Disconnection time formula?
- Thread starter skenk
- Start date
Indeed - you just beat me to it.
What I previously wrote represents what I believe virtually everyone seems to think about "which one is the error" - in other words, they believe that when ADS via an OPD is possible (i.e. in TN installations) one should not rely upon an RCD to provide that protection.
I would add that both of these regs appear unchanged in the 18th (411.4.4 having become 411.4.5).
Kind Regards, John
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It's still contradictory. Chapter 41 is all about protection from electric shock - so, when 415.1.2 (not 515.1.2) says that RCDs cannot be used as the sole method of protection, it presumably means protection against electric shock, and overcurrent protection will not provide that (unless it disconnects an L-E fault with the required disconnection times - which takes us back to not relying on the RCD).
Kind Regards, John
Not sure I see a difference.
On the last course I did, a BS7909 course taken by James Eade, we looked at long runs of cable (unit 3 - power distribution) and the resultant problematic fault currents. It was accepted that RCD's would take care of an earth fault, and volt-drop aside, the question was asked whether a 4mm cable at length of 250m would survive (why would this even be covered if the EFLI limits were absolute for L-N faults?). The fault current of 77A would take around 15 seconds to trip a C16 breaker (from looking up the graph). Using t = (k2*S2)/I2 (which as you have pointed out becomes inaccurate after about 5 seconds) the time given is 26 seconds. So even though it's not accurate at times longer than 5 seconds, it is still useful to see that the (inaccurate) overheat time is much longer than the disconnection time.
What value of k were you using? ... with k=115, I get roughly t=36s (not 26s - was that a typo?).
Fair enough. The thing I haven't pointed out explicitly is that when current duration gets longer, the equation you are using will effectively over-estimate the conductor temperature, since it is assuming that there has been no time for any heat to have been lost from the conductors. In your example, the true "overheat time" is probably a lot longer than the disconnection time than you thought.
Kind Regards, John
Perhaps because, as has been pointed out by John, such faults do not give rise to electric shock hazard.
Possibly it is just a mathematical exercise for you.
Then the cable will be alright (except obviously at the fault).
4mm² can carry 35A (possibly more) anyway.
77A equates to a Zn of 2.99Ω.
The Zs would not be compliant (1.37 for C16 before the derating factors).
So, even if you are relying on an RCD for fault current, it is obvious that other things might need looking at.
Is it?
Is it not just apparent without any calculations?
Whether it means electric shock or overcurrent. How can it mean overcurrent referring to RCDs?
Actually the rest of 411.4.4 just seem to be statements of fact rather than any regulation.
I'm not convinced that the problem you have feared is every going to arise.
I have not attempted to consider the general case, theoretically, but I have looked at the example you give. I have assumed a circuit protected by a C16 which has an L-N loop impedance of 0,35 at the DB, and have looked at lengths of 4mm cable from zero up to 500m. For each length of cable, I have plotted the C16 disconnection time (determined very approximately from the graph) and also a 'minimum estimate' of what you are calling the 'overheating time' -i.e. the critical time derived by using an adiabatic equation (using k=115), which, as I have said, is expected to overestimate the conductor temperature (hence underestimate the 'overheating time') for disconnection times above about 5 sec. ....
If I've done it right (and I admit that it has been done in haste, and not very thoroughly checked) it would appear that the (even the 'underestimate of) 'overheat time' is always well in excess of the C16 disconnect time.
KInd Regards, John
I didn't mean to imply that it does. My point was that 415.1.2 says that RCDs cannot be used as the sole means of protection (we assume against electric shock, since it's in Chapter 41). Skenk was trying to make a point of the fact that it refers to 411, which says that RCDs can be used for ADS provided that there is also overcurrent protection, and I was pointing out that such would not make the two regs any less contradictory, since 'overcurrent protection' would not provide protection against electric shock (unless it effectively became fault protection, by resulting in the required disconnection times for fault protection).
Kind Regards, John
Yes, sorry 36seconds
Yes I had already noticed that, so in this example we can see that the 4mm cable, aside from volt drop, will actually comply.
Yes for 4mm, but if you look at 2.5mm cable and 1.5mm cable in the otherwise same scenario things look a bit different! Both (depending on install method etc) have a CCC in excess of 16 amps.
Indeed - but, as my graph illustrates (if I've got in right), in the case of 4mm² cable and a C16 (and an L-N fault loop of 0.35 at the DB), it will always comply, no matter how long the cable. I rather suspect that the same would be true with most credible combinations of cable size and MCB type/rating.
Kind Regards, John
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