MCB selection and continuity

[Alessandro]

I hope this is the correct forum. I am not electrically qualified yet but I am not a DIYer either.

Question about selection of MCB:
For the purpose of ascertaining the minimum acceptable value of I_cn I need the highest value of I_pf, or: the lowest value of Z_e. This means that if the measured value in a TN-S system is, say, 0.50 ohms, I should preferably use this value when calculating I_pf for the purpose of selecting a device with an acceptable value of I_cn.

On the other hand, when I am considering disconnection times I would logically opt for the lowest value of I_pf, or: the highest value of Z_e. In the TN-S system above, I should preferably use the maximum acceptable value of 0.80 ohms (obtained by enquiry). By doing so, I can guarantee that the installation will remain safe even in the worst case scenario if Z_e changes over the years and reaches the maximum allowed.

Is my thinking correct?

Question about continuity:
On-Site Guide page 97, Figure 10.3.1(i) shows a measurement of R1 + R2 performed at the switch, but I don't understand why the switch line's resistance is ignored. I can easily imagine how, in a real installation, the length of the line conductor between the switch and the luminaire can be considerable. Also, the book "Installing Electrotechnical Systems & Equipment" states, at page 101, that "on a lighting circuit the value of R1 should include the switch wire at the luminaires". Then again one of my tutors said that it wouldn't make a real difference, it doesn't matter.

What is best practice? Is it not reasonably practicable to just take the time to add the resistance of the switch line to R1? What do you normally do?

Thanks.

 

[Evans Electric]

When did it all become so complicated ? :C   

You ask what I would normaly do ..............  if its a 2 ---4 bed housing job , couple of circuits wired in 1.00mm  with 6A MCBs !!

Commercial , work out load , choose cable size and MCB to suit  .

Don't worry about the resistance of the switch wire on house bashing , no one cares , certainly not me ,  just stick a 6A in . 

 

[Alessandro]

When did it all become so complicated ? :C   

You ask what I would normaly do ..............  if its a 2 ---4 bed housing job , couple of circuits wired in 1.00mm  with 6A MCBs !!

Commercial , work out load , choose cable size and MCB to suit  .

Don't worry about the resistance of the switch wire on house bashing , no one cares , certainly not me ,  just stick a 6A in . 

Thanks for your reply Evans.

I was thinking more in theoretical terms, I am curious to see if my thinking about the selection of MCB is generally acceptable. I was considering both: 1) disconnection time, and 2) breaking capacity.

I am asking because I've always seen one Z_e and never the use of two separate Z_e, i.e. the actual Z_e and the maximum allowable, where the actual one is usually less than the maximum Z_e.

I was thinking that maybe designers or inspectors carrying out the testing of an installation do similar considerations.

About the switch line, I wasn't talking about selection of MCB but the testing value to record on the Schedule of Test Results.

 

[Lusk]

Re: Continuity.  Testing the last light on a circuit, I'd be testing on the switched Line at the lamp, because that's the furthest point.

 

[Evans Electric]

I agree with Lusk ,  to test the last light on the circuit and you've covered the bases,  you know the earth wire is continuous to the end also.  

And measure the Zs there too . I can see what the tutor means ,  a reading at the switch is hardly any different to the light , assuming a short switchwire ....only difference is , you havn't proved the earth connection at the light ......but then ...you havn't proved it at all the others either unless you tested at every single fitting , and we're no expected to do that AFAIK . 

Much like testing socket circuit ,  write down a load of numbers at the board to satisfy the certificate but do you know that every socket has the two earths actually connected to the faceplate , do you know there are no damaged sockets , is polarity correct at every one , are there any open connectors hiding under the floorboards  , or a box with bare live wires twisted together, sticking out, but no front , as we've found a few times.

 

[Alessandro]

Re: Continuity.  Testing the last light on a circuit, I'd be testing on the switched Line at the lamp, because that's the furthest point.

I was thinking that I should still test at the switch to cover the whole length of R2, and obtain R1+R2 at the switch. Then measure the switch line with a long lead and add the value to R1+R2. I have the feeling though that nobody would ever do such a thing.

 

[binky]

the switch line will give the highest value of R1+R2, unless the light is switch fed in which case you should test at the light. I nearly always test at the switch, it's a lot easier than disturbing some light fittings.

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