Car Chargers

[chris day]

Hello guys

I have a car charger installation coming up and will be installed as per the below.

TN-S supply to building
35mm thermosetting main tails supply existing 3 phase DB.
X 5 chargers to be supplied by by 4mm SWA cabling on tray protected by a 32a 30ma RCCBO. the chargers are located outside the building.

1st question is because we have a TN-S supply i dont need to make special arrangements like you would under a PME system?

2nd question what special considerations may i have missed?

 

[1Justin]

I'd be interested in views on this since I have a car charger to move soon for one of my customers.

The IET Code of Practice 2013 reads (5.2.2 on page 21), "Unless it can be guaranteed by the relevant DNO that the TN-S system is TN-S back to source, and will not be converted to TN-C-S as part of any ongoing network upgrades, such installations shall be treated as TN-C-S systems (PME supply)" .

Now I have certainly come across car charging stations on outside walls of TNCS properties which haven't been installed with independent TT earth. I have advised the householders on those occasions.

It also seems to be quite common to see installation pictures in trade magazines such as "Professional Electrician" with proud smartly suited smiley installers alongside, on what are obviously modern estate houses, showing (in photos at least).. no obvious evidence of TT earthing conductor.

 

[Midwest]

I was once told by a DNO engineer, that most TN-S supplies, at some point back to source, will have been converted to TN-C-S (cable repairs).

Not having read anything of Section 722, or any other guidance, are 'We' saying that such a charging point on a PME supply, has to be converted to TT. Can't believe that?

 

[Flanders]

I beleave you have to have an independent earth sourse ie earth rod but you can still use the earth from the pme supply aswell, so as i understand it you can connect an earth rod the the MET to achieve this.

 

[Richard Burns]

The requirements are that if you have a TNS supply that cannot be guaranteed to be TNS in eternity, or if you have a TNCS supply then the options are to convert the car charging point (or the entire installation) to a TT system. There is an exclusion for domestic properties that permits TNCS (or TNS) supplies to not be converted where this is impracticable.
If the car charging point only is TT then measures must be taken to ensure that the two earthing systems do not overlap and that there is no access to extraneous parts from different earthing systems.

If it is decided to have the TNCS supply to the car charging point then a very low resistance earthing point can be installed on the installation MET, but this is normally prohibitively difficult as the resistance levels required for the rod/tape etc are in the region of 1-5 ohms (based on the calculation in BS7671).

 

[Midwest]

I've read some of Section 722, and tried to understand reg 722.411.4.1, specifically the exception of the regulation NOT being applied for a charging point at a PME supplied dwelling. Please have a read. Annex 722, item A722.3 gives guidance a formula to decide the effectiveness of the earth electrode resistance, which I believe Richard referred to in an earlier discussion on this topic;

Electric Vehicle Charging & PME - http://www.electriciansforums.co.uk/threads/electric-vehicle-charging-pme.88166/

I wonder if Uksparks went on his course? I've PM'd him, perhaps he can relate what was said on the course.

Having Googled that reg, there's suggestions that the reg can be ignored (dwelling with PME), following a risk assessment of the site?

Sufficed to say, I don't think I would be carrying out such installations without going on the course, to understand this reg with a bit more clarity.

 

[Richard Burns]

Regulation 722.411.4.1 states that you cannot use PME for an external charging point where the vehicle is also outside without complying with at least one of the three requirements.
It then says that the regulation does not apply to a single domestic house if none of three requirements if practicable.

The first requirement is relating to three phase supplies, multiple EV points and maintaining a maximum 70V difference from true earth if the supply is disconnected on the earthed conductor (PEN).

The third requirement asks for a voltage operated earth disconnector for which there is no product standard.

Both of those are in applicable to domestic properties.

The second requirement asks for an Ra on an earth electrode connected to the MET that will not permit the voltage difference from true earth to MET to exceed 70V when the earthed conductor of the supply is disconnected.
it gives the formula shown which if you calculate for a standard dwelling gives:
View attachment 35697
70* Uo = 70*230 = 16100
Uo -70 = 230-70 = 160

16100/160 is slightly over 100

Therefore you have 100/I inst = 100/ maximum demand for installation
So for a 100A maximum demand is 100/100 or approximately 1 ohm.

 

[Midwest]

722.411.4.1 also says 'The requirement of this regulation need not be applied for the charging point at a dwelling, if none of (i), (ii) or (iii) is reasonably practicable'.

Edit, which you mentioned. There most be some risk assessment process, to do this. That's why I wondered what Uksparks learnt form his course, might assist with this seemingly open deviation from the reg?

 

[Richard Burns]

Sorry about the size of the equation it looked smaller whilst I was editing it!

I have realised that I did not actually complete my post, I should have said more about the value of Ra and I should have completed my initial statement by pointing out that in all circumstances the three options are not practicable in a domestic situation and so a TN arrangement can be used.

In response to your following post it is not a deviation from the regs, it is an exclusion in the regs; a bit like saying "this regulation does not apply to..."

The 1st ed of the COP gives an example risk assessment form for assessing the level of hazard when either converting the charging point to a TT system or converting the installation to a TT system, and there is the local presence of a PME system (very likely in a domestic situation, nearby houses, etc.)

There are requirements to check that simultaneous accessibility between PME earthed and TT earthed parts is suitably low and if not to make such changes as are necessary to reduce the chances of such contact. It also mentions adjacent properties and outdoor electrical installations.
There is a pretty label to put on the consumer unit and or charging point to ensure the TT is not connected to the PME.

Generally it appears that a TT installation in its entirety is preferred (for whatever reason) for outdoor charging points, alternatively the charging point should be positioned in a building (garage or such) such that the vehicle will be charged only within the building and therefore the risk of exposure to true earth is minimised.

 

[Midwest]

I'll read your post tomorrow when I'm more compos mentis