Author Topic: How is Neutral Wire Neutral in Mains Electricity?  (Read 17963 times)

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Offline MadScientist

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #100 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.

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
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Offline richard.cs

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #101 on: February 03, 2020, 10:30:14 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.

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

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).
...
A subset of this type therefore use a functional earth connection as a alternate place to pass tripping coil current to.

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
 

Offline paulca

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #102 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?
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Offline richard.cs

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #103 on: February 03, 2020, 03:07:52 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?

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.
 

Offline Monkeh

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #104 on: February 03, 2020, 03:54:14 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."

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?

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

Offline SeanB

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #105 on: February 03, 2020, 08:02:45 pm »
With regards to that SWA cable, the same construction is used for 11kV, 33kV and 66kV cables, with only a small increase in the number of  overlapped oiled paper insulating sleeve windings used. They do have a very robust overload capability, in that you can have the cable running for decades with the temperature at the melting point of the pitch pour in the seal, provided that your cable ends do not get wet during this time.

I regularly see these in excavations, still powered, and with the pitch covering mostly gone, leaving the steel tapes exposed and corroding away, but the lead keeps it in more or less working condition, till it cracks and lets water in. Then the cable can and will excavate itself, I saw a failed 66kV joint, that blew itself out of the ground, leaving a 6m deep hole where the cable was buried, and turning the cast joint box into an art object. Current practise is not to lead wipe the joints, more due to the lack of any training on the art of lead wiping, and the older experienced people having mostly retired. Instead they use a cast resin joint, generally taking the paper cable to a PVC or HDPE insulated cable for further use, as it is something the electricians are trained to work with, though the core then changes from copper to aluminium in higher current uses, unless the length is short.

As to RCD units, the type with a ground connection was used for a few years here, as the earth connection allowed the unit to detect the loss of supply neutral, tripping on loss of neutral using a 120VAC MOV device, and also incorporating a 480VAC MOV from line to earth as well, which meant you had to disconnect the outputs before doing insulation tests, and you could not back feed them.. the current approved types are smaller, and are strictly 2 wires in and out, and are strictly 30mA RCD, requiring external overload protection. Required on all socket outlets, so typically, as it is rated for use as a disconnector, it is the single main input breaker, so if it trips you are in darkness.

Thankfully with the current Eishkom issues everybody has at ready reach some torches, a cellphone with torch or some form of lighting, as you will have regular rotational load shedding, and you might not get power back for a few hours to a few days if the bit turn on surge into a very cold load kills something upstream.
 

Offline Electro Detective

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #106 on: February 12, 2020, 09:54:17 am »
Re RCD mA faith:
 
If the fault current is high enough, fast enough and on a peaky? and or distorted top or bottom cycle/s, there is an extremely high possibilty that ALL the 10, 15, 30 and 100ma 'rated' RCDs will pop

Been there, sometimes the 'faster ones trip first' thinking in theory and in practice ~works~  and you think you got it nailed  :-+

but on repeat tests under the exact same test 'fault simulation' conditions, the whole lot go off, and you never know when   ???

i.e. it's unpredictable and not worth the embarrassment and inconvenience to assume a 10ma fast RCD will pop before the 30ma one at the switchboard 60 meters away.

If you need your lab or bench 'RCD safe' and isolated from upstream/downstream RCDs, then that entire area/zone should be on an large-ish isolated transformer (aka 'Medical Isolation' style)
with the Secondary earth/neutral/chassis bonded together, so the RCD can be plugged into that Seconday output and all the gear into the RCD via a power strip/s board/s etc.

If TESTED and done properly with correct input and output polarity checked, the RCD will trip on faults or tests, but any RCDs elsewhere won't know or care.

This is how I roll with suss scenarios or 'what ifs..'
It saves on a lot of 'reset' walking in the dark with a flat torch, getting rained on, stepping on cat, dog, bird poo..again..  :rant:
and avoids mumbling feeble 'power failure/blackout' excuses no one buys anymore  :-[

 

Offline richard.cs

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Re: How is Neutral Wire Neutral in Mains Electricity?
« Reply #107 on: February 12, 2020, 10:16:23 am »
If the fault current is high enough, fast enough and on a peaky? and or distorted top or bottom cycle/s, there is an extremely high possibilty that ALL the 10, 15, 30 and 100ma 'rated' RCDs will pop

This is well known behaviour. Standard practice here (UK) is to use devices with delays as well as different currents. e.g. instant tripping (well, 40 ms or so) 30 mA devices, with a 100 mA device upstream which has a 100 ms delay, and then if further layers are needed 300 mA 300 ms, etc. It's the delay not the current rating that ensures they trip in order, the increasing trip currents are more about having multiple sub-circuits on each one. Only the <=30 mA instant trip types are considered to offer protection to humans.
 


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