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?

 

[brianmoooore]

I sometimes think I exist in two parallel universes. One where electricians such as the one referred to in this post operate, and one like on here, where we discuss things like derating factors for cables passing through a few inches of insulation or the differences between an EIC and an EICR.

 

[James]

I agree,
There have been a few occasions where a customers of mine have called me in a somewhat annoyed / distressed state to tell me another "electrician" has come in to do some work and condemned or stated my previous work was not up to scratch etc.

because I keep a good ongoing relationship with over 90% of my customers, it normally goes one of 2 ways.

I suggest a meeting in person at the customers premises with me, the other "electrician" and the customer.
If there has been any suggestion that it is unsafe, I ask that a company director be there if possible and minutes are taken.

either
1
the other spark wont commit to a meeting or fails to turn up.
I walk the customer through whatever has been alleged to be improper, point out the way it has been installed and any relevant standards.
show them the relevant regs in the blue book and the other spark never gets asked to come back.

2
the other guy turns up, we walk the installation, he points out what is wrong.
i take a couple of notes, we go to a meeting room and I ask him to justify his opinion by pointing out the relevant regulations in the book on the table.
he cant find them because he is working to a set of standards such as, well that's the way i was taught and it must be done like that. or the reg is not relevant in that particular case etc.

somewhat worrying is a couple of times these other sparks have been registered with NIC or other trade scams, for some people it doesn't make them better of more professional, just gives them a nice sticker and boosts there ego to the point where they think they are better than me.

Note:
I have met sparks that are better than me, but none of them have started procedures by telling me I am wrong.

 

[Rockingit]

I agree,
There have been a few occasions where a customers of mine have called me in a somewhat annoyed / distressed state to tell me another "electrician" has come in to do some work and condemned or stated my previous work was not up to scratch etc.

because I keep a good ongoing relationship with over 90% of my customers, it normally goes one of 2 ways.

I suggest a meeting in person at the customers premises with me, the other "electrician" and the customer.
If there has been any suggestion that it is unsafe, I ask that a company director be there if possible and minutes are taken.

either
1
the other spark wont commit to a meeting or fails to turn up.
I walk the customer through whatever has been alleged to be improper, point out the way it has been installed and any relevant standards.
show them the relevant regs in the blue book and the other spark never gets asked to come back.

2
the other guy turns up, we walk the installation, he points out what is wrong.
i take a couple of notes, we go to a meeting room and I ask him to justify his opinion by pointing out the relevant regulations in the book on the table.
he cant find them because he is working to a set of standards such as, well that's the way i was taught and it must be done like that. or the reg is not relevant in that particular case etc.

somewhat worrying is a couple of times these other sparks have been registered with NIC or other trade scams, for some people it doesn't make them better of more professional, just gives them a nice sticker and boosts there ego to the point where they think they are better than me.

Note:
I have met sparks that are better than me, but none of them have started procedures by telling me I am wrong.

I always (try - hope!) start from a position of asking to understand why someone has approached something from a certain angle and then discuss / educate. It's near impossible to have an all-encompassing knowledge of everything electrical these days and anyone who claims to have gets an instant red flag from me, but for example I know that I know very little about Ch 8 stuff, so I'm not going to make a judgment call on anything around that without having a solid basis first. It's also very annoying that a lot of 'sausage machine' electricians will look at something and flag it up as wrong, simply because it's not standard to their way of thinking because they don't possess a full grasp of knowledge.

 

[mainline]

show them the relevant regs in the blue book and the other spark never gets asked to come back.

The other electrician could have pointed to the book and said that's outdated for a start

 

[James]

The other electrician could have pointed to the book and said that's outdated for a start

If you think that's outdated, you have not seen my hairstyle!!