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Smart EQ ForFour
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Discussion Starter · #1 ·
I recently got my first EV and have been charging on the included granny lead at home, which is more than sufficient for the amount of driving we do.

I’ve been considering getting a proper charge point installed but have held out purely due to cost, which is difficult to justify given that the granny lead is sufficient.

However, I’ve recently learned more about PEN fault risk from various YouTube videos and posts on here, and so was wondering:

Does using a 10A granny charger regularly carry the same PEN fault safety risk as if I were using a full 32A EVSE with no earth rod/PEN fault protection? Or does the fact that it is lower current somehow mitigate this risk? Intuitively to me it doesn’t seem that it should make a difference but I’m no expert.
 

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I recently got my first EV and have been charging on the included granny lead at home, which is more than sufficient for the amount of driving we do.

I’ve been considering getting a proper charge point installed but have held out purely due to cost, which is difficult to justify given that the granny lead is sufficient.

However, I’ve recently learned more about PEN fault risk from various YouTube videos and posts on here, and so was wondering:

Does using a 10A granny charger regularly carry the same PEN fault safety risk as if I were using a full 32A EVSE with no earth rod/PEN fault protection? Or does the fact that it is lower current somehow mitigate this risk? Intuitively to me it doesn’t seem that it should make a difference but I’m no expert.

The open PEN fault risk is the same with a granny lead as it is with a non-open PEN fault protected charge point. All outlets intended for charging EVs (not just charge points) are supposed to comply with BS7671:2018 Section 722, but the reality is that many do not, and anyway, there is an argument that an outlet installed to run a Class 2 appliance (like a lawnmower) doesn't need open PEN fault protection (as there are no exposed conductive parts), so the outlet could be said to be compliant with the regs if it wasn't put in specifically for car charging.

The regs do not apply to the granny lead itself, as it's not installed electrical equipment, it's classed as an appliance. That doesn't change anything in terms of the possible risk, but does mean that there is no obligation on granny lead manufacturers to put in any protection measures that may be required by the wiring regs.

In practice, the view regarding appliances that connect to any outdoor exposed conductive part is that the outlet supplying them should have open PEN fault protection. This goes back way before EVs, and includes things like metal framed greenhouses, metal buildings, caravans and hot tubs, all of which really need open PEN fault protection and have done for many years (not that it's ever been formalised in the wiring regs, though, it was always just a "good practice" thing, or sometimes in the MIs for things like hot tubs and caravan hook up boxes).

In terms of risk, no one has yet really done much about the risk that granny leads present, because the assumption is that they are really a "get out of jail" device, that won't be used very often. The theory is reasonable, we get around 300 to 500 open PEN faults a year, and each only usually lasts a very short time. The probability of someone using a granny lead at the same time as their supply has an open PEN fault, and at the same time as someone is standing on the ground and touching the car, is assumed to be very low, much lower than for a fixed charge point that may be in use for several hours every day. Of course, if someone chooses to use a granny lead as their only, or primary, means of charging then the risk is actually greater than that from a fixed charge point, just because the charge rate is lower so the time plugged in is much longer.
 

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Smart EQ ForFour
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Discussion Starter · #7 ·
Got it thanks.

One more risk related question then: how does the risk of going without open PEN fault protection compare with the risk of going without DC leakage protection in the RCD?
 

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That's very dependent on a number of factors, very hard to quantify as a DC leak will only happen due to a fault on the car, and in some cases the car will detect it and disconnect the pack contactors. In most cases the HV system is isolated from the rest of the car's electrical system, and it monitors insulation resistance.
One issue is that a DC fault can de-sensitise an upstream RCD to other faults, but this is primarily a problem if that RCD is shared with other loads, which it usually won't be in most installations.
 

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Is the RCD more likely to provide enough protection if it’s a 10A current as opposed to a 32A current?
the question is whether the rcd will even trip! Remember the rcd trips because of the current impalance... if there isnt an earth, then the current still flows through the neutral and so it then the rcd cant see any imbalance and wont trip.

from another source...

In the UK we call this a PEN conductor fault. It's called a PEN conductor when it's used as a combined Protective Earth & Neutral, as in a TN-C-S earthing arrangement.
The property earthing is bonded to the incoming neutral cable before the consumer unit ("fuse board" / breaker panel). This relies on the fact that the neutral side of the supply transformer is itself earthed (this is why it's called neutral; its potential with respect to the earth is ~0V).
If the neutral supply to your property is broken, then the return current will pass through the internal neutral wiring in your home, back through any protective devices like breakers and RCDs, and then jump across your earth bond, back through the earth wiring in your home, and then to e.g. a water line or gas line bond, or in your case, coaxial; through the earth itself, and then back to the neutral side of the supply transformer, completing the circuit.

This has 3 important implications:
1) The return conductor may not be adequately-sized to carry the current you're asking it to, but your circuit breakers are rated in relation to the fixed internal wiring of your property, so they won't trip. This risks melting and subsequently breaking the conductor,

leading to:
2) The exposed metalwork in your house becomes live. Screws at lightswitches, metal ceiling lighting pendants, copper water and gas lines, the works. All of it.

This leads to:
3) An RCD (GFCI) or a device that contains an RCD (like an RCBO) will not save you when you come into contact with any of it, because the current is first passing back through that RCD on your internal neutral wiring, so there is no current imbalance from its perspective.

So yes, very dangerous.
In the UK, we use concentric cabling for our street supplies, so it is very hard to accidentally sever the outer neutral conductor without also severing the inner line conductor.
As mentioned previously, this is a supplier side fault (!) and the volumes of faults are low... but it does happen and can happen, so this is why they insist on the the protection.
 

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the question is whether the rcd will even trip! Remember the rcd trips because of the current impalance... if there isnt an earth, then the current still flows through the neutral and so it then the rcd cant see any imbalance and wont trip.
It wouldn't trip on the PEN fault itself, but should protect a person coming into contact with the now-live car body from conducting enough current to be dangerous. They'd certainly know about it though!
 

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It wouldn't trip on the PEN fault itself, but should protect a person coming into contact with the now-live car body from conducting enough current to be dangerous. They'd certainly know about it though!
they WILL have felt it... and maybe felt the consequence (ie gone into fribulation) because the rcd is unlikely to trip.
If the RCD works, then you dont need PEN fault detection.
 

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This is very interesting, how many people are using granny chargers with the view they just work but without understanding the risk they run?
 

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42k miles on public charging. Am I an expert yet?
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What I wonder about is how many public charging stations haven't got proper PEN fault detection because they were installed before it was required and aren't properly maintained...
 

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What I wonder about is how many public charging stations haven't got proper PEN fault detection because they were installed before it was required and aren't properly maintained...

Almost all have it from what I've seen, in fact I've not seen one that hasn't had it. This isn't anything new, as it's been normal practice to consider the impact of a PEN fault ever since we first adopted TN-C-S/PME. Before it was added to Section 722 in the 18th Edition, it was just assumed that installers would consider the risk, as they already do for things like hot tubs, caravan hook up points, etc, and install them as TT, which is how every public charge point I've seen is installed. All the 18th Edition did was make it explicit in the regs for any power outlet installed to charge a vehicle. I'm near-certain that was done largely because quite a lot of installers of charge points were ignoring their obligation to consider the open PEN fault risk. The same issue applies to DC tolerant earth leakage protection, as there's been a need for this since the IET guidance was published around 2013 (which was adopted as a condition for grant funded charge points). That was also being ignored, so the IET added it to Section 722 in the regs to make it abundantly clear that it needed to be done.
 

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42k miles on public charging. Am I an expert yet?
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Almost all have it from what I've seen, in fact I've not seen one that hasn't had it. This isn't anything new, as it's been normal practice to consider the impact of a PEN fault ever since we first adopted TN-C-S/PME. Before it was added to Section 722 in the 18th Edition, it was just assumed that installers would consider the risk, as they already do for things like hot tubs, caravan hook up points, etc, and install them as TT, which is how every public charge point I've seen is installed. All the 18th Edition did was make it explicit in the regs for any power outlet installed to charge a vehicle. I'm near-certain that was done largely because quite a lot of installers of charge points were ignoring their obligation to consider the open PEN fault risk. The same issue applies to DC tolerant earth leakage protection, as there's been a need for this since the IET guidance was published around 2013 (which was adopted as a condition for grant funded charge points). That was also being ignored, so the IET added it to Section 722 in the regs to make it abundantly clear that it needed to be done.
Ahh, well that's encouraging then :)
 
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