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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

    Let's not forget the inspection phase of both initial verification and periodic inspection and testing.

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  • 0

    Let's not forget the inspection phase of both initial verification and periodic inspection and testing.

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  • 0 in reply to Chris Pearson

    It is also worth remembering that cables are not damaged by short temperature rises above 70C conductor temperature. Whilst this is often forgotten we can look at why fairly easily. You may look at the adiabatic temperature but this says that at this point zero heat is transferred to the insulation (that is what adiabatic means) and that the heat capacity of the conductor is much less than the insulation (a curiosity of copper and plastics). The damage to the plastic comes from melting (150C or so) burning (500C) and plasticiser loss (long period at high temperature, or very long at room temperature). To melt and be able to be displaced, the bulk plastic must melt, not a film close to the conductor.

    All these may be classed as "damage" but the effect is simply that the practical life of the cable is reduced, and it takes a very severe "overload" for a long time to melt a cable. It is very unusual to find a fixed installation cable that has melted, particularly in domestic installations, whereas many loose or poor connections are found, but rarely lead to actual fires. your 4.5 Ohm example is extreme, and as Mike suggests would melt quite quickly if the circuit current is significant.

    One may measure the PSSC at any point in an installation, the test you are suggesting on the neutral conductor, which would lead to an unexpectedly low figure, and which test is available on multifunction instruments.

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