Is there something wrong with my calcs?
- Thread starter Duncan Bennett
- Start date
D
Darkwood
Not checked them myself what Im getting at is the requirements of extending the equipotential zone with regards to additional earthing that may be required depending on the cable size chosen the csa of the armouring and whether a separate core would suffice, as I don't have the incoming N size I could tell you what is needed and as I don't know your final choice of CSA of the SWA nor can I say whether it will require additional earthing to allow for the bonding of the steel... that is if you choose to extend the equip' zone or whether settle to TT at the shed end.
B
Besoeker
Circular logic there.
The conductor current rating and voltage drop are two different constraints.
But, surely, if I can meet the required disconnection times then the install method is valid?
@besoeker. The conductor current rating is not an issue at all; 1.5mm would handle the amperage, the sizing has only to do with the VD on the length of the run :-(
and the bonding unless he TT's the workshop
B
Besoeker
Believe it or not, I do know that......
W
wozz
Your calculated Zs does it have any tempreture correction?
i believe its 10oC you should be using for the calc in the ground (you will have to check but I think it was 10)
of the constant temperature of the ground all year round
for when you Bury the swa or you will need to add a bit more for the temperature changes throughout the year
W
wozz
No the temperature correction for loaded conductors when calculating Zs. Unless it was corrected before he put it into the equation.
I didn't think this could get so complicated, lol.
I'm using table 4E4A on page 348 of 7671. 16mm 2 core swa. Reference method E (the cable is being run on the surface) - It shows CCC of 115amps.
Then calculating the VD from table 4E4B 16mm 2 core single phase at 2.9mV.
I'm running from a TNC-S supply and have calculated a loop impedance of 1.1915 ohms.
I'm running 3 x twin sockets close to the board through individual 16amp breakers for which Table 41.3 allows a max. 2.87 ohms to meet the disconnection times.
I've admitted - many times - on this board, that I am primarily a domestic installer. But i do come here to get educated when I'm not 100% sure of the odd job which falls my way. But, I am having a problem understanding why I can't export the equipotential earthing out to this cable. I do so when I run to garden huts etc. and while this particular building is 170m away, I can't see there being a problem so long as I can match disconnection times. As for a fault with the DNO supply, wouldn't this effect the farmhouse, milking shed and out-buildings which are all drawn from the same supply?
Apologies in advance if I seem dumb, but I can't get my head around it.
I'm using table 4E4A on page 348 of 7671. 16mm 2 core swa. Reference method E (the cable is being run on the surface) - It shows CCC of 115amps.
Then calculating the VD from table 4E4B 16mm 2 core single phase at 2.9mV.
I'm running from a TNC-S supply and have calculated a loop impedance of 1.1915 ohms.
I'm running 3 x twin sockets close to the board through individual 16amp breakers for which Table 41.3 allows a max. 2.87 ohms to meet the disconnection times.
I've admitted - many times - on this board, that I am primarily a domestic installer. But i do come here to get educated when I'm not 100% sure of the odd job which falls my way. But, I am having a problem understanding why I can't export the equipotential earthing out to this cable. I do so when I run to garden huts etc. and while this particular building is 170m away, I can't see there being a problem so long as I can match disconnection times. As for a fault with the DNO supply, wouldn't this effect the farmhouse, milking shed and out-buildings which are all drawn from the same supply?
Apologies in advance if I seem dumb, but I can't get my head around it.
D
Darkwood
Firstly watch your terminology here we never discussed 'exporting' we were discussing extending the equipotential zone (2 very different things).
Secondly I never said you can't extend it either what I did say is if you choose this method to bond the structural steel work in the shed then it may influence the cable you choose and may mean you up the number of cores... this all depends on the size of the DNO supply, if you choose to isolate earthing at the load end and TT the shed then this is an alternative method that may reduce costs.
As info is limited I cannot say, we are not discussing the other shed etc and I can't confirm there compliance, its often the case that this set-up is wired up incorrectly due to lack of understanding but in the rare event of a N/E DNO loss the full current of the DNO TX may source alternative route to return to the TX - this could see circulating currents around your earthing system that are related to other businesses fed from the same TX.. this is why the bonding requirements of a TNCS are greater for larger supplies where TNS has a lower plateau for max size.
I understand your stepping into new areas out of your comfort zone but this is why such titles as 'domestic installer' that can be attained with short courses are strictly limited to staying within the domestic realm .... things get a whole lot more complex when DNO supplies increase in size and can influence your whole design just to ensure compliance.
You will gather by the nature of this Thread that not many picked up on this as been something you need to consider when designing your install, a slighty worrying thing TBH but yes what seems like a simple job with just a larger distance than your used to can easily get very complex and often these installs are done incorrectly by migrating domestic sparks widening there field, I'm not saying their is a need to redesign the install to allow for extending the equip' zone all I saying is you have to ensure the armour can accommodate the additional requirements to bond the steel which will be in addition to the required earthing size needed for the sub-main.
Depending on size of the DNO supply this could mean...
-No extra measures need to be taken and the Armour can accommodate both circuit and bonding needs.
-An extra core may be needed to allow for the bonding requirements.
-Due to the nature of a larger DNO supply the cable choice may still be too small to comply with additional core so a separate earth may need to be run.
In the worst case scenario where you have a large DNO supply to the complex your bonding requirements may be 50mm^2 which is additional to the requirements of the submain earthing. If you decide your 16mm satisfies VD etc then an extra 16mm core comes no where near the 50mm needed so an independent 50mm is run - Note! this is a worst case scenario!
Alternatively you can earth the Armouring as normal and isolate it at the shed end and TT the shed, not forgetting to take measures into the design that you would normally associate with a TT set-up.
Edit... just to add here the local DNO may require greater measures than that of the regulations due to their system layout and on occasion they may request even greater earthing arrangements for equip' bonding etc... but I've only come across this once where my installed complied to regs but they requested a 35mm additional earth to allow for their requirements.
Secondly I never said you can't extend it either what I did say is if you choose this method to bond the structural steel work in the shed then it may influence the cable you choose and may mean you up the number of cores... this all depends on the size of the DNO supply, if you choose to isolate earthing at the load end and TT the shed then this is an alternative method that may reduce costs.
As info is limited I cannot say, we are not discussing the other shed etc and I can't confirm there compliance, its often the case that this set-up is wired up incorrectly due to lack of understanding but in the rare event of a N/E DNO loss the full current of the DNO TX may source alternative route to return to the TX - this could see circulating currents around your earthing system that are related to other businesses fed from the same TX.. this is why the bonding requirements of a TNCS are greater for larger supplies where TNS has a lower plateau for max size.
I understand your stepping into new areas out of your comfort zone but this is why such titles as 'domestic installer' that can be attained with short courses are strictly limited to staying within the domestic realm .... things get a whole lot more complex when DNO supplies increase in size and can influence your whole design just to ensure compliance.
You will gather by the nature of this Thread that not many picked up on this as been something you need to consider when designing your install, a slighty worrying thing TBH but yes what seems like a simple job with just a larger distance than your used to can easily get very complex and often these installs are done incorrectly by migrating domestic sparks widening there field, I'm not saying their is a need to redesign the install to allow for extending the equip' zone all I saying is you have to ensure the armour can accommodate the additional requirements to bond the steel which will be in addition to the required earthing size needed for the sub-main.
Depending on size of the DNO supply this could mean...
-No extra measures need to be taken and the Armour can accommodate both circuit and bonding needs.
-An extra core may be needed to allow for the bonding requirements.
-Due to the nature of a larger DNO supply the cable choice may still be too small to comply with additional core so a separate earth may need to be run.
In the worst case scenario where you have a large DNO supply to the complex your bonding requirements may be 50mm^2 which is additional to the requirements of the submain earthing. If you decide your 16mm satisfies VD etc then an extra 16mm core comes no where near the 50mm needed so an independent 50mm is run - Note! this is a worst case scenario!
Alternatively you can earth the Armouring as normal and isolate it at the shed end and TT the shed, not forgetting to take measures into the design that you would normally associate with a TT set-up.
Edit... just to add here the local DNO may require greater measures than that of the regulations due to their system layout and on occasion they may request even greater earthing arrangements for equip' bonding etc... but I've only come across this once where my installed complied to regs but they requested a 35mm additional earth to allow for their requirements.
Last edited by a moderator:
Darkwood, thanks for taking the time to deliver such a detailed answer. The very complexity of the arrangement, and your concerns over the size of the DNO supply (which I must admit I haven't even considered) informs me that I might well be out of my depth on this one. I think I'd probably be wise to hold my hands up and advise the customer to seek an industrial installer for this one. Again, though, thanks for taking the time to type out such a comprehensive answer - the very reason why I visit these boards in the first place. Cheers, Dunc.
D
Darkwood
Don't let my input shy you away, we all learn somewhere... you could just isolate supply earth at load end and TT the shed and forget any concerns about additional earthing, if your still not familiar with this concept then how about getting in a experienced guy and working along side him to pick up the jist of it all, he earns from it, you gain knowledge and widen your field of experience.
Only you can know if you are truly out off your depth but their was a time I couldn't wire a plug ... if I didn't take the path I did and stick to it I might have turned out a postman or worse a Plumber!!! :willy_nilly:
Remember there may not be any need for additional earthing here but I lack info so can't guide any further.
Only you can know if you are truly out off your depth but their was a time I couldn't wire a plug ... if I didn't take the path I did and stick to it I might have turned out a postman or worse a Plumber!!! :willy_nilly:
Remember there may not be any need for additional earthing here but I lack info so can't guide any further.
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