Shared Neutral (not borrowed) 3 phase

[Richard Tawn]

Nothing to do with the containment. Multiple circuits can be run through the same conduit/trunking etc...

as they can and are through the same dis board ! its actually about isolation from the outside world, I read somewhere but cant find it at the moment regarding depth of cables in walls or conduit type and special locations which seemed to justify the install.

 

[Tony S]

Phase angles....

In = ±√((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

 

[steptoe]

Balanced or unbalanced it makes no odds.

Think of it this way, you have a 50A submain to a DB, and that submain is full of single phase circuits. Do you you fit a neutral on the submain that is 3x the phase conductor?

Balanced or unbalanced just means it either needs or doesn't need a neutral. If it is unbalanced it does, but the phase angles means that each phase is only live when the other 2 aren't (over simplified), so the neutral is only used for any one phase at a given time.

Its so fkn simple when put like that,

Its basic stuff,

Thinking about it as being Star, only one leg is relevant to centre point at any one time,

as the 120degrees waveform means one wave + , one - , and one baseline, (I know that's wrong, but that's how I've just pictured it) makes complete sense now.

 

[binky]

In = ±√((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

 wish i hadn't asked now

I just stuck 16A in for Ia Ib and Ic and got 0, which would suggest no current in the neutral if all 3 phases are being used at the same time?

Tried 20 16 and 12 for the above and get 6.9A which would suggest even an unbalanced load would pt less down neutral than a single phase? Very interesting if I've done my maths right???

 

[Andy™]

This is why 3.5 core cables have gone away, even on the DNO side.

im surprised the DNO are not using aluminium coated string for the neutral

 

[Tony S]

 wish i hadn't asked now

I just stuck 16A in for Ia Ib and Ic and got 0, which would suggest no current in the neutral if all 3 phases are being used at the same time?

Tried 20 16 and 12 for the above and get 6.9A which would suggest even an unbalanced load would pt less down neutral than a single phase? Very interesting if I've done my maths right???

 

[binky]

do I get a Blue Peter badge for being a good boy? :slap

 

[Rob.]

im surprised the DNO are not using aluminium coated string for the neutral

DNO's are now resorting to fitting K type transformers with the star point connection being rated at 200% of the Tx's output. All due to the increasing amount of harmonics on networks. 

Harmonics are a mine field when you delve into them. All to do with skin effect. 

 

[Tony S]

im surprised the DNO are not using aluminium coated string for the neutral

The lecy boards came unstuck with aluminium wavecon cable. Damage the outer serving and water can get in, aluminium + current + water = failed neutrals and claims for compensation.

 

[Tony S]

do I get a Blue Peter badge for being a good boy? :slap

PM your E-Mail address and I'll send you a copy of the spreadsheet.

 

[Richard Tawn]

I don't mean to detract from the sparkling discussion over load imbalance,

In fact this installation ended up this way to get 3 machines off of one phase in the first place !

Should there be a vote as to the legitimacy regarding current(pun) regs ?

seems mixed opinion has been observed !

 

[Rob.]

Unfortunately this will be a grey area in he regs I imagine. I see it as being one circuit, as it is supplied from one OCPD.

Not fully versed anymore on BS7671 as my line of work has very little to do with it. Hence me hanging around on forums like this to keep my hand in on changes etc...

From a theoretical point of view there is no issue. 

 

[Badger]

It is an interesting question ...one which I've never considered before , however   lets say you are asked to install  three separate commando sockets  ...each on a different phase      surely  you would  feed them from  3  X  SP  MCBs  , each socket having it's own neutral .        You wouldn't consider using a TP  MCB...............or would you? 

Yes I would install three commandos on three single phase mcbs and separate neutrals for the circuits I would say to use a three phase supply to feed three sockets single phase down one neutral on a ton much is well g as the load would not be balanced between the phases and the neutral current 

 

[Evans Electric]

Another point to consider is ........we assume  we have 3 X  16A   x 240V     Commando type sockets.      So  the design criteria is for 3  X  16A  Commandos  . 

Anything could be plugged in ....... so the load is variable..........assume  a 16A load plugged into each socket .  

To protect the cables and the sockets from overload  do you fit a 16A  MCB    or a 50A  and wire the circuit in 10mm     or do you wire it correctly  with individual breakers  and L & N to each.    Or alter the design criteria  and wire  all three as a  single phase  radial  if you know the total load will be no greater than 16A .   

 

[Andy™]

they must be protected by a OCPD that match, or is lower than, the rating of the socket. except 16a, which is in the OSG that it can be on a 20a circuit

 

[Tony S]

Using Binky's figures we get this:

 

 

[Sidewinder]

It is a single 3 phase circuit supplying 3 single phase loads.

Not a good design, but, it could be called compliant.

I would say however, that it does not comply with regulations:

132.6

132.10

134.1

314.1

314.2

314.3

529.1

361.1

There are probably others.

 

[Marvo]

I'm glad there's finally sdome actual regs given for consideration, thanks SW.     ***edit*** where's the scoob button gone by the way?

My copy of the UK regs is several ammendments behind current but here's another one for consideration. RCD protection built into the socket outlet would be only supplimentary protection and wouldn't fulfil the requirement of disconnection of all line conductors.....

531.2.1
RCDs
An RCD shall be capable of disconnecting all the line conductors of the circuit at substantially the same time.

 

[Sidewinder]

RCD protection is only ever supplementary protection.

I thought the install was industrial, so it would not require an RCD on anything other than the socket outlet.

This is where the semantics of what is a circuit and what is not come in, and what does require disconnection and what does not.

 

[Andy™]

if there was an RCD on the circuit then it would be at the board and would then work on all line conductors. if its fitted as part of the socket then its not part of the circuits protection so that reg wouldnt apply