AC vs DC, live vs neutral

[JAMJAM]

Hi. I've read much about ac vs dc but can't find a 'simple' answer. If current alternates 50 times per second in a uk mains circuit, why do live and neutral not swap 50 time per second? On these forums, I've read that it does, so that copper does not migrate as it would in a dc circuit. I've also read that it doesn't and live is always live, neutral always neutral. Slightly related, if Neutral is always earthed, how does that work if current alternates? Again, much debate on here about the logic / value of earthing the neutral. Don't worry, I'm not going to fiddle but I want to understand as my electrician can't explain and made the metal casing of a power socket live! I don't use him any more btw. Thanks.

 

[brianmoooore]

It's the earthing of one of the conductors that determines that it's the neutral. The other {live} conductor swings from 230V positive to 230V negative and back again 50 times a second.

 

[Rockingit]

If you think about a simple 360° sine wave (the traditional AC wave form, positive 180° first and then rolling through 180° negative), the waveform is only actually peaking for a fraction (technically we're into calculus at this point) of a moment. Leaving aside the complications of RMS [Root Mean Square] values for now, what we measure using the unit of Volt(s) is the difference between two points - Potential Difference. So for the entire time our sine wave is swooping along, it's reference point is both mathematical and practical zero - or to put it another way, it's neutral to either the +ve or -ve phase of the wave. By then connecting that neutral to all the other neutrals around us (which is the principle of earthing) we know that there is no [Potential] difference between that neutral and any other so by doing so we sort of calibrate our live waveform - ie, if you were to take a probe on a meter to the live on one house, and the other probe to the live on another house you should (theoretically) have a reading of 0v - because there is no potential difference between them.

On a three phase system, things change again because now we have 3 lots of sine wave all interpolating at 120°, so when looked at simply (again, ignoring RMS..) you near as damn it always have something at the top, and something at the bottom, so the potential difference there is (simplified..) +230 to -230 which gives us 460V (it's actually lower) and mathematically nothing is ever zero because as soon as one waveform hits zero the one behind it is roughly 160v ahead of it already, followed by another. This is the reason (well, one of a few) why we don't even need a neutral to make a true 3ph circuit work.

 

[JAMJAM]

It's the earthing of one of the conductors that determines that it's the neutral. The other {live} conductor swings from 230V positive to 230V negative and back again 50 times a second.

Thanks, brianmoooore. So, if the sparky got it wrong and earthed the "live" (or brown wire) by mistake, would that become the neutral and the blue wire become the live?

 

[brianmoooore]

The neutral and earth are normally connected together either before or where the power enters the property, so earthing the wrong wire with the power on would likely result in a big bang.
When power I have flowing in a circuit there is a voltage drop in the neutral, so earthing the neutral at the far end of a circuit might result in a small spark.

 

[JAMJAM]

Thanks again, brianmoooore. I don't think I asked the right question or you're thinking "what idiot would do that?" . I understand TT, TNS, etc., but sparky says there is no TT and supplier says TNS is not supported. The house has 4 consumer units and MICC cable. Sparky who did the EICR says the MICC sheath has incorrectly been used as cpc (code 2!) and I see the suggestion gets ridiculed on Code 3 for usign Sheath of MICC as CPC - https://www.electriciansforums.net/threads/code-3-for-usign-sheath-of-micc-as-cpc.166120/ . I just wondered how a socket casing on a spur can be live - it is (apparently) earthed. Maybe the earth wire just goes nowhere?

 

[Rockingit]

Thanks again, brianmoooore. I don't think I asked the right question or you're thinking "what idiot would do that?" . I understand TT, TNS, etc., but sparky says there is no TT and supplier says TNS is not supported. The house has 4 consumer units and MICC cable. Sparky who did the EICR says the MICC sheath has incorrectly been used as cpc (code 2!) and I see the suggestion gets ridiculed on Code 3 for usign Sheath of MICC as CPC - https://www.electriciansforums.net/threads/code-3-for-usign-sheath-of-micc-as-cpc.166120/ . I just wondered how a socket casing on a spur can be live - it is (apparently) earthed. Maybe the earth wire just goes nowhere?

There’s only one of two reasons what you’re describing could have happened - either there was a PEN fault on the supply external to you or - and I’m increasingly convinced of this - whatever idiot you engaged as an ‘electrician’ managed to wire Live to an earth with an existing fault in the earth (cpc) path. I’d be getting trading standards and the HSE involved and asking some very serious questions. I’d also not be trusting ANY other work they’ve done for you and getting a reputable inspector to check it over. Post up your location and there’s bound to be someone close to you on here.

 

[littlespark]

Sparky who did the EICR says the MICC sheath has incorrectly been used as cpc

Are you sure that’s what was said?

The copper sheath of MICC is designed to be the cpc.


I’d suggest a second electrician to find the fault, and isolate the affected circuit until then.

 

[JAMJAM]

There’s only one of two reasons what you’re describing could have happened - either there was a PEN fault on the supply external to you or - and I’m increasingly convinced of this - whatever idiot you engaged as an ‘electrician’ managed to wire Live to an earth with an existing fault in the earth (cpc) path. I’d be getting trading standards and the HSE involved and asking some very serious questions. I’d also not be trusting ANY other work they’ve done for you and getting a reputable inspector to check it over. Post up your location and there’s bound to be someone close to you on here.

OK, thanks Rockingit! I got another sparky to look. Turns out (and it's complex for me) the idiot who caused it didn't understand solenoids and managed to wire two external floods in series, so 120v wasn't enough to fire them up! I figured that out with a simple tester when nothing worked. There were 6 x 500w floods on the relay circuit. That was fixed but an actual fault elsewhere caused a short to 'earth' but because he'd not earthed the circuit, neither the RCD nor the 16A breaker tripped. Not until I provided the earth by touching the casing. The idiot's been sacked and is no longer 'qualified'. The circuit is now earthed (ie connected) but should I have an extra TT earth added or just get the cpc resistance checked?

 

[JAMJAM]

Are you sure that’s what was said?

The copper sheath of MICC is designed to be the cpc.


I’d suggest a second electrician to find the fault, and isolate the affected circuit until then.

So, what he actual wrote was "5.7 Many circuits used the sheath of cable as a circuit protective conductor. Zs readings exceed the maximum permitted Zs values." But if the earth is (was) missing, would that account for excessive Zs? I don't pretend to really understand Zs / Ze but wouldn't R2 (and therefore Zs) be more or less infinite if there was no path to earth?