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R1 + RN Values - Why do they not seem to be important when testing and why s there not a max value so circuit breakers disconnect at quick as possible?

MC

My question is about short circuit faults and R1 + RN values and how they seem to not be very important when testing, especially on radial circuits.

I first came to look at this when looking into using RCD's for fault protection on TT circuits. After reading up on this I then wondered if there were any maximum values required for R1 + RN as we want the circuit to disconnect before any damage to the insulation of the cable occurs. I understand in a normal situation that the fault current will be high as the resistance in R1 + RN normally is very low and low resistance causes high current, which then causes instant tripping of the circuit breaker. (I know in this next part I'm making up the perfect storm but just go with it) What happens if a radial circuit has been installed with a high resistance joint in neutral conductor of say around 4.5 ohms at the first socket in the radial and then somewhere close to the last socket there is a line to neutral short. If I am correct (which I sure I could not be) with the high resistance in the neutral and the short further up it would cause around 48.42 amps of current to flow (230/4.75 = 48.42 - the extra 0.25 ohms if for the line conductor). If this was a 4mm radial it could take around 200s for a 32 amp type B circuit breaker to disconnect which seems a long time for a conductor to be overloaded.

I have been using the table on page 370 of BS7671 to look at disconnection times. 

I think my questions in short  are - Why do R1 + RN values not have a maximum value as surely in the event of a short circuit we want them to disconnect instantly just like we do when there is a short to earth (I understand we want it to trip quickly when there is a fault as someone could get a shock)?  How come we do not test for PFCC at circuits to make sure circuit breakers will trip quickly enough to prevent times longer than 5s disconnection times of circuit breakers when a short does occur? 

I am sure there is an answer and it is probably a really simple one which I have completely over looked but any help would be great.

(Please go easy on me as I am a first timer). 

Thanks for any help and time given to help me understand this.

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  • 0
    gkenyon said:
    Agreed ... but let's take the example of a simple fuse. What if a designer has assumed a minimum prospective fault current, permitting a lower value of S to be selected due to the fact that there might be high inrush currents (requiring a higher rating of OCPD) ... and there is no requirement for protection against overload current, but there is a requirement for protection against fault current ?

    As does happen in many situations and a good reason to check the design and measure R1+Rn and add to the line to neutral loop at the board to establish a rough idea of possible Ipf or otherwise directly measure KA.

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  • 0
    gkenyon said:
    Agreed ... but let's take the example of a simple fuse. What if a designer has assumed a minimum prospective fault current, permitting a lower value of S to be selected due to the fact that there might be high inrush currents (requiring a higher rating of OCPD) ... and there is no requirement for protection against overload current, but there is a requirement for protection against fault current ?

    As does happen in many situations and a good reason to check the design and measure R1+Rn and add to the line to neutral loop at the board to establish a rough idea of possible Ipf or otherwise directly measure KA.

Children
  • 0 in reply to lyledunn

     Having just said why we are not obliged to, I'd be in favour of testing Rn in more critical cases where loss of power would be more than just a mild inconvenience.

    (Thinking of things like doctors and dentists and those IT heavy locations where customers tell you they cannot turn it off when it comes to PIR time ;-) rather than the single socket off the landing lights that only ever provides power to the TV amp in the loft of no 42 subway street  )

    This could be a done as an LN volt drop test like a Zs test on existing live systems, By testing the live fall and neutral rise relative to the CPC when a significant load is applied, the R1 and Rn could be compared.

    Such a test could be an early warning of a number  of possible weaknesses that might be missed by normal process.

    Mike

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