Shared neutral

[Carlosfandango]

I was at a job today replacing a db and wiring supplies for an ASHP. The house used to be owned by a company who let them out (there's another 6 houses) as holiday cottages. The supplies were from a central board in cupboard on the site. The houses have been sold off and part of the deal was that they would pay for individual dno supplies and meters in outside boxes at each house . A firm was employed to run supplies from these meter boxes to the existing db's in the house. They have run a 3 core Swa terminated in a tp 63amp mcb using brown for live, black for off peak live, grey for neutral and the armour as the cpc. To me this isn't correct but I'm looking for of you guys expertise. Inside the house it was terminated into a 4 pole isolator and split into seperate db's. I was removing the need for the off peak supply so I fitted a switch fuse and used the Swa using a core as L,N&cpc but the other properties are as described above.

 

[Knobhead]

Is that necessary for SP? I know it is for TP but if I've understood the OP correctly this is going to be a standard 100 SP service with a second switched feed from the meter.

In a word......... Yes

 

[Lucien Nunes]

What if, for arguments sake the domestic is pulling 50amps and the off peak pulling 50amps.

They can't, as the total of the two is flowing through a 63A OCPD.

 

[Carlosfandango]

It's a 63amp triple pole fed from the 100amp fuse.

 

[7029 dave]

It's a 63amp triple pole fed from the 100amp fuse.

It's 63amp at your switchfuse. That's it, 100amp is not going to flow. (ping)

 

[Carlosfandango]

It's 63amp at your switchfuse. That's it, 100amp is not going to flow. (ping)

No, 100amp fuse to 63amp TP mcb to isolator to board.
Or am I being really dumb here?

 

[7029 dave]

Okay, is the 63amp tp mcb at the front end of this sub main? if so that is your OCPD.

 

[davesparks]

No, 100amp fuse to 63amp TP mcb to isolator to board.
Or am I being really dumb here?

With the neutral connected through the mcb it will limit the current in the two lives to a total of 63A

 

[Carlosfandango]

With the neutral connected through the mcb it will limit the current in the two lives to a total of 63A

Yeah, took me a bit to click!!
So not ideal but not against any regs then?? Is that the general opinion?

 

[Engineer54]

But the armour of the SWA cable being used as a main earth, to cater as both CPC and for Main Bonding purposes does ''NOT'' comply!!!

 

[old_sparky_RE]

Here goes, don't often get to calc something up out here...
16mm Copper = 1.15 Ohms/km
3c PVC SWA = 3.2 Ohms/km (Armour CSA = 50mm) (I'm Assuming 70degree)
1.15 / 3.2 = 0.3594
16 x .3594 = 5.75
50 / 5.75 = 8.7mm (CPC = 8.7mm)

16mm passes as an EARTH WIRE (Table 54.7 BGB)
FAILS as a Bonding conductor (GN 8 - Table 5.1 and Table 54.8 BGB)

Like I said its been a while, out here put in the biggest CPC/EC you can get as the high Z reading internal at the Genny's is enough to fail ring circuits only 50 feet away (unless you use Type 'B' 3A MCB's all over!!!!)

Now to wait for incomming from E54 !! :89:

 

[Engineer54]

I don't know what you are expecting, i'm not even going to check your calculation, because i already know the armouring of a 3 core 16mm SWA cable will not comply as a bonding conductor, only as a CPC!!

As for your Ze, or should i say Ra, it wouldn't be a bad idea to get that high value down to sub 1 ohm. So what are the relevant internal impedances of these gen sets your using over there, because something just doesn't sound right to me?? lol!!

 

[old_sparky_RE]

Ra are anywhere around 0.5 - 150 Ohms depending on the site and the amount of copper banged in the earth! (reading gained from disconnected EC at the MET using Method 1)

The internal Zgen (impedance) of the generators before the distribution system (T/S - MDP - SDP & then CDP + circuit resistance) can be enough of a high Zs to fail a Type B 32A MCB, this morning one refurbed genny (FG Wilson) had a Z of 0.5 Ohms at the incomming side of MCCB (genny running, MCCB switched off incomming earths disconnected) then you add the resistance of the distribution network, joints/connections through the system etc. It all pushes the Z up.

But enough of my gripe it makes life a challenge.

 

[Guest55]

If the on peak and off peak supplies originate from the same single dno fuse then it cant really be considered a "shared neutral".
What should be of more important design consideration is achieving proper discrimination between all the various protection devices in series from origin to final circuit.

 

[Carlosfandango]

But the armour of the SWA cable being used as a main earth, to cater as both CPC and for Main Bonding purposes does ''NOT'' comply!!!

Please correct me if I'm interpreting this wrong but does a 16mm Swa (70 deg) not comply for main bonding purposes as it provides a copper equivelant of 22.1mm?

 

[Richard Burns]

Please correct me if I'm interpreting this wrong but does a 16mm SWA (70 deg) not comply for main bonding purposes as it provides a copper equivalent of 22.1mm?

For the use as a protective conductor you use the formula using the respective k values for the appropriate materials.
In this case 143 for the copper line conductor and 46 for the steel armour.
Then the steel csa for 16mm² 3 core SWA is 50 mm² so the copper equivalent = 46/143 *50 = 16.1 mm² this is suitable as a circuit protective conductor.

For use as a bonding conductor you compare the resistivity values for copper and steel which have a ratio of 1:8.5 (1.7*10-8Ωm vs ~14.3 *10-8Ωm)
Then the steel csa for 16mm² 3 core SWA is 50 mm² so the copper equivalent = 50/8.5 = 5.88 mm²
which is not suitable for the 16mm² minimum size for a bonding conductor (from table 54.7)