Earthing for an EV Charging Point

[Simonslimline]

If the extra bracket has been inserted before the sqrt then your answer is correct. Correct equation should be confirmed. When I was doing structural analysis, stress calculations, for a company making actuators primarily for the aircraft and rail sectors, MS Excel was seriously frowned upon as an analytical tool. The very good reason for this is that the method of gaining the result is hidden from the user and is not expressed in the form that it would be mathematically presented.

I did calculate it using 2 other methods and it came out the same.

Thanks for the input though .

 

[uksparks]

Ok so just got home and done the calc for Im and got 53.57

ok, so for the next part to Calculate RAev:

70/53.57-(70/230)x(8+25+68) = -29.43

Which is below Zero.

Also, section A722.2 on the load balance side of things

(53.57 x 230) / (8+25+68) = 122v, now it does say it needs to be less than 70v

Damn this is confusing.

 

[spark 68]

Right UK,
I worked out the part below the line first to see if it was less than zero.



53.57-(70/230) X (8+25+68)

70/230 = 0.304
8+25+68 =101
0.304 X 101 =30.704

53.57 - 30.704 = 22.866

now 22.866 is greater than zero, so we divide 70 by that
70/22.866 = 3.06

you need an Ra at the rods below 3 ohms in reality to ensure the value remains stable in all conditions, 3 ohms is really the maximum value of Ra

 

[uksparks]

Excellent thank you, I must have typed something wrong on the calculator, as having just done it again, we get the same answer.

 

[uksparks]

Ok so I think I have it, ill speak out loud...

So looking at A722.1, to work out the max neutral current for Im

Im = √((70-0.5(30+8))²+(0.866(30-8))²
Im = 54.4A

Ok, so to then use that on the load balance A722.2

Less than or equal to 70v = 54.4 x 230 / (70 + 30 + 8) = 116v
116v is obviously greater than 70v, so this does not comply with 722.411.4 (i)


Ok, so lets work out the Earth Electrode to try to comply with 722.411.4.1(ii)

To check RAev is valid, 54.4 – (70/230) x (70+30+8) must exceed zero, which is equals 21.5

RAev must be less than or equal to zero to not need an earth rod, so to now work that out

RAev = 70 / (54.4 – (70/230) x (70+30+8))
RAev = 3.25 Ohms

So to comply with 722.411.4.1(ii), I would need to ensure that there is an earth electrode connected to the PME of the installation as a backup with a max Ra of 3.25 Ohms.

Now, the point of this is, is it acceptable to test the steel of the building and use that as the earth electrode?

What do you think? Does what I am saying sound correct?

 

[Engineer54]

Now, the point of this is, is it acceptable to test the steel of the building and use that as the earth electrode?

Well it needs to be connected to the MET whatever happens!! lol!!

As i stated in another post here the DNO can provide a ''Sudo'' TN-S supply from a PME network distribution cable, so why not ask them if that's possible for this building??

 

[driverman]

I'm intrigued what is a "Sudo" TN-S supply? I did Google but to no avail. Cheers

 

[Engineer54]

I'm intrigued what is a "Sudo" TN-S supply? I did Google but to no avail. Cheers

Not really sure myself, it was brought to my notice by a member here (can't remember name though)that works for, or used to work for one of the DNO's. He assured us that the PME broken neutral scenario is circumvented, when they provide such a supply. He related to it in his posts, as a TN-S supply, but i can't see for the life of me how that could be achieved from a PME Network cable, so i called it a ''Sudo TN-S'' supply!! lol!!

I do remember asking why the DNO don't provide this type of supply to caravan sites and petrol stations, but never did get a reply, ...none that i remember anyway!!

 

[spark 68]

I'm intrigued what is a "Sudo" TN-S supply? I did Google but to no avail. Cheers

I may be wrong here lol, as I only had a quick read through some UKPN pdfs about Earthing, anyway the way I read it was that on say a small new build Industrial estates, the small unit types, was that they were planning to put just one CNE link at the substation/local LV transformer distribution level as a PME supply, but each unit was a sort of SNE (pseudo TNS) with no CNE link in each Unit.

This was more to do with cutting down circulating N-E Currents IIRC, via shared steelwork and services, if I can find the document I will post it up so we can all have a proper look.

 

[spark 68]

Not really sure myself, it was brought to my notice by a member here (can't remember name though)that works for, or used to work for one of the DNO's. He assured us that the PME broken neutral scenario is circumvented, when they provide such a supply. He related to it in his posts, as a TN-S supply, but i can't see for the life of me how that could be achieved from a PME Network cable, so i called it a ''Sudo TN-S'' supply!! lol!!

I do remember asking why the DNO don't provide this type of supply to caravan sites and petrol stations, but never did get a reply, ...none that i remember anyway!!

See post #69 above, as far as I can recall this "hybrid" system still had all of the requirements (bonding cable sizes etc.) as PME, and was effectively treat as a PME system, so this would not be any use for caravans etc. as I say it was more to do with cutting down circulating E currents rather than anything, at least as far as I read it.

I may have read it wrong, as I was looking for something else, and just quickly read it out of general interest.

 

[uksparks]

Hi,

Ok so the unit has arrived, and its all plastic, i was under the impression it was metal... so on this basis, does this change your thoughts on anything to do with the earthing and proximity to the building?

And a few photos for you to see, its really nicely made actually. In the top there's 4 LED's for the light.

 

[spark 68]

It doesn't really matter what we think , the regs are what you/we follow.

Are those metal screws/fixings ECPs ?

What, if any, do the supplied MIs say ?

As I say I think a lot of he precautions in BS7671 are to ensure minimum risk of the Vehicles bodywork becoming live in the event of a broken suppliers N where PME supply is concerned.
It could be even the regs are far behind recent technological developments, but we are are not really in a position to question them, or we would be foolhardy to ignore them assuming we know better,they are what they are as written in the BYB

 

[uksparks]

I totally agree.

having said that, the calculations above are a nonsense really because the figure that determines if you need an Earth rod or not changes between you need one and you don't need one depending on the loading of the phases, so whilst it may comply for example at the point it's installed, next week when the empty office downstairs is populated with 20 people and 20 computers, the figures are going to be wrong and next week of you calculate it, it might need a rod or vise versa etc.

 

[uksparks]

It doesn't really matter what we think , the regs are what you/we follow.

Are those metal screws/fixings ECPs ?

What, if any, do the supplied MIs say ?

As I say I think a lot of he precautions in BS7671 are to ensure minimum risk of the Vehicles bodywork becoming live in the event of a broken suppliers N where PME supply is concerned.
It could be even the regs are far behind recent technological developments, but we are are not really in a position to question them, or we would be foolhardy to ignore them assuming we know better,they are what they are as written in the BYB

There is no instructions or paperwork with the unit at all.

yes they are metal and connected to the earthed chassis, ok so there is a tiny amount of exposed metal.

 

[spark 68]

I totally agree.

having said that, the calculations above are a nonsense really because the figure that determines if you need an Earth rod or not changes between you need one and you don't need one depending on the loading of the phases, so whilst it may comply for example at the point it's installed, next week when the empty office downstairs is populated with 20 people and 20 computers, the figures are going to be wrong and next week of you calculate it, it might need a rod or vise versa etc.

Probably

I get the feeling on both threads you are avoiding doing what you should be doing, I know the reasons why, because you took this job on without realising the full implications a lot of us have been here before

This is not dig BTW as I have tried to help you as best I am able, you do it whichever way you want, you are signing it off not us , just please don't try to "steer" it the way you would wish/like it to be , you are now fully armed with all of the information to do it compliantly, only you can decide on which is the easiest/cheapest correct way. Apologies in advance if I have got hold of the wrong end stick here.


Please don't take this post the wrong way, it has been a very interesting thread, and one I have learned a bit from too

 

[uksparks]

I know and I accept that I am probably being guilty of doing what you suggest.

Ok ok so I am coming to terms with the fact that I'm going to have to put earth rods in.

without trying to sound like I don't want to do that, do you feel it acceptable if I measure the Ra of the steel work that is connected to the MET to satisfy the requirement of the additional earth rods, this is in the basis it falls lower than that of the calculation of 3 ohms etc.

i am sure you would agree that there is little point in putting rods in if the steel is doing just as good a job.

 

[uksparks]

I do very much appreciate all the feedback and support on this, it's a newish area for me that I'd like to get more involved with, so would like to get on on the right foot.

 

[spark 68]

I think it is a newish area for most of us

Previous to this thread I had only a cursory look at AMD 2, no where near the more in-depth look since

 

[Knobhead]

I have an Excel spreadsheet as well. Wanted to show the way to calculate it though. Either way its the same answer

Here you go

I[SUB]n[/SUB] = √((I[SUB]a[/SUB]²+I[SUB]b[/SUB]²+I[SUB]c[/SUB]²)-((I[SUB]a[/SUB]*I[SUB]b[/SUB])+(I[SUB]a[/SUB]*I[SUB]c[/SUB])+(I[SUB]b[/SUB]*I[SUB]c[/SUB])))

I'm glad someone has made use of the spreadsheet.

 

[spark 68]

Ok ok so I am coming to terms with the fact that I'm going to have to put earth rods in.

without trying to sound like I don't want to do that, do you feel it acceptable if I measure the Ra of the steel work that is connected to the MET to satisfy the requirement of the additional earth rods, this is in the basis it falls lower than that of the calculation of 3 ohms etc.

i am sure you would agree that there is little point in putting rods in if the steel is doing just as good a job.

My stance here is that if you are going additional Earth rods to the PME MET then they should be certified separately and as standalone, others may differ here.

Another option might be if you can get access to the re-bar at the concrete base of the building and get a reading from here, others on here have more experience of Ufer Earthing than me.

I think it would be virtually impossible to take a reading of the building's steelwork alone because of the various difficulties of separating the services and parallel paths, hence my stance earlier wrt separate rods.

Some might think this doesn't, matter but my own opinion is that how can you be sure it meets the requirements for the exemptions you are proposing using if you cannot accurately quantify it ?