Single phase, 2 phase, 3 phase etc

Risteard said:

westie101 said:

Which is totally daft as the neutral in the UK is connected to earth any way so cannot be live

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If it couldn't be live then it wouldn't carry any current in ordinary circumstances. (Ignoring the fact that in a balanced three-phase system it will carry no current.)

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This begs the question as to the definition of "live". You seem to be regarding it as meaning that it "can carry current in ordinary circumstances". BS7671 says "...intended to be energised in normal use, including a neutral conductor but, by convention, not a PEN conductor." ... which, in turn, begs the question as to what is meant by "energised" (which it does not define).

None of this alters the fact that the great majority of the world's population probably know what they mean by a "live" conductor/wire/part - and that understanding of theirs would very rarely include a neutral conductor!

Kind Regards, John

Risteard said:

Ironically, despite his insistence that correct terminology should be used, he is one of the greatest serial offenders for abusing the term "live" and using it in a completely incorrect context.

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Old habits, I'm afraid.

I will try harder.

But at least I'm clued up enough to know which country I live in.

My semi-laymans understanding of "live" is "if I touch it i'll get an electric shock". We can think of that more formally as "at a dangerous voltage relative to the local earth reference".

So how do neutral conductors fit with that definition? well that depends on how much confidence we have in them!

At the one extreme you have portable appliances built for sale on the world market. Here while you have two input terminals labeled "L" and "N" or two wires coloured brown and blue the reality is you have no idea whether the "neutral" connection is actually a neutral. Maybe the appliance will be used in a country like germany where plugs are unpolarised or a country like france where while the plugs are polarised they are apparently less than steller about wiring them up the right way round. Maybe the appliance is being used on a 240/127 3 phase supply or a 240/120 split phase supply and connected between two phases so there is no neutral. You just don't know so your only option is to treat both wires as "live" and design accordingly. In this world fuses in both the "L" and "N" connections are considered good practice.

At the other extreme you have practices like those used by UK DNOs. Their strategy is to make damn sure the neutral stays tied to earth and hence there is no need to regard it as "live" and in some cases it is even possible to use it as a protective earth. Protective devices in the neutral are strictly forbidden, switches/isolators in the neutral are at the very least strongly discouraged. Cables are designed so that in the event they are accidentally cut the live will most likely be severed before the neutral.

Somewhere in between you have our electrical installations. Single pole switches and protective devices in the neutral are forbidden but double pole switches and protective devices (with the associated risk that neutral may end up breaking before line and hence becoming "live") are quite common. Single pole protective devices are the norm for overcurrent protection. Our cables have the neutral insulated to the same standard as the "line". Local isolation switches are typically double pole but it's common practice to use single pole MCBs to isolate circuits at a CU/DB and therefore potentially rely on the integrity of the neutral upstream of the CU/DB and through any isolators or RCDs when working on a circuit.

My guess would be that BS7671 picked up some unfortunate terminology from a standard that originated in a world (either portable appliances or countries with different practices) where there was much less trust in the neutral. Anyone fancy looking at older versions of the wiring regs to see what terminology they used?

westie101 said:

(c)no impedance is inserted in any connection with earth of a low voltage network other than that required for the operation of switching devices or of instruments or equipment for control, telemetry or metering.

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That is a surprise as at Point of Aye gas terminal there was a huge resistor between the DNO earth and the plants earth. I will admit I sunk around 200 earth rods each at 8Ω or less and an earth pit next the the resistor with less than 2Ω. It did have it's own gas turbine generators and clearly there has to be a limit on how much current an earth wire can carry so if the DNO lost it's earth with the plant having as near as one could measure 0Ω to earth some limiting device was clearly required.

As to multi-supplies I would have expected the DNO to have refused to supply them. When at one job we wanted a second supply they were very insistence that the buildings were never joined.

I would expect at one time each of the supplies were to separate clients and really once all buildings were joined there should only be one supply.

As to single, twin, split and three phase I am sure somewhere there is a twin phase supply but it is rather rare. More often split phase.

plugwash said:

My semi-laymans understanding of "live" is "if I touch it i'll get an electric shock". We can think of that more formally as "at a dangerous voltage relative to the local earth reference".

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With TN systems the BS7671 does allow you not to isolate the neutral. With TT systems neutral is regarded as live. I have had some nasty belts from neutral mainly due to shared neutrals but even without that if a neutral is disconnected then the appliance side will become live. This is why isolators must be linked so switching the neutral also switches the lines.

Around the world we do have some odd systems. In Algeria we had three phase 110 volt so 190 volt phase to phase in USA it's 120 volt but the way the phases are arranged alters from split phase to the hot phase in where the earth is at a centre point on one phase winding but the output of the transformer is delta. France also uses split phase hence why we get hobs which allow you to use both phases on the split phase supply.

In Algeria where I worked in Hassi R'Mel getting any earth was hard and you were lucky to get 100Ω on main generators earth rod but in UK unusual for any earth rod to be over 60Ω.

If you want to talk about non UK supplies there is a Electrics Outside of the UK section.

plugwash said:

My semi-laymans understanding of "live" is "if I touch it i'll get an electric shock". We can think of that more formally as "at a dangerous voltage relative to the local earth reference". ... So how do neutral conductors fit with that definition? well that depends on how much confidence we have in them!
My guess would be that BS7671 picked up some unfortunate terminology from a standard that originated in a world (either portable appliances or countries with different practices) where there was much less trust in the neutral. Anyone fancy looking at older versions of the wiring regs to see what terminology they used?

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That's quite possibly true, but what I don't really understand is why they felt the need to produce this definition of "Live" at all - I'm not sure what it achieves, beyond the potential confusion to those countless millions (possibly billions) of people who use the word to mean what an electrician would call 'line' or 'phase'.

Kind Regards, John

Note that it's a definition of "live conductor/part" not a definition of "live".

My guess would be that the writers of a standard in a world where neutral conductor needed to be treated as hazardous made the expediant descision to include neutrals in their definition of "live conductor/part" so they didn't have to repeatedly explain that requirements for live parts also applied to parts connected to the neutral.

You should treat the Neutral as possibly live since it will become live if there is a break in the Neutral between you and the incoming Neutral and there is an appliance connected between a live wire ( phase or line ) and the Neutral that is no longer connected to the incoming Neutral

And of ciurse there may be a network fault where the incoming Neutral is no longer held at ( or near ) ground potential.

And of ciurse there may be a network fault where the incoming Neutral is no longer held at ( or near ) ground potential.

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The only neutral faults we experience are on CNE (PME, TNCS) systems, where it seems the neutral should not be classed as live

That is a surprise as at Point of Aye gas terminal there was a huge resistor between the DNO earth and the plants earth

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The restriction is for between DNO networks and earth, not between a DNO and a 3rd party earth

The DNO definition of Live, so live conductor is "Electrically Charged"
So to us a neutral is not live in any way.
Yes it can become that in fault conditions, but in reality that is hardly common so classing it as a live conductor seems rather confusing

westie101 said:

And of ciurse there may be a network fault where the incoming Neutral is no longer held at ( or near ) ground potential.

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The only neutral faults we experience are on CNE (PME, TNCS) systems, where it seems the neutral should not be classed as live

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The fault can create the situation where the metal is neutral by name but far from neutral by nature

That is a surprise as at Point of Aye gas terminal there was a huge resistor between the DNO earth and the plants earth

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I recall something similar as a petrol refinary. The theory was that in the event of there being a potential difference between the two earth then the resistor would pass enough current to cause safety devices to operate while still keeping the difference in earth potentials low enough to be considered safe. Using a large resistor would limit current to prevent the bond cable from earth[1] to earth[2] being burnt out by excessive current and thus allowing the potential difference to increase beyond safe limits.

Having "earthed" pipe work even just a few volts above the potential of the ground is not wanted in a petrol refinery.

westie101 said:

And of ciurse there may be a network fault where the incoming Neutral is no longer held at ( or near ) ground potential.

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The only neutral faults we experience are on CNE (PME, TNCS) systems, where it seems the neutral should not be classed as live

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Systems other than TN-C-S surely aren't immune from neutral faults?! As bernard has said, a break anywhere in the neutral (anywhere downstream of the transformer) of any supply system will result in the neutral within a consumer's installation rising to 'live potential' if (as is almost inevitable) there are any connected L-E loads (in any installations using that neutral).

However, the fact that a neutral conductor may become 'live' under certain fault conditions does not, IMO, make it any more reasonable to call it a 'live conductor'. Let's face it, "under certain fault conditions" (usually two faults needed) a CPC, Earthing or Bonding Conductor could also 'become live' - but, thankfully, no-one has (yet!) suggested that they should also be called 'live conductors'!

Kind Regards, John

Systems other than TN-C-S surely aren't immune from neutral faults?! As bernard has said, a break anywhere in the neutral (anywhere downstream of the transformer) of any supply system will result in the neutral within a consumer's installation rising to 'live potential' if (as is almost inevitable) there are any connected L-E loads (in any installations using that neutral).

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Well I'll tell you what, in 43 years in the industry, and still counting. I am only aware of a few instances except in PME systems, of the neutral coming live, in each of those cases it is where the earthing has been stolen from substations!
I do not, or have not heard of any instances where the neutral has faulted in that time.

So I'll stick with my industry standard of not classing the neutral as a live as it is not, normally, electrically charged
What happens after the cut-out is another thing

westie101 said:

Systems other than TN-C-S surely aren't immune from neutral faults?! As bernard has said, a break anywhere in the neutral (anywhere downstream of the transformer) of any supply system will result in the neutral within a consumer's installation rising to 'live potential' if (as is almost inevitable) there are any connected L-E loads (in any installations using that neutral).

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Well I'll tell you what, in 43 years in the industry, and still counting. I am only aware of a few instances except in PME systems, of the neutral coming live, in each of those cases it is where the earthing has been stolen from substations! ... I do not, or have not heard of any instances where the neutral has faulted in that time.

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Are you perhaps talking just from the viewpoint of the DNO side of a fault? I'm seriously struggling to understand why the fact that a conductor is a CNE makes it any more susceptible to mechanical damage than if it were just a neutral conductor - and a break in any neutral conductor (CNE or not) will have the consequences (for consumers) that I mentioned.

westie101 said:

So I'll stick with my industry standard of not classing the neutral as a live as it is not, normally, electrically charged

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As you will realise, that makes total sense to me - and would as a generalisation (i.e. on both sides of the cutout).

Kind Regards, John

westie101 said:

Yes it can become that in fault conditions, but in reality that is hardly common so classing it as a live conductor seems rather confusing

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As I've said, I would personally say "totally confusing" (to most people). Furthermore, as I've said, under certain (2 fault) fault conditions, even a CPC can become 'live'!

Kind Regards, John