Ring Main

[Spark123]

Hi,
I am looking into installing a ring main in plastic PVC conduit around a room. The room is approx 75m² floor surface area. I will be looking to install 4 double sockets. The wire will be 2.5mm² for both live conductors and CPC. The conductor lengths will be approx 50m end to end.
The earth fault loop impedance (measured) at the consumer unit in the room is 0.3ohms. Am I OK to install the above protected by a 32A type C RCBO? Square D do not appear to manufacture a type B in the KQ range.

Paul F.

 

[Lectrician]

4 sockets?? 75m/sq or less??

Go for a 4mm radial - in conduit this will prove easier, and cable wise pretty much the same cost, as you don't need a return leg.

To determined the Zs of the circuit, we would need to know the circuit length, not area covered.

 

[ban-all-sheds]

Paul - are you taking the name "spark" in vain?

You know Ze, you can look up R1+R2 for the circuit,find out your Zs, and look to see if it's going to give you 0.4s disconnect with a 32A Type C...

 

[Lectrician]

And if its and RCBO - thats all out the window anyway!

 

[Spark123]

Sorry, I'm not used to using type C breakers/RCBO in ring mains. Also I have a lot of 2.5mm² cable in red and black in stock which I would like to use before all out harmonization kicks in.
Just thought I would ask for a second opinion. If I have my calculations correct, a type C breaker requires 320A to disconnect in 0.4s. The 50m+50m (r1+r2)/4 = (R1+R2) 0.045ohms. Ze is 0.3ohms, hence Zs will be 0.345ohms. 230/0.345=666A which should be adequate. Please check my maths and give me a well deserved kick!!

 

[pdcelec]

And if its and RCBO - thats all out the window anyway!

Assuming it meets the criteria of table 41B1

 

[mapj1]

I estimate, 320A 230V , 0.71 ohms total absolute max...
you are so well in you don't need to check in any further detail for an MCB. By fag packet is fine, its just not near enough to the limit to reach for the calculator or temperature tables for finer checking.
For an RCBO, as mentioned, if you put RCD in method of protection, then actually Rloop can be nearer 1500ohms..
Course its OK.

 

[Spark123]

Thanks for your replies, they have put my mind at rest.

 

[pdcelec]

For an RCBO, as mentioned, if you put RCD in method of protection, then actually Rloop can be nearer 1500ohms..

Not if the earth loop does not comply with table 41B2

 

[mapj1]

Oh yes it can, but only because the RCD part of the specification provides the primary means of protection agains eletric shock, not the MCB part. Otherwise there would be no TT installations !

 

[pdcelec]

Table 41B2 clearly states the maximum ELI's for a rcbo to BS61009. TT systems could be protected by RCD's to BS 4293

 

[ban-all-sheds]

Oh yes it can, but only because the RCD part of the specification provides the primary means of protection agains eletric shock, not the MCB part. Otherwise there would be no TT installations !

But this isn't a TT installation, it's TN-something (probably TN-C-S given that low Ze), and it is preferable for RCBOs to operate in overcurrent mode when providing indirect shock protection because there is a risk that the electronics that make the RCD function work will not be supplied with enough voltage to work properly if the neutral is lost or if the voltage is collapsing because of a fault.

That's why tables 41B2 and 41C do not differentiate between MCBs and RCBOs - they have the same EFLI requirements - pdelec is quite correct.

 

[Spark123]

The system looks like TN-S, i.e. the earth from the supply company is clamped to the outside of the cable below the knockout, although I agree with bas that the readings are low enough to be TN-C-S.

 

[mapj1]

I agree with the reading of the words following to the letter of the regs, but not the spirit of the regs. If this were a TT installation, you are implying RCBOs could not be used.


Actually this is neither a dangerous nor an uncommon practice, in terms of discrimination it is probably better than one RCD spanning many MCBs.
In this case though its academic - as you say it is low enough Ze.

PS round here the undergound TNS has a lower L-E loop impedance than L-N so a low reading is not always guaranteeing a TNC/PME supply.
I guess there is more armour cross-section than reduced neutral core.

 

[Lectrician]

One town here has huge fault current levels to E, but very poor to N. Lovely lead sheathed cables burried in nice wet soil.