Amendment 3 2024, BS7671 2018.

[Julie.]

Do you have a mechanism in mind where it wouldn't detect an imbalance between the line and neutral lines which are AC ?
Unless it specifically calculated the phase angle (why do that ?) then it would see the same imbalance whichever way the device is inserted.

So ?
The detection circuit is normally energised 100% of the time. If you mean the trip circuit, when it's tripped, the fault will disappear. The only exception might be a N-E fault with a single pole device that doesn't switch the neutral, but who fits those ?

The problem is that the electronic circuit is, say energised and has a detection feed from the output, when it's the correct way round and it trips or is switched off, the detection circuit is dead, but the main circuit is still energised. All good.

However if the wrong way round, when it trips, the detection circuit remains live, but the main circuit is disconnected! This can cause a reverse supply to the circuit. If it wasn't designed for this condition, it can damage the electronics.

Once damaged the rcd may not work - yet outwardly all you did was switch it off and back on again.

Only the manufacturer would know if keeping the detection circuit energised whilst the main control circuit is not causes a problem or not.

It isn't just RCDs BTW, RCBOs and especially combined AFDDs may be especially susceptible to this issue.

In reality if a manufacturer dictates their device is not suitable for bidirectional applications, they don't have to explain why; we need to follow their recommendations.

 

[SparkySy]

The problem is that the electronic circuit is, say energised and has a detection feed from the output, when it's the correct way round and it trips or is switched off, the detection circuit is dead,

Sorry @Julie.
I have not looked in depth at how bi-directional RCD's overcome this issue, as the detection circuit would surely have to come from one or other side of the device.
Do you have any knowledge of how this is achieved?

Thanks
Sy

 

[Julie.]

Sorry @Julie.
I have not looked in depth at how bi-directional RCD's overcome this issue, as the detection circuit would surely have to come from one or other side of the device.
Do you have any knowledge of how this is achieved?

Thanks
Sy

Well it's a pretty common issue with the cheaper microprocessors out there.

If there is no supply to the main controller, but there is to one of the i/o ports the current can flow into the controller and destroy it. If the supply is there the relative voltage across the protection diodes is not a problem.

If however you can limit the current to a much lower value, although the current backfeeds it isn't enough to cause damage.

So a redesign of the detection circuit may be sufficient, or changing to a microcontroller which doesn't suffer from this problem may be required.

In effect it's merely an electronic design flaw/feature the solution of which comes down to the actual implementation itself.

As a hobby I do design stuff using microcontrollers (often better ones like atmega4808 or atmega328, or stm32... but i have use things like the attiny range which are "cheap and cheerful".)

When I use an i/o port I tend to use a potential divider into a zener diode, then into the i/o port. The zener limits the voltage (usually to 4V9) and given the voltage a 10k resistance limits the current into the controller to 0.49mA. Of course this adds a zener and a resistor in addition to any potential divider on every i/o port - which for me is not a problem, and probably overkill for the better devices, but adds up for every production device.

 

[SparkySy]

thanks @Julie
That is an incredibly detailed answer.

I have forgotten a lot of electronics from my college days through lack of use unfortunately, but that makes sense!