It seems AMD3 is trying to mitigate tip of the larger problem. The problem of "unidirectional" and "bidirectional" RC devices stems from the way electronics are designed on the secondary winding inside. But the problem is not just the design but the fact that the functionality of modern RCD/RCBO in UK relies on electronics at all. As many of you may already be aware - this type of design was disallowed on mainland continent and most of RCBOs sold nowadays in UK are illegal to use in EU because the electronics will not function without neutral (eg. in the event of PEN fault). The device that's supposed to be part of secondary protection and fire protection will do absolutely nothing if the fault starts by burning through neutral. Just a few years ago just about any RCBO with electronic release would have a flylead connected to MET ensuring the device was still functioning without neutral under fault conditions, but with time even that failsafe was withdrawn from most of the designs.

So - don't add unidirectional and bidirectional malarky to the regs - just ban shoddy electronic designs from UK markets all together before it becomes larger issue. Most of the world already did.

Martin Korczak said:

As many of you may already be aware - this type of design was disallowed on mainland continent and most of RCBOs sold nowadays in UK are illegal to use in EU because the electronics will not function without neutral (eg. in the event of PEN fault). The device that's supposed to be part of secondary protection and fire protection will do absolutely nothing if the fault starts by burning through neutral.

Is this correct? I'm sceptical because the terms in BS EN 61009-1 relating to voltage dependant operation aren't BS national departures.

The overcurrent functions of an RCBO are not dependent upon voltage - they are essentially an MCB in these respects.

The RCD characteristic however is. And you're correct that there's no guarantee this will work if the L-N voltage drops below 0.8 nominal. 

But it would need multiple faults for this to become a hazard. For example a PEN fault is a fault condition so if this was to coincide with a secondary fault that was unable to clear then yes that would be immediately dangerous.

I can see some justification in domestic where the risk of existing faults is sufficiently high - hence the elevated requirements for AP in the first place. But then the advantage of AP by 30mA RCDs [as opposed to e.g. the traditional supplementary bonding approach] is that a problem raises its head and the occupants hopefully ring their electrician about an annoying RCBO.

FEs are a bit of a pain. I've always wondered why they don't put a supercapacitor or something in - but would have its own problems.

Some also argue that we should have upfront PEN protection - I'm not convinced but I think smart meters should report undervoltage to the DNO.

Ultimately protective devices are there for fault conditions not dodgy wiring - which aren't the same thing.

Keir Stitt said:

but I think smart meters should report undervoltage to the DNO.

I think that would be a good idea ... overvoltage too.

Keir Stitt said:

but I think smart meters should report undervoltage to the DNO.

I think that would be a good idea ... overvoltage too.