3 sp circuits off a tp isolator

[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...

 

[Silly Sausage]

Draw it out, two ways of doing it, it's an unbalanced star!!!
OP best go back and make sure all his Neutral connections are secure, floating Star point is another box of frogs!

 

[johnboy6083]

I'm aware that the DNO abide by the ESQCR regs (or should do) and not BS7671.
however I don't see how a standard cutout with three BS88 fuses is any different in its theory to what we have in the OP.
i can see where I think you are coming from.
are you along the lines of the neutral current from say the far sockets will sum with the neutral currents of the middle sockets and then the near sockets.
Current always sums in series, (the way the neutrals are wired) but only if they are in phase. I understand that they will not always be 120 degrees exactly out of phase because of load characteristics, but the regs have a built in safety margin to allow for the unknowns. It's not always wise to rely on them, but we are talking about assessing an existing install, not installing from new.

 

[netblindpaul]

At last Archy!
Also there are other issues.
I was not going to say this, but, I did suggest this when I said that the currents could not balance without affecting the other 1ph circuits.
I can't believe how difficult it can be to get people to understand?
Also from the star point to the protective device there could be other issues...
Are there not?...

 

[D Skelton]

Paul mate, no one is arguing the point that the circuit is badly designed! We all agree, just not on the neutral overload point.

 

[netblindpaul]

I still maintain that the circuit can be overloaded on the N conductor, as no-one has proven otherwise with 3 off socket outlet circuits as the point of use.
I'm not talking about PF, here either, we are at least talking harmonics & distorted N currents, which you cannot ignore.
Whilst the simplification I posted earlier I agree may have been a little "too" simple at 60A due to the 3 x 20A phase currents, no one has proven as yet that this cannot happen, & I don't believe that anyone can, when the final loads are unknown!