Hi all,
Bit of a long post here, constructive criticism welcomed!!
No it's not a secret!!! It is a bit of a favour i am doing for someone!! If you can, imagine a group of 5 or 6 small buildings in a field. The origin of the supply is going to be in a purpose built brick "structure" just off the road as it were.
Trouble is, the buildings are a rather long way from the road in two little groups, the nearest about 70 meters from the origin, and the furthest about 110 meters!!...
Now then, they have already got some of the buildings connected up to an adjacent building with an existing supply, others are powered by generators!!! Now the numpties that designed the installation as it is now, were obviously "arithmetically challenged" and presumably incapable of designing anything.
Now, I am only a newbie, and it is not for me to criticise what other vastly more experienced persons have done [at considerable expense i might add] but you really would laugh!!!
First off they ran a "sub main" or "distribution circuit" [call it what you will] about 70 yards long from the existing building in 2.5 swa... and that is not counting the unknown distance as the cabling wends its happy way from the board they took their supply from, to the origin of the supply in the very large existing building...
Anyway, by the time they got to the end of the cable and stuck their meter on it, they obviously found that the Zs was such that they could only fit a very small MCB, so now, if you plug in a kettle and switch on a heater both at the same time the MCB trips!!
Ok, they could/should perhaps have TT'ed it and solved the Zs problem, but then that leaves the volt drop....
I measured the supply impedance at the end of the circuit [at the socket where the kettle ws plugged in as it happens] and found it was such that, you exceeded the permitted volt drop, once you got above about 5 amps!!!!
And then they wonder why the lights are dim.....
So, nice guy that i am, i decided to sort all this out for them....
First off, i could not just increase the existing cable size, as demand has grown in the existing building over the years and the supply would just be too small, so it was a case of a fresh supply being arranged.
They [the end user] wanted to do this years ago, but another numpty told them the DNO would charge about 30 grand!!! In fact, they would not believe me at first when i told them what it would actually cost, so i said "let me apply for you and see what they say" When they got the price [about 2 grand] they thought all their christmasses had come at once!!]
I also arranged that the new supply would be a three phase one, because although yet more numpties told them this was not needed, i pointed out that, although they could have a single phase supply if they really wanted, it would be massively over loaded from the start, and that, although i could sort the volt drop with proper sized cabling, they would not be able to run much more than they were already, and so it would all be an expensive pointless exercise...
They soon came around to my way of thinking!!!
So, back to the cable.. This is a rather strange installation in that volt drop was always going to be the big problem, compounded by a few other factors;
1, Diversity.. This was not really going to be possible to allow much for...mainly, because although most of the time the loading might be only a few light bulbs, the rest of the installation comprises a lot of heaters, thermostatically controlled water heaters, and electric kettles!!! Even worse, these are not going to be used in an "airy fairy" or "random" type of way as they would be, in say, a large house. As i say, most of the time the loading will only be a few light bulbs, but then three times a day, all at EXACTLY the same time, the whole lot will all be switched on, in all 6 little buildings, and all at the same time!!
If, after their past experiences, when all this lot is switched on, there is even a hint of lights dimming, they would not be happy bunnies....
2, "Future proofing" I know what they are like!!! Originally, when i first got involved with all this, i asked them "Are you going to install any additional equipment or put up any more buildings" and "No, absolutely definitely not" came the reply. Next thing, all this "limitless" power has gone to their heads, and it is not even installed yet!!!
All of a sudden there are two more water heaters already planned, and only today i find out that yet another building will soon be popping up. They seem to think that this "extra" building does not matter, but as soon as it is up they will want lights, heaters....
3, What they really want, from an "end users" point of view, is a "fit and forget" system. What i mean is this; They do not, at some unspecified time in the future, want to have to modify or "reinforce" this installation. They would MUCH rather spend the money now and be able to forget about it.... They simply want to be able to plug in or connect up anything they like and it will simply work!!!
4, So, back to the cable.. The conductor sizes were decided upon from a volt drop point of view. But why 5 cores??? Well, first off, the supply is TNCS, so the armouring of this great long sub main [bearing in mind that it is a hell of a long way from the MET at the origin, to the BEMT's in the buildings] has to double as both CPC and also bonding conductor.
Problem is, as it is TNCS, and the cable armour is going to "double" as both CPC and bonding conductor, the "normal" situation as regards CPC sizing, namely the adiabatic equation, and sizing the CPC from the point of view of thermal constraints, is overshadowed somewhat, by the requirement to provide for the correct bonding conductor size, which as we all know, for a supply with a neutral conductor of up to 35mm CSA is 10mm. Now, as the armour will be used for a bonding conductor, i need to provide for, in steel armouring terms, the "copper equivalent" of 10mm csa. The resistivity of steel being what it is, i need to provide some 80mm of armouring csa...
A four core cable armour csa, is insufficient for this, so i would have to run an "extra" core to act as bonding conductor. Next problem is that you are not allowed, or at least it is frowned upon, to try to "top up" the armour size with a small additional cable. You have to run a separate cable of the full size required..
I know that in theory there can be problems [involving cable heating] using an "internal" cable core, or indeed the armour itself in this way [as a PME bond] and that it is better to actually run a separate core, but in this instance, as the cable will be, for 99 percent of the time only lightly loaded. i think all should be well.
So, five core it is!! As it happens, interestingly enough, the armour csa of the five core WOULD be sufficient on its own, but you might as well connect up the "extra" core as it is there, and besides, think of the super low R1 + R2 and consequent mega low Zs!!!
Now then, i know what people are going to say, why bother with all this, why not just use th DNO supplied earth to protect the submain itself, and then TT the rest on a building by building basis and forget all about the DNO supplied earthing arrangements.
Even better, now prepare to laugh!!!! cos you are all going to fall off your seats with this one, never mind earthing, why do i want to provide for bonding for anyway!!!! ha ha ha THERE ARE NO EXTRANEOUS PARTS ANYWAY.. Waaaaaaa HA HA, so what the heck do i want to provide for bonding for!!!! ha ha ha.
Simple answer is where earthing is concerned;
1, The soil is very dry and crumbly, i doubt you would get a consistantly reliable Ra from your earth rod in the first place.
2, Do i really want to rely entirely on RCD protection, these not being the most reliable of things, when i could have "proper" earthing, AND rcd's as well.
3, I know TNCS has its fair share of critics [and for good reason too may i add!!] But, there are NO extraneous parts anyway, so who cares..
Which brings us neatly on to bonding, why would i want to provide for any in the first place, if there are no extraneous parts??
Well, two reasons;
1, The extra core will reduce Zs greatly, which is always going to help on such a long run of cable
2, What if someone ever DOES introduce extraneous parts.
And finally, remember i mentioned "future proofing" If in the future someone comes along that is a "PME fan" they can use the DNO supplied earthing arrangements if they choose, as i will have provided for them, and if they go and install extraneous parts, they can bond them to the provision for bonding that i have provided too.
If conversely, a "TT fan" comes along, they can just disconnect my provision for earthing and bonding, and bang in a rod!!! [the rcd's will already be there anyway]
So, my system aims to be as "future proof" and as flexible as possible. It will cater for TT fans, PME fans, the "end user" [who still cannot decide on exactly what it is that they want] will, more or less, be able to connect up anything they like, within reason, all volt drop problems will be banished and everyone will be happy.
I know this will not be the cheapest way of doing things, but the end user has said reapeatedly, "never mind the cost, we just want to be able to install it, use it, and forget about it"
I am merely giving them what they want; a flexible system, that will do what they want now, and be easily adaptable to future possible changes.
Finally [i said that a few paragraphs ago!] the DNO have been along, they know EXACTLY what is being done, and are entirely happy, with no objections whatsoever!!
Everyone is happy!!!!!
john...
Bit of a long post here, constructive criticism welcomed!!
No it's not a secret!!! It is a bit of a favour i am doing for someone!! If you can, imagine a group of 5 or 6 small buildings in a field. The origin of the supply is going to be in a purpose built brick "structure" just off the road as it were.
Trouble is, the buildings are a rather long way from the road in two little groups, the nearest about 70 meters from the origin, and the furthest about 110 meters!!...
Now then, they have already got some of the buildings connected up to an adjacent building with an existing supply, others are powered by generators!!! Now the numpties that designed the installation as it is now, were obviously "arithmetically challenged" and presumably incapable of designing anything.
Now, I am only a newbie, and it is not for me to criticise what other vastly more experienced persons have done [at considerable expense i might add] but you really would laugh!!!
First off they ran a "sub main" or "distribution circuit" [call it what you will] about 70 yards long from the existing building in 2.5 swa... and that is not counting the unknown distance as the cabling wends its happy way from the board they took their supply from, to the origin of the supply in the very large existing building...
Anyway, by the time they got to the end of the cable and stuck their meter on it, they obviously found that the Zs was such that they could only fit a very small MCB, so now, if you plug in a kettle and switch on a heater both at the same time the MCB trips!!
Ok, they could/should perhaps have TT'ed it and solved the Zs problem, but then that leaves the volt drop....
I measured the supply impedance at the end of the circuit [at the socket where the kettle ws plugged in as it happens] and found it was such that, you exceeded the permitted volt drop, once you got above about 5 amps!!!!
And then they wonder why the lights are dim.....
So, nice guy that i am, i decided to sort all this out for them....
First off, i could not just increase the existing cable size, as demand has grown in the existing building over the years and the supply would just be too small, so it was a case of a fresh supply being arranged.
They [the end user] wanted to do this years ago, but another numpty told them the DNO would charge about 30 grand!!! In fact, they would not believe me at first when i told them what it would actually cost, so i said "let me apply for you and see what they say" When they got the price [about 2 grand] they thought all their christmasses had come at once!!]
I also arranged that the new supply would be a three phase one, because although yet more numpties told them this was not needed, i pointed out that, although they could have a single phase supply if they really wanted, it would be massively over loaded from the start, and that, although i could sort the volt drop with proper sized cabling, they would not be able to run much more than they were already, and so it would all be an expensive pointless exercise...
They soon came around to my way of thinking!!!
So, back to the cable.. This is a rather strange installation in that volt drop was always going to be the big problem, compounded by a few other factors;
1, Diversity.. This was not really going to be possible to allow much for...mainly, because although most of the time the loading might be only a few light bulbs, the rest of the installation comprises a lot of heaters, thermostatically controlled water heaters, and electric kettles!!! Even worse, these are not going to be used in an "airy fairy" or "random" type of way as they would be, in say, a large house. As i say, most of the time the loading will only be a few light bulbs, but then three times a day, all at EXACTLY the same time, the whole lot will all be switched on, in all 6 little buildings, and all at the same time!!
If, after their past experiences, when all this lot is switched on, there is even a hint of lights dimming, they would not be happy bunnies....
2, "Future proofing" I know what they are like!!! Originally, when i first got involved with all this, i asked them "Are you going to install any additional equipment or put up any more buildings" and "No, absolutely definitely not" came the reply. Next thing, all this "limitless" power has gone to their heads, and it is not even installed yet!!!
All of a sudden there are two more water heaters already planned, and only today i find out that yet another building will soon be popping up. They seem to think that this "extra" building does not matter, but as soon as it is up they will want lights, heaters....
3, What they really want, from an "end users" point of view, is a "fit and forget" system. What i mean is this; They do not, at some unspecified time in the future, want to have to modify or "reinforce" this installation. They would MUCH rather spend the money now and be able to forget about it.... They simply want to be able to plug in or connect up anything they like and it will simply work!!!
4, So, back to the cable.. The conductor sizes were decided upon from a volt drop point of view. But why 5 cores??? Well, first off, the supply is TNCS, so the armouring of this great long sub main [bearing in mind that it is a hell of a long way from the MET at the origin, to the BEMT's in the buildings] has to double as both CPC and also bonding conductor.
Problem is, as it is TNCS, and the cable armour is going to "double" as both CPC and bonding conductor, the "normal" situation as regards CPC sizing, namely the adiabatic equation, and sizing the CPC from the point of view of thermal constraints, is overshadowed somewhat, by the requirement to provide for the correct bonding conductor size, which as we all know, for a supply with a neutral conductor of up to 35mm CSA is 10mm. Now, as the armour will be used for a bonding conductor, i need to provide for, in steel armouring terms, the "copper equivalent" of 10mm csa. The resistivity of steel being what it is, i need to provide some 80mm of armouring csa...
A four core cable armour csa, is insufficient for this, so i would have to run an "extra" core to act as bonding conductor. Next problem is that you are not allowed, or at least it is frowned upon, to try to "top up" the armour size with a small additional cable. You have to run a separate cable of the full size required..
I know that in theory there can be problems [involving cable heating] using an "internal" cable core, or indeed the armour itself in this way [as a PME bond] and that it is better to actually run a separate core, but in this instance, as the cable will be, for 99 percent of the time only lightly loaded. i think all should be well.
So, five core it is!! As it happens, interestingly enough, the armour csa of the five core WOULD be sufficient on its own, but you might as well connect up the "extra" core as it is there, and besides, think of the super low R1 + R2 and consequent mega low Zs!!!
Now then, i know what people are going to say, why bother with all this, why not just use th DNO supplied earth to protect the submain itself, and then TT the rest on a building by building basis and forget all about the DNO supplied earthing arrangements.
Even better, now prepare to laugh!!!! cos you are all going to fall off your seats with this one, never mind earthing, why do i want to provide for bonding for anyway!!!! ha ha ha THERE ARE NO EXTRANEOUS PARTS ANYWAY.. Waaaaaaa HA HA, so what the heck do i want to provide for bonding for!!!! ha ha ha.
Simple answer is where earthing is concerned;
1, The soil is very dry and crumbly, i doubt you would get a consistantly reliable Ra from your earth rod in the first place.
2, Do i really want to rely entirely on RCD protection, these not being the most reliable of things, when i could have "proper" earthing, AND rcd's as well.
3, I know TNCS has its fair share of critics [and for good reason too may i add!!] But, there are NO extraneous parts anyway, so who cares..
Which brings us neatly on to bonding, why would i want to provide for any in the first place, if there are no extraneous parts??
Well, two reasons;
1, The extra core will reduce Zs greatly, which is always going to help on such a long run of cable
2, What if someone ever DOES introduce extraneous parts.
And finally, remember i mentioned "future proofing" If in the future someone comes along that is a "PME fan" they can use the DNO supplied earthing arrangements if they choose, as i will have provided for them, and if they go and install extraneous parts, they can bond them to the provision for bonding that i have provided too.
If conversely, a "TT fan" comes along, they can just disconnect my provision for earthing and bonding, and bang in a rod!!! [the rcd's will already be there anyway]
So, my system aims to be as "future proof" and as flexible as possible. It will cater for TT fans, PME fans, the "end user" [who still cannot decide on exactly what it is that they want] will, more or less, be able to connect up anything they like, within reason, all volt drop problems will be banished and everyone will be happy.
I know this will not be the cheapest way of doing things, but the end user has said reapeatedly, "never mind the cost, we just want to be able to install it, use it, and forget about it"
I am merely giving them what they want; a flexible system, that will do what they want now, and be easily adaptable to future possible changes.
Finally [i said that a few paragraphs ago!] the DNO have been along, they know EXACTLY what is being done, and are entirely happy, with no objections whatsoever!!
Everyone is happy!!!!!
john...
