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?

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.

  • Welcome.

    Well the first thing to realise is  that the times for ADS for an earth fault should be far faster than it needs to be to protect the cables - it is set to be a fraction of a human heartbeat time, the exact time being a function of the exposed voltage, as while a large L-E fault current is flowing, the "earthed" case of the faulty item will not be at the same "earth" voltage as the user's feet or whatever - so there is an electrocution time to think of, as there may be someone hanging onto it at the time,

    In contrast, an L-N fault that is not a dead short is more like an overload condition, and the time constant to consider is the cable getting warmer than it should several seconds to minutes. Also you are looking at loss of equipment or cables, rather than loss of life. As both the fuse curves and the cable damage mechanisms are thermal, they sort of track each other - a modest overload will blow the fuse slowly, but then the cable will not overheat very quickly either. In general as fuses are in sand filled ceramic cylinders, which is about as badly cooled as if can be, the cable is if anything better cooled than the fuse wire...

    MCBs once you get off the 'instant trip' part of the curve have a similar time/current relationship to a traditional fuse of a the same rating.

    Now you are quite right that neutral resistance is not tested, and perhaps it should be, as cooked neutral connections are certainly not unknown. Sadly ther test  limits would be hard to set to be certain of success, as the sort of resistance you get localized at one poor joint can be simultaneously high enough to give overheating problems, and yet low enough to be masked by the resistance of a reasonable length of cable,  - the cable shape gives a surface area to sweat off a few tens of watts per metre of length but while  50 -100 watts would go un-noticed or a slight warm up on a 10m run of SWA , when concentrated in a small screw terminal the same energy loss, will quickly get it to the temperature where it changes colour and oxidises.

    In that sense a quick tweak  of all current carrying terminals is a worthwhile last step before commissioning, instrument tests passed or not !

    In your rather extreme example of the 4.5 ohms, consider how hot that resistance will get if it is all in one space, and then also that the voltage drop will probably be noticed as lamps run dim and motors struggle to start...

    Mike.

  • Thanks very much for your reply Mike,

    Everything you have said makes perfect sense, and maybe I am slightly underestimating what a cable can take before it becomes damaged by being overloaded or having a short circuit. I am so use to looking at meeting Zs values which are always so low for obvious reasons.

    I know there is an equation in BS7671 - 434.5.2 - which helps you work out what size CSA a cable needs to be to take a certain fault current for a certain amount of time and I have had a play around with this to see what sort of fault current they can take for a certain amount of time. 

    I just thought that for a simple test of R1+RN to check the neutral is fine and setting values close to or slightly higher then the Zs values given in BS7671 so the cables never end up getting overloaded for a period of time would have been something that would been done as good practice. As the the neutral CSA is the same CSA as the line conductor I thought a value worked out by the IET would be easy to meet and would guarantee disconnection without overload ever being a risk, as we meet these figures with a smaller CSA of CPC in certain situations. We also already end to end tests on rings which proves continuity and allows us to check all three conductors.

    I am more then probably looking far too deep into this,

    Thanks again for the reply and if you have anymore thoughts or views on this I am all ears,

    Michael

  • Hmm, the neutral is not always the same impedance as the line - in some 3 phase systems it may be reduced or indeed totally absent for loads that are delta wired, but I would agree that for most single phase situations it would be reasonable to expect  it to be within a few % of the R1 value.

    There is also a desire to only insist on testing things that actually add value or safety in return for the effort of testing before energizing,  - and a high resistance neutral is likely to become  pretty obvious soon after energizing, while not actually that dangerous, I agree that something like an automated  plug in tester could do an L-N loop test as easily as an L-E one, and probably at the same time.  there is the question of where you set the pass/fail limit - with heart fibrillation it is a bit of a fudge in terms of assumptions about skin resistance and contact area, but at least the curren time curve is well defined by human physiology/ However  for L-N you need to make assumptions about cooling time constants and how localized the resistance is as well.

    BTW a Zs or R1-R2 test does not find slightly poor live connections either - just verifies the fuse  would blow if there was a bolted fault. In the UK and many other places an R2 only test with a wander lead can be used so show that a system is safe, so the R1 part is not always tested either .

    Now there will be occasions that  it is a very good  idea to go beyond the minimum set of tests, but there is no requirement to do so each time, or to record the readings.

    Cable heat up times are a funny thing - if you don't already know it, you may find the John Ward Videos are quite educational - over the last 6 otr 7 years he has done a few hundred - there are a couple that suit this topic..

    Overloading some twin and earth - a lot of overload !!

    coiled cables are bad for long term loads...

    Mike.

  • 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? 

    Because the limiting value is not dependent on I, but rather I2t. It's entirely possible a fault at the end nearest the OCPD performs "worse" than the "far end" ... but also a very long disconnection time might also be acceptable depending on the circuit arrangement.

  • Another advantage of testing R1  and Rn is to confirm polarity and Ud. One of the biggest landlords in the country used to insist that both R1 and Rn were measured and recorded on their bespoke documentation before dropping the requirement in favour of the templates in 7671. 

  • Because the limiting value is not dependent on I, but rather I2t.

    At the risk of being a bit of a nit picker, but to give the full picture, that is only really true for very short duration events, where the cable is run at tens to hundreds times it's steady state current rating. The reasoning is that I2t *R is energy, not power, and is only really correct if there the heat arrives so fast  is no time for the heat to start to soak into the surroundings.
    A stick of dynamite  going off , eating a mars bar or  large-ish church candle burning are all  similar total energy, but the rate of delivery alters what happens next in terms of heating and airflow. (!)

    Realise that I2t is the same as joules per ohm

    This is sort of saying you may ignore the open plughole in the bath and still fill it to overflowing if you fill  it fast enough...
    Clearly a low enough current is OK to persist for ever, that is the cable rating, and between the two extremes of the steady state (heat arriving matches heat escaping, so no more temperature rise after equilibrium is reached) and the adiabatic (so fast no heat can escape) there is a rather soggy region of a few times the nominal current rating,  where cable damage is possible but requires an overload to last for many minutes, for things to get  really hot enough to do any damage.

    Actually a modest overload can sometimes last for hours for heavy things like substation transformers without injury,  and the sums required to be sure of exactly how long is safe  require information that is not usually available.

    It is this sort  of understanding that allows the DNOs to play with the after diversity maximum demand and use thinner cables than BS7671 might suggest, without anything bad happening.

    Mike.

  • 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 ?


    This doesn't mean I'm not in favour of testing for Rn, just that setting a maximum value in the way you discussed is perhaps not such a good idea in all cases?

  • 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.

  •  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

  • Just to add that testing in effect serves two different purposes - it verifies the design (e.g. the cable isn't too long) and it verifies the installation (wires connected to the right terminals etc.) If R1+R2 is OK, then from a design perspective the cable length is OK and (excepting vary rate instances of reduced c.s.a. N) then the design for the N side must be OK too. It's nigh on impossible to create a resistance of several Ohms by an installation error - either it's connected or it isn't. A loose connection either rattles between connected and not connected or poses a very small additional resistance (usually very small fraction of an Ohm). That small fraction of an Ohm, on a circuit carrying heavy currents for long durations may well overheat - eventually oxidising metal and charring insulation which can lead to lead to higher resistances - but that really only gives you a means to detect the problem after the event, which isn't a great help for testing that's usually done before the installation/changes are put into service.

        - Andy.