3 sp circuits off a tp isolator

[Guest55]

is it over ?
are we done yet ?
any more calcs to throw in ?

phew!
;-D

 

[lewlec]

Ok I may have missed some thing here, say you have a tp&n supply connected to a switch fuse with say a 4 core swa feeding a tp&n db and all loads on that db are single phase, would that be incorrect? Would you oversize your neutral?

 

[netblindpaul]

No it's not a tp+n supply & it's not to a db, it 3 off final circuits fed from the same 3p mcb, with a common N to all circuits, and the cct's are socket outlet circuits.

 

[Guest55]

*facepalm*

 

[D Skelton]

What local distribution network, all of these things can be found on a small domestic install these days, I'm not going to back down and no one has yet given proof that the N cannot be overloaded in the scenario given by the OP.
So until this is done I stand by my arguments, I DESIGN, I DO NOT GUESS values, so the N can be overloaded, & if I was being paid for my consultancy then I would be bothered to calculate things, as I am not, it;s down to you lot to prove otherwise, which, as yet, you have failed to do.

I never said never, I just said given the OP, it was highly unlikely.

On another note, PF is not cumulative in the sense that if a system is made up of loads at a PF of .9, the overall PF is still .9. The average domestic install is expected to average out at roughly .9 - .95, three of those domestic installs across three phases and the slightly negative effects caused by the non-linear loads within each dwelling become meaningless. The exact reason for this can be found within our discussion regarding our three 20A socket circuits.

We all suffer from a lack of understanding at times (myself included! ) you have since seen sense but still seem to be fixated on being theoretically correct. It seems to me that you're just not looking at the bigger picture.

 

[netblindpaul]

I am not talking about PF, I am mainly talking about distortion of the supply, I have never said anything about PF.
Supply distortion these days has a much bigger affect on installs than PF as this is normally taken care of, but, distortion IS cumulative.

 

[Silly Sausage]

Fantastic thread guys.............well done to all.

Great stuff

Hahaha...must have a proper read tomorrow!
The original Q just appears to be an unbalanced Star with a Neutral, what's the problem?!?! :sifone::smilielol5:

 

[D Skelton]

Is it? On a large scale you would not be wrong to expect that distortion would equal itself out across the phases.

 

[44ALLAN]

Hahaha...must have a proper read tomorrow!
The original Q just appears to be an unbalanced Star with a Neutral, what's the problem?!?! :sifone::smilielol5:

If you sort it out let me know mate.................I've agreed with at least 10 differing versions up till now lol

 

[netblindpaul]

TO be honest, whist there seems to be a lack of belief from posters here with regard to what I am saying, there seems to be a significant lack of understanding of what I am trying to put across as well, I don;t know how else to describe things.

Thing is see, I'm not a spark, I operate mostly as an Engineer, & consultant thus I am paid for my opinions, & I don't start throwing arguments about unless I can back them up, because that is how I make my living, and I'm doing OK, thank you and have been for a few years now, based solely on my own personal opinion, which is hen backed up by fact once the money starts flowing, why should I do otherwise when it is my living.
I'm not proposing anything as stupid as payment here, I'm here for the fun of it. but, I will never post anything that I cannot back up, and no one has managed to prove with science that my arguments are incorrect yet, once they do I will give an apology, until then I stand by them.

In the OP, the N conductor CAN be overloaded.
The circuit is not acceptable.
If anyone can PROVE otherwise in all scenarios then I will bow down, until then I await proof.

 

[johnboy6083]

I actually enjoy decent threads like this, where we all have different takes, but with no ill manners to be seen.

 

[netblindpaul]

Well I thought I got pretty ****ty at one point so I WILL apologise for that, but, I stand by my engineering arguments even if my attitude stunk!
If it did, sorry.

 

[D Skelton]

In the OP, the N conductor CAN be overloaded.
The circuit is not acceptable.
If anyone can PROVE otherwise in all scenarios then I will bow down, until then I await proof.

And pigs CAN fly, albeit thrown out of a hot air baloon strapped to a hang glider!

You are theroetically correct, just not realistic.

 

[johnboy6083]

I don't doubt your conviction Paul, but in my mind, I can't see the theoretical difference between the OPs scenario, and the scenario of a feeder feeding houses on different phases in the street.

 

[Silly Sausage]

I think it depends on how it's wired (to state the obvious!)
Trying some sketches as we speak.
Can't see the Neutral carrying 60A though...

 

[44ALLAN]

Set the circuit up and clamp it mate then tell us what the neutral was taking...........I'll trust you.

3 single phase circuits feeding 3 heaters(all on different phases.....or should that be lines !) through same mcb with one neutral for all 3 circuits.

 

[netblindpaul]

Ahh, now johnny, there you have a good point, they don't comply with BS7671 as a start! #
They don't have to!
Also, the scenario is again different, because they will approximate phase balance across their feeder, and, the N conductor at each house will be protected load end by the fuse in the head, & the N conductor in the street will be either earthed "PME" or protected supply end by a suitable OCPD, which will prevent unbalanced currents overloading the N.
In the OP this is not the situation.

 

[netblindpaul]

OK maybe not 60A in all scenario's but it could take that depending on the loads...

Also no one has come back with a calculation that can prove that the N cannot be overloaded regardless of the connected loads yet.

 

[johnboy6083]

The Sqrt(((A^2)+(B^2)+(C^2))-((AB)+(AC)+(BC))) should prove that. I'm fully aware that not all loads are linear, and socket outlet circuits will always be prone to this, whatever configuration they are installed in.

You are absolutely correct in saying that we cannot guarantee the nature of the load, but this is true for most installs nowadays.


This is an interesting debate though.

 

[netblindpaul]

Sorry johnny, not convinced!

Also the DNO argument is not along the same lines.

You have to look at the origin over current devices, and the circuit conductor sizes in relation to the prospective loads...