I have observed some unusual behaviour between my domestic 240VAC supply and an EV charger which has over-voltage protection.

I'm posting this question here in search of a possible explanation or for guidance to investigate further.

  1. Background

I have an electric vehicle charger. As per standard design, the actual charger is inside the car, but the "charger" just routes the 250 VAC straight through a connector to the car. It also signals to the car the maximum supported charging current by using the duty cycle of a square wave, in this case signalling to the car that it may draw 15A maximum, because it uses a wall socket plug. The charger is not one after those higher power types which are wired in.

The "charger" also reads the car status so it knows when to stop charging, once the car is fully charged.

I have not reverse-engineered the charger to see which chips it uses, but part of its design includes detecting over-voltage conditions.

When over-voltage or other errors happen, it flashes error codes via front-panel LEDs. Very likely has a small microcontroller.

The manual says it can take 240V input max, but the UK tolerance for mains supply is 230 volts -6%, +10%, allowing a maximum of 253V. I'm assuming the Chinese manufacturers know this when they say 240V max.

  1. Question

When I plugged in the charger, it showed a double red flash error code, meaning "over voltage error, the supply is too high". So I measured the voltage and found it was 245 volts which is more than 240 as per the manual, but also well within the UK allowed range.

If your domestic supply actually is too high i.e above the tolerance then the supply company will send out an engineer and work with you to get it down, they don't want you frying your appliances. If your supply is simply above 240 and within tolerance then this remedy will not happen.

Now for the weird part; I have discovered that the charger behaves exactly the same, consistently showing the over voltage error with the three or four other plug sockets in my home, but there is one single mains socket where it works perfectly.

This is my question: how can one socket work perfectly and yet other sockets in the same house trigger the over voltage error consistently?

To eliminate a once-off fault in the charger, I sent it back and had a replacement sent out, which behaved in exactly the same way. It worked fine when plugged in to the magical one wall socket, but flashed the over-voltage error when plugged into any of the others.

So my question is, what could cause one mains socket to work fine, while another three sockets in the same room trigger the EV charger over-voltage condition?

How can this be?

  • 1
    \$\begingroup\$ We can't guess what it is. If there is lower voltage due to other loads drawing current, have you checked for bad contacts that heat up? Or rather had an electrician to figure it out for you safely? This is more about using electrical devices than designing them. \$\endgroup\$
    – Justme
    Commented May 27 at 19:57
  • \$\begingroup\$ check the voltage at the magic socket, before and during charging \$\endgroup\$
    – jsotola
    Commented May 27 at 21:12

1 Answer 1


The most likely reason - assuming that your home has a single-phase supply - is cable length / distance from the fuseboard and that extra length is causing a voltage drop. At 7.4 mΩ/m (double-check this) 15 A would cause a voltage drop of 7.4 × 15 × 2 = 0.222 V/m loop.

I would expect the charger to measure the voltage before starting a charge though and the no-load voltage on that socket should be the same as all the others ()unless there is another load on that branch) and I would expect it to error.

For your own understanding try measuring the voltage at each socket under load.


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