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.

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.

Thanks very much, I will those a watch and hopefully this will help my understanding much more. 

I have watched many of his videos and they are extremely good.