Electronics > Beginners

How is Neutral Wire Neutral in Mains Electricity?

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MadScientist:

--- Quote ---In UK practice RCBOs are often half the width of an RCD, and sometimes do not switch neutral (this is a rubbish plan but very common). It may be that due the the space constraints these are more likely to have electronic tripping, but in any case it seems far more common for RCBOs to have a functional earth than for RCDs to have them. Also some RCBOs have earth tails that are essentially just there for EMC filtering.
--- End quote ---

I think you are mixing up MCB with RCBOs

An RCBO is a residual disconnect device and an overload trip in one. They are very common compared to just RCDs. I’ve never seen an RCD or RCBO with an earth connection , what would be the point

richard.cs:

--- Quote from: MadScientist on February 03, 2020, 09:55:30 am ---
--- Quote ---In UK practice RCBOs are often half the width of an RCD, and sometimes do not switch neutral (this is a rubbish plan but very common). It may be that due the the space constraints these are more likely to have electronic tripping, but in any case it seems far more common for RCBOs to have a functional earth than for RCDs to have them. Also some RCBOs have earth tails that are essentially just there for EMC filtering.
--- End quote ---

I think you are mixing up MCB with RCBOs

An RCBO is a residual disconnect device and an overload trip in one. They are very common compared to just RCDs. I’ve never seen an RCD or RCBO with an earth connection , what would be the point

--- End quote ---

UK RCBOs are usually single-width to fit in the space an MCB would normally take up, and often have a functional earth tail. I understand other countries often have both double pole MCBs and RCBOs in double width, but we generally only have switches and RCDs like that. Commonly our RCBOs have a solid neutral connection, it passes through the device for residual current sensing but doesn't necessarily have a switch contact.

Here's a couple of RCBOs with a functional earth connections (the thin white wire), in single-width the same as our MCBs:
https://willrose-electrical.co.uk/product/hager-rcbo-10-amp-30ma-type-b-10a-106361-b10-adn-range-adn110/
https://www.sparksdirect.co.uk/schneider-ez9d16810-easy9-10a-rcd-1-pole-neutral-6ka-30ma-type-b-rcbo-with-overcurrent-protection#.Xjf0h2j7SUk

As I said before the functional earth is used to sink current when the upstream neutral is O/C and an electronic tripping mechanism is in use (i.e. one where the differential current is a signal input but not the source of tripping energy).

--- Quote from: richard.cs on January 31, 2020, 02:57:08 pm ---...
A subset of this type therefore use a functional earth connection as a alternate place to pass tripping coil current to.

--- End quote ---

RCBOs are less common here compared to having a group of MCBs on the output of an RCD, like this:
https://www.designingbuildings.co.uk/w/images/7/70/Consumer_unit_photo.JPG

paulca:
On RCDs.  I asked the Spark who installed my RJ45 cables how much it would cost to put an RCD on my plug-ring and he said, "There is no point, that main RCD will always trip first."

However, it turns out the main RCDs (in the UK anyway) are actually quite high current, 30mA / 30ms.

I understand the 30mA is due to earlier much more sensitive RCDs nuisance tripping regularly and increasingly due to PCs and switch mode power supplies.

Is it worth installing more sensitive RCDs in key areas such as my electronics bench and my fish tank?

richard.cs:

--- Quote from: paulca on February 03, 2020, 02:33:43 pm ---On RCDs.  I asked the Spark who installed my RJ45 cables how much it would cost to put an RCD on my plug-ring and he said, "There is no point, that main RCD will always trip first."

However, it turns out the main RCDs (in the UK anyway) are actually quite high current, 30mA / 30ms.

I understand the 30mA is due to earlier much more sensitive RCDs nuisance tripping regularly and increasingly due to PCs and switch mode power supplies.

Is it worth installing more sensitive RCDs in key areas such as my electronics bench and my fish tank?

--- End quote ---

Possibly. If you installed 10 mA RCDs (the only widely available lower value in the UK) then tripping would occur at a lower current which might be desirable for some faults. If you were working on your bench and found yourself in a fault path with sufficiently high impedance that only 15 mA flowed (a painful but not usually harmful current) the 30 mA RCD would allow that continue indefinitely whereas the 10 mA one would trip within a few tens of milliseconds. In the majority of faults however the current would be >>30 mA and either or both RCDs would trip so you're not preventing the bench causing nuisance trips to the rest of the house.

It sounds like you have a single 30 mA RCD covering the whole house? This is safe but a bit of a nuisance from a stray tripping perspective. There are other changes you can make to achieve discrimination, one example would be changing that one to a 100 mA time delay type and then putting separate 30 mA RCDs (or RCBOs) onto all the circuits that need 30 mA protection.

Monkeh:

--- Quote from: paulca on February 03, 2020, 02:33:43 pm ---On RCDs.  I asked the Spark who installed my RJ45 cables how much it would cost to put an RCD on my plug-ring and he said, "There is no point, that main RCD will always trip first."
--- End quote ---

Don't employ him again, he doesn't pay attention.


--- Quote ---However, it turns out the main RCDs (in the UK anyway) are actually quite high current, 30mA / 30ms.

I understand the 30mA is due to earlier much more sensitive RCDs nuisance tripping regularly and increasingly due to PCs and switch mode power supplies.

Is it worth installing more sensitive RCDs in key areas such as my electronics bench and my fish tank?

--- End quote ---

30mA is perfectly normal and sufficient to protect you. The 100mA device you have is not supposed to provide human protection.

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