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Topic Title: EV charging, PME, and 3 phases
Topic Summary: Is there a flaw in 722.411.4.1?
Created On: 12 June 2018 01:48 PM
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 12 June 2018 01:48 PM
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chrispearson

Posts: 530
Joined: 15 February 2018

Mike's recent posting in another thread made for an interesting read and got me thinking.

Originally posted by: mapj1

And as a point of note many of those who plug a car into the mains to charge it take the same risk too, and probably don't realise it.

HSL report on the safety of charging cars from PME networks (dropbox link, ignore the adverts, you don't need to join) has some figures.


Until the end of the year, we can get away with the "reasonably practical" exception, but then we shall have to rely on one of the sub-paragraphs. Let's look at (i).

The HSL paper tells me two things: (a) the risk arising from a lost neutral in a PME supply may be reduced by faster charging; so if you can top up using a rapid charger giving 3 x 32 A, the duration of risk will be reduced by an order of magnitude compared with pulling 10 A through a 13 A socket. (b) the risk requires contact with the vehicle and the ground when it is plugged in at the same time as a lost neutral occurs. It seems to me that this will be most likely when plugging-in or unplugging.

In my scenario there will be one phase to the kitchen, one to the rest of the house, and one to the garage, which is as balanced as I can get it.

When I put some figures into the equations in A722.1 and A722.2 with, let's say, 32 A x 3 being drawn by my Tesla, quite a wide range of imbalance indoors did not raise the voltage between the MET and true earth above 70 V. Happy days!

In summer, when the family is out, there may be virtually no load - just those little LEDs everywhere, so better to think about the winter.

So I come home to a nice warm house where the central heating is on, and the PIR-controlled lights illuminate my way, but at the moment I plug in, the loads do not keep the MET/true earth voltage below 70 V. Then the car draws its current and all is well. However, when I go out to unplug, now with TVs, ovens, cookers, kettles all drawing current, the car is no longer drawing any significant current, so the MET/true earth voltage will have risen again. At this stage, a lost neutral may have been noticed, but not necessarily just because the PIR-controlled lights have not come on.

So I suggest that the first flaw in 722.411.4.1(i) and A722.2 is that the situation is a dynamic one.

The second issue is what is the meaning of "not likely"? Clearly it does not say "will never" so is it on the balance of probabilities; is it for the majority of the time; or does it have some other meaning.

Personally, I found the HSL paper very reassuring particularly because the chance of an open neutral in my neighbourhood (all underground) must be vanishingly remote.
 12 June 2018 01:52 PM
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mapj1

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Nothing for it, you need to have 3 cars and put them all on charge together one per phase to balance the load

Or just send the least important member of your team out to do the plugging and unplugging, rather like we were told only those who already had had children should clear up fallout, in the time of "protect and survive", attack drills, and all that.

It could all be handled with suitable procedure. (method statement)
"approach slowly and gently rub the car with the back of the hand, and only if no sensation is felt, proceed to unplug the connector."

-------------------------
regards Mike


Edited: 12 June 2018 at 02:14 PM by mapj1
 12 June 2018 02:06 PM
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AJJewsbury

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So I suggest that the first flaw in 722.411.4.1(i) and A722.2 is that the situation is a dynamic one.

I presume you'd have to look for the worst case. Where you have only single phase loads which may be switched on/off independently then (i) isn't going to be much use for you. If however you had some large 3-phase loads (or groups of single phase loads that were always switched together so they were effectively a 3-phase load) then the rest of the installation would in effect create an artificial N for you and then option (i) might be useful.

More of a flaw to my mind is the assumption that the broken supply CNE affects only your installation. A break further upstream could find you trying to balance your neighbour's loads as well as your own (which presumably are unknown and subject to future alteration). So perhaps (i) is only really an option for private supplies (which would more likely be TN-S anyway).

the risk arising from a lost neutral in a PME supply may be reduced by faster charging

I would want to qualify that I think. The risk persists when the car is plugged in, as distinct from when it's charging. If you plug it in when you get home from work and unplug it when you leave for work in the morning the fact it took 2 hrs rather than 8 hrs to charge doesn't really make a difference. If you unplugged it as soon as charging was complete, then you'd reducing the time available for any passers by to get a shock from the car; but you'd still be spending a similar time plugging it in and unplugging it.

- Andy.
 12 June 2018 08:06 PM
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chrispearson

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Originally posted by: AJJewsburySo perhaps (i) is only really an option for private supplies (which would more likely be TN-S anyway).


Mike, Andy, thank you for your contributions.

In respect of an upstream fault, if it is as upstream as you can get, i.e. the neutral conductor has fallen of the transformer, wouldn't the dozens of consumers' diversity across the loads fix that?

On the basis of your responses I was thinking that (i) is only any use domestically in a block of flats where there is communal charging and sufficient load spread across the phases such that the equation works.

However, on further contemplation this afternoon and evening, it occurs to me that the connexion must have some form of switch. It's rather like we were taught to switch off before removing a plug (top ). So if the isolater is open when you plug in and unplug, there is no risk.

Alternatively you have a TT island (if you haven't fitted a reasonably practical alternative before 31 Dec 18). Any concerns about RCD reliability can be overcome by pressing the test button before plugging in. It isn't onerous - I have done this with ordinary RCD protected 13 A sockets for years.
 12 June 2018 08:20 PM
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mapj1

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However, on further contemplation this afternoon and evening, it occurs to me that the connexion must have some form of switch

However, unlike most RCDs or similar switching arrangements it must also break the CPC connection not just phase(s) and neutral.
If we have a switch in the CPC, we must be very sure, by design, that the CPC is interlocked and makes first, breaks last, easy with a plug and socket, less easy in switch gear or contactors

-------------------------
regards Mike
 12 June 2018 10:50 PM
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AJJewsbury

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In respect of an upstream fault, if it is as upstream as you can get, i.e. the neutral conductor has fallen of the transformer, wouldn't the dozens of consumers' diversity across the loads fix that?

Quite possibly, but it can't be guaranteed - indeed the amount of reported equipment damage due to overvoltage from a simple broken N in polyphase systems (whether a DNO's CNE or simple N within an installation) suggests that there's quite frequently sufficient imbalance to move N very significantly away from 0V.

On the basis of your responses I was thinking that (i) is only any use domestically in a block of flats where there is communal charging and sufficient load spread across the phases such that the equation works.

Again it's hard to guarantee - the flats may be evenly spread across the phases, but it's difficult to consistently ensure balance even then. Say if each flat draw perhaps 0.5 or 1A off peak - it would only need one flat to switch on an electric shower the throw the balance for the entire building off significantly.

- Andy.

(edited formatting)

Edited: 13 June 2018 at 08:37 AM by AJJewsbury
 12 June 2018 11:32 PM
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mapj1

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Results of one phase balance survey here

The bit on how much extra capacity the network would have if it was actually balanced (upto about 50%) is quite telling.
Or skip to page 25 for some plots of a substation's results.

If you have a 400Amps per phase substation, and it is imbalanced by 20% or so on one phase relative to the same kVAif it had been equally shared, that may be called a 'good balance' but it's quite a lot of amps to get back to the star point.

Then there is the problem of non-sinusoidal waveforms from switching supplies or inverters, such that cancellation is not complete even if the currents have the same RMS values on each phase.
For example if you add 3 sinewaves 120 degrees apart the sum is zero, but if you draw peaks of current on the crests, as you tend to do with rectifiers feeding smoothing capacitors, then as there are 2 pulses per cycle, for each phase, but never at the same time, so the neutral simply sees 6 pulses per cycle 3 of each polarity, - you can think of this as a third harmonic effect if you like, and even quite small amounts of it can cause problems with neutrals running hotter rather than cooler than the phase cables .
There are limits like these for how much waveform distortion is permitted for CE marked equipment, but the effect is still not insignificant, and can be as high as a couple of amps of 3F waveform for each electronic item such as PC, PC monitors, radio, or TV receivers, with input power P 600 W or less.
So on a housing estate, substation with perhaps 50 houses per phase, there may be a few tens of amps of un-cancellable waveform distortion to consider as well.
(its also why the substations have to be delta on the HV side and star on the secondary - if they were star-star then an HV neutral would be needed.)

-------------------------
regards Mike


Edited: 13 June 2018 at 01:17 AM by mapj1
 13 June 2018 10:37 AM
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Zoomup

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"Personally, I found the HSL paper very reassuring particularly because the chance of an open neutral in my neighbourhood (all underground) must be vanishingly remote."

A lost neutral occurred in my old neighbourhood in an aluminium conductor underground 1970s' cable. The incident happened about 5 years ago. The jointer said that he had found more than one breakdown in underground cables as the aluminium compresses and fails at joints.

Z.
 13 June 2018 05:22 PM
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chrispearson

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Originally posted by: mapj1

However, unlike most RCDs or similar switching arrangements it must also break the CPC connection not just phase(s) and neutral.


Ah yes, silly me!

So the discussion above means that sub-paragraph (i) is not likely to be reasonably practicable for an ordinary dwelling.

I assume that only a foundation electrode would be capable of satisfying sub-paragraph (ii) so that is ruled out too.

Given that (iii) requires an earth rod, it seems to confer no advantage over having a TT island.

TT it is then.
 13 June 2018 05:25 PM
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chrispearson

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Originally posted by: Zoomup

A lost neutral occurred in my old neighbourhood in an aluminium conductor underground 1970s' cable. The incident happened about 5 years ago. The jointer said that he had found more than one breakdown in underground cables as the aluminium compresses and fails at joints.


Compressed by what? Vehicles driving over them?
 13 June 2018 05:27 PM
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arg

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Originally posted by: chrispearson

Given that (iii) requires an earth rod, it seems to confer no advantage over having a TT island.



(iii) also has the major disadvantage of requiring a device that nobody actually makes.
 13 June 2018 06:01 PM
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mapj1

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looking at page 400-404 I think TT is almost the only option, as it only requires an earth rod of physically practical dimensions, as opposed to an earth impedance that is better than the one at the substation that may pull the whole network off balance if ever there is an L_E event.
also think the chaps at DCODE have a funny idea of the operation of copyright, but it is quite useful.

-------------------------
regards Mike
 13 June 2018 06:50 PM
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chrispearson

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Originally posted by: mapj1

... also think the chaps at DCODE have a funny idea of the operation of copyright, but it is quite useful.


Very useful!

I see that the phrase "not likely" in "i" has become "does not".

I think that I might try drilling through the concrete outside my garage and try to insert an earth rod. If that fails, I shall put in an EV point before 31 Dec 18.
 13 June 2018 08:23 PM
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Zoomup

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Originally posted by: chrispearson

Originally posted by: Zoomup



A lost neutral occurred in my old neighbourhood in an aluminium conductor underground 1970s' cable. The incident happened about 5 years ago. The jointer said that he had found more than one breakdown in underground cables as the aluminium compresses and fails at joints.




Compressed by what? Vehicles driving over them?


I believe that the jointer was saying that any compressed aluminium joint that is crimped can fail after a while, as the aluminium can compress but become loose after time unlike copper which seems to be stronger and more reliable when crimped. The underground joint was too deep to be affected by traffic, and was actually located under a pavement. Whether aluminium oxide on the surface of the conductor contributed to the higher resistance of the joint I do not know. I know that I struggled at first to know just what was happening when I visited the house, as a supply was present but when under load the supply failed. A very high resistance neutral joint underground on the incoming cable was the reason. U.K.P.N. repaired the underground cable.

Z.
 13 June 2018 08:50 PM
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mapj1

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You may find the pics in theLV cable jointer's manual in the UKPN library.
chaps 1 and 2
help explain how underground joints are made. In some cases its not much more complex than a glorified hose clip. The problem is often that with water ingress aluminium corrodes to a high resistance state.

-------------------------
regards Mike
 13 June 2018 11:26 PM
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gkenyon

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Originally posted by: mapj1

looking at page 400-404 I think TT is almost the only option, as it only requires an earth rod of physically practical dimensions, as opposed to an earth impedance that is better than the one at the substation that may pull the whole network off balance if ever there is an L_E event.

also think the chaps at DCODE have a funny idea of the operation of copyright, but it is quite useful.
Provided there are no exposed-conductive-parts or extraneous-conductive-parts of the original TN-C-S system (or indeed any other installation, including the neighbour's and the public street furniture installations) within arm's reach of the charging point, AND the vehicle whilst on charge.

Of course, the "simultaneous contact" issue happens whatever supply system you use, unless each charging point is provided with its own isolated TN-S system (with the protective conductor of the secondary isolated from the primary protective conductor).

Which, if you want a "rule of thumb", makes the "isolation transformer to each individual charging point" option the star of the class at the moment (price excluded ).

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 14 June 2018 02:59 PM
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arg

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Originally posted by: mapj1

also think the chaps at DCODE have a funny idea of the operation of copyright, but it is quite useful.


That unofficial copy with changebars drew my attention to two significant changes in 722 (which were in fact all in the DPC, but I hadn't noticed them):


1) RCDs. Now effectively required to be Type B. Previously Type A was permitted unless DC residual current was "known to be greater than 6mA". Now only permitted if there is also "appropriate equipment that ensures disconnection of the supply in case of DC fault current above 6mA". I suppose that permits use of a Type A in conjunction with a chargepoint that has built-in DC detection that isn't strictly a Type B, but I don't know of any on the market at present. It might also permit the arrangement of Type A per outlet and an overall Type B (time delayed?) upstream, though it is not entirely clear.

Maybe a legitimate safety enhancement, though I'm intrigued as to why EV chargers (as distinct from any other kind of high power switching converter) are considered at risk of generating pure DC leakage currents.


2) Overcurrent - new clause 722.533.101. "Each charging point shall be supplied individually by a final circuit protected by (some sort of breaker)", where the definitions make clear that "charging point" is for just one car and a helpful note points out that "EV charging equipment may have multiple charging points".

This seems rather curious - it seems to require that if you have, for example, a post with two sockets on it, you must run two final circuits to it (that the unit can't be a "twin" with a single set of input terminals). It might be reasonable to insist that each outlet has overcurrent protection to no more than its rating (though you'd expect that to be in the equipment standard rather than BS7671, and anyhow this clause doesn't say anything about the rating of the breaker). Even if that were reasonable, you have the case of something like a twin 32A unit with power sharing such that the total is limited to 32A - you are now required to run two final circuits to it, with two separate 32A breakers, even though your upstream breaker on the submain is probably also 32A.

There's also internal inconsistency in the text, since 722.311 still says "where the final circuit supplies more than one charging point no diversity shall be allowed", while this new 722.533.101 doesn't allow a final circuit to supply more than one charging point.

Also curious that you are required to use breakers rather than fuses - which might be a sensible design choice, but seems unreasonable to mandate it.

This clause seems like a mistake to me - it offers no safety improvement while adding some inconvenient and potentially costly extra requirements.
 14 June 2018 04:39 PM
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mapj1

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let us imagine that the battery charger in the car was only a bridge rectifier off the mains, and then a series regulation element and a battery.
I realise there is a bit more to it than that, but humour me that this is the simplest model.
Unless the battery and all the traction wiring in the car is double insulated, then a single fault condition between say one side of the battery and chassis, would cause a fault that looks rather like a diode and a current limiting element between live and earth. Now this I would expect to fire a type A RCD, and probably quite a few AC types too, as the current is bursty on positive half cycles only. If we now add a circuit that in the interest of good power factor effectively keeps the current flowing even when well below the AC peak, then a short to earth downstream of that, may create a near DC. but as you say, many inverter designs for motor controls etc have that problem.

But some cars also have the option to push power back into the mains, and be used as local storage. If this should fail to chop at 50hz as per design, could the battery drive a large DC back either between N and E or L and E (more likely to be spotted)? I'm not sure either how credible that fault sequence is, but DC in the NE loop would not operate an MCB, and could blind a normal RCD

Really though it will be more like
this diagram
and the choice of 6mA is really quite an odd choice of threshold.

-------------------------
regards Mike


Edited: 14 June 2018 at 10:18 PM by mapj1
 14 June 2018 05:16 PM
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AJJewsbury

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RCDs. Now effectively required to be Type B

But only where the socket is a 'vehicle connector' type (to BS EN 62196) - not when it's conventional BS 4343/BS EN 60309 one. Which I suspects means it only applies to wall mounted EVSE - which typically (hopefully) has in-built appropriate RCD protection.

Maybe a legitimate safety enhancement, though I'm intrigued as to why EV chargers (as distinct from any other kind of high power switching converter) are considered at risk of generating pure DC leakage currents.

I'm only guessing, but perhaps the presence of large d.c. source - i.e. the vehicle battery? (especially considering the possibility of EVs back feeding into the grid).

There's also internal inconsistency in the text, since 722.311 still says "where the final circuit supplies more than one charging point no diversity shall be allowed", while this new 722.533.101 doesn't allow a final circuit to supply more than one charging point.

I almost agree - the only hole I can see is that BS 7671's slightly curious definition of a final circuit does permit a final circuit to supply another circuit - e.g. you could have a circuit that supplies both a socket outlet and a DB - the fact that it supplies a socket deems it to be a final circuit even though it also supplies a DB. Similar logic to a ring final circuit supplying sockets and FCUs (each of which in turn start further circuits) - the ring is still counted as a final circuit. So logically it is possible to have a final circuit supplying two or more final circuits which individually supply EVSE - thus complying with the words. I somehow doubt that's what the authors had in mind, and I doubt any designer would want such an arrangement, but the wording makes it possible.

The regs seem to have a habit of causing extra complexity by trying to treat final and non-final circuits differently - often a simple sweeping statement would (to my mind) do the job far more succinctly (e.g. no diversity is allowable for EV charge points unless load control effectively limits the maximum load to a lower value).

Also curious that you are required to use breakers rather than fuses - which might be a sensible design choice, but seems unreasonable to mandate it.

I don't think it does - it mentions BS EN 60269 which I think is the new standard for fuses, superseding BS 88, BS 1361 etc. It does seem to prohibit rewireable fuses (BS 3036) though.

- Andy.
 14 June 2018 05:58 PM
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arg

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Originally posted by: AJJewsbury

RCDs. Now effectively required to be Type B


But only where the socket is a 'vehicle connector' type (to BS EN 62196) - not when it's conventional BS 4343/BS EN 60309 one. Which I suspects means it only applies to wall mounted EVSE - which typically (hopefully) has in-built appropriate RCD protection.


I haven't yet found one that does claim it - nearly all the makers instructions tell you to put an upstream Type A. I suspect this may be because those that do have internal RCD functions are self-resetting, or otherwise can't claim full conformance to EN61008. Those designs that I've opened up don't look like they have a DC function (with just a single simple sense coil) but it may be there's something more cunning in there.

Hopefully we will see some appearing in due course.

The exemption for other socket types is a two-edged sword though: it encourages people to use portable EVSE (which may or may not have DC-sensitive RCD in them), and normalises the idea of using ordinary sockets for EV charging - which is a bad thing as the average driver isn't easily able to tell the difference between a commando socket installed for EV charging and one to run the xmas lights.


Also curious that you are required to use breakers rather than fuses - which might be a sensible design choice, but seems unreasonable to mandate it.


I don't think it does - it mentions BS EN 60269 which I think is the new standard for fuses, superseding BS 88, BS 1361 etc. It does seem to prohibit rewireable fuses (BS 3036) though.


Ah yes, my mistake on that aspect.
IET » Wiring and the regulations » EV charging, PME, and 3 phases

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