and happens quicker. But, say the 30A current is flowing in a 4.0mm2 cable which gets warm, does the presence of insulation cause a heating increase? Like an oven increases in temperature and keeps getting hotter until the stat cuts the power. Does insulation trap heat and increase heat or can it maintain a constant temperature?

you are clearly not understanding the basic principles that set the rating of a cable.

Every cable, under a particular load value, will dissipate a certain amount of power in that cable, so it will heat up.

It's all about keeping that heat rise within acceptable limits so the cable is not damaged.

Clearly, to most people, a single cable, clipped to a wall, with nothing around it and no insulation anywhere, will dissipate heat quite well, so it will not warm up much and it will be fine.

Put the SAME cable inside a conduit with other cables also carrying current, and it's obvious that the cable will heat up a lot more. So when installed like that it has to be derated so it can only be allowed to pass a smaller current, so it does not heat up to the point of damaging the cable.

Now cover the whole lot in insulation, and it's heat dissipation becomes lower again, so now it can carry an even smaller current before it woudl heat up to the point of damaging the cable.

If you can't grasp that concept and think heat can't possibly be a problem, then you might not be the best person to be designing circuits and selecting the correct cable size for a particular installation method.

See Step's very good example in post #21

 

[Manator]

All metals have a conductive potential and all resistance is increased under heat. Even the background heat within the space where the cable is installed will effect its resistance which is why we have the ambient temperature as a guide when doing any testing.

The electric "voltage" is passed down a conductor on the back of the electrons this is a really inefficient way of passing any current down a conductor, but is a compromise, and the effects results in heat being generated. Under normal conditions this heat is quickly dissipated by the cable in free air, and a length of approximately 1m could dissipate enough heat in a 20m cable run to negate the need to over de-rate.

However now enclose that cable in insulation or as Prodave says in a conduit or trunking alongside other cables and we have problems.

Problem A

The cable is unable to freely dissipate any heat

Problem B

The cable not only generates its own heat but is collecting heat from other cables

Problem C

The resistance is increased along the cable length

Problem D

You pay more for electricity, and maintenance required more often.

Problem E

Increased risk of fire, and or electrical failure of the protective device for the circuit.

 

[Steve3948]

I totally agree with all that is being said, but I think what Unphased was trying to put over was that if you take a lighting circuit be it 1.0mm

 

[Manator]

Well said Steve, I know exactly what you are saying. Cables can run overloaded for some considerable amount of time, the heat generated is equal to the Joules Law principle, as such, unless the heat generated is far in excess of the cables tolerance heat damage is unlikely to be visible, however constant changes in this temperature and excessive high temperatures have proven to cause eventual breakdown in the conductor and the insulation.

Lighting circuits are pretty safely installed though I must admit to using more and more 2.5mm rather than the 1.5mm.

Fixed loads should always be calculated just like you say.

 

[unphased]

Hello. Thank you for allowing me to continue. If we are to debate "my views" can we all please stop assuming that I do not know the basics or that I seem to show "a basic lack of understanding". May I assure everyone that I totally do have a basic understanding. My views on the regs are born out of a late introduction in to this trade and having been familiar with BSs and Codes of pracice written in a totally different way. BS7671 just doesn't sit right with me on some concepts. One of which happens to be the treatment of cables in contact with or completely surrounded with, insulation. Steve3938 has understood where I am coming from so thank you for that Steve. :x I think that the treatment of the cable is overkill as to the reality of its behaviour in the real situation. Please don't assume I misunderstand that cables will warm up when electrical current is flowing. Where I stand is that the degree in which cables heat when in contact with insulation is not as onerous as the regs would have us believe.

523.9 requires the cable to be derated by 0.5 times the rated current carrying capacity even if it is only covered by 0.5m. So, what this says is that the cable if it is 10m long, say, and happens to pass through 0.5m of insualtion the entire cable has to be designed for double the currrent carrying capacity. This just does not sit right with me and I cannot see the need to increase a cable to double the current carrying capacity when passing through 0.5m of insulation. Table 52.2 is even more of a challenge on the belief. What table 52.2 says is that if a cable passes through 50mm (thats 2 inches) , 50mm it needs to be derated by 0.88. To me that is totally ridiculous. A cable may be 15m long, but if it passes through 50mm of insulation it needs derating? Sorry, guys, this is just beyond my common sense belief that this is really necessary. I know its in the regs but really!!! It just doesn't make sense. It would make more sense to me if derating is required for much longer lengths than the 50mm to 500mm and I would dearly love to know where thaose lengths were derived. This has to be overkill. Surely?

So when cables are trailing through mountains of insulation in a loft, I CAN see that there is a need to consider the insulation. I just really can't see why an entire run of cable would need to be derated by a factor of 0.88 when passing through 50mm of insulation. That is a challenge on my belief.

 

[Zeespark]

Unphased, BS7671 is always written with a view to erring on the side of caution.

AndyGuinness

 

[Manator]

Now that is what we like to see, a well written fully personal view that simplifies what you are trying to say.

From now on people will be giving their views and I now guess more people will agree with you! ( I will stay on the fence for this one )

 

[unphased]

Hi Zeespark. Yes I agree with you. But there is caution and there is "just can't see the need for it". I also understand that in all standards where design is involved ther is usually a safety factor built in, nominally 3, so anything which works will be multiplied by 3 to make damned sure it works. This helps to mitigate errors and assures a high degree of safety.

 

[Zeespark]

From your example, you need to be able to distinguish whether the insulation is of the rockwool type, or the solid type stuff which you need to cut with a knife to fit. From My own experience the latter derates cables faster than the wool type.

AndyGuinness

 

[Manator]

I know some of you may think I am a little "sad", but I do have some research papers about the effects on cable when passed through and used within insulation materials, I will try and dig them out because I do not remember the final synopsis.

I do remember there is a measure where the de-rating can be ignored, and also how quickly a length of open cable can act like a sort of heat exchanger and sufficiently cool the cable enough to be to a lesser degree de-rated less. This is just from memory mind, so when I find the papers I will try and give defined answer, than the garbage I have just sprouted.